BASH(1)

📦

version 5.3.9 · 10 versions

🎀 NAME 🎀

hiii bestie 💖 welcome to bash(1) — that's section 1 of the General Commands Manual, the one you open when you want the whole lore drop.

bash — GNU Bourne-Again SHell 👑

✨ SYNOPSIS ✨

bash [options] [command_string | file]

like, that's the whole vibe in one line. options, then either a command string or a file. slay.

💖 COPYRIGHT 💖

(iconic behavior tbh, it's been serving since 1989 🫶)

🦋 DESCRIPTION 🦋

okay so. Bash is a command language interpreter that executes commands read from the standard input, from a string, or from a file. That's it, that's the job. 💅

It's a reimplementation and extension of the Bourne shell — aka the historical Unix command language interpreter, the OG, the blueprint. And bash also swiped some genuinely useful features from the Korn and C shells (ksh and csh), so it's like a collab album. 🌈

POSIX is the name for a family of computing standards based on Unix. Bash is intended to be a conformant implementation of the Shell and Utilities portion of the IEEE POSIX specification (IEEE Standard 1003.1). Bash POSIX mode (from here on out we're calling it posix mode) changes the shell's behavior where its default operation differs from the standard, so it strictly conforms to the standard. No halfsies. ✨ See SEE ALSO below for a reference to a document that details how posix mode affects bash's behavior. And yes, bash can be configured to be POSIX-conformant by default — some builds just wake up like that. 💅

👑 OPTIONS 👑

okay so first thing: all of the single-character shell options documented in the description of the set builtin command — including -o — can be used as options when the shell is invoked. Not some of them. All of them. 😤

In addition, bash interprets the following options when it is invoked:

-c — if the -c option is present, then commands are read from the first non-option argument command_string. And if there are arguments after the command_string? The first argument gets assigned to $0, and any remaining arguments get assigned to the positional parameters. The assignment to $0 sets the name of the shell, which is used in warning and error messages. so it's literally how bash introduces itself 😭
-i — if the -i option is present, the shell is interactive. bestie mode: on.
-l — make bash act as if it had been invoked as a login shell (see INVOCATION below).
-r — if the -r option is present, the shell becomes restricted (see RESTRICTED SHELL below). 🚧
-s — if the -s option is present, or if no arguments remain after option processing, the shell reads commands from the standard input. This option allows the positional parameters to be set when invoking an interactive shell or when reading input through a pipe.
-D — print a list of all double-quoted strings preceded by $ on the standard output. These are the strings that are subject to language translation when the current locale is not C or POSIX. This implies the -n option; no commands will be executed. so it's a little peek behind the curtain 👀
[-+]O [shopt_option] — shopt_option is one of the shell options accepted by the shopt builtin (see SHELL BUILTIN COMMANDS below). If shopt_option is present, -O sets the value of that option; +O unsets it. If shopt_option is not supplied, bash prints the names and values of the shell options accepted by shopt on the standard output. And if the invocation option is +O, the output is displayed in a format that may be reused as input. 👑
-- — a -- signals the end of options and disables further option processing. Any arguments after the -- are treated as a shell script filename (see below) and arguments passed to that script. An argument of - is equivalent to --.

Bash also interprets a number of multi-character options. 🫶 heads up: these options must appear on the command line before the single-character options to be recognized. order matters, bestie, it's giving strict.

--debugger — arrange for the debugger profile to be executed before the shell starts. Turns on extended debugging mode (see the description of the extdebug option to the shopt builtin below).
--dump-po-strings — equivalent to -D, but the output is in the GNU gettext “po” (portable object) file format.
--dump-strings — equivalent to -D.
--help — display a usage message on standard output and exit successfully. 🥺 (it's helping!)
--init-file file, --rcfile file — execute commands from file instead of the standard personal initialization file ~/.bashrc if the shell is interactive (see INVOCATION below).
--login — equivalent to -l.
--noediting — do not use the GNU readline library to read command lines when the shell is interactive.
--noprofile — do not read either the system-wide startup file /etc/profile or any of the personal initialization files ~/.bash_profile, ~/.bash_login, or ~/.profile. By default, bash reads these files when it is invoked as a login shell (see INVOCATION below).
--norc — do not read and execute the personal initialization file ~/.bashrc if the shell is interactive. This option is on by default if the shell is invoked as sh.
--posix — enable posix mode; change the behavior of bash where the default operation differs from the POSIX standard to match the standard. ✨
--restricted — the shell becomes restricted (see RESTRICTED SHELL below).
--rpm-requires — produce the list of files that are required for the shell script to run. This implies -n and is subject to the same limitations as compile time error checking checking; Command substitutions, Conditional expressions and eval builtin are not parsed so some dependencies may be missed. 👀
--verbose — equivalent to -v.
--version — show version information for this instance of bash on the standard output and exit successfully. 💅

🌸 ARGUMENTS 🌸

okay so here's what happens with the leftovers:

If arguments remain after option processing, and neither the -c nor the -s option has been supplied, the first argument is treated as the name of a file containing shell commands — a shell script. 📜

When bash is invoked in this fashion, $0 is set to the name of the file, and the positional parameters are set to the remaining arguments. Bash reads and executes commands from this file, then exits.

Bash's exit status is the exit status of the last command executed in the script. If no commands are executed, the exit status is 0. 🌟

And the search order? Bash first attempts to open the file in the current directory, and, if no file is found, searches the directories in PATH for the script. no cap. 🧚

💫 INVOCATION 💫

ok so first things first, bestie, let's talk about how you even start bash, because it genuinely changes everything about what it does next. 🦋

A login shell is one whose first character of argument zero is a -, or one started with the --login option.

An interactive shell is one started without non-option arguments (unless -s is specified) and without the -c option, and whose standard input and standard error are both connected to terminals (as determined by isatty(3)), or one started with the -i option. Bash sets PS1 and $- includes i if the shell is interactive, so a shell script or a startup file can test this state. 👑

The following paragraphs describe how bash executes its startup files. If any of the files exist but cannot be read, bash reports an error. Tildes are expanded in filenames as described below under Tilde Expansion in the EXPANSION section.

When bash is invoked as an interactive login shell, or as a non-interactive shell with the --login option, it first reads and executes commands from the file /etc/profile, if that file exists. After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and reads and executes commands from the first one that exists and is readable. The --noprofile option may be used when the shell is started to inhibit this behavior. ✨

When an interactive login shell exits, or a non-interactive login shell executes the exit builtin command, bash reads and executes commands from the files ~/.bash_logout and /etc/bash.bash_logout, if the files exists. 🫶

When an interactive shell that is not a login shell is started, bash reads and executes commands from ~/.bashrc, if that file exists. The --norc option inhibits this behavior. The --rcfile file option causes bash to use file instead of ~/.bashrc.

When bash is started non-interactively, to run a shell script, for example, it looks for the variable BASH_ENV in the environment, expands its value if it appears there, and uses the expanded value as the name of a file to read and execute. Bash behaves as if the following command were executed:

if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi

but does not use the value of the PATH variable to search for the filename. 💅

If bash is invoked with the name sh, it tries to mimic the startup behavior of historical versions of sh as closely as possible, while conforming to the POSIX standard as well. When invoked as an interactive login shell, or a non-interactive shell with the --login option, it first attempts to read and execute commands from /etc/profile and ~/.profile, in that order. The --noprofile option inhibits this behavior. When invoked as an interactive shell with the name sh, bash looks for the variable ENV, expands its value if it is defined, and uses the expanded value as the name of a file to read and execute. Since a shell invoked as sh does not attempt to read and execute commands from any other startup files, the --rcfile option has no effect. A non-interactive shell invoked with the name sh does not attempt to read any other startup files. 😭

When invoked as sh, bash enters posix mode after reading the startup files.

When bash is started in posix mode, as with the --posix command line option, it follows the POSIX standard for startup files. In this mode, interactive shells expand the ENV variable and read and execute commands from the file whose name is the expanded value. No other startup files are read. 🌸

Bash attempts to determine when it is being run with its standard input connected to a network connection, as when executed by the historical and rarely-seen remote shell daemon, usually rshd, or the secure shell daemon sshd. If bash determines it is being run non-interactively in this fashion, it reads and executes commands from ~/.bashrc, if that file exists and is readable. Bash does not read this file if invoked as sh. The --norc option inhibits this behavior, and the --rcfile option makes bash use a different file instead of ~/.bashrc, but neither rshd nor sshd generally invoke the shell with those options or allow them to be specified. 🙈

If the shell is started with the effective user (group) id not equal to the real user (group) id, and the -p option is not supplied, no startup files are read, shell functions are not inherited from the environment, the SHELLOPTS, BASHOPTS, CDPATH, and GLOBIGNORE variables, if they appear in the environment, are ignored, and the effective user id is set to the real user id. If the -p option is supplied at invocation, the startup behavior is the same, but the effective user id is not reset. 💀

bestie this one is a whole mood, it's giving "security mode engaged" 🔒

🌈 DEFINITIONS 🌈

The following definitions are used throughout the rest of this document.

blank — a space or tab.
whitespace — a character belonging to the space character class in the current locale, or for which isspace(3) returns true.
word — a sequence of characters considered as a single unit by the shell. Also known as a token.
name — a word consisting only of alphanumeric characters and underscores, and beginning with an alphabetic character or an underscore. Also referred to as an identifier.
metacharacter — a character that, when unquoted, separates words. One of the following:

|  & ; ( ) < > space tab newline

control operator — a token that performs a control function. It is one of the following symbols:

|| & && ; ;; ;& ;;& ( ) | |& <newline>

👑 RESERVED WORDS 👑

Reserved words are words that have a special meaning to the shell. The following words are recognized as reserved when unquoted and either the first word of a command (see SHELL GRAMMAR below), the third word of a case or select command (only in is valid), or the third word of a for command (only in and do are valid):

! case  coproc  do done elif else esac fi for function if in select then
until while { } time [[ ]]

🎀 SHELL GRAMMAR 🎀

This section describes the syntax of the various forms of shell commands.

Simple Commands

A simple command is a sequence of optional variable assignments followed by blank-separated words and redirections, and terminated by a control operator. The first word specifies the command to be executed, and is passed as argument zero. The remaining words are passed as arguments to the invoked command.

The return value of a simple command is its exit status, or 128+n if the command is terminated by signal n. ⭐

Pipelines

A pipeline is a sequence of one or more commands separated by one of the control operators | or |&. The format for a pipeline is:

[time [-p]] [ ! ] command1 [ [|⎪|&] command2 ... ]

The standard output of command1 is connected via a pipe to the standard input of command2. This connection is performed before any redirections specified by the command1 (see REDIRECTION below). If |& is the pipeline operator, command1's standard error, in addition to its standard output, is connected to command2's standard input through the pipe; it is shorthand for 2>&1 |. This implicit redirection of the standard error to the standard output is performed after any redirections specified by the command1. 💖

The return status of a pipeline is the exit status of the last command, unless the pipefail option is enabled. If pipefail is enabled, the pipeline's return status is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands exit successfully. If the reserved word ! precedes a pipeline, the exit status of that pipeline is the logical negation of the exit status as described above. If a pipeline is executed synchronously, the shell waits for all commands in the pipeline to terminate before returning a value. ✨

If the time reserved word precedes a pipeline, the shell reports the elapsed as well as user and system time consumed by its execution when the pipeline terminates. The -p option changes the output format to that specified by POSIX. When the shell is in posix mode, it does not recognize time as a reserved word if the next token begins with a -. The value of the TIMEFORMAT variable is a format string that specifies how the timing information should be displayed; see the description of TIMEFORMAT below under Shell Variables.

When the shell is in posix mode, time may appear by itself as the only word in a simple command. In this case, the shell displays the total user and system time consumed by the shell and its children. The TIMEFORMAT variable specifies the format of the time information. 🕐

Each command in a multi-command pipeline, where pipes are created, is executed in a subshell, which is a separate process. See COMMAND EXECUTION ENVIRONMENT for a description of subshells and a subshell environment. If the lastpipe option is enabled using the shopt builtin (see the description of shopt below), and job control is not active, the last element of a pipeline may be run by the shell process. 🦋

Lists

A list is a sequence of one or more pipelines separated by one of the operators ;, &, &&, or ||, and optionally terminated by one of ;, &, or newline.

Of these list operators, && and || have equal precedence, followed by ; and &, which have equal precedence.

A sequence of one or more newlines may appear in a list instead of a semicolon to delimit commands.

If a command is terminated by the control operator &, the shell executes the command in the background in a subshell. The shell does not wait for the command to finish, and the return status is 0. These are referred to as asynchronous commands. Commands separated by a ; are executed sequentially; the shell waits for each command to terminate in turn. The return status is the exit status of the last command executed. 💅

AND and OR lists are sequences of one or more pipelines separated by the && and || control operators, respectively. AND and OR lists are executed with left associativity. An AND list has the form:

command1 && command2

command2 is executed if, and only if, command1 returns an exit status of zero (success). 🔥

An OR list has the form:

command1 || command2

command2 is executed if, and only if, command1 returns a non-zero exit status. The return status of AND and OR lists is the exit status of the last command executed in the list.

Compound Commands

A compound command is one of the following. In most cases a list in a command's description may be separated from the rest of the command by one or more newlines, and may be followed by a newline in place of a semicolon.

(list) — list is executed in a subshell (see COMMAND EXECUTION ENVIRONMENT below for a description of a subshell environment). Variable assignments and builtin commands that affect the shell's environment do not remain in effect after the command completes. The return status is the exit status of list. 🌸

`{ list; }`

okay so this is a group command, and the vibe is simple: list runs in the current shell environment. not a subshell, not a copy, the real thing. but bestie, list must be terminated with a newline or a semicolon — no exceptions, that's the whole deal. the return status is just the exit status of list. periodt. 💅

🚨 okay important plot twist: unlike the metacharacters ( and ), the { and } here are reserved words, so they have to appear wherever a reserved word is allowed to be recognized. and since they don't cause a word break, you have to separate them from list with whitespace or another shell metacharacter. yes this is why your script exploded. no cap. 😭

`((expression))`

so this evaluates the arithmetic expression using the rules down in ARITHMETIC EVALUATION. if the value of the expression is non-zero, the return status is 0. otherwise the return status is 1. that's it, that's the whole personality ✨

and here's the lil detail nobody tells you: the expression gets the same expansions as if it were inside double quotes, EXCEPT unescaped double quote characters in expression are not treated specially and get removed. and since that can potentially leave you with empty strings, this command treats those as expressions that evaluate to 0. 💀

`[[ expression ]]`

evaluate the conditional expression expression and hand back a status of zero (true) or non-zero (false). the expressions are built from the primaries described down below in CONDITIONAL EXPRESSIONS. 🦋

and the glow-up here? the words between [[ and ]] do not undergo word splitting and pathname expansion. but the shell does perform tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal on those words. also: conditional operators like -f must be unquoted to be recognized as primaries. iykyk. 🙈

when used with [[, the < and > operators sort lexicographically using the current locale.

when the == and != operators are used, the string to the right of the operator is considered a pattern and matched according to the rules described below under Pattern Matching, as if the extglob shell option were enabled. the = operator is equivalent to ==. if the nocasematch shell option is enabled, the match happens without regard to the case of alphabetic characters. the return value is 0 if the string matches (==) or does not match (!=) the pattern, and 1 otherwise. and if any part of the pattern is quoted, that quoted portion is matched as a string: every character in it matches itself instead of having any special pattern matching meaning. 🎀

an additional binary operator, =~, is available, with the same precedence as == and !=. when you use it, the string to the right of the operator is considered a POSIX extended regular expression and matched accordingly (using the POSIX regcomp and regexec interfaces usually described in regex(3)). the return value is 0 if the string matches the pattern, and 1 otherwise. if the regular expression is syntactically incorrect, the conditional expression's return value is 2. and if the nocasematch shell option is enabled, the match is performed without regard to the case of alphabetic characters. ✨

if any part of the pattern is quoted, the quoted portion is matched literally, as above. if the pattern is stored in a shell variable, quoting the variable expansion forces the entire pattern to be matched literally. and pls be careful with bracket expressions in regular expressions, since normal quoting and pattern characters lose their meanings between brackets. 🙈

the match succeeds if the pattern matches any part of the string. so anchor your pattern using the ^ and $ regular expression operators to force it to match the entire string. 🌸

the array variable BASH_REMATCH records which parts of the string matched the pattern. the element of BASH_REMATCH with index 0 contains the portion of the string matching the entire regular expression. substrings matched by parenthesized subexpressions within the regular expression are saved in the remaining BASH_REMATCH indices. the element of BASH_REMATCH with index n is the portion of the string matching the *n*th parenthesized subexpression.

💀 BIG WARNING BESTIE: bash sets BASH_REMATCH in the global scope. declaring it as a local variable will lead to unexpected results. do not do it. i'm begging.

expressions may be combined using the following operators, listed in decreasing order of precedence:

( expression ) — returns the value of expression. you can use this to override the normal precedence of operators. 👑
! expression — true if expression is false.
expression1 && expression2 — true if both expression1 and expression2 are true.
expression1 || expression2 — true if either expression1 or expression2 is true.

and the slay detail: the && and || operators do not evaluate expression2 if the value of expression1 is already sufficient to determine the return value of the entire conditional expression. efficiency queen. 💅

`for name [ [ in word ... ] ; ] do list ; done`

first, expand the list of words following in, generating a list of items. then the variable name is set to each element of this list in turn, and list is executed each time. 🔁

if the in word is omitted, the for command executes list once for each positional parameter that is set (see PARAMETERS below). the return status is the exit status of the last command that executes. and if the expansion of the items following in results in an empty list, no commands are executed, and the return status is 0. 🌈

`for (( expr1 ; expr2 ; expr3 )) [;] do list ; done`

first, evaluate the arithmetic expression expr1 according to the rules described below under ARITHMETIC EVALUATION. then repeatedly evaluate the arithmetic expression expr2 until it evaluates to zero. each time expr2 evaluates to a non-zero value, execute list and evaluate the arithmetic expression expr3. 🔥

if any expression is omitted, it behaves as if it evaluates to 1. the return value is the exit status of the last command in list that is executed, or non-zero if any of the expressions is invalid.

use the break and continue builtins (see SHELL BUILTIN COMMANDS below) to control loop execution. 🎀

`select name [ in word ] ; do list ; done`

first, expand the list of words following in, generating a list of items, and print the set of expanded words to the standard error, each preceded by a number. if the in word is omitted, print the positional parameters (see PARAMETERS below). 🦋

select then displays the PS3 prompt and reads a line from the standard input. here's the vibe check on what that line can be:

if the line consists of a number corresponding to one of the displayed words, then select sets the value of name to that word.
if the line is empty, select displays the words and prompt again.
if EOF is read, select completes and returns 1.
any other value sets name to null.

the line read is saved in the variable REPLY. 📝 the list is executed after each selection until a break command is executed. the exit status of select is the exit status of the last command executed in list, or zero if no commands were executed. 💜

`case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac`

a case command first expands word, and tries to match it against each pattern in turn, proceeding from first to last, using the matching rules described under Pattern Matching below. a pattern list is a set of one or more patterns separated by |, and the ) operator terminates the pattern list. 🌸

word is expanded using tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution and quote removal. each pattern examined is expanded using tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal. if the nocasematch shell option is enabled, the match is performed without regard to the case of alphabetic characters. a clause is a pattern list and an associated list. 🎀

when a match is found, case executes the corresponding list. and now the iconic trio of terminators:

if the ;; operator terminates the case clause, the case command completes after the first match.
using ;& in place of ;; causes execution to continue with the list associated with the next pattern list.
using ;;& in place of ;; causes the shell to test the next pattern list in the statement, if any, and execute any associated list if the match succeeds, continuing the case statement execution as if the pattern list had not matched. 🤯

the exit status is zero if no pattern matches. otherwise, it's the exit status of the last command executed in the last list executed. 👑

`if list; then list; [ elif list; then list; ] ... [ else list; ] fi`

the if list is executed. if its exit status is zero, the then list is executed. otherwise, each elif list is executed in turn, and if its exit status is zero, the corresponding then list is executed and the command completes. otherwise, the else list is executed, if present. 🌈

the exit status is the exit status of the last command executed, or zero if no condition tested true. 💖

`while list-1; do list-2; done`

`until list-1; do list-2; done`

the while command continuously executes the list list-2 as long as the last command in the list list-1 returns an exit status of zero. the until command is identical to the while command, except that the test is negated: list-2 is executed as long as the last command in list-1 returns a non-zero exit status. 🌀

the exit status of the while and until commands is the exit status of the last command executed in list-2, or zero if none was executed. ✨

🧚 Coprocesses 🧚

a coprocess is a shell command preceded by the coproc reserved word. a coprocess is executed asynchronously in a subshell, as if the command had been terminated with the & control operator, with a two-way pipe established between the executing shell and the coprocess. 🔥

the syntax for a coprocess is:

coproc [NAME] command [redirections]

this creates a coprocess named NAME. command may be either a simple command or a compound command (see above). NAME is a shell variable name. if NAME is not supplied, the default name is COPROC. 👑

The recommended form to use for a coprocess is

coproc NAME { command [redirections]; }

This form is preferred because simple commands result in the coprocess always being named COPROC, and it is simpler to use and more complete than the other compound commands. 💅

If command is a compound command, NAME is optional. The word following coproc determines whether that word is interpreted as a variable name: it is interpreted as NAME if it is not a reserved word that introduces a compound command. If command is a simple command, NAME is not allowed — this is to avoid confusion between NAME and the first word of the simple command. 🙈

When the coprocess is executed, the shell creates an array variable (see Arrays below) named NAME in the context of the executing shell. The standard output of command is connected via a pipe to a file descriptor in the executing shell, and that file descriptor is assigned to NAME[0]. The standard input of command is connected via a pipe to a file descriptor in the executing shell, and that file descriptor is assigned to NAME[1]. This pipe is established before any redirections specified by the command (see REDIRECTION below). The file descriptors can be utilized as arguments to shell commands and redirections using standard word expansions. Other than those created to execute command and process substitutions, the file descriptors are not available in subshells. 😭

The process ID of the shell spawned to execute the coprocess is available as the value of the variable NAME_PID. The wait builtin may be used to wait for the coprocess to terminate. 🫶

Since the coprocess is created as an asynchronous command, the coproc command always returns success. The return status of a coprocess is the exit status of command.

🧚 Shell Function Definitions

A shell function is an object that is called like a simple command and executes a compound command with a new set of positional parameters. Shell functions are declared as follows:

fname () compound-command [redirection]
function fname [()] compound-command [redirection]

This defines a function named fname. The reserved word function is optional. If the function reserved word is supplied, the parentheses are optional. The body of the function is the compound command compound-command (see Compound Commands above). That command is usually a list of commands between { and }, but may be any command listed under Compound Commands above. If the function reserved word is used, but the parentheses are not supplied, the braces are recommended. compound-command is executed whenever fname is specified as the name of a simple command. When in posix mode, fname must be a valid shell name and may not be the name of one of the POSIX special builtins. In default mode, a function name can be any unquoted shell word that does not contain $. ✨

Any redirections (see REDIRECTION below) specified when a function is defined are performed when the function is executed.

The exit status of a function definition is zero unless a syntax error occurs or a readonly function with the same name already exists. When executed, the exit status of a function is the exit status of the last command executed in the body. (See FUNCTIONS below.)

💬 COMMENTS

In a non-interactive shell, or an interactive shell in which the interactive_comments option to the shopt builtin is enabled (see SHELL BUILTIN COMMANDS below), a word beginning with # introduces a comment. A word begins at the beginning of a line, after unquoted whitespace, or after an operator. The comment causes that word and all remaining characters on that line to be ignored. An interactive shell without the interactive_comments option enabled does not allow comments. The interactive_comments option is enabled by default in interactive shells. 🎀

🎀 QUOTING

Quoting is used to remove the special meaning of certain characters or words to the shell. Quoting can be used to disable special treatment for special characters, to prevent reserved words from being recognized as such, and to prevent parameter expansion. 💅

Each of the metacharacters listed above under DEFINITIONS has special meaning to the shell and must be quoted if it is to represent itself.

When the command history expansion facilities are being used (see HISTORY EXPANSION below), the history expansion character, usually !, must be quoted to prevent history expansion. ⚠️

There are four quoting mechanisms: the escape character, single quotes, double quotes, and dollar-single quotes.

A non-quoted backslash (\) is the escape character. It preserves the literal value of the next character that follows, removing any special meaning it has, with the exception of <newline>. If a \<newline> pair appears, and the backslash is not itself quoted, the \<newline> is treated as a line continuation (that is, it is removed from the input stream and effectively ignored).

Enclosing characters in single quotes preserves the literal value of each character within the quotes. A single quote may not occur between single quotes, even when preceded by a backslash. 😭

Enclosing characters in double quotes preserves the literal value of all characters within the quotes, with the exception of $, `, \, and, when history expansion is enabled, !. When the shell is in posix mode, the ! has no special meaning within double quotes, even when history expansion is enabled. The characters $ and ` retain their special meaning within double quotes. The backslash retains its special meaning only when followed by one of the following characters: $, `, ", \, or <newline>. Backslashes preceding characters without a special meaning are left unmodified.

A double quote may be quoted within double quotes by preceding it with a backslash. If enabled, history expansion will be performed unless an ! appearing in double quotes is escaped using a backslash. The backslash preceding the ! is not removed.

The special parameters * and @ have special meaning when in double quotes (see PARAMETERS below).

Character sequences of the form $'string' are treated as a special variant of single quotes. The sequence expands to string, with backslash-escaped characters in string replaced as specified by the ANSI C standard. Backslash escape sequences, if present, are decoded as follows:

\a — alert (bell) 🔔
\b — backspace
\e, \E — an escape character
\f — form feed
\n — new line
\r — carriage return
\t — horizontal tab
\v — vertical tab
\\ — backslash
\' — single quote
\" — double quote
\? — question mark
\nnn — the eight-bit character whose value is the octal value nnn (one to three octal digits).
\xHH — the eight-bit character whose value is the hexadecimal value HH (one or two hex digits).
\uHHHH — the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHH (one to four hex digits).
\UHHHHHHHH — the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHHHHHH (one to eight hex digits).
\cx — a control-x character.

The expanded result is single-quoted, as if the dollar sign had not been present. 💜

🌸 Translating Strings

A double-quoted string preceded by a dollar sign ($"string") causes the string to be translated according to the current locale. The gettext infrastructure performs the lookup and translation, using the LC_MESSAGES, TEXTDOMAINDIR, and TEXTDOMAIN shell variables. If the current locale is C or POSIX, if there are no translations available, or if the string is not translated, the dollar sign is ignored, and the string is treated as double-quoted as described above. This is a form of double quoting, so the string remains double-quoted by default, whether or not it is translated and replaced. If the noexpand_translation option is enabled using the shopt builtin, translated strings are single-quoted instead of double-quoted. See the description of shopt below under SHELL BUILTIN COMMANDS. 🌈

🦋 PARAMETERS

A parameter is an entity that stores values. It can be a name, a number, or one of the special characters listed below under Special Parameters. A variable is a parameter denoted by a name. A variable has a value and zero or more attributes. Attributes are assigned using the declare builtin command (see declare below in SHELL BUILTIN COMMANDS). The export and readonly builtins assign specific attributes.

A parameter is set if it has been assigned a value. The null string is a valid value. Once a variable is set, it may be unset only by using the unset builtin command (see SHELL BUILTIN COMMANDS below).

A variable is assigned to using a statement of the form

name=[value]

If value is not given, the variable is assigned the null string. All values undergo tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, and quote removal (see EXPANSION below). If the variable has its integer attribute set, then value is evaluated as an arithmetic expression even if the $((...)) expansion is not used (see Arithmetic Expansion below). Word splitting and pathname expansion are not performed. Assignment statements may also appear as arguments to the alias, declare, typeset, export, readonly, and local builtin commands (declaration commands). When in posix mode, these builtins may appear in a command after one or more instances of the command builtin and retain these assignment statement properties. ✨

In the context where an assignment statement is assigning a value to a shell variable or array index, the += operator appends to or adds to the variable's previous value. This includes arguments to declaration commands such as declare that accept assignment statements. When += is applied to a variable for which the integer attribute has been set, the variable's current value and value are each evaluated as arithmetic expressions, and the sum of the results is assigned as the variable's value. The current value is usually an integer constant, but may be an expression. When += is applied to an array variable using compound assignment (see Arrays below), the variable's value is not unset (as it is when using =), and new values are appended to the array beginning at one greater than the array's maximum index (for indexed arrays) or added as additional key-value pairs in an associative array. When applied to a string-valued variable, value is expanded and appended to the variable's value. 💖

Okay so a variable can get the nameref attribute slapped onto it with the -n option to the declare or local builtin commands (go read the declare and local writeups below, they're right there), and that literally creates a nameref — a reference to another variable. It's giving indirect manipulation, and it's iconic. Whenever the nameref variable is referenced, assigned to, unset, or has its attributes modified (other than using or changing the nameref attribute itself, obviously), the operation is actually performed on the variable that the nameref variable's value points at. Sneaky. Elegant. Slay.

Namerefs are commonly used inside shell functions to refer to a variable whose name got passed in as an argument to the function. For instance, if a variable name is passed to a shell function as its first argument, running

declare -n ref=$1

inside the function creates a local nameref variable ref whose value is the variable name passed as the first argument. References and assignments to ref, plus changes to its attributes, are treated as references, assignments, and attribute modifications to the variable whose name was passed as $1. Main character behavior, honestly.

And if the control variable in a for loop has the nameref attribute, the list of words can be a list of shell variables, and a name reference is established for each word in the list, in turn, when the loop is executed. Chef's kiss.

Heads up bestie: array variables cannot be given the nameref attribute. Periodt. But — plot twist — nameref variables can reference array variables and subscripted array variables. 🙈

Namerefs can be unset using the -n option to the unset builtin. Otherwise, if unset is executed with the name of a nameref variable as an argument, the variable referenced by the nameref variable is what gets unset. Yeah. Read that twice. 💀

When the shell starts, it reads its environment and creates a shell variable from each environment variable that has a valid name, as described below (see ENVIRONMENT).

🌸 Positional Parameters 🌸

A positional parameter is a parameter denoted by one or more digits, other than the single digit 0. They get assigned from the shell's arguments when it's invoked, and you can reassign them using the set builtin command. You cannot assign to positional parameters with assignment statements though — that's not the vibe. And the positional parameters get temporarily replaced when a shell function is executed (see FUNCTIONS below).

When a positional parameter consisting of more than a single digit is expanded, it must be enclosed in braces (see EXPANSION below). Without braces, a digit following $ can only refer to one of the first nine positional parameters ($1-$9) or the special parameter $0 (see the next section). No cap.

👑 Special Parameters 👑

The shell treats several parameters specially, and these parameters may only be referenced — assignment to them is not allowed, sorry babe, them's the rules. Special parameters are denoted by one of the following characters.

* — ($*) Expands to the positional parameters, starting from one. When the expansion is not within double quotes, each positional parameter expands to a separate word. In contexts where word expansions are performed, those words are subject to further word splitting and pathname expansion. When the expansion occurs within double quotes, it expands to a single word with the value of each parameter separated by the first character of the IFS variable. That is, "$*" is equivalent to "$1c$2c...", where c is the first character of the value of the IFS variable. If IFS is unset, the parameters are separated by spaces. If IFS is null, the parameters are joined without intervening separators.
@ — ($@) Expands to the positional parameters, starting from one. In contexts where word splitting is performed, this expands each positional parameter to a separate word; if not within double quotes, these words are subject to word splitting. In contexts where word splitting is not performed, such as the value portion of an assignment statement, this expands to a single word with each positional parameter separated by a space. When the expansion occurs within double quotes, and word splitting is performed, each parameter expands to a separate word. That is, "$@" is equivalent to "$1" "$2" ... If the double-quoted expansion occurs within a word, the expansion of the first parameter is joined with the expansion of the beginning part of the original word, and the expansion of the last parameter is joined with the expansion of the last part of the original word. When there are no positional parameters, "$@" and $@ expand to nothing (i.e., they are removed).
# — ($#) Expands to the number of positional parameters in decimal.
? — ($?) Expands to the exit status of the most recently executed command. Your little status checker. 🔥
- — ($-) Expands to the current option flags as specified upon invocation, by the set builtin command, or those set by the shell itself (such as the -i option).
$ — ($$) Expands to the process ID of the shell. In a subshell, it expands to the process ID of the parent shell, not the subshell. Plot twist!
! — ($!) Expands to the process ID of the job most recently placed into the background, whether executed as an asynchronous command or using the bg builtin (see JOB CONTROL below).
0 — ($0) Expands to the name of the shell or shell script. This is set at shell initialization. If bash is invoked with a file of commands, $0 is set to the name of that file. If bash is started with the -c option, then $0 is set to the first argument after the string to be executed, if one is present. Otherwise, it is set to the filename used to invoke bash, as given by argument zero.

🦋 Shell Variables 🦋

The shell sets following variables:

_ (that's an underscore, bestie) — okay so this one is lowkey a shape-shifter, it means different things depending on the vibe. At shell startup, _ is set to the pathname used to invoke the shell or shell script being executed, as passed in the environment or argument list. After that, it expands to the last argument to the previous simple command executed in the foreground, after expansion. It's also set to the full pathname used to invoke each command executed and placed in the environment exported to that command. And when checking mail, $_ expands to the name of the mail file currently being checked. It's giving main character energy, fr fr. 💅
BASH — expands to the full filename used to invoke this instance of bash. Iconic. ✨
BASHOPTS — a colon-separated list of enabled shell options. Each word in the list is a valid argument for the -s option to the shopt builtin command (see SHELL BUILTIN COMMANDS below). The options appearing in BASHOPTS are those reported as on by shopt. If this variable is in the environment when bash starts up, the shell enables each option in the list before reading any startup files. If this variable is exported, child shells will enable each option in the list. This variable is read-only. No cap. 🔥
BASHPID — expands to the process ID of the current bash process. This differs from $$ under certain circumstances, such as subshells that do not require bash to be re-initialized. Assignments to BASHPID have no effect. If BASHPID is unset, it loses its special properties, even if it is subsequently reset. So don't even try to assign to it, bestie. 💀
BASH_ALIASES — an associative array variable whose members correspond to the internal list of aliases as maintained by the alias builtin. Elements added to this array appear in the alias list; however, unsetting array elements currently does not remove aliases from the alias list. If BASH_ALIASES is unset, it loses its special properties, even if it is subsequently reset. It's giving "I contain multitudes" energy. 🎀
BASH_ARGC — an array variable whose values are the number of parameters in each frame of the current bash execution call stack. The number of parameters to the current subroutine (shell function or script executed with . or source) is at the top of the stack. When a subroutine is executed, the number of parameters passed is pushed onto BASH_ARGC. The shell sets BASH_ARGC only when in extended debugging mode (see the description of the extdebug option to the shopt builtin below). Setting extdebug after the shell has started to execute a script, or referencing this variable when extdebug is not set, may result in inconsistent values. Assignments to BASH_ARGC have no effect, and it may not be unset. So hands off, okay? 🙅‍♀️
BASH_ARGV — an array variable containing all of the parameters in the current bash execution call stack. The final parameter of the last subroutine call is at the top of the stack; the first parameter of the initial call is at the bottom. When a subroutine is executed, the shell pushes the supplied parameters onto BASH_ARGV. The shell sets BASH_ARGV only when in extended debugging mode (see the description of the extdebug option to the shopt builtin below). Setting extdebug after the shell has started to execute a script, or referencing this variable when extdebug is not set, may result in inconsistent values. Assignments to BASH_ARGV have no effect, and it may not be unset. Same vibe as BASH_ARGC, bestie. 🦋
BASH_ARGV0 — when referenced, this variable expands to the name of the shell or shell script (identical to $0; see the description of special parameter 0 above). Assigning a value to BASH_ARGV0 sets $0 to the same value. If BASH_ARGV0 is unset, it loses its special properties, even if it is subsequently reset. So you can change your whole identity with this one. 😌
BASH_CMDS — an associative array variable whose members correspond to the internal hash table of commands as maintained by the hash builtin. Adding elements to this array makes them appear in the hash table; however, unsetting array elements currently does not remove command names from the hash table. If BASH_CMDS is unset, it loses its special properties, even if it is subsequently reset. It's the backstage pass to your command history. 🎟️
BASH_COMMAND — expands to the command currently being executed or about to be executed, unless the shell is executing a command as the result of a trap, in which case it is the command executing at the time of the trap. If BASH_COMMAND is unset, it loses its special properties, even if it is subsequently reset. It's like, "what am I doing right now?" energy. 🤔
BASH_EXECUTION_STRING — the command argument to the -c invocation option. Short and sweet. 💖
BASH_LINENO — an array variable whose members are the line numbers in source files where each corresponding member of FUNCNAME was invoked. ${BASH_LINENO[$i]} is the line number in the source file (${BASH_SOURCE[$i+1]}) where ${FUNCNAME[$i]} was called (or ${BASH_LINENO[$i-1]} if referenced within another shell function). Use LINENO to obtain the current line number. Assignments to BASH_LINENO have no effect, and it may not be unset. So it's the receipts of where everything went down. 🧾
BASH_LOADABLES_PATH — a colon-separated list of directories in which the enable command looks for dynamically loadable builtins. It's the plug-in directory, basically. 🔌
BASH_MONOSECONDS — each time this variable is referenced, it expands to the value returned by the system's monotonic clock, if one is available. If there is no monotonic clock, this is equivalent to EPOCHSECONDS. If BASH_MONOSECONDS is unset, it loses its special properties, even if it is subsequently reset. Time is fake, but this is real. ⏰
BASH_REMATCH — an array variable whose members are assigned by the =~ binary operator to the [[ conditional command. The element with index 0 is the portion of the string matching the entire regular expression. The element with index n is the portion of the string matching the *n*th parenthesized subexpression. So it's the regex match receipts. 🔍
BASH_SOURCE — an array variable whose members are the source filenames where the corresponding shell function names in the FUNCNAME array variable are defined. The shell function ${FUNCNAME[$i]} is defined in the file ${BASH_SOURCE[$i]} and called from ${BASH_SOURCE[$i+1]}. Assignments to BASH_SOURCE have no effect, and it may not be unset. It's the origin story of your functions. 📖
BASH_SUBSHELL — incremented by one within each subshell or subshell environment when the shell begins executing in that environment. The initial value is 0. If BASH_SUBSHELL is unset, it loses its special properties, even if it is subsequently reset. It's like the Inception of shells. 🌀
BASH_TRAPSIG — set to the signal number corresponding to the trap action being executed during its execution. See the description of trap under SHELL BUILTIN COMMANDS below for information about signal numbers and trap execution. It's the "which trap am I in?" indicator. 🪤
BASH_VERSINFO — a readonly array variable whose members hold version information for this instance of bash. The values assigned to the array members are as follows:
- BASH_VERSINFO[0] — The major version number (the release).
- BASH_VERSINFO[1] — The minor version number (the version).
- BASH_VERSINFO[2] — The patch level.
- BASH_VERSINFO[3] — The build version.
- BASH_VERSINFO[4] — The release status (e.g., beta).
- BASH_VERSINFO[5] — The value of MACHTYPE.
BASH_VERSION — expands to a string describing the version of this instance of bash (e.g., 5.2.37(3)-release). Slay. 💅
COMP_CWORD — an index into ${COMP_WORDS} of the word containing the current cursor position. This variable is available only in shell functions invoked by the programmable completion facilities (see Programmable Completion below). So it's only for the completion girlies. ✨
COMP_KEY — the key (or final key of a key sequence) used to invoke the current completion function. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Programmable Completion below). Keyboard smash but make it purposeful. ⌨️
COMP_LINE — the current command line. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Programmable Completion below). It's the whole line, bestie. 📝
COMP_POINT — the index of the current cursor position relative to the beginning of the current command. If the current cursor position is at the end of the current command, the value of this variable is equal to ${#COMP_LINE}. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Programmable Completion below). Cursor location, but make it a variable. 📍
COMP_TYPE — set to an integer value corresponding to the type of attempted completion that caused a completion function to be called: TAB, for normal completion, ?, for listing completions after successive tabs, !, for listing alternatives on partial word completion, @, to list completions if the word is not unmodified, or %, for menu completion. This variable is available only in shell functions and external commands invoked by the programmable completion facilities (see Programmable Completion below). So many moods of completion. 🎭
COMP_WORDBREAKS — the set of characters that the readline library treats as word separators when performing word completion. If COMP_WORDBREAKS is unset, it loses its special properties, even if it is subsequently reset. It's the boundaries of your words, literally. 🚧
COMP_WORDS — an array variable (see Arrays below) consisting of the individual words in the current command line. The line is split into words as readline would split it, using COMP_WORDBREAKS as described above. This variable is available only in shell functions invoked by the programmable completion facilities (see Programmable Completion below). Word by word, we're breaking it down. 🧩
COPROC — an array variable (see Arrays below) created to hold the file descriptors for output from and input to an unnamed coprocess (see Coprocesses above). It's the BFF of your coprocess. 👯‍♀️
DIRSTACK — an array variable (see Arrays below) containing the current contents of the directory stack. Directories appear in the stack in the order they are displayed by the dirs builtin. Assigning to members of this array variable may be used to modify directories already in the stack, but the pushd and popd builtins must be used to add and remove directories. Assigning to this
DIRSTACK — okay so real talk: if you try to assign to this one, it will not move you to a new directory. Nope. And the second you unset it, it drops all its special ✨magic✨ properties — even if you set it again later. Gone. No takebacks. 💀

EPOCHREALTIME — every single time you reference this one, it expands to the number of seconds since the Unix Epoch (peep time(3)) as a floating-point value with micro-second granularity. Assignments to EPOCHREALTIME are ignored — like, the shell is not listening bestie. 🙈 If EPOCHREALTIME is unset, it loses its special properties, even if it is subsequently reset.
EPOCHSECONDS — same energy, different unit: each time you reference it, it expands to the number of seconds since the Unix Epoch (see time(3) again). Assignments to EPOCHSECONDS are ignored. If EPOCHSECONDS is unset, it loses its special properties, even if it is subsequently reset.
EUID — expands to the effective user ID of the current user, initialized at shell startup. This one is readonly, so don't even try. 💅
FUNCNAME — an array variable holding the names of all the shell functions currently chilling in the execution call stack. The element with index 0 is the name of any currently-executing shell function. The bottom-most element (the one with the highest index) is “main”. This variable exists only when a shell function is executing. Assignments to FUNCNAME have no effect. If FUNCNAME is unset, it loses its special properties, even if it is subsequently reset.

You can pair this icon with BASH_LINENO and BASH_SOURCE 🫶 — each element of FUNCNAME has corresponding elements in BASH_LINENO and BASH_SOURCE to describe the call stack. For instance, ${FUNCNAME[$i]} was called from the file ${BASH_SOURCE[$i+1]} at line number ${BASH_LINENO[$i]}. The caller builtin displays the current call stack using this information.

GROUPS — an array variable containing the list of groups of which the current user is a member. Assignments to GROUPS have no effect. If GROUPS is unset, it loses its special properties, even if it is subsequently reset.
HISTCMD — the history number, aka the index in the history list, of the current command. Assignments to HISTCMD have no effect. If HISTCMD is unset, it loses its special properties, even if it is subsequently reset.
HOSTNAME — automatically set to the name of the current host. Free real estate. 🏠
HOSTTYPE — automatically set to a string that uniquely describes the type of machine on which bash is executing. The default is system-dependent.
LINENO — each time this parameter is referenced, the shell substitutes a decimal number representing the current sequential line number (starting with 1) within a script or function. When you're not in a script or function, the value substituted is not guaranteed to be meaningful — so like, no promises bestie. If LINENO is unset, it loses its special properties, even if it is subsequently reset.
MACHTYPE — automatically set to a string that fully describes the system type on which bash is executing, in the standard GNU cpu-company-system format. The default is system-dependent.
MAPFILE — an array variable (see Arrays below) created to hold the text read by the mapfile builtin when no variable name is supplied.
OLDPWD — the previous working directory as set by the cd command. Your lil "wait, where was I" button. 🦋
OPTARG — the value of the last option argument processed by the getopts builtin command (see SHELL BUILTIN COMMANDS below).
OPTIND — the index of the next argument to be processed by the getopts builtin command (see SHELL BUILTIN COMMANDS below).
OSTYPE — automatically set to a string that describes the operating system on which bash is executing. The default is system-dependent.
PIPESTATUS — an array variable (see Arrays below) containing a list of exit status values from the commands in the most-recently-executed foreground pipeline, which may consist of only a simple command (see SHELL GRAMMAR above). Bash sets PIPESTATUS after executing multi-element pipelines, timed and negated pipelines, simple commands, subshells created with the ( operator, the [[ and (( compound commands, and after error conditions that result in the shell aborting command execution. The receipts of your whole pipeline, preserved. 👑
PPID — the process ID of the shell's parent. This variable is readonly.
PWD — the current working directory as set by the cd command.
RANDOM — each time this parameter is referenced, it expands to a random integer between 0 and 32767. Assigning a value to RANDOM initializes (seeds) the sequence of random numbers. Seeding the random number generator with the same constant value produces the same sequence of values. If RANDOM is unset, it loses its special properties, even if it is subsequently reset.
READLINE_ARGUMENT — any numeric argument given to a readline command that was defined using “bind -x” (see SHELL BUILTIN COMMANDS below) when it was invoked.
READLINE_LINE — the contents of the readline line buffer, for use with “bind -x” (see SHELL BUILTIN COMMANDS below).
READLINE_MARK — the position of the mark (saved insertion point) in the readline line buffer, for use with “bind -x” (see SHELL BUILTIN COMMANDS below). The characters between the insertion point and the mark are often called the region.
READLINE_POINT — the position of the insertion point in the readline line buffer, for use with “bind -x” (see SHELL BUILTIN COMMANDS below).
REPLY — set to the line of input read by the read builtin command when no arguments are supplied.
SECONDS — each time this parameter is referenced, it expands to the number of seconds since shell invocation. If a value is assigned to SECONDS, the value returned upon subsequent references is the number of seconds since the assignment plus the value assigned. The number of seconds at shell invocation and the current time are always determined by querying the system clock at one-second resolution. If SECONDS is unset, it loses its special properties, even if it is subsequently reset. It's giving stopwatch, it's giving timer, we love her. ⏱️
SHELLOPTS — a colon-separated list of enabled shell options. Each word in the list is a valid argument for the -o option to the set builtin command (see SHELL BUILTIN COMMANDS below). The options appearing in SHELLOPTS are those reported as on by set -o. If this variable is in the environment when bash starts up, the shell enables each option in the list before reading any startup files. If this variable is exported, child shells will enable each option in the list. This variable is read-only.
SHLVL — incremented by one each time an instance of bash is started. Nested shells? The number goes up. Shell-ception. 🌀
SRANDOM — each time it is referenced, this variable expands to a 32-bit pseudo-random number. The random number generator is not linear on systems that support /dev/urandom or arc4random(3), so each returned number has no relationship to the numbers preceding it. The random number generator cannot be seeded, so assignments to this variable have no effect. If SRANDOM is unset, it loses its special properties, even if it is subsequently reset.
UID — expands to the user ID of the current user, initialized at shell startup. This variable is readonly.

The shell uses the following variables. In some cases, bash assigns a default value to a variable; these cases are noted below.

`BASH_COMPAT` 💅

bestie this one is the time machine of variables fr. its value sets the shell's compatibility level. like, see SHELL COMPATIBILITY MODE below for a full rundown of all the compatibility levels and what each one actually does.

the value can be a decimal number (like 4.2) or an integer (like 42) — whichever one matches the compatibility level you're chasing. and if BASH_COMPAT is unset, or set to the empty string? then the compatibility level just snaps back to the default for the current version. iconic, low-effort.

but here's the part where it gets a lil spicy: if you set BASH_COMPAT to something that is not one of the valid compatibility levels, the shell prints an error message at you and then sets the compatibility level to the default for the current version anyway. so basically it claps back and then moves on 🙈.

a subset of the valid values line up with the compatibility levels described below under SHELL COMPATIBILITY MODE. for example, 4.2 and 42 are both valid values that correspond to the compat42 shopt option and set the compatibility level to 42. the current version is also a valid value, no cap.

`BASH_ENV` ✨

if this parameter is set when bash is executing a shell script, its expanded value gets interpreted as a filename containing commands that initialize the shell before it reads and executes commands from the script. so it's like a lil pre-game routine for your shell 🎀.

the value of BASH_ENV is subjected to parameter expansion, command substitution, and arithmetic expansion before it's interpreted as a filename. and heads up: PATH is not used to search for the resultant filename. so don't be out here expecting it to go hunting for it, bestie.

`BASH_XTRACEFD` 🔥

if you set this to an integer corresponding to a valid file descriptor, bash writes the trace output generated when set -x is enabled to that file descriptor instead of the standard error. tracing, but make it custom 💅.

the file descriptor gets closed when BASH_XTRACEFD is unset or assigned a new value. unsetting BASH_XTRACEFD, or assigning it the empty string, causes the trace output to be sent to the standard error instead.

⚠️ okay real talk, note this: setting BASH_XTRACEFD to 2 (the standard error file descriptor) and then unsetting it will result in the standard error being closed. that's not a vibe, that's a cautionary tale.

`CDPATH` 🦋

the search path for the cd command. it's a colon-separated list of directories where the shell looks for directories specified as arguments to cd.

a sample value is ".:~:/usr". cute, right?

`CHILD_MAX` 🌸

sets the number of exited child status values for the shell to remember.

bash will not let this value be decreased below a POSIX-mandated minimum, and there's a maximum value (currently 8192) that this may not exceed. the minimum value is system-dependent, so it varies depending on your setup.

`COLUMNS` 🫶

used by the select compound command to figure out the terminal width when it's printing selection lists.

it gets automatically set if the checkwinsize option is enabled, or in an interactive shell upon receipt of a SIGWINCH. so it kinda just keeps itself updated, main character behavior honestly.

`COMPREPLY` 😭

an array variable from which bash reads the possible completions generated by a shell function invoked by the programmable completion facility (see Programmable Completion below).

each array element contains one possible completion. one element = one option. periodt.

`EMACS` 💀

if bash finds this variable in the environment when the shell starts with value "t", it assumes the shell is running in an Emacs shell buffer and disables line editing.

that's it. that's the whole personality. a single t and suddenly no line editing 🙈.

`ENV` 🍓

expanded and executed similarly to BASH_ENV (see INVOCATION above) when an interactive shell is invoked in posix mode.

`EXECIGNORE` 🧚

a colon-separated list of shell patterns (see Pattern Matching) defining the set of filenames to be ignored by command search using PATH.

files whose full pathnames match one of these patterns are not considered executable files for the purposes of completion and command execution via PATH lookup. so basically: ghosted by the shell 👻.

important details so you don't get got:

this does not affect the behavior of the [, test, and [[ commands.
full pathnames in the command hash table are not subject to EXECIGNORE.
use this variable to ignore shared library files that have the executable bit set, but are not executable files.

the pattern matching honors the setting of the extglob shell option.

`FCEDIT` 💜

the default editor for the fc builtin command.

`FIGNORE` ⭐

a colon-separated list of suffixes to ignore when performing filename completion (see READLINE below).

a filename whose suffix matches one of the entries in FIGNORE is excluded from the list of matched filenames. a sample value is ".o:~".

`FUNCNEST` 🌈

if set to a numeric value greater than 0, this defines a maximum function nesting level.

function invocations that exceed this nesting level cause the current command to abort. so it's the "okay that's enough recursion for today" variable 🛑.

`GLOBIGNORE` 💖

a colon-separated list of patterns defining the set of file names to be ignored by pathname expansion.

if a file name matched by a pathname expansion pattern also matches one of the patterns in GLOBIGNORE, it gets removed from the list of matches. bye bestie 💅.

the pattern matching honors the setting of the extglob shell option.

`GLOBSORT` 👑

okay this one is the glow-up for your globbing. it controls how the results of pathname expansion are sorted. the value of this variable specifies the sort criteria and the sort order for the results of pathname expansion.

if this variable is unset or set to the null string, pathname expansion uses the historical behavior of sorting by name, in ascending lexicographic order as determined by the LC_COLLATE shell variable.

if it is set, a valid value begins with an optional +, which is ignored, or -, which reverses the sort order from ascending to descending, followed by a sort specifier.

the valid sort specifiers are:

name — sorts the files on name
numeric — sorts names in numeric rather than lexicographic order
size — sorts on file size
mtime — sorts on modification time
atime — sorts on access time
ctime — sorts on inode change time
blocks — sorts on number of blocks

if any of the non-name keys compare as equal (e.g., if two files are the same size), sorting uses the name as a secondary sort key.

for example, a value of -mtime sorts the results in descending order by modification time (newest first). chef's kiss 🎀

the numeric specifier treats names consisting solely of digits as numbers and sorts them using their numeric value (so "2" sorts before "10", for example). when using numeric, names containing non-digits sort after all the all-digit names, and are sorted by name using the traditional behavior.

a sort specifier of nosort disables sorting completely; bash returns the results in the order they are read from the file system, ignoring any leading -. chaos mode, but like, on purpose 🔥.

So picking up exactly where we were: if the sort specifier is missing, it defaults to name — so a value of + is equivalent to the null string, and a value of - sorts by name in descending order (reverse alphabet, we love to see it). And any invalid value just restores the historical sorting behavior. no notes.

HISTCONTROL — a colon-separated list of values that controls how commands get saved on the history list. If the list of values includes ignorespace, then lines which begin with a space character are not saved in the history list. A value of ignoredups makes lines matching the previous history entry not get saved. A value of ignoreboth is shorthand for ignorespace and ignoredups together (a two-for-one, iconic). A value of erasedups causes all previous lines matching the current line to be removed from the history list before that line is saved. Any value that's not in the above list is just ignored, bestie.

If HISTCONTROL is unset, or doesn't include a valid value, bash saves all lines read by the shell parser on the history list — subject to the value of HISTIGNORE. And here's the multi-line compound command tea: if the first line of a multi-line compound command was saved, the second and subsequent lines are not tested, and are added to the history regardless of the value of HISTCONTROL. If the first line was not saved, then the second and subsequent lines of the command are not saved either. all or nothing energy fr.

HISTFILE — the name of the file in which command history is saved (see HISTORY below). Bash assigns a default value of ~/.bash_history. If HISTFILE is unset or null, the shell does not save the command history when it exits. tragic, honestly.
HISTFILESIZE — the maximum number of lines contained in the history file. When this variable is assigned a value, the history file is truncated, if necessary, to contain no more than the number of history entries that total no more than that number of lines — by removing the oldest entries. If the history list contains multi-line entries, the history file may contain more lines than this maximum, so bash doesn't leave partial history entries lying around. The history file is also truncated to this size after writing it when a shell exits, or by the history builtin. If the value is 0, the history file is truncated to zero size. Non-numeric values and numeric values less than zero inhibit truncation. The shell sets the default value to the value of HISTSIZE after reading any startup files.
HISTIGNORE — a colon-separated list of patterns used to decide which command lines should be saved on the history list. If a command line matches one of the patterns in the value of HISTIGNORE, it is not saved on the history list. Each pattern is anchored at the beginning of the line and must match the complete line (bash does not implicitly append a * — no free passes here). Each pattern is tested against the line after the checks specified by HISTCONTROL are applied. On top of the normal shell pattern matching characters, & matches the previous history line. A backslash escapes the &; the backslash is removed before attempting a match. And same as before: if the first line of a multi-line compound command was saved, the second and subsequent lines are not tested, and are added to the history regardless of the value of HISTIGNORE. If the first line was not saved, the second and subsequent lines of the command are not saved either. The pattern matching honors the setting of the extglob shell option.

HISTIGNORE subsumes some of the function of HISTCONTROL. A pattern of & is identical to ignoredups, and a pattern of [ ]* is identical to ignorespace. Combine those two patterns, separating them with a colon, and you get the functionality of ignoreboth. iykyk.

HISTSIZE — the number of commands to remember in the command history (see HISTORY below). If the value is 0, commands are not saved in the history list. Numeric values less than zero result in every command being saved on the history list (there is no limit — unhinged, love that for us). The shell sets the default value to 500 after reading any startup files.
HISTTIMEFORMAT — if this variable is set and not null, its value is used as a format string for strftime(3) to print the time stamp associated with each history entry displayed by the history builtin. If this variable is set, the shell writes time stamps to the history file so they may be preserved across shell sessions. This uses the history comment character to distinguish timestamps from other history lines.
HOME — the home directory of the current user; the default argument for the cd builtin command. The value of this variable is also used when performing tilde expansion. our beloved. 💜
HOSTFILE — contains the name of a file in the same format as /etc/hosts that should be read when the shell needs to complete a hostname. The list of possible hostname completions may be changed while the shell is running; the next time hostname completion is attempted after the value is changed, bash adds the contents of the new file to the existing list. If HOSTFILE is set, but has no value, or does not name a readable file, bash attempts to read /etc/hosts to obtain the list of possible hostname completions. When HOSTFILE is unset, bash clears the hostname list.
IFS — the Internal Field Separator, used for word splitting after expansion and to split lines into words with the read builtin command. Word splitting is described below under EXPANSION. The default value is <space><tab><newline>. respect her or she will ruin your day. ✨
IGNOREEOF — controls the action of an interactive shell on receipt of an EOF character as the sole input. If set, the value is the number of consecutive EOF characters which must be typed as the first characters on an input line before bash exits. If the variable is set but does not have a numeric value, or the value is null, the default value is 10. If it is unset, EOF signifies the end of input to the shell.
INPUTRC — the filename for the readline startup file, overriding the default of ~/.inputrc (see READLINE below).
INSIDE_EMACS — if this variable appears in the environment when the shell starts, bash assumes that it is running inside an Emacs shell buffer and may disable line editing, depending on the value of TERM.
LANG — used to determine the locale category for any category not specifically selected with a variable starting with LC_.
LC_ALL — this variable overrides the value of LANG and any other LC_ variable specifying a locale category.
LC_COLLATE — this variable determines the collation order used when sorting the results of pathname expansion, and determines the behavior of range expressions, equivalence classes, and collating sequences within pathname expansion and pattern matching.
LC_CTYPE — this variable determines the interpretation of characters and the behavior of character classes within pathname expansion and pattern matching.
LC_MESSAGES — this variable determines the locale used to translate double-quoted strings preceded by a $.
LC_NUMERIC — this variable determines the locale category used for number formatting.
LC_TIME — this variable determines the locale category used for data and time formatting.
LINES — used by the select compound command to determine the column length for printing selection lists. Automatically set if the checkwinsize option is enabled, or in an interactive shell upon receipt of a SIGWINCH.
MAIL — if the value is set to a file or directory name and the MAILPATH variable is not set, bash informs the user of the arrival of mail in the specified file or Maildir-format directory.
MAILCHECK — specifies how often (in seconds) bash checks for mail. The default is 60 seconds. When it is time to check for mail, the shell does so before displaying the primary prompt. If this variable is unset, or set to a value that is not a number greater than or equal to zero, the shell disables mail checking.
MAILPATH — a colon-separated list of filenames to be checked for mail. The message to be printed when mail arrives in a particular file may be specified by separating the filename from the message with a ?. When used in the text of the message, $_ expands to the name of the current mailfile. For example:

MAILPATH='/var/mail/bfox?"You have mail":~/shell-mail?"$_ has mail!"'

Bash can be configured to supply a default value for this variable (there is no value by default), but the location of the user mail files that it uses is system dependent (e.g., /var/mail/$USER).

OPTERR — if set to the value 1, bash displays error messages generated by the getopts builtin command (see SHELL BUILTIN COMMANDS below). OPTERR is initialized to 1 each time the shell is invoked or a shell script is executed.
PATH — the search path for commands. It is a colon-separated list of directories in which the shell looks for commands (see COMMAND EXECUTION below). A zero-length (null) directory name in the value of PATH indicates the current directory. A null directory name may appear as two adjacent colons, or as an initial or trailing colon. The default path is system-dependent, and is set by the administrator who installs bash. A common value is

/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:/sbin

POSIXLY_CORRECT — if this variable is in the environment when bash starts, the shell enters posix mode before reading the startup files, as if the --posix invocation option had been supplied. If it is set while the shell is running, bash enables posix mode, as if the command set -o posix had been executed. When the shell enters posix mode, it sets this variable if it was not already set.
PROMPT_COMMAND — if this variable is set, and is an array, the value of each set element is executed as a command prior to issuing each primary prompt. If this is set but not an array variable, its value is used as a command to execute instead.
PROMPT_DIRTRIM
PROMPT_DIRTRIM — if you set this to a number bigger than zero, that's how many trailing directory components bash keeps around when it expands the \w and \W prompt escapes (peep PROMPTING below for the tea). Anything it trims off gets replaced with an ellipsis, so your prompt doesn't turn into a whole novel. 💅
PS0 — bash expands this value (same way as the others, see PROMPTING below) and shows it in interactive shells after it reads a command but before that command actually runs. Sneaky little pre-show moment. ✨
PS1 — the main character of prompts. Its value gets expanded (yes, PROMPTING below) and used as the primary prompt string. The default is “\s-\v\$ ”.
PS2 — expanded just like PS1, but it's the secondary prompt string. The default is “> ”. You know, the one that shows up when you're clearly not done typing. 🙈
PS3 — this value is the prompt for the select command (see SHELL GRAMMAR above).
PS4 — expanded like PS1, and its value gets printed before every command bash displays during an execution trace. The first character of the expanded PS4 gets replicated a bunch of times as needed, so you can tell how many levels of indirection deep you are. The default is “+ ”.
SHELL — expands to the full pathname of the shell. If it's not set when the shell starts, bash assigns it the full pathname of the current user's login shell.
TIMEFORMAT — this is the format string that decides how the timing info for pipelines prefixed with the time reserved word gets displayed. The % character kicks off an escape sequence that expands to a time value or some other info. Here are the escapes and what they mean — the brackets are just there to show what's optional, bestie:
- %% — a literal %.
- %[p][l]R — the elapsed time in seconds.
- %[p][l]U — the number of CPU seconds spent in user mode.
- %[p][l]S — the number of CPU seconds spent in system mode.
- %P — the CPU percentage, computed as (%U + %S) / %R.

That optional p is a digit setting the precision, aka how many fractional digits come after the decimal point. A value of 0 means no decimal point and no fraction get output at all. time prints at most six digits after the decimal point, and any p greater than 6 gets changed to 6. If you don't specify p, time prints three digits after the decimal point.

That optional l asks for a longer format that includes minutes, shaped like MMmSS.FFs. The value of p is what decides whether the fraction is included or not.

If this variable isn't set, bash acts like it had the value $'\nreal\t%3lR\nuser\t%3lU\nsys\t%3lS'. If the value is null, bash shows you no timing information at all. And a trailing newline gets added when the format string is displayed.

TMOUT — set it to a value greater than zero and the read builtin uses that value as its default timeout. The select command terminates if input doesn't show up after TMOUT seconds when input is coming from a terminal. In an interactive shell, the value is interpreted as the number of seconds to wait for a line of input after it prints the primary prompt. Bash terminates after waiting that many seconds if a complete line of input doesn't arrive. ⏰
TMPDIR — if set, bash uses its value as the name of the directory where it creates temporary files for the shell's own use.
auto_resume — this one controls how the shell interacts with you and with job control. If it's set, simple commands made of only a single word, no redirections, are treated as candidates for resuming an existing stopped job. There is no ambiguity allowed; if more than one job begins with or contains that word, this picks the most recently accessed job. The name of a stopped job here means the command line used to start it, exactly as jobs displays it. If you set it to the value exact, the word has to match the name of a stopped job exactly; if you set it to substring, the word just needs to match a substring of a stopped job's name. That substring value gives you functionality analogous to the %? job identifier (see JOB CONTROL below). If you set it to any other value (like prefix), the word has to be a prefix of a stopped job's name, which gives you functionality analogous to the %string job identifier.
histchars — the two or three characters that control history expansion, quick substitution, and tokenization (see HISTORY EXPANSION below). The first character is the history expansion character, the one that begins a history expansion, normally “!”. The second character is the quick substitution character, normally “^” — when it shows up as the first character on the line, history substitution repeats the previous command, swapping one string for another. The optional third character is the history comment character, normally “#”, which means the rest of the line is a comment when it appears as the first character of a word. The history comment character disables history substitution for the remaining words on the line. It does not necessarily make the shell parser treat the rest of the line as a comment. 🎀

🍓 Arrays 🍓

Bash gives you one-dimensional indexed arrays and associative array variables. Any variable can be used as an indexed array, and the declare builtin explicitly declares one. There's no maximum limit on how big an array can be, and no requirement that members be indexed or assigned contiguously. Indexed arrays are referenced using arithmetic expressions that have to expand to an integer (see ARITHMETIC EVALUATION below) and they're zero-based; associative arrays are referenced using arbitrary strings. Unless it says otherwise somewhere, indexed array indices have to be non-negative integers.

The shell performs parameter and variable expansion, arithmetic expansion, command substitution, and quote removal on indexed array subscripts. Since that can potentially leave you with empty strings, subscript indexing treats those as expressions that evaluate to 0. 💅

For associative array subscripts, the shell performs tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, and quote removal. Empty strings cannot be used as associative array keys.

Bash automatically creates an indexed array if any variable gets assigned using the syntax

name[subscript]=value .

The subscript is treated as an arithmetic expression that has to evaluate to a number greater than or equal to zero. To explicitly declare an indexed array, use

declare -a name

(see SHELL BUILTIN COMMANDS below).

declare -a name[subscript]

is also accepted; the subscript is just ignored. 😌

Associative arrays get created using

declare -A name

Attributes can be specified for an array variable using the declare and readonly builtins. Each attribute applies to all members of an array.

Arrays are assigned using compound assignments of the form name=(value1 ... valuen), where each value may be of the form [subscript]=string. Indexed array assignments don't need anything but string. Each value in the list is expanded using the shell expansions described below under EXPANSION, but values that are valid variable assignments — including the brackets and subscript — do not undergo brace expansion and word splitting, same as with individual variable assignments.

When you're assigning to indexed arrays, if the optional brackets and subscript are supplied, that index is the one that gets assigned to; otherwise the index of the element assigned is the last index assigned to by the statement plus one. Indexing starts at zero, bestie. ✨

When you're assigning to an associative array, the words in a compound assignment can be either assignment statements, for which the subscript is required, or a list of words that gets interpreted as a sequence of alternating keys and values: name=( key1 value1 key2 value2 ...). Those are treated identically to name=( [key1]=value1 [key2]=value2 ...). The first word in the list is what determines how the remaining words get interpreted; all assignments in a list have to be the same type. When you're using key/value pairs, the keys can't be missing or empty; a final missing value is treated like the empty string.

This syntax is also accepted by the declare builtin. Individual array elements can be assigned to using the name[subscript]=value syntax we introduced above.

When assigning to an indexed array, if name is subscripted by a negative number, that number gets interpreted as relative to one greater than the maximum index of name, so negative indices count back from the end of the array, and an index of -1 references the last element. 👑

The “+=” operator appends to an array variable when you're assigning using the compound assignment syntax; see PARAMETERS above.

An array element is referenced using ${name[subscript]}. The braces are required to avoid conflicts with pathname expansion. If subscript is @ or *, the word expands to all members of name, unless noted in the description of a builtin or word expansion. These subscripts differ only when the word appears within double quotes. If the word is double-quoted, ${name[*]} expands to a single word with the value of each array member separated by the first character of the IFS special variable, and ${name[@]} expands each element of name to a separate word. When there are no array members, ${name[@]} expands to nothing. If the double-quoted expansion happens within a word, the expansion of the first parameter is joined with the beginning part of the expansion of the original word, and the expansion of the last parameter is joined with the last part of the expansion of the original word. This is analogous to the expansion of the special parameters * and @ (see Special Parameters above).

${#name[subscript]} expands to the length of ${name[subscript]}. If subscript is * or @, the expansion is the number of elements in the array. 🌸

If the subscript used to reference an element of an indexed array evaluates to a number less than zero, it's interpreted as relative to one greater than the maximum index of the array, so negative indices count back from the end of the array, and an index of -1 references the last element.

Referencing an array variable without a subscript is equivalent to referencing the array with a subscript of 0. Any reference to a variable using a valid subscript is valid; bash creates an array if necessary. No cap. 🦋

An array variable counts as set the moment some subscript has been assigned a value. And yes, the null string totally counts as a valid value. Empty is still a vibe 💅

You can also grab the keys (the indices) and not just the values. ${!name[@]} and ${!name[*]} expand out to the indices that got assigned in the array variable name. When you're inside double quotes, the treatment is basically the same as how the special parameters @ and * behave inside double quotes.

The unset builtin is your girl for destroying arrays:

unset name[subscript] unsets the array element at index subscript — works for both indexed and associative arrays.
Negative subscripts on indexed arrays get interpreted the way described above.
Unsetting the last element of an array variable does not unset the variable itself. She's still there, bestie.
unset name, where name is an array, removes the whole entire array.
unset name[subscript] acts different depending on whether name is indexed or associative, when subscript is * or @:
- If name is an associative array, this unsets the element whose subscript is * or @.
- If name is an indexed array, unset removes all of the elements but does not remove the array itself.

Heads up: when you pass a variable name with a subscript as an argument to a command — like with unset — and you're not using the word expansion syntax described above (e.g., unset a[4]), that argument is subject to pathname expansion. Quote the argument if you don't want that (e.g., unset 'a[4]'). Lowkey a trap, fr fr.

The declare, local, and readonly builtins each take a -a option to specify an indexed array, and a -A option to specify an associative array. If you supply both, -A wins, periodt. The read builtin takes a -a option to assign a list of words read from standard input to an array. The set and declare builtins display array values in a way that lets them be reused as assignments. Other builtins accept array name arguments too (e.g., mapfile) — go check the descriptions of individual builtins below for the deets. And the shell ships with a bunch of builtin array variables. 👑

✨ EXPANSION ✨

Expansion happens on the command line after it's been split into words. The shell performs these expansions: brace expansion, tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, word splitting, pathname expansion, and quote removal.

The order of expansions is:

brace expansion;
tilde expansion, parameter and variable expansion, arithmetic expansion, and command substitution (done left-to-right);
word splitting;
pathname expansion;
quote removal.

On systems that can support it, there's one extra expansion available: process substitution. It's performed at the same time as tilde, parameter, variable, and arithmetic expansion and command substitution.

Quote removal is always performed last. It removes quote characters present in the original word — not ones that resulted from one of the other expansions, unless they've been quoted themselves.

Only brace expansion, word splitting, and pathname expansion can increase the number of words in the expansion; all the other expansions take a single word and give you back a single word. The only exceptions are the expansions of "$@" and "${name[@]}", and, in most cases, $* and ${name[*]} as explained above (see PARAMETERS). 🦋

🎀 Brace Expansion 🎀

Brace expansion is the mechanism that generates arbitrary strings sharing a common prefix and suffix, where either one of those can be empty. It's similar to pathname expansion, except the filenames generated don't have to exist. Iconic.

Patterns to be brace expanded are formed from:

an optional preamble, followed by
either a series of comma-separated strings or a sequence expression between a pair of braces, followed by
an optional postscript.

The preamble gets prefixed to each string inside the braces, and then the postscript gets appended to each resulting string, expanding left to right.

Brace expansions may be nested. The results of each expanded string are not sorted — brace expansion preserves left-to-right order. For example, a{d,c,b}e expands into “ade ace abe”.

A sequence expression takes the form x..y[..incr], where x and y are either integers or single letters, and incr, an optional increment, is an integer.

When integers are supplied, the expression expands to each number between x and y, inclusive. If either x or y begins with a zero, each generated term will contain the same number of digits, zero-padding where necessary.
When letters are supplied, the expression expands to each character lexicographically between x and y, inclusive, using the C locale.
Note that both x and y must be of the same type (integer or letter).
When the increment is supplied, it's used as the difference between each term. The default increment is 1 or -1 as appropriate.

Brace expansion is performed before any other expansions, and any characters special to other expansions are preserved in the result. It is strictly textual. Bash does not apply any syntactic interpretation to the context of the expansion or to the text between the braces.

A correctly-formed brace expansion must contain unquoted opening and closing braces, and at least one unquoted comma or a valid sequence expression. Any incorrectly formed brace expansion is left unchanged. 💀

A { or , may be quoted with a backslash to stop it from being considered part of a brace expression. And to avoid conflicts with parameter expansion, the string ${ is not considered eligible for brace expansion, and it inhibits brace expansion until the closing }.

This construct is typically used as shorthand when the common prefix of the strings to be generated is longer than in the example above:

mkdir /usr/local/src/bash/{old,new,dist,bugs}

chown root /usr/{ucb/{ex,edit},lib/{ex?.?*,how_ex}}

Brace expansion introduces a slight incompatibility with historical versions of sh. sh doesn't treat opening or closing braces specially when they appear as part of a word, and preserves them in the output. Bash removes braces from words as a consequence of brace expansion. For example, a word entered to sh as “file{1,2}” appears identically in the output. Bash outputs that word as “file1 file2” after brace expansion. Start bash with the +B option, or disable brace expansion with the +B option to the set command (see SHELL BUILTIN COMMANDS below), for strict sh compatibility. 🌈

🌸 Tilde Expansion 🌸

If a word begins with an unquoted tilde character (~), all of the characters preceding the first unquoted slash (or all characters, if there's no unquoted slash) are considered a tilde-prefix. If none of the characters in the tilde-prefix are quoted, the characters in the tilde-prefix following the tilde are treated as a possible login name.

If this login name is the null string, the tilde is replaced with the value of the shell parameter HOME.
If HOME is unset, the tilde expands to the home directory of the user executing the shell instead.
Otherwise, the tilde-prefix is replaced with the home directory associated with the specified login name.

If the tilde-prefix is a ~+, the value of the shell variable PWD replaces the tilde-prefix. If the tilde-prefix is a ~-, the shell substitutes the value of the shell variable OLDPWD, if it's set. If the characters following the tilde in the tilde-prefix consist of a number N, optionally prefixed by a + or a -, the tilde-prefix is replaced with the corresponding element from the directory stack, as it would be displayed by the dirs builtin invoked with the characters following the tilde in the tilde-prefix as an argument. If the characters following the tilde in the tilde-prefix consist of a number without a leading + or -, tilde expansion assumes +.

The results of tilde expansion are treated as if they were quoted, so the replacement is not subject to word splitting and pathname expansion. ✨

If the login name is invalid, or the tilde expansion fails, the tilde-prefix is left unchanged.

Bash checks each variable assignment for unquoted tilde-prefixes immediately following a : or the first =, and performs tilde expansion in those cases. Consequently, you can use filenames with tildes in assignments to PATH, MAILPATH, and CDPATH, and the shell assigns the expanded value. 💖

Bash also performs tilde expansion on words satisfying the conditions of variable assignments (as described above under PARAMETERS) when they appear as arguments to simple commands. Bash does not do this — except for the declaration commands listed above — when in posix mode.

💖 Parameter Expansion 💖

The $ character introduces parameter expansion, command substitution, or arithmetic expansion. The parameter name or symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to be expanded from characters immediately following it that could be interpreted as part of the name.

When braces are used, the matching ending brace is the first } not escaped by a backslash or within a quoted string, and not within an embedded arithmetic expansion, command substitution, or parameter expansion.

The basic form of parameter expansion is

${parameter}

which substitutes the value of parameter. The braces are required when parameter is a positional parameter with more than one digit, or when parameter is followed by a character that isn't to be interpreted as part of its name. The parameter is a shell parameter as described above (PARAMETERS) or an array reference (Arrays).

If the first character of parameter is an exclamation point (!), and parameter is not a nameref, it introduces a level of indirection. Bash uses the value formed by expanding the rest of parameter as the new parameter; this new parameter is then expanded and that value is used in the rest of the expansion, rather than the expansion of the original parameter. This is known as indirect expansion. The value is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion. If parameter is a nameref, this expands to the name of the parameter referenced by parameter instead of performing the complete indirect expansion, for compatibility. The exceptions to this are the expansions of ${!prefix*} and ${!name[@]} described below. The exclamation point must immediately follow the left brace in order to introduce indirection. ⭐

In each of the cases below, word is subject to tilde expansion, parameter expansion, command substitution, and arithmetic expansion.

When not performing substring expansion, using the forms documented below (e.g., :-), bash tests for a parameter that is unset or null. Omitting the colon tests only for a parameter that is unset. 💅

okay bestie, the parameter expansion menu continues and this is the part where it gets actually unhinged 💅 every single one of these bad boys takes your parameter and does something to it. buckle up.

${parameter:-word} — Use Default Values. If parameter is unset or null, the expansion of word gets swapped in instead. Otherwise, you just get the value of parameter. the classic "if you're not here, here's a backup" move ✨
${parameter:=word} — Assign Default Values. If parameter is unset or null, the expansion of word gets assigned to parameter, and that expansion becomes the final value of parameter. but heads up — positional parameters and special parameters may not be assigned this way. they simply will not let you 💀
${parameter:?word} — Display Error if Null or Unset. If parameter is null or unset, the shell writes the expansion of word (or a message to that effect if word isn't present) to the standard error and, if it's not interactive, exits with a non-zero status. An interactive shell does not exit, but it also does not execute the command associated with the expansion. Otherwise, the value of parameter is substituted. it's giving "bestie we have a problem and I'm telling everyone" 😭
${parameter:+word} — Use Alternate Value. If parameter is null or unset, nothing gets substituted. Otherwise, the expansion of word is substituted. and the value of parameter is not used 🫶
${parameter:offset}, ${parameter:offset:length} — Substring Expansion. Expands to up to length characters of the value of parameter starting at the character specified by offset. If parameter is @ or *, an indexed array subscripted by @ or *, or an associative array name, the results differ as described below. If :length is omitted (the first form above), this expands to the substring of the value of parameter starting at the character specified by offset and extending to the end of the value. If offset is omitted, it's treated as 0. If length is omitted, but the colon after offset is present, it's treated as 0. length and offset are arithmetic expressions (see ARITHMETIC EVALUATION below).
- If offset evaluates to a number less than zero, the value is used as an offset in characters from the end of the value of parameter. If length evaluates to a number less than zero, it is interpreted as an offset in characters from the end of the value of parameter rather than a number of characters, and the expansion is the characters between offset and that result. note that a negative offset must be separated from the colon by at least one space to avoid being confused with the :- expansion. don't get them mixed up, they are NOT the same thing 🙈
- If parameter is @ or *, the result is length positional parameters beginning at offset. A negative offset is taken relative to one greater than the greatest positional parameter, so an offset of -1 evaluates to the last positional parameter (or 0 if there are no positional parameters). it is an expansion error if length evaluates to a number less than zero.
- If parameter is an indexed array name subscripted by @ or *, the result is the length members of the array beginning with ${parameter[offset]}. A negative offset is taken relative to one greater than the maximum index of the specified array. it is an expansion error if length evaluates to a number less than zero.
- Substring expansion applied to an associative array produces undefined results. we do not do that here 💀
- Substring indexing is zero-based unless the positional parameters are used, in which case the indexing starts at 1 by default. If offset is 0, and the positional parameters are used, $0 is prefixed to the list. iykyk
${!prefix*}, ${!prefix@} — Names matching prefix. Expands to the names of variables whose names begin with prefix, separated by the first character of the IFS special variable. When @ is used and the expansion appears within double quotes, each variable name expands to a separate word. like a little friend group of variables that all start the same way 🎀
${!name[@]}, ${!name[*]} — List of array keys. If name is an array variable, expands to the list of array indices (keys) assigned in name. If name is not an array, expands to 0 if name is set and null otherwise. When @ is used and the expansion appears within double quotes, each key expands to a separate word. the receipts of which slots are actually filled 🧚
${#parameter} — Parameter length. Substitutes the length in characters of the expanded value of parameter. If parameter is * or @, the value substituted is the number of positional parameters. If parameter is an array name subscripted by * or @, the value substituted is the number of elements in the array. If parameter is an indexed array name subscripted by a negative number, that number is interpreted as relative to one greater than the maximum index of parameter, so negative indices count back from the end of the array, and an index of -1 references the last element. counting queen 👑
${parameter#word}, ${parameter##word} — Remove matching prefix pattern. The word is expanded to produce a pattern just as in pathname expansion, and matched against the expanded value of parameter using the rules described under Pattern Matching below. If the pattern matches the beginning of the value of parameter, then the result of the expansion is the expanded value of parameter with the shortest matching pattern (the # case) or the longest matching pattern (the ## case) deleted. If parameter is @ or *, the pattern removal operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with @ or *, the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list. single # = shy, double ## = greedy. periodt. 💅
${parameter%word}, ${parameter%%word} — Remove matching suffix pattern. The word is expanded to produce a pattern just as in pathname expansion, and matched against the expanded value of parameter using the rules described under Pattern Matching below. If the pattern matches a trailing portion of the expanded value of parameter, then the result of the expansion is the expanded value of parameter with the shortest matching pattern (the % case) or the longest matching pattern (the %% case) deleted. If parameter is @ or *, the pattern removal operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with @ or *, the pattern removal operation is applied to each member of the array in turn, and the expansion is the resultant list. same energy, other end of the string 🦋
${parameter/pattern/string}, ${parameter//pattern/string}, ${parameter/#pattern/string}, ${parameter/%pattern/string} — Pattern substitution. The pattern is expanded to produce a pattern and matched against the expanded value of parameter as described under Pattern Matching below. The longest match of pattern in the expanded value is replaced with string. string undergoes tilde expansion, parameter and variable expansion, arithmetic expansion, command and process substitution, and quote removal.
- In the first form above, only the first match is replaced. If there are two slashes separating parameter and pattern (the second form above), all matches of pattern are replaced with string. If pattern is preceded by # (the third form above), it must match at the beginning of the expanded value of parameter. If pattern is preceded by % (the fourth form above), it must match at the end of the expanded value of parameter.
- If the expansion of string is null, matches of pattern are deleted and the / following pattern may be omitted.
- If the patsub_replacement shell option is enabled using shopt, any unquoted instances of & in string are replaced with the matching portion of pattern.
- Quoting any part of string inhibits replacement in the expansion of the quoted portion, including replacement strings stored in shell variables. Backslash escapes & in string; the backslash is removed in order to permit a literal & in the replacement string. Backslash can also be used to escape a backslash; \\ results in a literal backslash in the replacement. users should take care if string is double-quoted to avoid unwanted interactions between the backslash and double-quoting, since backslash has special meaning within double quotes. Pattern substitution performs the check for unquoted & after expanding string; shell programmers should quote any occurrences of & they want to be taken literally in the replacement and ensure any instances of & they want to be replaced are unquoted. this is the part that eats people alive, ngl 💀
- Like the pattern removal operators, double quotes surrounding the replacement string quote the expanded characters, while double quotes enclosing the entire parameter substitution do not, since the expansion is performed in a context that doesn't take any enclosing double quotes into account.
- If the nocasematch shell option is enabled, the match is performed without regard to the case of alphabetic characters. case-blind, as a treat
- If parameter is @ or *, the substitution operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with @ or *, the substitution operation is applied to each member of the array in turn, and the expansion is the resultant list.
${parameter^pattern}, ${parameter^^pattern}, ${parameter,pattern}, ${parameter,,pattern} — Case modification. This expansion modifies the case of alphabetic characters in parameter. First, the pattern is expanded to produce a pattern as described below under Pattern Matching. Bash then examines characters in the expanded value of parameter against pattern as described below. If a character matches the pattern, its case is converted. the pattern should not attempt to match more than one character. one character per character bestie, we are NOT greedy here 🌈✨

okay bestie so we're still in parameter expansion land and it's giving ✨case modification✨ — literally so useful. Using ^ converts lowercase letters matching pattern to uppercase; , converts matching uppercase letters to lowercase. The ^ and , variants examine the first character in the expanded value and convert its case if it matches pattern; the ^^ and ,, variants examine all characters in the expanded value and convert each one that matches pattern. If pattern is omitted, it is treated like a ?, which matches every character.

If parameter is @ or *, the case modification operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with @ or *, the case modification operation is applied to each member of the array in turn, and the expansion is the resultant list.

💅 ${parameter@operator} 💅 — Parameter transformation. The expansion is either a transformation of the value of parameter or information about parameter itself, depending on the value of operator. Each operator is a single letter:

U — The expansion is a string that is the value of parameter with lowercase alphabetic characters converted to uppercase.
u — The expansion is a string that is the value of parameter with the first character converted to uppercase, if it is alphabetic.
L — The expansion is a string that is the value of parameter with uppercase alphabetic characters converted to lowercase.
Q — The expansion is a string that is the value of parameter quoted in a format that can be reused as input.
E — The expansion is a string that is the value of parameter with backslash escape sequences expanded as with the $'...' quoting mechanism.
P — The expansion is a string that is the result of expanding the value of parameter as if it were a prompt string (see PROMPTING below).
A — The expansion is a string in the form of an assignment statement or declare command that, if evaluated, recreates parameter with its attributes and value.
K — Produces a possibly-quoted version of the value of parameter, except that it prints the values of indexed and associative arrays as a sequence of quoted key-value pairs (see Arrays above). The keys and values are quoted in a format that can be reused as input.
a — The expansion is a string consisting of flag values representing parameter's attributes.
k — Like the K transformation, but expands the keys and values of indexed and associative arrays to separate words after word splitting.

If parameter is @ or *, the operation is applied to each positional parameter in turn, and the expansion is the resultant list. If parameter is an array variable subscripted with @ or *, the operation is applied to each member of the array in turn, and the expansion is the resultant list.

The result of the expansion is subject to word splitting and pathname expansion as described below.

✨ Command Substitution ✨

Command substitution allows the output of a command to replace the command itself. There are two standard forms:

$(command)

or (deprecated)

`command`.

Bash performs the expansion by executing command in a subshell environment and replacing the command substitution with the standard output of the command, with any trailing newlines deleted. Embedded newlines are not deleted, but they may be removed during word splitting. The command substitution $(cat file) can be replaced by the equivalent but faster $(< file).

With the old-style backquote form of substitution, backslash retains its literal meaning except when followed by $, `, or \. The first backquote not preceded by a backslash terminates the command substitution. When using the $(command) form, all characters between the parentheses make up the command; none are treated specially.

There is an alternate form of command substitution:

${c command;}

which executes command in the current execution environment and captures its output, again with trailing newlines removed.

The character c following the open brace must be a space, tab, newline, or |, and the close brace must be in a position where a reserved word may appear (i.e., preceded by a command terminator such as semicolon). Bash allows the close brace to be joined to the remaining characters in the word without being followed by a shell metacharacter as a reserved word would usually require.

Any side effects of command take effect immediately in the current execution environment and persist in the current environment after the command completes (e.g., the exit builtin exits the shell).

This type of command substitution superficially resembles executing an unnamed shell function: local variables are created as when a shell function is executing, and the return builtin forces command to complete; however, the rest of the execution environment, including the positional parameters, is shared with the caller.

If the first character following the open brace is a |, the construct expands to the value of the REPLY shell variable after command executes, without removing any trailing newlines, and the standard output of command remains the same as in the calling shell. Bash creates REPLY as an initially-unset local variable when command executes, and restores REPLY to the value it had before the command substitution after command completes, as with any local variable.

Command substitutions may be nested. To nest when using the backquoted form, escape the inner backquotes with backslashes.

If the substitution appears within double quotes, bash does not perform word splitting and pathname expansion on the results.

🌈 Arithmetic Expansion 🌈

Arithmetic expansion evaluates an arithmetic expression and substitutes the result. The format for arithmetic expansion is:

$((expression))

The expression undergoes the same expansions as if it were within double quotes, but unescaped double quote characters in expression are not treated specially and are removed. All tokens in the expression undergo parameter and variable expansion, command substitution, and quote removal. The result is treated as the arithmetic expression to be evaluated. Since the way Bash handles double quotes can potentially result in empty strings, arithmetic expansion treats those as expressions that evaluate to 0. Arithmetic expansions may be nested.

The evaluation is performed according to the rules listed below under ARITHMETIC EVALUATION. If expression is invalid, bash prints a message to standard error indicating failure, does not perform the substitution, and does not execute the command associated with the expansion.

🦋 Process Substitution 🦋

Process substitution allows a process's input or output to be referred to using a filename. It takes the form of <(list) or >(list). The process list is run asynchronously, and its input or output appears as a filename. This filename is passed as an argument to the current command as the result of the expansion.

If the >(list) form is used, writing to the file provides input for list. If the <(list) form is used, reading the file obtains the output of list. No space may appear between the < or > and the left parenthesis, otherwise the construct would be interpreted as a redirection.

Process substitution is supported on systems that support named pipes (FIFOs) or the /dev/fd method of naming open files.

When available, process substitution is performed simultaneously with parameter and variable expansion, command substitution, and arithmetic expansion.

🎀 Word Splitting 🎀

The shell scans the results of parameter expansion, command substitution, and arithmetic expansion that did not occur within double quotes for word splitting. Words that were not expanded are not split.

The shell treats each character of IFS as a delimiter, and splits the results of the other expansions into words using these characters as field terminators.

An IFS whitespace character is whitespace as defined above (see Definitions) that appears in the value of IFS. Space, tab, and newline are always considered IFS whitespace, even if they don't appear in the locale's space category.

If IFS is unset, field splitting acts as if its value were <space><tab><newline>, and treats these characters as IFS whitespace. If the value of IFS is null, no word splitting occurs, but implicit null arguments (see below) are still removed.

Word splitting begins by removing sequences of IFS whitespace characters from the beginning and end of the results of the previous expansions, then splits the remaining words.

If the value of IFS consists solely of IFS whitespace, any sequence of IFS whitespace characters delimits a field, so a field consists of characters that are not unquoted IFS whitespace, and null fields result only from quoting.

If IFS contains a non-whitespace character, then any character in the value of IFS that is not IFS whitespace, along with any adjacent IFS whitespace characters, delimits a field. This means that adjacent non-IFS-whitespace delimiters produce a null field. A sequence of IFS whitespace characters also delimits a field.

Explicit null arguments ("" or '') are retained and passed to commands as empty strings. Unquoted implicit null arguments, resulting from the expansion of parameters that have no values, are removed. Expanding a parameter with no value within double quotes produces a null field, which is retained and passed to a command as an empty string.

When a quoted null argument appears as part of a word whose expansion is non-null, word splitting removes the null argument portion, leaving the non-null expansion. That is, the word “-d''” becomes “-d” after word splitting and null argument removal.

⭐ Pathname Expansion ⭐

After word splitting, unless the -f option has been set, bash scans each word for the characters *, ?, and [. If one of these characters appears, and is not quoted, then the word is regarded as a pattern, and replaced with a sorted list of filenames matching the pattern (see Pattern Matching below) subject to the value of the GLOBSORT shell variable.

okay so bestie, if no matching filenames are found, and the shell option nullglob is not enabled, the word is left unchanged — like, it just vibes there. If nullglob is set and there are zero matches? the word gets removed. If the failglob shell option is set and nothing matches, bash prints an error message and does not execute the command. And if the shell option nocaseglob is enabled, the match happens without regard to the case of alphabetic characters. 💅

When a pattern is used for pathname expansion, the character . at the start of a name or immediately following a slash must be matched explicitly, unless the shell option dotglob is set. In order to match the filenames . and .., the pattern must begin with . (for example, .?), even if dotglob is set. If the globskipdots shell option is enabled, the filenames . and .. never match, even if the pattern begins with a .. When not matching pathnames, the . character is not treated specially.

When matching a pathname, the slash character must always be matched explicitly by a slash in the pattern, but in other matching contexts it can be matched by a special pattern character as described below under Pattern Matching. 🦋

See the description of shopt below under SHELL BUILTIN COMMANDS for a description of the nocaseglob, nullglob, globskipdots, failglob, and dotglob shell options.

The GLOBIGNORE shell variable may be used to restrict the set of file names matching a pattern. If GLOBIGNORE is set, each matching file name that also matches one of the patterns in GLOBIGNORE is removed from the list of matches. If the nocaseglob option is set, the matching against the patterns in GLOBIGNORE is performed without regard to case. The filenames . and .. are always ignored when GLOBIGNORE is set and not null. However, setting GLOBIGNORE to a non-null value has the effect of enabling the dotglob shell option, so all other filenames beginning with a . match. To get the old behavior of ignoring filenames beginning with a ., make .* one of the patterns in GLOBIGNORE. The dotglob option is disabled when GLOBIGNORE is unset. The GLOBIGNORE pattern matching honors the setting of the extglob shell option.

The value of the GLOBSORT shell variable controls how the results of pathname expansion are sorted, as described above under Shell Variables.

✨ Pattern Matching ✨

Any character that appears in a pattern, other than the special pattern characters described below, matches itself. The NUL character may not occur in a pattern. A backslash escapes the following character; the escaping backslash is discarded when matching. The special pattern characters must be quoted if they are to be matched literally. 💅

The special pattern characters have the following meanings:

* — Matches any string, including the null string. When the globstar shell option is enabled, and * is used in a pathname expansion context, two adjacent *s used as a single pattern match all files and zero or more directories and subdirectories. If followed by a /, two adjacent *s match only directories and subdirectories.
? — Matches any single character.
[...] — Matches any one of the characters enclosed between the brackets. This is known as a bracket expression and matches a single character. A pair of characters separated by a hyphen denotes a range expression; any character that falls between those two characters, inclusive, using the current locale's collating sequence and character set, matches. If the first character following the [ is a ! or a ^ then any character not within the range matches. To match a -, include it as the first or last character in the set. To match a ], include it as the first character in the set.

The sorting order of characters in range expressions, and the characters included in the range, are determined by the current locale and the values of the LC_COLLATE or LC_ALL shell variables, if set. To obtain the traditional interpretation of range expressions, where [a-d] is equivalent to [abcd], set the value of the LC_COLLATE or LC_ALL shell variables to C, or enable the globasciiranges shell option.

Within a bracket expression, character classes can be specified using the syntax [:class:], where class is one of the following classes defined in the POSIX standard:

alnum alpha ascii blank cntrl digit graph lower print
punct space upper word xdigit

A character class matches any character belonging to that class. The word character class matches letters, digits, and the character _.

Within a bracket expression, an equivalence class can be specified using the syntax [=c=], which matches all characters with the same collation weight (as defined by the current locale) as the character c.

Within a bracket expression, the syntax [.symbol.] matches the collating symbol symbol.

If the extglob shell option is enabled using the shopt builtin, the shell recognizes several extended pattern matching operators. In the following description, a pattern-list is a list of one or more patterns separated by a |. Composite patterns may be formed using one or more of the following sub-patterns:

?(pattern-list) — Matches zero or one occurrence of the given patterns.
*(pattern-list) — Matches zero or more occurrences of the given patterns.
+(pattern-list) — Matches one or more occurrences of the given patterns.
@(pattern-list) — Matches one of the given patterns.
!(pattern-list) — Matches anything except one of the given patterns.

The extglob option changes the behavior of the parser, since the parentheses are normally treated as operators with syntactic meaning. To ensure that extended matching patterns are parsed correctly, make sure that extglob is enabled before parsing constructs containing the patterns, including shell functions and command substitutions. 😭

When matching filenames, the dotglob shell option determines the set of filenames that are tested: when dotglob is enabled, the set of filenames includes all files beginning with ., but . and .. must be matched by a pattern or sub-pattern that begins with a dot; when it is disabled, the set does not include any filenames beginning with . unless the pattern or sub-pattern begins with a .. If the globskipdots shell option is enabled, the filenames . and .. never appear in the set. As above, . only has a special meaning when matching filenames.

Complicated extended pattern matching against long strings is slow, especially when the patterns contain alternations and the strings contain multiple matches. Using separate matches against shorter strings, or using arrays of strings instead of a single long string, may be faster. 🐢

🌸 Quote Removal 🌸

After the preceding expansions, all unquoted occurrences of the characters \, ', and " that did not result from one of the above expansions are removed. ✨

💅 REDIRECTION 💅

okay so before a command even gets to run, its input and output can get redirected using a special lil notation that the shell itself interprets. redirection is the whole reason commands' file handles can be duplicated, opened, closed, and made to point at totally different files — and it can also change which files the command reads from and writes to. and when you use it with the exec builtin? it modifies file handles in the current shell execution environment, not just some baby subshell. 💅

the following redirection operators can come before a simple command, appear anywhere within it, or follow the command. and they get processed in the order they show up, left to right. periodt.

each redirection that can be preceded by a file descriptor number can also be preceded by a word shaped like {varname}. in that case — for every redirection operator except >&- and <&- — the shell allocates a file descriptor greater than or equal to 10 and assigns it to varname. if {varname} comes before >&- or <&-, then the value of varname tells you which file descriptor to close. and if {varname} is supplied at all, the redirection persists beyond the scope of the command, which lets the shell programmer manage the file descriptor's lifetime manually without ever touching the exec builtin. the varredir_close shell option manages this behavior. iykyk 🫶

in the descriptions below, if the file descriptor number is omitted and the first character of the redirection operator is “<”, the redirection refers to standard input (file descriptor 0). if the first character of the redirection operator is “>”, the redirection refers to standard output (file descriptor 1). simple.

the word following the redirection operator — unless noted otherwise — gets put through brace expansion, tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, quote removal, pathname expansion, and word splitting. if it expands to more than one word, bash reports an error. 😭 so don't do that.

🎀 the order matters, bestie 🎀

the order of redirections is significant. for example, this command:

ls > dirlist 2>&1

directs both standard output and standard error to the file dirlist. but this one:

ls 2>&1 > dirlist

directs only the standard output to the file dirlist, because standard error was pointed at standard output before standard output got redirected to dirlist. the way that one character of reordering changes everything… it's giving chaos. 💀

🌸 special filenames 🌸

bash handles several filenames specially when they're used in redirections, as described below. if the operating system bash is running on provides these special files, bash uses them; otherwise it emulates them internally with the behavior described here.

/dev/fd/fd — if fd is a valid integer, duplicate file descriptor fd.
/dev/stdin — file descriptor 0 is duplicated.
/dev/stdout — file descriptor 1 is duplicated.
/dev/stderr — file descriptor 2 is duplicated.
/dev/tcp/host/port — if host is a valid hostname or Internet address, and port is an integer port number or service name, bash attempts to open the corresponding TCP socket.
/dev/udp/host/port — if host is a valid hostname or Internet address, and port is an integer port number or service name, bash attempts to open the corresponding UDP socket.

a failure to open or create a file causes the redirection to fail. that's just how it is. 🙈

⚠️ redirections using file descriptors greater than 9 should be used with care, because they may conflict with file descriptors the shell uses internally. not me casually stealing the shell's internal fds… 💀

✨ Redirecting Input ✨

redirecting input opens the file whose name results from the expansion of word for reading on file descriptor n, or on the standard input (file descriptor 0) if n isn't specified.

the general format for redirecting input is:

[n]<word

✨ Redirecting Output ✨

redirecting output opens the file whose name results from the expansion of word for writing on file descriptor n, or on the standard output (file descriptor 1) if n isn't specified. if the file doesn't exist, it gets created; if it does exist, it gets truncated to zero size. yes, zero. gone. 🫣

the general format for redirecting output is:

[n]>word

if the redirection operator is >, and the noclobber option to the set builtin command has been enabled, the redirection fails if the file whose name results from the expansion of word exists and is a regular file. if the redirection operator is >|, or the redirection operator is > and the noclobber option to the set builtin is not enabled, bash attempts the redirection even if the file named by word exists. so >| is your "i said what i said" operator. 💅

✨ Appending Redirected Output ✨

redirecting output in this fashion opens the file whose name results from the expansion of word for appending on file descriptor n, or on the standard output (file descriptor 1) if n isn't specified. if the file doesn't exist, it is created. (no truncating here — we're adding, not deleting. growth mindset. 🌱)

the general format for appending output is:

[n]>>word

✨ Redirecting Standard Output and Standard Error ✨

this construct redirects both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to the file whose name is the expansion of word.

there are two formats for redirecting standard output and standard error:

&>word

and

>&word

of the two forms, the first is preferred. this is semantically equivalent to:

>word 2>&1

when using the second form, word may not expand to a number or -. if it does, other redirection operators apply (see Duplicating File Descriptors below) for compatibility reasons. just a lil historical quirk, don't worry about it bestie. 🎀

✨ Appending Standard Output and Standard Error ✨

this construct appends both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to the file whose name is the expansion of word.

the format for appending standard output and standard error is:

&>>word

this is semantically equivalent to:

>>word 2>&1

(see Duplicating File Descriptors below).

✨ Here Documents ✨

this type of redirection tells the shell to read input from the current source until it reads a line containing only delimiter (with no trailing blanks). all of the lines read up to that point then become the standard input (or file descriptor n if n is specified) for a command. it's like a lil care package of text. 🎁

the format of here-documents is:

[n]<<[-]word
        here-document
delimiter

the shell does not perform parameter and variable expansion, command substitution, arithmetic expansion, or pathname expansion on word.

if any part of word is quoted, the delimiter is the result of quote removal on word, and the lines in the here-document are not expanded. if word is unquoted, the delimiter is word itself, and the here-document text is treated similarly to a double-quoted string: all lines of the here-document are subjected to parameter expansion, command substitution, and arithmetic expansion, the character sequence \<newline> is treated literally, and \ must be used to quote the characters \, $, and `; however, double quote characters have no special meaning. 🌈

if the redirection operator is <<-, then the shell strips all leading tab characters from input lines and the line containing delimiter. this allows here-documents within shell scripts to be indented in a natural fashion. aesthetic AND functional, we love to see it. 💖

if the delimiter is not quoted, the \<newline> sequence is treated as a line continuation: the two lines are joined and the backslash-newline is removed. this happens while reading the here-document, before the check for the ending delimiter, so joined lines can form the end delimiter. sneaky but valid. 👑

✨ Here Strings ✨

a variant of here documents, the format is:

[n]<<<word

the word undergoes tilde expansion, parameter and variable expansion, command substitution, arithmetic expansion, and quote removal. pathname expansion and word splitting are not performed. the result is supplied as a single string, with a newline appended, to the command on its standard input (or file descriptor n if n is specified). one string, one newline, no drama. ⭐

✨ Duplicating File Descriptors ✨

the redirection operator

[n]<&word

is used to duplicate input file descriptors. if word expands to one or more digits, file descriptor n is made to be a copy of that file descriptor. it is a redirection error if the digits in word do not specify a file descriptor open for input. if word evaluates to -, file descriptor n is closed. if n is not specified, this uses the standard input (file descriptor 0).

the operator

[n]>&word

is used similarly to duplicate output file descriptors. if n is not specified, this uses the standard output (file descriptor 1). it is a redirection error if the digits in word do not specify a file descriptor open for output. if word evaluates to -, file descriptor n is closed. as a special case, if n is omitted, and word does not expand to one or more digits or -, this redirects the standard output and standard error as described previously. special case behavior, iconic honestly. 👑

✨ Moving File Descriptors ✨

the redirection operator

[n]<&digit-

moves the file descriptor digit to file descriptor n, or to the standard input (file descriptor 0) if n is not specified. digit is closed after being duplicated to n. it's not a copy, bestie, it's a whole relocation. 📦

similarly, the redirection operator

[n]>&digit-

moves the file descriptor digit to file descriptor n, or to the standard output (file descriptor 1) if n is not specified.

✨ Opening File Descriptors for Reading and Writing ✨

the redirection operator

[n]<>word

opens the file whose name is the expansion of word for both reading and writing on file descriptor n, or on file descriptor 0 if n is not specified. if the file does not exist, it is created. the duality of it… reading and writing? she contains multitudes. 🦋

✨ ALIASES ✨

okay so bestie, aliases are basically little nickname swaps for commands 💅. An alias lets a string get substituted for a word when that word is in a position where it can be the first word of a simple command. Aliases have names and matching values, and you set/unset them with the alias and unalias builtin commands (peep SHELL BUILTIN COMMANDS below for the tea ☕).

When the shell reads an unquoted word in the right position, it checks that word to see if it matches an alias name. If it matches, the shell replaces the word with the alias value, and then reads that value as if it had been read instead of the word. And no cap: the shell doesn’t look at any characters following the word before attempting alias substitution. It’s giving instant swap energy ✨.

The characters /, $, `, and = — plus any of the shell metacharacters or quoting characters listed above — may not appear in an alias name. The replacement text, though? It can contain any valid shell input, including shell metacharacters. The first word of the replacement text is tested for aliases, but a word that is identical to an alias being expanded is not expanded a second time. This means you can alias ls to ls -F, for instance, and bash does not try to recursively expand the replacement text. Iconic, honestly 🎀.

If the last character of the alias value is a blank, the shell checks the next command word following the alias for alias expansion. So yes, trailing space matters, bestie 👀.

Aliases are created and listed with the alias command, and removed with the unalias command.

There is no mechanism for using arguments in the replacement text. If arguments are needed, use a shell function (see FUNCTIONS below) instead. periodt.

Aliases are not expanded when the shell is not interactive, unless the expand_aliases shell option is set using shopt (see the description of shopt under SHELL BUILTIN COMMANDS below).

ngl, the rules concerning the definition and use of aliases are somewhat confusing. Bash always reads at least one complete line of input, and all lines that make up a compound command, before executing any of the commands on that line or the compound command. Aliases are expanded when a command is read, not when it is executed. Therefore, an alias definition appearing on the same line as another command does not take effect until the shell reads the next line of input, and an alias definition in a compound command does not take effect until the shell parses and executes the entire compound command. The commands following the alias definition on that line, or in the rest of a compound command, are not affected by the new alias. This behavior is also an issue when functions are executed. Aliases are expanded when a function definition is read, not when the function is executed, because a function definition is itself a command. As a consequence, aliases defined in a function are not available until after that function is executed. To be safe, always put alias definitions on a separate line, and do not use alias in compound commands. This is the part where you listen to me, bestie 😭.

For almost every purpose, shell functions are preferable to aliases. fr fr.

🫶 FUNCTIONS 🫶

A shell function, defined as described above under SHELL GRAMMAR, stores a series of commands for later execution. When the name of a shell function is used as a simple command name, the shell executes the list of commands associated with that function name. Functions are executed in the context of the calling shell; there is no new process created to interpret them (contrast this with the execution of a shell script). It’s lowkey main character behavior: same shell, same vibe, no fork drama 🦋.

When a function is executed, the arguments to the function become the positional parameters during its execution. The special parameter # is updated to reflect the new positional parameters. Special parameter 0 is unchanged. The first element of the FUNCNAME variable is set to the name of the function while the function is executing. Cute, right? ✨

All other aspects of the shell execution environment are identical between a function and its caller with these exceptions: the DEBUG and RETURN traps (see the description of the trap builtin under SHELL BUILTIN COMMANDS below) are not inherited unless the function has been given the trace attribute (see the description of the declare builtin below) or the -o functrace shell option has been enabled with the set builtin (in which case all functions inherit the DEBUG and RETURN traps), and the ERR trap is not inherited unless the -o errtrace shell option has been enabled. Got it? good, because that sentence is a mouthful 💀.

Variables local to the function are declared with the local builtin command (local variables). Ordinarily, variables and their values are shared between the function and its caller. If a variable is declared local, the variable’s visible scope is restricted to that function and its children (including the functions it calls). So local is giving boundaries, and we love that for you 💜.

In the following description, the current scope is a currently-executing function. Previous scopes consist of that function’s caller and so on, back to the “global” scope, where the shell is not executing any shell function. A local variable at the current scope is a variable declared using the local or declare builtins in the function that is currently executing.

Local variables “shadow” variables with the same name declared at previous scopes. For instance, a local variable declared in a function hides variables with the same name declared at previous scopes, including global variables: references and assignments refer to the local variable, leaving the variables at previous scopes unmodified. When the function returns, the global variable is once again visible. Shadowing is real, and it’s not even scary 🌈.

The shell uses dynamic scoping to control a variable’s visibility within functions. With dynamic scoping, visible variables and their values are a result of the sequence of function calls that caused execution to reach the current function. The value of a variable that a function sees depends on its value within its caller, if any, whether that caller is the global scope or another shell function. This is also the value that a local variable declaration shadows, and the value that is restored when the function returns. It’s giving call-stack ancestry realness 👑.

For example, if a variable var is declared as local in function func1, and func1 calls another function func2, references to var made from within func2 resolve to the local variable var from func1, shadowing any global variable named var.

The unset builtin also acts using the same dynamic scope: if a variable is local to the current scope, unset unsets it; otherwise the unset will refer to the variable found in any calling scope as described above. If a variable at the current local scope is unset, it remains so (appearing as unset) until it is reset in that scope or until the function returns. Once the function returns, any instance of the variable at a previous scope becomes visible. If the unset acts on a variable at a previous scope, any instance of a variable with that name that had been shadowed becomes visible (see below how the localvar_unset shell option changes this behavior). okay that’s a lot of scoping lore, but you ate 💅.

The FUNCNEST variable, if set to a numeric value greater than 0, defines a maximum function nesting level. Function invocations that exceed the limit cause the entire command to abort. So FUNCNEST is the no more recursion, bestie button 🛑.

If the builtin command return is executed in a function, the function completes and execution resumes with the next command after the function call. If return is supplied a numeric argument, that is the function’s return status; otherwise the function’s return status is the exit status of the last command executed before the return. Any command associated with the RETURN trap is executed before execution resumes. When a function completes, the values of the positional parameters and the special parameter # are restored to the values they had prior to the function’s execution. Clean exit, no crumbs ✨.

The -f option to the declare or typeset builtin commands lists function names and definitions. The -F option to declare or typeset lists the function names only (and optionally the source file and line number, if the extdebug shell option is enabled). Functions may be exported so that child shell processes (those created when executing a separate shell invocation) automatically have them defined with the -f option to the export builtin. The -f option to the unset builtin deletes a function definition. Function management? We love to see it 🎀.

Functions may be recursive. The FUNCNEST variable may be used to limit the depth of the function call stack and restrict the number of function invocations. By default, bash imposes no limit on the number of recursive calls. So go off, but maybe set FUNCNEST unless you want your shell to spiral 😭.

🧮 ARITHMETIC EVALUATION 🧮

okay bestie, plot twist: bash can literally do math 💅 not always, only under certain circumstances — specifically the let and declare builtin commands, the (( compound command, the arithmetic for command, the [[ conditional command, and Arithmetic Expansion. That's the guest list, no plus-ones.

Evaluation is done in the largest fixed-width integers available, with no check for overflow 😭 though division by 0 is trapped and flagged as an error. So like. Overflow? Not bash's problem. Divide by zero? Bash will absolutely call you out.

The operators and their precedence, associativity, and values are the same as in the C language. Below is the operator list grouped into levels of equal-precedence operators, and the levels are listed in order of decreasing precedence. Highest-clout operators first, iykyk 👑

id++, id-- — variable post-increment and post-decrement
++id, --id — variable pre-increment and pre-decrement
-, + — unary minus and plus
!, ~ — logical and bitwise negation
** — exponentiation
*, /, % — multiplication, division, remainder
+, - — addition, subtraction
<<, >> — left and right bitwise shifts
<=, >=, <, > — comparison
==, != — equality and inequality
& — bitwise AND
^ — bitwise exclusive OR
| — bitwise OR
&& — logical AND
|| — logical OR
expr?expr:expr — conditional operator
=, *=, /=, %=, +=, -=, <<=, >>=, &=, ^=, |= — assignment
expr1, expr2 — comma

Shell variables are allowed as operands, bestie — parameter expansion is performed before the expression is evaluated. But within an expression, shell variables may also be referenced by name without using the parameter expansion syntax. Which means you can use x, where x is a shell variable name, in an arithmetic expression, and the shell will evaluate its value as an expression and use the result. A shell variable that is null or unset evaluates to 0 when referenced by name in an expression. Zero. Nothing. Nada. She's giving empty but the math says 0 ✨

The value of a variable is evaluated as an arithmetic expression when it is referenced, or when a variable which has been given the integer attribute using declare -i is assigned a value. A null value evaluates to 0. And a shell variable does not need its integer attribute turned on to be used in an expression — no prerequisites, she's welcome as she is 🫶

Integer constants follow the C language definition, without suffixes or character constants. So:

Constants with a leading 0 are interpreted as octal numbers
A leading 0x or 0X denotes hexadecimal
Otherwise, numbers take the form [base#]n, where the optional base is a decimal number between 2 and 64 representing the arithmetic base, and n is a number in that base
If base# is omitted, then base 10 is used
When specifying n, if a non-digit is required, the digits greater than 9 are represented by the lowercase letters, the uppercase letters, @, and _, in that order
If base is less than or equal to 36, lowercase and uppercase letters may be used interchangeably to represent numbers between 10 and 35

Operators are evaluated in precedence order. Sub-expressions in parentheses are evaluated first and may override the precedence rules above — parentheses are literally the "actually, let me speak" of arithmetic 🎀

🦋 CONDITIONAL EXPRESSIONS 🦋

Conditional expressions are used by the [[ compound command and the test and [ builtin commands to test file attributes and perform string and arithmetic comparisons. The test and [ commands determine their behavior based on the number of arguments — see the descriptions of those commands for any other command-specific actions, bestie.

Expressions are formed from the unary or binary primaries listed below. Unary expressions are often used to examine the status of a file or shell variable. Binary operators are used for string, numeric, and file attribute comparisons. Different jobs, same glow-up ✨

Bash handles several filenames specially when they're used in expressions. If the operating system on which bash is running provides these special files, bash will use them; otherwise it will emulate them internally with this behavior: if any file argument to one of the primaries is of the form /dev/fd/n, then bash checks file descriptor n. And if the file argument to one of the primaries is one of /dev/stdin, /dev/stdout, or /dev/stderr, bash checks file descriptor 0, 1, or 2, respectively. Sneaky little shortcut, we love her 💅

Unless otherwise specified, primaries that operate on files follow symbolic links and operate on the target of the link, rather than the link itself. So she's looking through the link, not at it. 👀

Also, a sorting tea: when used with [[, or when the shell is in posix mode, the < and > operators sort lexicographically using the current locale. When the shell is not in posix mode, the test command sorts using ASCII ordering. Not the same vibe, don't mix them up.

🎀 The file primaries 🎀

-a file — True if file exists.
-b file — True if file exists and is a block special file.
-c file — True if file exists and is a character special file.
-d file — True if file exists and is a directory.
-e file — True if file exists.
-f file — True if file exists and is a regular file.
-g file — True if file exists and is set-group-id.
-h file — True if file exists and is a symbolic link.
-k file — True if file exists and its “sticky” bit is set.
-p file — True if file exists and is a named pipe (FIFO).
-r file — True if file exists and is readable.
-s file — True if file exists and has a size greater than zero.
-t fd — True if file descriptor fd is open and refers to a terminal.
-u file — True if file exists and its set-user-id bit is set.
-w file — True if file exists and is writable.
-x file — True if file exists and is executable.
-G file — True if file exists and is owned by the effective group id.
-L file — True if file exists and is a symbolic link.
-N file — True if file exists and has been modified since it was last accessed.
-O file — True if file exists and is owned by the effective user id.
-S file — True if file exists and is a socket.

🌸 The variable primaries 🌸

-o optname — True if the shell option optname is enabled. See the list of options under the description of the -o option to the set builtin below.
-v varname — True if the shell variable varname is set (has been assigned a value). If varname is an indexed array variable name subscripted by @ or *, this returns true if the array has any set elements. If varname is an associative array variable name subscripted by @ or *, this returns true if an element with that key is set.
-R varname — True if the shell variable varname is set and is a name reference.

🍓 The string primaries 🍓

-z string — True if the length of string is zero.
string, -n string — True if the length of string is non-zero.

💜 The string comparisons 💜

string1 == string2, string1 = string2 — True if the strings are equal. = should be used with the test command for POSIX conformance. When used with the [[ command, this performs pattern matching as described above (Compound Commands).
string1 != string2 — True if the strings are not equal.
string1 < string2 — True if string1 sorts before string2 lexicographically.
string1 > string2 — True if string1 sorts after string2 lexicographically.

🌈 The file comparisons 🌈

file1 -ef file2 — True if file1 and file2 refer to the same device and inode numbers.
file1 -nt file2 — True if file1 is newer (according to modification date) than file2, or if file1 exists and file2 does not.
file1 -ot file2 — True if file1 is older than file2, or if file2 exists and file1 does not.

⭐ The arithmetic comparisons ⭐

arg1 OP arg2 — OP is one of -eq, -ne, -lt, -le, -gt, or -ge. These arithmetic binary operators return true if arg1 is equal to, not equal to, less than, less than or equal to, greater than, or greater than or equal to arg2, respectively. arg1 and arg2 may be positive or negative integers. When used with the [[ command, arg1 and arg2 are evaluated as arithmetic expressions (see ARITHMETIC EVALUATION above). Since the expansions the [[ command performs on arg1 and arg2 can potentially result in empty strings, arithmetic expression evaluation treats those as expressions that evaluate to 0.

🧚 SIMPLE COMMAND EXPANSION 🧚

When the shell executes a simple command, it performs the following expansions, assignments, and redirections, from left to right, in the following order. Order matters, bestie, do not skip steps 😤

The words that the parser has marked as variable assignments (those preceding the command name) and redirections are saved for later processing. Not now. Later. She's patient 💅
The words that are not variable assignments or redirections are expanded. If any words remain after expansion, the first word is taken to be the name of the command and the remaining words are the arguments.
Redirections are performed as described above under REDIRECTION.
The text after the = in each variable assignment undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal before being assigned to the variable.

If no command name results, the variable assignments affect the current shell environment. In the case of such a command (one that consists only of assignment statements and redirections), assignment statements are performed before redirections. Otherwise, the variables are added to the environment of the executed command and do not affect the current shell environment.

If any of the assignments attempts to assign a value to a readonly variable, an error occurs, and the command exits with a non-zero status. Readonly said no and she meant it 🚫

If no command name results, redirections are performed, but do not affect the current shell environment. A redirection error causes the command to exit with a non-zero status.

If there is a command name left after expansion, execution proceeds as described below. Otherwise, the command exits. If one of the expansions contained a command substitution, the exit status of the command is the exit status of the last command substitution performed. And if there were no command substitutions, the command exits with a zero status. Clean exit, no drama, periodt ✨

✨ COMMAND EXECUTION ✨

Okay so bestie, vibe check: after a command has been split into words, if that ends up being a simple command plus maybe a lil optional list of arguments, the shell does these things next. 💅

If the command name contains no slashes, the shell attempts to locate it. If there's a shell function by that name, that function is invoked as described above in FUNCTIONS. If the name doesn't match a function, the shell searches for it in the list of shell builtins. If a match is found, that builtin is invoked. 👑

If the name is neither a shell function nor a builtin, and contains no slashes, bash searches each element of PATH for a directory containing an executable file by that name. Bash uses a hash table to remember the full pathnames of executable files (see hash under SHELL BUILTIN COMMANDS below). Bash performs a full search of the directories in PATH only if the command is not found in the hash table. If the search is unsuccessful, the shell searches for a defined shell function named command_not_found_handle. If that function exists, it is invoked in a separate execution environment with the original command and the original command's arguments as its arguments, and the function's exit status becomes the exit status of that subshell. If that function is not defined, the shell prints an error message and returns an exit status of 127. 😭

If the search is successful, or if the command name contains one or more slashes, the shell executes the named program in a separate execution environment. Argument 0 is set to the name given, and the remaining arguments to the command are set to the arguments given, if any. ✨

If this execution fails because the file is not in executable format, and the file is not a directory, it is assumed to be a shell script, a file containing shell commands, and the shell creates a new instance of itself to execute it. Bash tries to determine whether the file is a text file or a binary, and will not execute files it determines to be binaries. This subshell reinitializes itself, so that the effect is as if a new shell had been invoked to handle the script, with the exception that the locations of commands remembered by the parent (see hash below under SHELL BUILTIN COMMANDS are retained by the child. 🫶

If the program is a file beginning with #!, the remainder of the first line specifies an interpreter for the program. The shell executes the specified interpreter on operating systems that do not handle this executable format themselves. The arguments to the interpreter consist of a single optional argument following the interpreter name on the first line of the program, followed by the name of the program, followed by the command arguments, if any. 🌈

🦋 COMMAND EXECUTION ENVIRONMENT 🦋

The shell has an execution environment, which consists of the following:

Open files inherited by the shell at invocation, as modified by redirections supplied to the exec builtin.
The current working directory as set by cd, pushd, or popd, or inherited by the shell at invocation.
The file creation mode mask as set by umask or inherited from the shell's parent.
Current traps set by trap.
Shell parameters that are set by variable assignment or with set or inherited from the shell's parent in the environment.
Shell functions defined during execution or inherited from the shell's parent in the environment.
Options enabled at invocation (either by default or with command-line arguments) or by set.
Options enabled by shopt.
Shell aliases defined with alias.
Various process IDs, including those of background jobs, the value of $$, and the value of PPID.

When a simple command other than a builtin or shell function is to be executed, it is invoked in a separate execution environment that consists of the following. Unless otherwise noted, the values are inherited from the shell.

The shell's open files, plus any modifications and additions specified by redirections to the command.
The current working directory.
The file creation mode mask.
Shell variables and functions marked for export, along with variables exported for the command, passed in the environment.
Traps caught by the shell are reset to the values inherited from the shell's parent, and traps ignored by the shell are ignored.

A command invoked in this separate environment cannot affect the shell's execution environment. 💀

A subshell is a copy of the shell process.

Command substitution, commands grouped with parentheses, and asynchronous commands are invoked in a subshell environment that is a duplicate of the shell environment, except that traps caught by the shell are reset to the values that the shell inherited from its parent at invocation. Builtin commands that are invoked as part of a pipeline, except possibly in the last element depending on the value of the lastpipe shell option, are also executed in a subshell environment. Changes made to the subshell environment cannot affect the shell's execution environment. 🎀

When the shell is in posix mode, subshells spawned to execute command substitutions inherit the value of the -e option from their parent shell. When not in posix mode, bash clears the -e option in such subshells. See the description of the inherit_errexit shell option below for how to control this behavior when not in posix mode. 👑

If a command is followed by a & and job control is not active, the default standard input for the command is the empty file /dev/null. Otherwise, the invoked command inherits the file descriptors of the calling shell as modified by redirections. 🌸

🌸 ENVIRONMENT 🌸

When a program is invoked it is given an array of strings called the environment. This is a list of name-value pairs, of the form name=value. ✨

The shell provides several ways to manipulate the environment. On invocation, the shell scans its own environment and creates a parameter for each name found, automatically marking it for export to child processes. Executed commands inherit the environment. The export, declare -x, and unset commands modify the environment by adding and deleting parameters and functions. If the value of a parameter in the environment is modified, the new value automatically becomes part of the environment, replacing the old. The environment inherited by any executed command consists of the shell's initial environment, whose values may be modified in the shell, less any pairs removed by the unset or export -n commands, plus any additions via the export and declare -x commands. 🫶

If any parameter assignments, as described above in PARAMETERS, appear before a simple command, the variable assignments are part of that command's environment for as long as it executes. These assignment statements affect only the environment seen by that command. If these assignments precede a call to a shell function, the variables are local to the function and exported to that function's children. 💅

If the -k option is set (see the set builtin command below), then all parameter assignments are placed in the environment for a command, not just those that precede the command name. 🔥

When bash invokes an external command, the variable _ is set to the full pathname of the command and passed to that command in its environment. 🌈

👑 EXIT STATUS 👑

The exit status of an executed command is the value returned by the waitpid system call or equivalent function. Exit statuses fall between 0 and 255, though, as explained below, the shell may use values above 125 specially. Exit statuses from shell builtins and compound commands are also limited to this range. Under certain circumstances, the shell will use special values to indicate specific failure modes. 😭

For the shell's purposes, a command which exits with a zero exit status has succeeded. So while an exit status of zero indicates success, a non-zero exit status indicates failure. Periodt. 💅

When a command terminates on a fatal signal N, bash uses the value of 128+N as the exit status. 💀

If a command is not found, the child process created to execute it returns a status of 127. If a command is found but is not executable, the return status is 126. 🫶

If a command fails because of an error during expansion or redirection, the exit status is greater than zero. ✨

Shell builtin commands return a status of 0 (true) if successful, and non-zero (false) if an error occurs while they execute. All builtins return an exit status of 2 to indicate incorrect usage, generally invalid options or missing arguments. 👑

The exit status of the last command is available in the special parameter $?. 🌸

Bash itself returns the exit status of the last command executed, unless a syntax error occurs, in which case it exits with a non-zero value. See also the exit builtin command below. 🎀

💖 SIGNALS 💖

okay bestie, signal time, and this is genuinely important so lock in 🔒

when bash is interactive, and there are no traps anywhere in sight, it straight-up ignores SIGTERM — that's literally so that kill 0 doesn't nuke an interactive shell 💀 — and it catches and handles SIGINT (which is the whole reason the wait builtin can be interrupted at all). when bash catches a SIGINT, it breaks out of whatever loops are running. and in all cases, no matter what, bash ignores SIGQUIT. if job control is in effect, bash also ignores SIGTTIN, SIGTTOU, and SIGTSTP. periodt.

the trap builtin is the thing that messes with the shell's signal handling, and it's described below 👇

non-builtin commands that bash runs get signal handlers set to the values the shell inherited from its parent — unless trap set them to be ignored, in which case the child process will ignore them too. when job control is not in effect, asynchronous commands ignore SIGINT and SIGQUIT on top of those inherited handlers. and commands run as a result of command substitution ignore the keyboard-generated job control signals SIGTTIN, SIGTTOU, and SIGTSTP. slay, that's a lot of signals, i know 😭

the shell exits by default when it gets a SIGHUP. before it dips out, an interactive shell resends that SIGHUP to all jobs, running or stopped. and it sends SIGCONT to stopped jobs to make sure they actually receive the SIGHUP (peep JOB CONTROL below for more on running vs stopped jobs, iykyk). if you want to keep the shell from sending the signal to one particular job, remove it from the jobs table with the disown builtin (see SHELL BUILTIN COMMANDS below) or mark it as not receiving SIGHUP using disown -h. your fave little save-a-job hack 💅

if the huponexit shell option has been set using shopt, then bash sends a SIGHUP to all jobs whenever an interactive login shell exits. noted 📝

if bash is waiting for a command to complete and it gets a signal that has a trap set on it, it will not execute the trap until the command completes. but — plot twist — if bash is waiting for an asynchronous command via the wait builtin and it gets a signal with a trap set, wait returns immediately with an exit status greater than 128, and right after that the shell runs the trap. the way that works is so specific and i love it 🫶

when job control is not enabled and bash is waiting for a foreground command to finish, the shell receives keyboard-generated signals like SIGINT (usually from ^C) that users normally mean to send to that command. this happens because the shell and the command are chilling in the same process group as the terminal, and ^C sends SIGINT to every process in that group. since bash doesn't turn on job control by default when the shell isn't interactive, this whole scenario is most common in non-interactive shells. classic 🌸

when job control is enabled and bash is waiting for a foreground command, the shell does not receive keyboard-generated signals, because it's not in the same process group as the terminal. this one shows up most in interactive shells, where bash tries to enable job control by default. see JOB CONTROL below for the process group lore. 🦋

when job control is not enabled and bash receives SIGINT while waiting for a foreground command, it waits until that foreground command terminates and then decides what to do about the SIGINT:

if the command terminates due to the SIGINT, bash concludes the user meant to send the SIGINT to the shell too, and acts on the SIGINT (like, running a SIGINT trap, exiting a non-interactive shell, or returning to the top level to read a new command). main character decision-making, honestly ✨
if the command does not terminate due to the SIGINT, then the program handled the SIGINT itself and didn't treat it as a fatal signal. in that case bash doesn't treat SIGINT as fatal either — it just assumes the SIGINT was part of the program's normal operation (e.g., emacs uses it to abort editing commands) or was deliberately discarded. however, bash will still run any trap set on SIGINT, same as it does with any other trapped signal it receives while waiting for the foreground command to complete, for compatibility. the compatibility clause really said "fine, i'll do it" 🙈

when job control is enabled, bash doesn't receive keyboard-generated signals like SIGINT while it's waiting for a foreground command. an interactive shell does not pay attention to the SIGINT, even if the foreground command terminates because of it — other than noting its exit status. if the shell is not interactive and the foreground command terminates due to the SIGINT, bash pretends it received the SIGINT itself (scenario 1 above), for compatibility. pretending is so valid of it 😌

🎀 JOB CONTROL 🎀

job control = the ability to selectively stop (suspend) the execution of processes and continue (resume) them later. a user typically uses this whole facility through an interactive interface that the operating system kernel's terminal driver and bash provide jointly. it's giving teamwork 💞

the shell associates a job with each pipeline. it keeps a table of currently executing jobs, and the jobs command will show you that table. every job has a job number, which jobs displays between brackets. job numbers start at 1. when bash starts a job asynchronously (in the background), it prints a line that looks like:

[1] 25647

which means this job is job number 1 and the process ID of the last process in the pipeline associated with this job is 25647. every process in a single pipeline is a member of the same job. bash uses the job abstraction as the basis for job control. iconic architecture, fr fr 👑

to make the user interface to job control possible, every process has a process group ID, and the operating system keeps track of a current terminal process group ID. that terminal process group ID is associated with the controlling terminal.

processes with the same process group ID are said to be part of the same process group. members of the foreground process group (the processes whose process group ID equals the current terminal process group ID) receive keyboard-generated signals like SIGINT. processes in the foreground process group are called foreground processes. background processes are the ones whose process group ID differs from the controlling terminal's — and those babies are immune to keyboard-generated signals 🛡️. only foreground processes are allowed to read from the controlling terminal, or write to it if the user says so with stty tostop. the system sends a SIGTTIN (SIGTTOU) signal to background processes that try to read from (or write to, when tostop is in effect) the terminal, and unless that signal is caught, it suspends the process. so rude, honestly 💀

if the operating system bash is running on supports job control, bash has facilities to use it. typing the suspend character (typically ^Z, Control-Z) while a process is running stops that process and hands control back to bash. typing the delayed suspend character (typically ^Y, Control-Y) makes the process stop when it tries to read input from the terminal, and returns control to bash. then you get to mess with the state of that job: use the bg command to continue it in the background, fg to continue it in the foreground, or kill to kill it. the suspend character takes effect immediately, and it has the extra side effect of discarding any pending output and typeahead. if you want to force a background process to stop, or stop a process that's not associated with the current terminal session, send it the SIGSTOP signal using kill. 💅

there are a bunch of ways to refer to a job in the shell. the % character introduces a job specification (jobspec).

job number n can be referred to as %n. a job can also be referred to using a prefix of the name used to start it, or using a substring that appears in its command line. for example, %ce refers to a job whose command name begins with ce. using %?ce, on the other hand, refers to any job containing the string ce in its command line. if the prefix or substring matches more than one job, bash reports an error. no ambiguity allowed, bestie 🚫

the symbols %% and %+ refer to the shell's notion of the current job. a single % (with no job specification attached) also refers to the current job. %- refers to the previous job. when a job starts in the background, a job stops while in the foreground, or a job is resumed in the background, it becomes the current job. the job that was the current job becomes the previous job. when the current job terminates, the previous job becomes the current job. if there's only a single job, %+ and %- can both be used to refer to that one job. in output that has to do with jobs (like the output of the jobs command), the current job is always marked with a +, and the previous job with a -. little markers, so cute 🎀

simply naming a job can bring it into the foreground: %1 is a synonym for fg %1, bringing job 1 from the background into the foreground. similarly, %1 & resumes job 1 in the background, equivalent to bg %1. shortcuts are self-care ✨

the shell learns immediately whenever a job changes state. normally bash waits until it's about to print a prompt before telling the user about changes in a job's status, so it doesn't interrupt any other output — though it will notify you of changes in a job's status after a foreground command in a list completes, before it executes the next command in the list. if the -b option to the set builtin command is enabled, bash reports status changes immediately. and bash executes any trap on SIGCHLD for each child that terminates. multitasking queen 👑

when a job terminates and bash notifies the user about it, bash removes the job from the table. it won't show up in jobs output anymore, but wait will report its exit status, as long as it's given the process ID associated with the job as an argument. when the table is empty, job numbers start over at 1. recycling, we love to see it ♻️

if a user tries to exit bash while jobs are stopped (or, if the checkjobs shell option has been enabled using the shopt builtin, while jobs are running), the shell prints a warning message, and if the checkjobs option is enabled, it lists the jobs and their statuses. you can then use the jobs command to inspect their status. if the user immediately tries to exit again, without an intervening command, bash does not print another warning, and it terminates any stopped jobs. "are you sure?" "yes." "okay bye" 😭

when the shell is waiting for a job or process using the wait builtin, and job control is enabled, wait will return when the job changes state. the -f option makes wait wait until the job or process terminates before returning. patience is a virtue, bestie 🧚

✨ PROMPTING ✨

ok bestie so when you're running bash interactively it's literally always yapping at you through prompts. Bash shows the primary prompt PS1 when it's ready to read a command, and the secondary prompt PS2 when it needs more input to finish a command. 🫶

Bash also peeks at the value of the array variable PROMPT_COMMAND right before printing each primary prompt. If any elements in PROMPT_COMMAND are set and non-null, Bash executes each value, in numeric order, just as if it had been typed on the command line. And Bash displays PS0 after it reads a command but before executing it.

Bash displays PS4 as described above before tracing each command when the -x option is enabled.

Bash lets you customize the prompt strings PS0, PS1, PS2, and PS4 by inserting a bunch of backslash-escaped special characters, which get decoded like this:

\a — An ASCII bell character (07). 🔔
\d — The date in "Weekday Month Date" format (e.g., "Tue May 26").
\D{format} — The format is passed to strftime(3) and the result is inserted into the prompt string; an empty format results in a locale-specific time representation. The braces are required. no skipping them fr
\e — An ASCII escape character (033).
\h — The hostname up to the first ".".
\H — The hostname.
\j — The number of jobs currently managed by the shell.
\l — The basename of the shell's terminal device name (e.g., "ttys0").
\n — A newline.
\r — A carriage return.
\s — The name of the shell: the basename of $0 (the portion following the final slash).
\t — The current time in 24-hour HH:MM:SS format.
\T — The current time in 12-hour HH:MM:SS format.
\@ — The current time in 12-hour am/pm format.
\A — The current time in 24-hour HH:MM format.
\u — The username of the current user.
\v — The bash version (e.g., 2.00).
\V — The bash release, version + patch level (e.g., 2.00.0)
\w — The value of the PWD shell variable ($PWD), with $HOME abbreviated with a tilde (uses the value of the PROMPT_DIRTRIM variable).
\W — The basename of $PWD, with $HOME abbreviated with a tilde.
\! — The history number of this command.
\# — The command number of this command.
\$ — If the effective UID is 0, a #, otherwise a $.
\nnn — The character corresponding to the octal number nnn.
\\ — A backslash.
\[ — Begin a sequence of non-printing characters, which could be used to embed a terminal control sequence into the prompt.
\] — End a sequence of non-printing characters.

The command number and the history number are usually different, and this trips people up literally every time 😭: the history number of a command is its position in the history list, which may include commands restored from the history file (see HISTORY below), while the command number is the position in the sequence of commands executed during the current shell session. After the string is decoded, it is expanded via parameter expansion, command substitution, arithmetic expansion, and quote removal, subject to the value of the promptvars shell option (see the description of the shopt command under SHELL BUILTIN COMMANDS below). This can have unwanted side effects if escaped portions of the string appear within command substitution or contain characters special to word expansion. 💀

🦋 READLINE 🦋

This is the library that handles reading input when you're using an interactive shell — unless the --noediting option is supplied at shell invocation. Line editing is also used when using the -e option to the read builtin. By default, the line editing commands are similar to those of emacs; a vi-style line editing interface is also available. You can enable line editing at any time using the -o emacs or -o vi options to the set builtin (see SHELL BUILTIN COMMANDS below). And to turn line editing off after the shell is running, use the +o emacs or +o vi options to the set builtin. 🎀

💜 Readline Notation 💜

This section uses Emacs-style editing concepts and uses its notation for keystrokes. Control keys are denoted by C-key, e.g., C-n means Control-N. Similarly, meta keys are denoted by M-key, so M-x means Meta-X. The Meta key is often labeled "Alt" or "Option".

On keyboards without a Meta key, M-x means ESC x, i.e., press and release the Escape key, then press and release the x key, in sequence. This makes ESC the meta prefix. The combination M-C-x means ESC Control-x: press and release the Escape key, then press and hold the Control key while pressing the x key, then release both. 🧚

On some keyboards, the Meta key modifier produces characters with the eighth bit (0200) set. You can use the enable-meta-key variable to control whether or not it does this, if the keyboard allows it. On many others, the terminal or terminal emulator converts the metafied key to a key sequence beginning with ESC as described in the preceding paragraph.

If your Meta key produces a key sequence with the ESC meta prefix, you can make M-key key bindings you specify (see Readline Key Bindings below) do the same thing by setting the force-meta-prefix variable.

Readline commands may be given numeric arguments, which normally act as a repeat count. Sometimes, however, it's the sign of the argument that's significant. Passing a negative argument to a command that acts in the forward direction (e.g., kill-line) makes that command act in a backward direction. Commands whose behavior with arguments deviates from this are noted below.

The point is the current cursor position, and mark refers to a saved cursor position. The text between the point and mark is referred to as the region. Readline has the concept of an active region: when the region is active, readline redisplay highlights the region using the value of the active-region-start-color variable. The enable-active-region variable turns this on and off. Several commands set the region to active; those are noted below. ✨

When a command is described as killing text, the text deleted is saved for possible future retrieval (yanking). The killed text is saved in a kill ring. Consecutive kills accumulate the deleted text into one unit, which can be yanked all at once. Commands which do not kill text separate the chunks of text on the kill ring.

🌸 Readline Initialization 🌸

Readline is customized by putting commands in an initialization file (the inputrc file). The name of this file is taken from the value of the INPUTRC shell variable. If that variable is unset, the default is ~/.inputrc. If that file does not exist or cannot be read, readline looks for /etc/inputrc. When a program that uses the readline library starts up, readline reads the initialization file and sets the key bindings and variables found there, before reading any user input. 🌈

There are only a few basic constructs allowed in the inputrc file. Blank lines are ignored. Lines beginning with a # are comments. Lines beginning with a $ indicate conditional constructs. Other lines denote key bindings and variable settings.

The default key-bindings in this section may be changed using key binding commands in the inputrc file. Programs that use the readline library, including bash, may add their own commands and bindings.

For example, placing

M-Control-u: universal-argument

C-Meta-u: universal-argument

into the inputrc would make M-C-u execute the readline command universal-argument.

Key bindings may contain the following symbolic character names: DEL, ESC, ESCAPE, LFD, NEWLINE, RET, RETURN, RUBOUT (a destructive backspace), SPACE, SPC, and TAB.

In addition to command names, readline allows keys to be bound to a string that is inserted when the key is pressed (a macro). The difference between a macro and a command is that a macro is enclosed in single or double quotes. 💅

💖 Readline Key Bindings 💖

The syntax for controlling key bindings in the inputrc file is simple. All that's required is the name of the command or the text of a macro and a key sequence to which it should be bound. The key sequence may be specified in one of two ways: as a symbolic key name, possibly with Meta- or Control- prefixes, or as a key sequence composed of one or more characters enclosed in double quotes. The key sequence and name are separated by a colon. There can be no whitespace between the name and the colon. 👑

When using the form keyname:function-name or macro, keyname is the name of a key spelled out in English. For example:

Control-u: universal-argument
Meta-Rubout: backward-kill-word
Control-o: "> output"

In the above example, C-u is bound to the function universal-argument, M-DEL is bound to the function backward-kill-word, and C-o is bound to run the macro expressed on the right hand side (that is, to insert the text "> output" into the line).

In the second form, "keyseq":function-name or macro, keyseq differs from keyname above in that strings denoting an entire key sequence may be specified by placing the sequence within double quotes. Some GNU Emacs style key escapes can be used, as in the following example, but none of the symbolic character names are recognized. ⭐

"\C-u": universal-argument
"\C-x\C-r": re-read-init-file
"\e[11~": "Function Key 1"

In this example, C-u is again bound to the function universal-argument. C-x C-r is bound to the function re-read-init-file, and ESC [ 1 1 ~ is bound to insert the text "Function Key 1".

The full set of GNU Emacs style escape sequences available when specifying key sequences is

\C- — A control prefix.
\M- — Adding the meta prefix or converting the following character to a meta character, as described below under force-meta-prefix.
\e — An escape character.
\\ — Backslash.
\" — Literal ", a double quote.
\' — Literal ', a single quote.

In addition to the GNU Emacs style escape sequences, a second set of backslash escapes is available:

\a — alert (bell)
\b — backspace
\d — delete
\f — form feed
\n — newline
\r — carriage return
\t — horizontal tab
\v — vertical tab
\nnn — The eight-bit character whose value is the octal value nnn (one to three digits).
\xHH — The eight-bit character whose value is the hexadecimal value HH (one or two hex digits).

When you're typing out the text of a macro, bestie, you gotta wrap it in single or double quotes to show it's a macro definition. No quotes? Bash is just gonna assume it's a function name, periodt. The backslash escapes we talked about above get expanded inside the macro body, and backslash will also quote any other character in the macro text — including " and '. 💅

Bash shows and edits your current readline key bindings with the bind builtin command. And if you wanna flip your editing mode mid-interactive-session? The -o emacs or -o vi options to the set builtin do exactly that (peep SHELL BUILTIN COMMANDS below, iykyk). ✨

🦋 Readline Variables 🦋

Readline keeps a whole lil collection of variables so you can customize its behavior even further. You can set one in the inputrc file with a statement shaped like this:

set variable-name value

...or you can just use the bind builtin command instead (again, SHELL BUILTIN COMMANDS below, we stay referencing it 💖).

Unless it says otherwise, readline variables take the values On or Off — and case does not matter here, so don't stress about your caps lock. 🙈 Unrecognized variable names? Silently ignored, no drama, no error. And when readline actually reads a variable value, these are all equivalent to On:

empty or null values
on (case-insensitive, so ON, On, oN — all slay)
1

Everything else? That's Off. No in-between, bestie. 💀

Wanna see what's currently set? The bind -V command lists the current readline variable names and values (yep — SHELL BUILTIN COMMANDS below 🫶).

The variables and their default values are:

active-region-start-color — a string variable that controls the text color and background when readline displays text in the active region (peep the description of enable-active-region below if you're lost 🫶). Bestie, this string must not take up any physical character positions on the display, so it should be made of nothing but terminal escape sequences. It gets output to the terminal before the text in the active region is displayed. This variable resets to its default whenever the terminal type changes. The default is the string that puts the terminal in standout mode, as obtained from the terminal's terminfo description. A sample value might be "\e[01;33m" ✨
active-region-end-color — a string variable that "undoes" the effects of active-region-start-color and restores "normal" terminal display appearance after text has been shown in the active region. Same deal as above: it must not take up any physical character positions on the display, so terminal escape sequences only. It's output to the terminal after the text in the active region is displayed. Reset to its default whenever the terminal type changes. The default is the string that restores the terminal from standout mode, as obtained from the terminal's terminfo description. A sample value might be "\e[0m" 💅
bell-style (default: audible) — controls what happens when readline wants to ring the terminal bell 🔔. Set it to none and readline never rings the bell. Set it to visible and readline uses a visible bell if one is available. Set it to audible and readline attempts to ring the terminal's bell. That's it, that's the whole vibe.
bind-tty-special-chars (default: On) — if set to On, readline attempts to bind the control characters that the kernel's terminal driver treats specially to their readline equivalents. Heads up: these override the default readline bindings described here 👀. Type stty -a at a bash prompt to see your current terminal settings, including the special control characters (usually cchars).
blink-matching-paren (default: Off) — if set to On, readline attempts to briefly move the cursor to an opening parenthesis when a closing parenthesis is inserted. Cute lil cursor hop 🎀
colored-completion-prefix (default: Off) — if set to On, when listing completions, readline displays the common prefix of the set of possible completions using a different color 🌈. The color definitions are taken from the value of the LS_COLORS environment variable. And plot twist: if there's a color definition in $LS_COLORS for the custom suffix ".readline-colored-completion-prefix", readline uses this color for the common prefix instead of its default. iykyk.
colored-stats (default: Off) — if set to On, readline displays possible completions using different colors to indicate their file type. Color definitions come from the value of the LS_COLORS environment variable. Aesthetic and functional, we love to see it.
comment-begin (default: "#") — the string that the readline insert-comment command inserts. This command is bound to M-# in emacs mode and to # in vi command mode. No cap.
completion-display-width (default: -1) — the number of screen columns used to display possible matches when performing completion. The value is ignored if it's less than 0 or greater than the terminal screen width. A value of 0 causes matches to be displayed one per line. The default value is -1.
completion-ignore-case (default: Off) — if set to On, readline performs filename matching and completion in a case-insensitive fashion. lowercase girlies stay winning 🙈
completion-map-case (default: Off) — if set to On, and completion-ignore-case is enabled, readline treats hyphens (-) and underscores (_) as equivalent when performing case-insensitive filename matching and completion. The chaos of - vs _ finally resolved fr fr.
completion-prefix-display-length (default: 0) — the maximum length in characters of the common prefix of a list of possible completions that is displayed without modification. When set to a value greater than zero, readline replaces common prefixes longer than this value with an ellipsis when displaying possible completions. And if a completion begins with a period, and readline is completing filenames, it uses three underscores instead of an ellipsis. Iconic behaviour, honestly.
completion-query-items (default: 100) — this determines when the user is queried about viewing the number of possible completions generated by the possible-completions command. It may be set to any integer value greater than or equal to zero. If the number of possible completions is greater than or equal to the value of this variable, readline asks whether or not the user wishes to view them; otherwise readline simply lists them on the terminal. A zero value means readline should never ask; negative values are treated as zero.
convert-meta (default: On) — if set to On, readline converts characters it reads that have the eighth bit set to an ASCII key sequence by clearing the eighth bit and prefixing it with an escape character (converting the character to have the meta prefix). The default is On, but readline sets it to Off if the locale contains characters whose encodings may include bytes with the eighth bit set. This variable is dependent on the LC_CTYPE locale category, and may change if the locale changes. It also affects key bindings — see the description of force-meta-prefix below 👇
disable-completion (default: Off) — if set to On, readline inhibits word completion. Completion characters are inserted into the line as if they had been mapped to self-insert. So basically, Tab becomes Just A Guy.
echo-control-characters (default: On) — when set to On, on operating systems that indicate they support it, readline echoes a character corresponding to a signal generated from the keyboard. 💖
editing-mode (default: emacs) — controls whether readline uses a set of key bindings similar to Emacs or vi. editing-mode can be set to either emacs or vi. Choose your fighter 🥊
emacs-mode-string (default: @) — if the show-mode-in-prompt variable is enabled, this string is displayed immediately before the last line of the primary prompt when emacs editing mode is active. The value is expanded like a key binding, so the standard set of meta- and control- prefixes and backslash escape sequences is available. The \1 and \2 escapes begin and end sequences of non-printing characters, which can be used to embed a terminal control sequence into the mode string. Very main character energy ✨
enable-active-region (default: On) — when this variable is set to On, readline allows certain commands to designate the region as active. When the region is active, readline highlights the text in the region using the value of the active-region-start-color variable, which defaults to the string that enables the terminal's standout mode. The active region shows the text inserted by bracketed-paste and any matching text found by incremental and non-incremental history searches. Highlighting that actually means something? Obsessed 🫶
enable-bracketed-paste (default: On) — when set to On, readline configures the terminal to insert each paste into the editing buffer as a single string of characters, instead of treating each character as if it had been read from the keyboard. This is called bracketed-paste mode; it prevents readline from executing any editing commands bound to key sequences appearing in the pasted text. Translation: pasting random text won't accidentally run commands on you 🙏
enable-keypad (default: Off) — when set to On, readline tries to enable the application keypad when it is called. Some systems need this to enable the arrow keys. Yes bestie, the arrows. Those ones. ⬆️⬇️
enable-meta-key (default: On) — when set to On, readline tries to enable any meta modifier key the terminal claims to support. On many terminals, the Meta key is used to send eight-bit characters; this variable checks for the terminal capability that indicates the terminal can enable and disable a mode that sets the eighth bit of a character (0200) if the Meta key is held down when the character is typed (a meta character). Deep lore, I know 💀
expand-tilde (default: Off) — if set to On, readline performs tilde expansion when it attempts word completion. Your ~ finally gets the respect it deserves 🏠
force-meta-prefix (default: Off) — if set to On, readline modifies its behavior when binding key sequences containing \M- or Meta- (see Key Bindings above) by converting a key sequence of the form \M-C or Meta-C to the two-character sequence ESC C (adding the meta prefix). If force-meta-prefix is set to Off (the default), readline uses the value of the convert-meta variable to determine whether to perform this conversion: if convert-meta is On, readline performs the conversion described above; if it is Off, readline converts C to a meta character by setting the eighth bit (0200). Two variables, one job, iconic team-up 🌸
history-preserve-point (default: Off) — if set to On, the history code attempts to place point at the same location on each history line retrieved with previous-history or next-history. Your cursor stays exactly where you left it. Character development 💅
history-size (default: unset) — set the maximum number of history entries saved in the history list. If set to zero, any existing history entries are deleted and no new entries are saved. If set to a value less than zero, the number of history entries is not limited. By default, bash sets the maximum number of history entries to the value of the HISTSIZE shell variable. And a lil warning: setting history-size to a non-numeric value will set the maximum number of history entries to 500. 🍓
horizontal-scroll-mode (default: Off) — setting this variable to On makes readline use a single line for display, scrolling the input horizontally on a single screen line when it becomes longer than the screen width rather than wrapping to a new line. This setting is automatically enabled for terminals of height 1. One-line girlies, this one's for you 🌈
input-meta (default: Off) — if set to On, readline enables eight-bit input (that is, it does not clear the eighth bit in the characters it reads), regardless of what the terminal claims it can support. The default is Off, but readline sets it to On if the locale contains characters whose encodings may include bytes with the eighth bit set. This variable is dependent on the LC_CTYPE locale category, and its value may change if the locale changes. Oh and psst — the name meta-flag is a synonym for input-meta 🧚
isearch-terminators (default: "C-[C-j") —

Okay so the chars that end an incremental search without then running that char as a command — that's the whole job here. And if this variable never got a value assigned? Then ESC and C-j are the ones that terminate an incremental search. That's it, that's the rule. 💅

keymap (default: emacs) — sets the current readline keymap, bestie. The valid keymap names are emacs, emacs-standard, emacs-meta, emacs-ctlx, vi, vi-command, and vi-insert. FYI vi is equivalent to vi-command, and emacs is equivalent to emacs-standard. Default is emacs, and the value of editing-mode also affects the default keymap. Chain reaction, iconic.
keyseq-timeout (default: 500) — how long readline waits for a character when it's reading an ambiguous key sequence. Ambiguous meaning: it could be a complete key sequence with the input read so far, OR it could take more input to finish a longer sequence. If nothing shows up within the timeout, readline goes with the shorter-but-complete key sequence. Value is in milliseconds, so 1000 = readline waits one whole second for more input. If you set this to a value less than or equal to zero, or to a non-numeric value, readline just waits until another key is pressed to decide which key sequence to complete. No time pressure, main character energy. ⏳
mark-directories (default: On) — set it to On and completed directory names get a slash appended. Cute, organized, slay.
mark-modified-lines (default: Off) — set to On and readline displays history lines that have been modified with a preceding asterisk (*). Little star for the edited ones ⭐.
mark-symlinked-directories (default: Off) — set to On and completed names that are symbolic links to directories get a slash appended — but only subject to the value of mark-directories. Teamwork.
match-hidden-files (default: On) — when set to On, this forces readline to match files whose names begin with a “.” (hidden files) when it's doing filename completion. Set it to Off and you have to type the leading “.” yourself in the filename you want completed. 🙈
menu-complete-display-prefix (default: Off) — set to On and menu completion shows the common prefix of the list of possible completions (which might be totally empty, ngl) before it cycles through the list.
output-meta (default: Off) — set to On and readline displays characters with the eighth bit set directly, instead of as a meta-prefixed escape sequence. Default is Off, but readline flips it to On if the locale contains characters whose encodings may include bytes with the eighth bit set. Heads up: this variable depends on the LC_CTYPE locale category, and its value may change if the locale changes. It's giving chameleon 🦋.
page-completions (default: On) — set to On and readline uses an internal pager resembling more(1) to show you a screenful of possible completions at a time. Very demure, very mindful scrolling.
prefer-visible-bell — see bell-style. That's the whole vibe check, go look over there 👀.
print-completions-horizontally (default: Off) — set to On and readline displays completions with matches sorted horizontally in alphabetical order, instead of down the screen.
revert-all-at-newline (default: Off) — set to On and readline will undo all changes to history lines before returning when executing accept-line. By default, history lines may be modified and retain individual undo lists across calls to readline. Undo history is lowkey a whole personality trait.
search-ignore-case (default: Off) — set to On and readline does incremental and non-incremental history list searches in a case-insensitive way. Search Makefile, find makefile, no cap.
show-all-if-ambiguous (default: Off) — this alters the default behavior of the completion functions. If set to On, words that have more than one possible completion cause the matches to be listed immediately instead of ringing the bell. 🔔➡️📋
show-all-if-unmodified (default: Off) — also alters the default behavior of the completion functions, in a way similar to show-all-if-ambiguous. If set to On, words that have more than one possible completion without any possible partial completion (translation: the possible completions don't share a common prefix) cause the matches to be listed immediately instead of ringing the bell.
show-mode-in-prompt (default: Off) — set to On and it adds a string to the beginning of the prompt indicating the editing mode: emacs, vi command, or vi insertion. The mode strings are user-settable (e.g., emacs-mode-string). Let everyone know which mode you're in, periodt.
skip-completed-text (default: Off) — set to On and this alters the default completion behavior when inserting a single match into the line. It's only active when you're performing completion in the middle of a word. If enabled, readline does not insert characters from the completion that match characters after point in the word being completed — so portions of the word following the cursor don't get duplicated. No double-typing, we don't do that here 🫶.
vi-cmd-mode-string (default: ((cmd))) — if the show-mode-in-prompt variable is enabled, this string is displayed immediately before the last line of the primary prompt when vi editing mode is active and in command mode. The value is expanded like a key binding, so the standard set of meta- and control- prefixes and backslash escape sequences is available. The \1 and \2 escapes begin and end sequences of non-printing characters, which you can use to embed a terminal control sequence into the mode string. Fancy.
vi-ins-mode-string (default: ((ins))) — if the show-mode-in-prompt variable is enabled, this string is displayed immediately before the last line of the primary prompt when vi editing mode is active and in insertion mode. The value is expanded like a key binding, so the standard set of meta- and control- prefixes and backslash escape sequences is available. The \1 and \2 escapes begin and end sequences of non-printing characters, which can be used to embed a terminal control sequence into the mode string. Twin behavior with vi-cmd-mode-string, obsessed. 👯
visible-stats (default: Off) — set to On and a character denoting a file's type as reported by stat(2) gets appended to the filename when listing possible completions. Little type-tags for your files, we love a labeled moment.

🧚 Readline Conditional Constructs

So readline implements a facility similar in spirit to the conditional compilation features of the C preprocessor — which lets key bindings and variable settings be performed as the result of tests. There are four parser directives available. Four! Let's go. ✨

$if — the $if construct allows bindings to be made based on the editing mode, the terminal being used, or the application using readline. The text of the test, after any comparison operator, extends to the end of the line; unless otherwise noted, no characters are required to isolate it.
- mode — the mode= form of the $if directive tests whether readline is in emacs or vi mode. You can use this together with the set keymap command, for instance, to set bindings in the emacs-standard and emacs-ctlx keymaps only if readline is starting out in emacs mode.
- term — the term= form can be used to include terminal-specific key bindings, maybe to bind the key sequences output by the terminal's function keys. The word on the right side of the = is tested against both the full name of the terminal and the portion of the terminal name before the first -. That's how xterm gets to match both xterm and xterm-256color, for instance. Sneaky and smart.
- version — the version test can be used to perform comparisons against specific readline versions. The version expands to the current readline version. The set of comparison operators includes =, ==, !=, <=, >=, <, and >. The version number supplied on the right side of the operator consists of a major version number, an optional decimal point, and an optional minor version (e.g., 7.1). If the minor version is omitted, it defaults to 0. The operator may be separated from the string version and from the version number argument by whitespace.
- application — the application construct is used to include application-specific settings. Each program using the readline library sets the application name, and an initialization file can test for a particular value. You could use this to bind key sequences to functions useful for a specific program. For instance, the following command adds a key sequence that quotes the current or previous word in bash:

$if Bash
# Quote the current or previous word
"\C-xq": "\eb\"\ef\""
$endif

variable — the variable construct provides simple equality tests for readline variables and values. The permitted comparison operators are =, ==, and !=. The variable name must be separated from the comparison operator by whitespace; the operator may be separated from the value on the right hand side by whitespace. String and boolean variables may be tested. Boolean variables must be tested against the values on and off.

$else — commands in this branch of the $if directive are executed if the test fails. The plan B branch 💀.
$endif — this command, as seen in the previous example, terminates an $if command. Closing the loop, as you do.
$include — this directive takes a single filename as an argument and reads commands and key bindings from that file. For example, the following directive would read /etc/inputrc:

$include  /etc/inputrc

🔍 Searching

Readline provides commands for searching through the command history (see HISTORY below) for lines containing a specified string. There are two search modes: incremental and non-incremental. Two flavors, both iconic, pick your fighter 🌈.

okay so incremental searches are literally the main character of history search — they start looking before you've even finished typing. every single character you type, readline is like "bestie I got u" and instantly shows the next history entry matching what you've typed so far. the whole point is that you only need as many characters as it takes to find the entry you want. in emacs editing mode, C-r searches backward through the history for a particular string, and C-s searches forward. ✨

the characters sitting in the value of the isearch-terminators variable are what end an incremental search. if that variable has never been assigned a value, then ESC and C-j are the ones that terminate it. and if you wanna bail entirely, C-g aborts the search and restores the original line. once the search is terminated, the history entry containing your search string becomes the current line. 🎀

wanna find other matching entries in the history list? just type C-r or C-s as appropriate. that searches backward or forward for the next entry matching the search string you've typed so far. any other key sequence bound to a readline command terminates the search and runs that command instead. for example, a newline terminates the search and accepts the line — so it executes the command straight out of the history list. a movement command will terminate the search, make the last line found the current line, and start editing. 🦋

readline remembers the last incremental search string. so if you type two C-rs with no intervening characters defining a new search string, readline just uses whatever search string it remembered. iconic behaviour honestly.

non-incremental searches are the chill ones — they read the entire search string before they even start looking for matching history entries. and that search string can be typed by you, or it can be part of the contents of the current line. 💅

Readline Command Names ✨

here's the list of command names and the default key sequences they're bound to. and heads up: command names with no key sequence next to them are unbound by default. 💀

in all of these descriptions, point = the current cursor position, and mark = a cursor position saved by the set-mark command. the text between point and mark is what we call the region. readline also has the concept of an active region — when the region is active, readline redisplay highlights it using the value of the active-region-start-color variable. the enable-active-region readline variable turns this whole thing on and off. several commands set the region to active, and those are noted below. 👑

Commands for Moving 🦋

beginning-of-line (C-a) — bounce to the start of the current line. on some keyboards this may also be bound to the Home key.
end-of-line (C-e) — bounce to the end of the line. on some keyboards this may also be bound to the End key.
forward-char (C-f) — move forward one character. on some keyboards this may also be bound to the right arrow key.
backward-char (C-b) — move back one character. on some keyboards this may also be bound to the left arrow key.
forward-word (M-f) — move forward to the end of the next word. words are made of alphanumeric characters (letters and digits).
backward-word (M-b) — move back to the start of the current or previous word. words are made of alphanumeric characters (letters and digits).
shell-forward-word (M-C-f) — move forward to the end of the next word. here words are delimited by non-quoted shell metacharacters.
shell-backward-word (M-C-b) — move back to the start of the current or previous word. again, words are delimited by non-quoted shell metacharacters.
previous-screen-line — tries to move point to the same physical screen column on the previous physical screen line. heads up: this will not do what you want if the current readline line doesn't take up more than one physical line, or if point isn't greater than the length of the prompt plus the screen width.
next-screen-line — tries to move point to the same physical screen column on the next physical screen line. same deal: this will not do what you want if the current readline line doesn't take up more than one physical line, or if the length of the current readline line isn't greater than the length of the prompt plus the screen width.
clear-display (M-C-l) — clear the screen and, if possible, the terminal's scrollback buffer too, then redraw the current line, leaving it at the top of the screen. 🧼
clear-screen (C-l) — clear the screen, then redraw the current line, leaving it at the top of the screen. give it a numeric argument and it'll refresh the current line without clearing the screen.
redraw-current-line — refresh the current line. that's it, that's the whole vibe. 🌸

🕰️ Commands for Manipulating the History 💅

okay bestie, this is the history squad — the readline commands that let you scroll through everything you've ever typed like it's your own personal group chat archive 🫶

accept-line (Newline, Return) — Take the line, no matter where your cursor is chilling. If it's not empty, it gets added to the history list, following whatever rules HISTCONTROL and HISTIGNORE are running. And if the line was a modified history line? It gets restored to its original state. iconic cleanup behavior ✨
previous-history (C-p) — Grab the previous command from the history list and scoot back through it. On some keyboards this might also be glued to the up-arrow key 👆
next-history (C-n) — Grab the next command from the history list and scoot forward. Might also be bound to the down-arrow key on some keyboards 👇
beginning-of-history (M-<) — Jump to the very first line in the history. ancient scrolls energy 📜
end-of-history (M->) — Jump to the end of your input history — i.e. the line you're currently typing. back to the present, bestie 🕐
operate-and-get-next (C-o) — Accept the current line for execution like you hit newline, and fetch the next line relative to it from the history so you can edit it. If you hand it a numeric argument, that argument says which history entry to use instead of the current line 👑
fetch-history — With a numeric argument, pull that exact entry out of the history list and make it your current line. No argument? Just move back to the first entry in the history list 💫
reverse-search-history (C-r) — Search backward, starting at the current line and moving "up" through the history as needed. This one is an incremental search, and it sets the region to the matched text and activates the region. the classic C-r glow-up 🔍
forward-search-history (C-s) — Search forward, starting at the current line and moving "down" through the history as needed. Also incremental, and also sets the region to the matched text and activates the region 🎀
non-incremental-reverse-search-history (M-p) — Search backward through the history starting at the current line, using a non-incremental search for a string you supply. The search string may match anywhere in a history line, no cap 🧚
non-incremental-forward-search-history (M-n) — Search forward through the history using a non-incremental search for a string you supply. Same deal — the string can match anywhere in a history line 💜
history-search-backward — Search backward through the history for the string of characters between the start of the current line and the point. Here the search string must match at the beginning of a history line. Non-incremental. Might be bound to the Page Up key on some keyboards ⬆️
history-search-forward — Search forward through the history for the string of characters between the start of the current line and the point. Must match at the beginning of a history line. Non-incremental. Might be bound to the Page Down key on some keyboards ⬇️
history-substring-search-backward — Search backward through the history for the string of characters between the start of the current line and the point. The search string may match anywhere in a history line. Non-incremental 🌸
history-substring-search-forward — Search forward through the history for the string of characters between the start of the current line and the point. May match anywhere in a history line. Non-incremental 🌈
yank-nth-arg (M-C-y) — Insert the first argument to the previous command (usually the second word on the previous line) at point. Give it an argument n and it inserts the nth word from the previous command — and btw the words in the previous command start counting at word 0. A negative argument inserts the nth word from the end of the previous command. Once n is computed, this uses the history expansion facilities to pull out that word, exactly as if you'd typed the !n history expansion 😤
yank-last-arg (M-., M-_) — Insert the last argument to the previous command (the last word of the previous history entry). With a numeric argument, it behaves exactly like yank-nth-arg. Successive calls to yank-last-arg keep moving back through the history list, inserting the last word of each line in turn (or the word specified by the argument to the first call). Any numeric argument you pass to those successive calls decides the direction you move through the history. A negative argument flips the direction (back or forward). It uses the history expansion facilities to extract that last word, as if you'd typed the !$ history expansion 💅
shell-expand-line (M-C-e) — Expand the line by doing shell word expansions. That means: alias and history expansion, $'string' and $"string" quoting, tilde expansion, parameter and variable expansion, arithmetic expansion, command and process substitution, word splitting, and quote removal. An explicit argument suppresses command and process substitution. Peep HISTORY EXPANSION below for the history expansion tea ☕
history-expand-line (M-^) — Perform history expansion on the current line. Again, see HISTORY EXPANSION below for the details 🔥
magic-space — Perform history expansion on the current line and insert a space. See HISTORY EXPANSION below ✨
alias-expand-line — Perform alias expansion on the current line. See ALIASES above for what that even means 🎀
history-and-alias-expand-line — Perform both history and alias expansion on the current line. double the expansion, double the slay 💖
insert-last-argument (M-., M-_) — Just a synonym for yank-last-arg, bestie 🫶
edit-and-execute-command (C-x C-e) — Pull up an editor on your current command line and execute the result as shell commands. Bash tries to invoke $VISUAL, then $EDITOR, then emacs — in that order, no exceptions 📝

✍️ Commands for Changing Text ✨

the glow-up squad — everything that edits what's on your line, character by character, word by word 💅

end-of-file (usually C-d) — The character that signals end-of-file, set by things like stty(1), for example. If this character gets read while there are no characters on the line and point is at the beginning of the line, readline interprets it as the end of input and returns EOF. that's the sign-off, periodt 🚪
delete-char (C-d) — Delete the character at point. If this function is bound to the same character as the tty EOF character — which C-d commonly is — then see the entry above for what actually happens 👀 This may also be bound to the Delete key on some keyboards.
backward-delete-char (Rubout) — Delete the character behind the cursor. Give it a numeric argument and the deleted text gets saved on the kill ring 💀✨
forward-backward-delete-char — Delete the character under the cursor — unless the cursor is at the end of the line, in which case it deletes the character behind the cursor instead. adaptive queen 👑
quoted-insert (C-q, C-v) — Add the next character you type to the line verbatim. This is how you insert characters like C-q, for example 🪄
tab-insert (C-v TAB) — Insert a tab character. that's it, that's the whole job 🌸
self-insert (a, b, A, 1, !, ...) — Insert the character you typed. the most basic icon in the game 🎀
bracketed-paste-begin — This function is meant to be bound to the "bracketed paste" escape sequence that some terminals send, and that binding is assigned by default. It lets readline insert pasted text as a single unit instead of treating each character like it came from the keyboard. The pasted characters get inserted as if each one were bound to self-insert rather than executing any editing commands. Bracketed paste sets the region to the inserted text and activates the region 📋💖
transpose-chars (C-t) — Drag the character before point forward over the character at point, moving point forward too. If point is at the end of the line, it transposes the two characters before point instead. Negative arguments do absolutely nothing here 🙈
transpose-words (M-t) — Drag the word before point past the word after point, moving point past that word too. If point is at the end of the line, it transposes the last two words on the line 🔄
shell-transpose-words (M-C-t) — Drag the word before point past the word after point, moving point past that word too. If the insertion point is at the end of the line, it transposes the last two words on the line. Word boundaries here are the same as shell-forward-word and shell-backward-word 🦋
upcase-word (M-u) — Uppercase the current (or following) word. With a negative argument, uppercase the previous word — but don't move point. stay put, bestie 🫶
downcase-word (M-l) — Lowercase the current (or following) word. With a negative argument, lowercase the previous word, and again point does not move 🌸
capitalize-word (M-c) — Capitalize the current (or following) word. With a negative argument, capitalize the previous word, and no, point still doesn't move 👑
overwrite-mode — Toggle overwrite mode. With an explicit positive numeric argument, switch to overwrite mode. With an explicit non-positive numeric argument, switch to insert mode. This command only affects emacs mode — vi mode does overwrite differently, iykyk. Also: every single call to readline() starts out in insert mode. In overwrite mode, characters bound to self-insert replace the text at point instead of shoving the text to the right. Characters bound to backward-delete-char replace the character before point with a space. By default this command is unbound, but it may be bound to the Insert key on some keyboards ⌨️✨

💅 Killing and Yanking 💅

okay bestie this is the ✂️ section, the cut and paste of readline, and it SLAPS once you know it.

kill-line (C-k) — kills everything from point to the end of the current line, periodt. Give it a negative numeric argument and it flips: it kills backward from the cursor to the start of the line instead. both directions, iconic
backward-kill-line (C-x Rubout) — kills backward to the beginning of the current line. Negative numeric argument? Then it kills forward from the cursor to the end of the line. mirror mirror
unix-line-discard (C-u) — kills backward from point to the beginning of the line, and (important!!) it saves the killed text on the kill-ring. recycling era
kill-whole-line — kills ALL characters on the current line, no matter where point happens to be. no survivors, respectfully
kill-word (M-d) — kills from point to the end of the current word, or if you're chillin' between words, to the end of the next word. Word boundaries are the same ones forward-word uses. 🦋
backward-kill-word (M-Rubout) — kills the word behind point. Word boundaries same as backward-word. 💀
shell-kill-word (M-C-d) — kills from point to the end of the current word, or if between words, to the end of the next word. Word boundaries same as shell-forward-word.
shell-backward-kill-word — kills the word behind point. Word boundaries same as shell-backward-word.
unix-word-rubout (C-w) — kills the word behind point using white space as the word boundary, and saves the killed text on the kill-ring. ✨
unix-filename-rubout — kills the word behind point using white space and the slash character as the word boundaries, saving the killed text on the kill-ring. path-aware queen
delete-horizontal-space (M-\) — deletes all the spaces and tabs around point. cleanup on aisle cursor
kill-region — kills the text in the current region.
copy-region-as-kill — copies the text in the region to the kill buffer so you can yank it immediately. no delete, just vibes
copy-backward-word — copies the word before point to the kill buffer. Word boundaries same as backward-word.
copy-forward-word — copies the word following point to the kill buffer. Word boundaries same as forward-word.
yank (C-y) — yanks the top of the kill ring into the buffer at point. 🫶
yank-pop (M-y) — rotates the kill ring and yanks the new top. only works right after a yank or a yank-pop — it's not a free-standing move, bestie.

🔢 Numeric Arguments 🔢

digit-argument (M-0, M-1, ..., M--) — adds this digit to the argument you're already accumulating, or starts a brand new argument. And M-- starts a negative argument. math girlies rise
universal-argument — another way to specify an argument. If this command is followed by one or more digits (optionally with a leading minus sign), those digits define the argument. If the command is followed by digits, executing universal-argument again ends the numeric argument — but is otherwise ignored. And as a special case: if this command is immediately followed by a character that's neither a digit nor a minus sign, the argument count for the next command gets multiplied by four. The argument count starts at one, so executing this function the first time makes the argument count four, a second time makes it sixteen, and so on. exponential slay 🎀

🧚 Completing 🧚

complete (TAB) — attempts to perform completion on the text before point. Bash tries completion by first checking for any programmable completions for the command word (see Programmable Completion below); otherwise it treats the text as a variable (if it begins with $), a username (if it begins with ~), a hostname (if it begins with @), or a command (including aliases, functions, and builtins) in turn. If none of those produces a match, it falls back to filename completion. try everything, queen
possible-completions (M-?) — lists the possible completions of the text before point. When displaying completions, readline sets the number of columns used for display to the value of completion-display-width, the value of the shell variable COLUMNS, or the screen width — in that order. 💖
insert-completions (M-*) — inserts all the completions of the text before point that possible-completions would have generated, separated by a space. spam that insert button
menu-complete — like complete, but it replaces the word to be completed with a single match from the list of possible completions. Run menu-complete over and over and it steps through the list of possible completions, inserting each match in turn. At the end of the list of completions, menu-complete rings the bell (subject to the setting of bell-style) and restores the original text. An argument of n moves n positions forward in the list of matches; a negative argument moves backward through the list. This command is intended to be bound to TAB, but it's unbound by default. sleeping beauty energy, someone go bind her
menu-complete-backward — identical to menu-complete, but moves backward through the list of possible completions, as if menu-complete had been given a negative argument. This command is unbound by default.
export-completions — performs completion on the word before point as described above and writes the list of possible completions to readline's output stream using this format, writing information on separate lines:
- the number of matches N;
- the word being completed;
- S:E, where S and E are the start and end offsets of the word in the readline line buffer; then
- each match, one per line

If there are no matches, the first line will be 0, and this command does not print any output after the S:E. If there is only a single match, this prints a single line containing it. If there is more than one match, this prints the common prefix of the matches — which may be empty — on the first line after the S:E, then the matches on subsequent lines. In this case, N will include the first line with the common prefix.

The user or application should be able to accommodate the possibility of a blank line. The intent is that the user or application reads N lines after the line containing S:E to obtain the match list. This command is unbound by default. format negotiation, very demure

delete-char-or-list — deletes the character under the cursor if it's not at the beginning or end of the line (like delete-char). At the end of the line, it behaves identically to possible-completions. This command is unbound by default.
complete-filename (M-/) — attempts filename completion on the text before point. 🍓
possible-filename-completions (C-x /) — lists the possible completions of the text before point, treating it as a filename.
complete-username (M-~) — attempts completion on the text before point, treating it as a username.
possible-username-completions (C-x ~) — lists the possible completions of the text before point, treating it as a username.
complete-variable (M-$) — attempts completion on the text before point, treating it as a shell variable.
possible-variable-completions (C-x $) — lists the possible completions of the text before point, treating it as a shell variable.
complete-hostname (M-@) — attempts completion on the text before point, treating it as a hostname.
possible-hostname-completions (C-x @) — lists the possible completions of the text before point, treating it as a hostname.
complete-command (M-!) — attempts completion on the text before point, treating it as a command name. Command completion tries to match the text against aliases, reserved words, shell functions, shell builtins, and finally executable filenames — in that order. priority list, we respect it
possible-command-completions (C-x !) — lists the possible completions of the text before point, treating it as a command name.
dynamic-complete-history (M-TAB) — attempts completion on the text before point, comparing the text against history list entries for possible completion matches. history said what it said
dabbrev-expand — attempts menu completion on the text before point, comparing the text against lines from the history list for possible completion matches.
complete-into-braces (M-{) — performs filename completion and inserts the list of possible completions enclosed within braces so the list is available to the shell (see Brace Expansion above). brace expansion cameo!!

⌨️ Keyboard Macros ⌨️

start-kbd-macro (C-x () — begins saving the characters you type into the current keyboard macro. 🎬
end-kbd-macro (C-x )) — stops saving the characters you type into the current keyboard macro and stores the definition. saved to the vault
call-last-kbd-macro (C-x e) — re-executes the last keyboard macro defined, by making the characters in the macro appear as if they were typed at the keyboard. it's giving ghostwriter
print-last-kbd-macro (()) — prints the last keyboard macro defined in a format suitable for the inputrc file. 💅

✨ Miscellaneous ✨

okay so these are the readline commands that live in the miscellaneous drawer, and honestly? they're giving main character energy 💅

re-read-init-file (C-x C-r) — read in the contents of the inputrc file, and incorporate any bindings or variable assignments found there. 💖
abort (C-g) — abort the current editing command and ring the terminal's bell (subject to the setting of bell-style). not me ringing bells mid-edit 😭
do-lowercase-version (M-A, M-B, M-x, ...) — if the metafied character x is uppercase, run the command that is bound to the corresponding metafied lowercase character. the behavior is undefined if x is already lowercase. periodt.
prefix-meta (ESC) — metafy the next character typed. ESC f is equivalent to Meta-f. lowkey iconic ✨
undo (C-_, C-x C-u) — incremental undo, separately remembered for each line. obsessed with a good undo 🫶
revert-line (M-r) — undo all changes made to this line. this is like executing the undo command enough times to return the line to its initial state. it's giving factory reset 💅
tilde-expand (M-&) — perform tilde expansion on the current word.
set-mark (C-@, M-<space>) — set the mark to the point. if a numeric argument is supplied, set the mark to that position.
exchange-point-and-mark (C-x C-x) — swap the point with the mark. set the current cursor position to the saved position, then set the mark to the old cursor position. a little swap situation 🔄
character-search (C-]) — read a character and move point to the next occurrence of that character. a negative argument searches for previous occurrences.
character-search-backward (M-C-]) — read a character and move point to the previous occurrence of that character. a negative argument searches for subsequent occurrences.
skip-csi-sequence — read enough characters to consume a multi-key sequence such as those defined for keys like Home and End. CSI sequences begin with a Control Sequence Indicator (CSI), usually ESC [. if this sequence is bound to \e[, keys producing CSI sequences have no effect unless explicitly bound to a readline command, instead of inserting stray characters into the editing buffer. this is unbound by default, but usually bound to ESC [. fr fr, no stray characters allowed 🙈
insert-comment (M-#) — without a numeric argument, insert the value of the readline comment-begin variable at the beginning of the current line. if a numeric argument is supplied, this command acts as a toggle: if the characters at the beginning of the line do not match the value of comment-begin, insert the value; otherwise delete the characters in comment-begin from the beginning of the line. in either case, the line is accepted as if a newline had been typed. the default value of comment-begin causes this command to make the current line a shell comment. if a numeric argument causes the comment character to be removed, the line will be executed by the shell. heads up bestie, that last bit means it actually runs 💀
spell-correct-word (C-x s) — perform spelling correction on the current word, treating it as a directory or filename, in the same way as the cdspell shell option. word boundaries are the same as those used by shell-forward-word. autocorrect but make it useful 🎀
glob-complete-word (M-g) — treat the word before point as a pattern for pathname expansion, with an asterisk implicitly appended, then use the pattern to generate a list of matching file names for possible completions.
glob-expand-word (C-x *) — treat the word before point as a pattern for pathname expansion, and insert the list of matching file names, replacing the word. if a numeric argument is supplied, append a * before pathname expansion.
glob-list-expansions (C-x g) — display the list of expansions that would have been generated by glob-expand-word and redisplay the line. if a numeric argument is supplied, append a * before pathname expansion.
dump-functions — print all of the functions and their key bindings to the readline output stream. if a numeric argument is supplied, the output is formatted in such a way that it can be made part of an inputrc file.
dump-variables — print all of the settable readline variables and their values to the readline output stream. if a numeric argument is supplied, the output is formatted in such a way that it can be made part of an inputrc file.
dump-macros — print all of the readline key sequences bound to macros and the strings they output to the readline output stream. if a numeric argument is supplied, the output is formatted in such a way that it can be made part of an inputrc file.
execute-named-command (M-x) — read a bindable readline command name from the input and execute the function to which it's bound, as if the key sequence to which it was bound appeared in the input. if this function is supplied with a numeric argument, it passes that argument to the function it executes. it's giving "run me by name" energy 👑
display-shell-version (C-x C-v) — display version information about the current instance of bash. iykyk, version checks are self-care 💖

🌈 Programmable Completion 🌈

okay so when a user attempts word completion for a command or an argument to a command for which a completion specification (a compspec) has been defined using the complete builtin (see SHELL BUILTIN COMMANDS below), readline invokes the programmable completion facilities. it's giving ✨magic✨ but make it shell.

First, bash identifies the command name. if a compspec has been defined for that command, the compspec is used to generate the list of possible completions for the word. if the command word is the empty string (completion attempted at the beginning of an empty line), bash uses any compspec defined with the -E option to complete. the -I option to complete indicates that the command word is the first non-assignment word on the line, or after a command delimiter such as ; or |. this usually indicates command name completion. periodt.

if the command word is a full pathname, bash searches for a compspec for the full pathname first. if there is no compspec for the full pathname, bash attempts to find a compspec for the portion following the final slash. if those searches do not result in a compspec, or if there is no compspec for the command word, bash uses any compspec defined with the -D option to complete as the default. if there is no default compspec, bash performs alias expansion on the command word as a final resort, and attempts to find a compspec for the command word resulting from any successful expansion.

if a compspec is not found, bash performs its default completion as described above under Completing. otherwise, once a compspec has been found, bash uses it to generate the list of matching words.

First, bash performs the actions specified by the compspec. this only returns matches which are prefixes of the word being completed. when the -f or -d option is used for filename or directory name completion, bash uses the shell variable FIGNORE to filter the matches. filter queen 💅

Next, programmable completion generates matches specified by a pathname expansion pattern supplied as an argument to the -G option. the words generated by the pattern need not match the word being completed. bash uses the FIGNORE variable to filter the matches, but does not use the GLOBIGNORE shell variable. not GLOBIGNORE, bestie, keep up 😭

Next, completion considers the string specified as the argument to the -W option. the string is first split using the characters in the IFS special variable as delimiters. this honors shell quoting within the string, in order to provide a mechanism for the words to contain shell metacharacters or characters in the value of IFS. each word is then expanded using brace expansion, tilde expansion, parameter and variable expansion, command substitution, and arithmetic expansion, as described above under EXPANSION. the results are split using the rules described above under Word Splitting. the results of the expansion are prefix-matched against the word being completed, and the matching words become possible completions. obsessed with a thorough expansion moment 🫶

after these matches have been generated, bash executes any shell function or command specified with the -F and -C options. when the command or function is invoked, bash assigns values to the COMP_LINE, COMP_POINT, COMP_KEY, and COMP_TYPE variables as described above under Shell Variables. if a shell function is being invoked, bash also sets the COMP_WORDS and COMP_CWORD variables. when the function or command is invoked, the first argument ($1) is the name of the command whose arguments are being completed, the second argument ($2) is the word being completed, and the third argument ($3) is the word preceding the word being completed on the current command line. there is no filtering of the generated completions against the word being completed; the function or command has complete freedom in generating the matches and they do not need to match a prefix of the word. freedom fr fr 🦋

any function specified with -F is invoked first. the function may use any of the shell facilities, including the compgen and compopt builtins described below, to generate the matches. it must put the possible completions in the COMPREPLY array variable, one per array element. one per element, no cap 💯

Next, any command specified with the -C option is invoked in an environment equivalent to command substitution. it should print a list of completions, one per line, to the standard output. backslash will escape a newline, if necessary. these are added to the set of possible completions.

after generating all of the possible completions, bash applies any filter specified with the -X option to the completions in the list. the filter is a pattern as used for pathname expansion; a & in the pattern is replaced with the text of the word being completed. a literal & may be escaped with a backslash; the backslash is removed before attempting a match. any completion that matches the pattern is removed from the list. a leading ! negates the pattern; in this case bash removes any completion that does not match the pattern. if the nocasematch shell option is enabled, bash performs the match without regard to the case of alphabetic characters. filter and negate, it's giving delete that vibe 💀

Finally, programmable completion adds any prefix and suffix specified with the -P and -S options, respectively, to each completion, and returns the result to readline as the list of possible completions. slay, we love a final touch ✨

if the previously-applied actions do not generate any matches, and the -o dirnames option was supplied to complete when the compspec was defined, bash attempts directory name completion.

if the -o plusdirs option was supplied to complete when the compspec was defined, bash attempts directory name completion and adds any matches to the set of possible completions. directories said "me too" and honestly? valid 💅

So default behaviour bestie: if a compspec gets found, whatever it spits out gets handed right back to the completion code as the full set of possible completions, periodt. And the default bash completions plus readline's default filename completion? Disabled. Gone. Not invited.

But if you passed the -o bashdefault option to complete back when you defined that compspec, and the compspec generates zero matches, bash will go ahead and attempt its default completions. And if the compspec and (if it got attempted) the default bash completions both generate no matches, and you'd supplied -o default to complete when defining the compspec, then programmable completion goes and performs readline's default completion instead. Chain reaction, but make it useful. ✨

The options you hand to complete and compopt can control how readline treats the completions. For instance — okay so — the -o fullquote option tells readline to quote the matches as if they were filenames. See the description of complete below for the whole tea. 🫖

When a compspec says it wants directory name completion, the programmable completion functions force readline to append a slash to completed names which are symbolic links to directories — and that's subject to the value of the mark-directories readline variable, regardless of the setting of the mark-symlinked-directories readline variable. Yeah, it overrides. She's got main character energy like that. 👑

Dynamically Modifying Completions

There is some support for dynamically modifying completions. This hits hardest when you combine it with a default completion specified with complete -D. Lowkey the move. 🦋

Shell functions executed as completion functions can indicate that completion should be retried by returning an exit status of 124. If a shell function returns 124, and it changes the compspec associated with the command on which completion is being attempted (that command comes in as the first argument when the function is executed), programmable completion restarts from the very beginning, with an attempt to find a new compspec for that command. So you can build up a set of completions dynamically as completion is attempted, instead of loading them all at once. Chef's kiss. 💅

For instance, assuming that there is a library of compspecs, each kept in a file corresponding to the name of the command, the following default completion function would load completions dynamically:

_completion_loader()
{
  . "/etc/bash_completion.d/$1.sh" \
    >/dev/null 2>&1 && return 124
}
complete -D -F _completion_loader \
    -o bashdefault -o default

🕰️ HISTORY 🕰️

When the -o history option to the set builtin is enabled, the shell gives you access to the command history — aka the list of commands you previously typed. Yes, the receipts. 🧾

The value of the HISTSIZE variable is used as the number of commands to save in a history list: the shell saves the text of the last HISTSIZE commands (default 500). So by default she's remembering your last 500 moments. Iconic.

The shell stores each command in the history list prior to parameter and variable expansion (see EXPANSION above) but after history expansion is performed — subject to the values of the shell variables HISTIGNORE and HISTCONTROL. Note the order bestie, it matters. ✨

On startup, bash initializes the history list by reading history entries from the file named by the HISTFILE variable (default ~/.bash_history). That file is referred to as the history file. The history file is truncated, if necessary, to contain no more than the number of history entries specified by the value of the HISTFILESIZE variable. And if HISTFILESIZE is unset, or set to null, a non-numeric value, or a numeric value less than zero, the history file is not truncated. Leave her alone then. 🙈

When the history file is read, lines beginning with the history comment character followed immediately by a digit are interpreted as timestamps for the following history line. These timestamps are optionally displayed depending on the value of the HISTTIMEFORMAT variable. When present, history timestamps delimit history entries, which makes multi-line entries possible. Slay, multi-line representation. 🌈

When a shell with history enabled exits, bash copies the last $HISTSIZE entries from the history list to $HISTFILE. If the histappend shell option is enabled (see the description of shopt under SHELL BUILTIN COMMANDS below), bash appends the entries to the history file — otherwise it overwrites the history file. Choose wisely.

If HISTFILE is unset or null, or if the history file is unwritable, the history is not saved. And after saving the history, bash truncates the history file to contain no more than HISTFILESIZE lines as described above.

If the HISTTIMEFORMAT variable is set, the shell writes the timestamp information associated with each history entry to the history file, marked with the history comment character, so timestamps are preserved across shell sessions. This uses the history comment character to distinguish timestamps from other history lines. And as above, when using HISTTIMEFORMAT, the timestamps delimit multi-line history entries. 🎀

The fc builtin command (see SHELL BUILTIN COMMANDS below) will list or edit and re-execute a portion of the history list. The history builtin can display or modify the history list and manipulate the history file. And when you're using command-line editing, search commands are available in each editing mode that provide access to the history list. So many doors into the archive bestie. 🚪✨

The shell allows control over which commands get saved on the history list. The HISTCONTROL and HISTIGNORE variables are used to save only a subset of the commands entered. If the cmdhist shell option is enabled, the shell attempts to save each line of a multi-line command in the same history entry, adding semicolons where necessary to preserve syntactic correctness. The lithist shell option modifies cmdhist by saving the command with embedded newlines instead of semicolons. See the description of the shopt builtin below under SHELL BUILTIN COMMANDS for info on setting and unsetting shell options. 💜

🕰️✨ HISTORY EXPANSION ✨🕰️

okay so bestie, the shell has a whole history expansion feature and it's lowkey giving csh energy — that's literally where it comes from. This section is the full tour of what syntax you get. 🎀

History expansion is on by default for interactive shells, and you can turn it off with the +H option to the set builtin command (peep SHELL BUILTIN COMMANDS below). Non-interactive shells do not do history expansion by default, but you can switch it on with set -H. ✨

What even is it for? History expansions pull words from the history list straight into your input stream, which makes it super easy to repeat commands, shove the arguments of a previous command into your current input line, or fix errors in previous commands at lightning speed. 💅

Timing matters here, so listen: history expansion happens immediately after a complete line is read, before the shell breaks it into words, and it's performed on each line individually. The shell attempts to inform the history expansion functions about quoting still in effect from previous lines. 🫶

It goes down in two parts:

First, figure out which history list entry you're substituting from.
Second, pick which portions of that entry get included into the current one.

The entry selected from the history is the event, and the portions of that entry that get acted upon are words. There are various modifiers available to manipulate the selected words. The entry is split into words in the same fashion as when reading input, so that several metacharacter-separated words surrounded by quotes are considered one word. The event designator selects the event, the optional word designator selects words from the event, and the various optional modifiers are there to manipulate the selected words. 🌸

History expansions are introduced by the appearance of the history expansion character, which is ! by default. They may appear anywhere in the input, but they do not nest. 🚫

Only backslash (\) and single quotes can quote the history expansion character — but plot twist, the history expansion character is also treated as quoted if it immediately precedes the closing double quote in a double-quoted string. 💀

Several characters inhibit history expansion if found immediately following the history expansion character, even if it's unquoted: space, tab, newline, carriage return, =, and the other shell metacharacters defined above.

There's also a special abbreviation for substitution, active when the quick substitution character (described above under histchars) is the first character on the line. It selects the previous history list entry, using an event designator equivalent to !!, and substitutes one string for another in that entry. It's described below under Event Designators. This is the only history expansion that does not begin with the history expansion character. 👑

Several shell options settable with the shopt builtin will modify history expansion behavior (see the description of the shopt builtin below). If the histverify shell option is enabled, and readline is being used, history substitutions are not immediately passed to the shell parser. Instead, the expanded line is reloaded into the readline editing buffer for further modification. If readline is being used, and the histreedit shell option is enabled, a failed history substitution is reloaded into the readline editing buffer for correction. 🧚

The -p option to the history builtin command shows what a history expansion will do before you actually use it. The -s option to the history builtin will add commands to the end of the history list without actually executing them, so that they're available for subsequent recall. 🙈

The shell allows control of the various characters used by the history expansion mechanism (see the description of histchars above under Shell Variables). And the shell uses the history comment character to mark history timestamps when writing the history file. ⭐

🎯 Event Designators

An event designator is a reference to an entry in the history list. It consists of the portion of the word beginning with the history expansion character and ending with the word designator if present, or the end of the word. Unless the reference is absolute, events are relative to the current position in the history list. 💖

! — Start a history substitution, except when followed by a blank, newline, carriage return, =, or, when the extglob shell option is enabled using the shopt builtin, (.
!n — Refer to history list entry n.
!-n — Refer to the current entry minus n.
!! — Refer to the previous entry. This is a synonym for !-1.
!string — Refer to the most recent command preceding the current position in the history list starting with string.
!?string[?] — Refer to the most recent command preceding the current position in the history list containing string. The trailing ? may be omitted if string is followed immediately by a newline. If string is missing, this uses the string from the most recent search; it is an error if there is no previous search string.
^string1^string2^ — Quick substitution. Repeat the previous command, replacing string1 with string2. Equivalent to !!:s^string1^string2^ (see Modifiers below).
!# — The entire command line typed so far.

🧩 Word Designators

Word designators are used to select the desired words from the event. They're optional — if the word designator isn't supplied, the history expansion uses the entire event. A : separates the event specification from the word designator. It may be omitted if the word designator begins with a ^, $, *, -, or %. Words are numbered from the beginning of the line, with the first word being denoted by 0 (zero). Words are inserted into the current line separated by single spaces. ✨

0 (zero) — The zeroth word. For the shell, this is the command word.
n — The *n*th word.
^ — The first argument: word 1.
$ — The last word. This is usually the last argument, but will expand to the zeroth word if there is only one word in the line.
% — The first word matched by the most recent ?string? search, if the search string begins with a character that is part of a word. By default, searches begin at the end of each line and proceed to the beginning, so the first word matched is the one closest to the end of the line.
x-y — A range of words; -y abbreviates 0-y.
* — All of the words but the zeroth. This is a synonym for 1-$. It is not an error to use * if there is just one word in the event; it expands to the empty string in that case.
x* — Abbreviates x-$.
x- — Abbreviates x-$ like x*, but omits the last word. If x is missing, it defaults to 0.

If a word designator is supplied without an event specification, the previous command is used as the event, equivalent to !!. 🫶

🛠️ Modifiers

After the optional word designator, the expansion may include a sequence of one or more of the following modifiers, each preceded by a :. These modify, or edit, the word or words selected from the history event. 🎀

h — Remove a trailing pathname component, leaving only the head.
t — Remove all leading pathname components, leaving the tail.
r — Remove a trailing suffix of the form .xxx, leaving the basename.
e — Remove all but the trailing suffix.
p — Print the new command but do not execute it.
q — Quote the substituted words, escaping further substitutions.
x — Quote the substituted words as with q, but break into words at blanks and newlines. The q and x modifiers are mutually exclusive; expansion uses the last one supplied.
s/old/new/ — Substitute new for the first occurrence of old in the event line. Any character may be used as the delimiter in place of /. The final delimiter is optional if it is the last character of the event line. A single backslash quotes the delimiter in old and new. If & appears in new, it is replaced with old. A single backslash quotes the &. If old is null, it is set to the last old substituted, or, if no previous history substitutions took place, the last string in a !?string[?] search. If new is null, each matching old is deleted.
& — Repeat the previous substitution.
g — Cause changes to be applied over the entire event line. This is used in conjunction with :s (e.g., :gs/old/new/) or :&. If used with :s, any delimiter can be used in place of /, and the final delimiter is optional if it is the last character of the event line. An a may be used as a synonym for g.
G — Apply the following s or & modifier once to each word in the event line.

🛠️💖 SHELL BUILTIN COMMANDS 💖🛠️

Unless otherwise noted, each builtin command documented in this section as accepting options preceded by - accepts -- to signify the end of the options. Now for the exceptions, bestie, pay attention:

The :, true, false, and test/[ builtins do not accept options and do not treat -- specially.
The exit, logout, return, break, continue, let, and shift builtins accept and process arguments beginning with - without requiring --.
Other builtins that accept arguments but are not specified as accepting options interpret arguments beginning with - as invalid options and require -- to prevent this interpretation. 💀

`:` [arguments]

: [arguments]

No effect; the command does nothing beyond expanding arguments and performing any specified redirections. The return status is zero. Literally the chillest builtin alive, it's giving "I'm just here to vibe" energy. 🧘‍♀️✨

`.` [-p path] filename [arguments] / `source` [-p path] filename [arguments]

. [-p path] filename [arguments]
source [-p path] filename [arguments]

The . command (aka source) reads and execute commands from filename in the current shell environment and returns the exit status of the last command executed from filename. 👑

If filename does not contain a slash, . searches for it. If the -p option is supplied, . treats path as a colon-separated list of directories in which to find filename; otherwise, . uses the entries in PATH to find the directory containing filename. filename does not need to be executable. 🦋

When bash is not in posix mode, it searches the current directory if filename is not found in PATH, but does not search the current directory if -p is supplied. And if the sourcepath option to the shopt builtin command is turned off, . does not search PATH at all. 🚫

If any arguments are supplied, they become the positional parameters when filename is executed. Otherwise the positional parameters are unchanged. 🌸

okay so if the -T option is enabled, . inherits any trap on DEBUG. If -T is not on, then any DEBUG trap string gets saved and restored around the call to ., and . actually unsets the DEBUG trap while it's executing. And if -T isn't set and the file you're sourcing changes the DEBUG trap, that new value sticks around after . finishes. No cap.

The return status is the status of the last command executed from filename (0 if no commands were executed), and non-zero if filename isn't found or can't be read. 💅

`alias` ✨

alias [-p] [name[=value] ...]

With no arguments or with the -p option, alias prints the list of aliases in the form alias name=value on standard output. 🫶

When arguments are supplied, you define an alias for each name whose value is given. And lowkey important: a trailing space in value causes the next word to be checked for alias substitution when the alias gets expanded during command parsing. <em>iykyk</em>

For each name in the argument list that has no value supplied, it prints the name and value of that alias.

alias returns true unless a name is given (without a matching =value) for which no alias has been defined. 🦋

`bg` 💤

bg [jobspec ...]

Resume each suspended job jobspec in the background, like it had been started with &. If jobspec isn't there, the shell uses its notion of the current job.

bg jobspec returns 0 unless it's run when job control is disabled — or, when run with job control enabled, any specified jobspec wasn't found or was started without job control. 😭

`bind` 🎀

bind [-m keymap] [-lsvSVX]
bind [-m keymap] [-q function] [-u function] [-r keyseq]
bind [-m keymap] -f filename
bind [-m keymap] -x keyseq[:] shell-command
bind [-m keymap] keyseq:function-name
bind [-m keymap] -p|-P [readline-command]
bind [-m keymap] keyseq:readline-command
bind readline-command-line

This one displays current readline key and function bindings, binds a key sequence to a readline function or macro or to a shell command, or sets a readline variable. it's giving control panel for your keyboard ⌨️✨

Each non-option argument is a key binding or command as it would appear in a readline initialization file such as .inputrc, but every binding or command has to be passed as a separate argument. Like:

'"\C-x\C-r": re-read-init-file'

In the following descriptions, output available to be re-read is formatted as commands that would appear in a readline initialization file or that would be supplied as individual arguments to a bind command.

Options, if supplied, have the following meanings:

-m keymap — Use keymap as the keymap to be affected by the subsequent bindings. Acceptable keymap names are emacs, emacs-standard, emacs-meta, emacs-ctlx, vi, vi-move, vi-command, and vi-insert. vi is equivalent to vi-command (vi-move is also a synonym); emacs is equivalent to emacs-standard. 💜
-l — List the names of all readline functions.
-p — Display readline function names and bindings in such a way that they can be used as an argument to a subsequent bind command or in a readline initialization file. If arguments remain after option processing, bind treats them as readline command names and restricts output to those names.
-P — List current readline function names and bindings. If arguments remain after option processing, bind treats them as readline command names and restricts output to those names.
-s — Display readline key sequences bound to macros and the strings they output in such a way that they can be used as an argument to a subsequent bind command or in a readline initialization file.
-S — Display readline key sequences bound to macros and the strings they output.
-v — Display readline variable names and values in such a way that they can be used as an argument to a subsequent bind command or in a readline initialization file.
-V — List current readline variable names and values.
-f filename — Read key bindings from filename.
-q function — Display key sequences that invoke the named readline function.
-u function — Unbind all key sequences bound to the named readline function.
-r keyseq — Remove any current binding for keyseq.
-x keyseq[: ]shell-command — Cause shell-command to be executed whenever keyseq is entered. The separator between keyseq and shell-command is either whitespace or a colon optionally followed by whitespace. If the separator is whitespace, shell-command must be enclosed in double quotes and readline expands any of its special backslash-escapes in shell-command before saving it. If the separator is a colon, any enclosing double quotes are optional, and readline does not expand the command string before saving it. Since the entire key binding expression must be a single argument, it should be enclosed in single quotes. When shell-command is executed, the shell sets the READLINE_LINE variable to the contents of the readline line buffer and the READLINE_POINT and READLINE_MARK variables to the current location of the insertion point and the saved insertion point (the mark), respectively. The shell assigns any numeric argument the user supplied to the READLINE_ARGUMENT variable. If there was no argument, that variable is not set. If the executed command changes the value of any of READLINE_LINE, READLINE_POINT, or READLINE_MARK, those new values will be reflected in the editing state. 🔥
-X — List all key sequences bound to shell commands and the associated commands in a format that can be reused as an argument to a subsequent bind command.

The return value is 0 unless an unrecognized option is supplied or an error occurred. 😌

`break` 💥

break [n]

Exit from within a for, while, until, or select loop. If n is specified, break exits n enclosing loops. n must be ≥ 1. If n is greater than the number of enclosing loops, all enclosing loops are exited.

The return value is 0 unless n is not greater than or equal to 1. 🫶

`builtin` 🧱

builtin shell-builtin [arguments]

Execute the specified shell builtin shell-builtin, passing it arguments, and return its exit status. This is useful when you're defining a function whose name is the same as a shell builtin, so you can keep the builtin's functionality inside the function. The cd builtin is commonly redefined this way. classic move tbh

The return status is false if shell-builtin is not a shell builtin command. 🙈

`caller` 📞

caller [expr]

Returns the context of any active subroutine call (a shell function or a script executed with the . or source builtins).

Without expr, caller displays the line number and source filename of the current subroutine call. If a non-negative integer is supplied as expr, caller displays the line number, subroutine name, and source file corresponding to that position in the current execution call stack. This extra info may be used, for example, to print a stack trace. The current frame is frame 0. ✨

The return value is 0 unless the shell is not executing a subroutine call or expr doesn't correspond to a valid position in the call stack. 😭

`cd` 📁💖

cd [-L] [-@] [dir]
cd -P [-e] [-@] [dir]

Change the current directory to dir. If dir isn't supplied, the value of the HOME shell variable is used as dir. If the variable CDPATH exists, and dir doesn't begin with a slash (/), cd uses it as a search path: the shell searches each directory name in CDPATH for dir. Alternative directory names in CDPATH are separated by a colon (:). A null directory name in CDPATH is the same as the current directory, i.e., “.”. 🦋

The -P option causes cd to use the physical directory structure by resolving symbolic links while traversing dir and before processing instances of .. in dir (see also the -P option to the set builtin command).

The -L option forces cd to follow symbolic links by resolving the link after processing instances of .. in dir. If .. appears in dir, cd processes it by removing the immediately previous pathname component from dir, back to a slash or the beginning of dir, and verifying that the portion of dir it has processed to that point is still a valid directory name after removing the pathname component. If it's not a valid directory name, cd returns a non-zero status. If neither -L nor -P is supplied, cd behaves as if -L had been supplied. 👑

If the -e option is supplied with -P, and cd can't successfully determine the current working directory after a successful directory change, it returns a non-zero status. 💀

On systems that support it, the -@ option presents the extended attributes associated with a file as a directory.

An argument of - is converted to $OLDPWD before attempting the directory change. 🎀

If cd uses a non-empty directory name from CDPATH, or if - is the first argument, and the directory change is successful, cd writes the absolute pathname of the new working directory to the standard output. ✨

If the directory change is successful, cd sets the value of the PWD environment variable to the new directory name, and sets the OLDPWD environment variable to the value of the current working directory before the change.

The return value is true if the directory was successfully changed; false otherwise. 🌸

command [-pVv] command [arg ...] — bestie, this one runs command with its args, but it suppresses the normal shell function lookup for command. So it's giving "we're skipping the vibes, going straight to the source." Only builtin commands, or commands found in the PATH named command, actually get executed. If you pass -p, the search for command happens using a default value for PATH that's guaranteed to find all of the standard utilities. Lowkey iconic.

If either -V or -v is supplied, command prints a description of command. The -v option displays a single word indicating the command or filename used to invoke command; the -V option produces a more verbose description. Ngl, -v is the quick vibe check and -V is the full paragraph.

If -V or -v is supplied, the exit status is zero if command was found, and non-zero if not. If neither option is supplied and an error occurred or command cannot be found, the exit status is 127. Otherwise, the exit status of the command builtin is just the exit status of command. Periodt.

compgen [-V varname] [option] [word] — this one generates possible completion matches for word according to the options, and writes the matches to standard output. The options may be any option accepted by the complete builtin, with the exceptions of -p, -r, -D, -E, and -I. (Those five are simply not invited to this party. 🙈)

If the -V option is supplied, compgen stores the generated completions into the indexed array variable varname instead of writing them to standard output. Cute, right? Saving them for later.

When using the -F or -C options, the various shell variables set by the programmable completion facilities, while available, will not have useful values. So like… they're there, but they're not serving. 😭

The matches will be generated in the same way as if the programmable completion code had generated them directly from a completion specification with the same flags. If word is specified, only those completions matching word will be displayed or stored.

The return value is true unless an invalid option is supplied, or no matches were generated.

complete [-abcdefgjksuv] [-o comp-option] [-DEI] [-A action] [-G globpat] [-W wordlist] [-F function] [-C command] [-X filterpat] [-P prefix] [-S suffix] name [name ...]
complete -pr [-DEI] [name ...] — specify how arguments to each name should be completed. This is the main character of tab-completion, fr fr.

If the -p option is supplied, or if no options or names are supplied, it prints existing completion specifications in a way that allows them to be reused as input. The -r option removes a completion specification for each name, or, if no names are supplied, all completion specifications. (Delete the whole group chat. 💀)

The -D option indicates that other supplied options and actions should apply to the "default" command completion — that is, completion attempted on a command for which no completion has previously been defined. The -E option indicates that other supplied options and actions should apply to "empty" command completion — that is, completion attempted on a blank line. The -I option indicates that other supplied options and actions should apply to completion on the initial non-assignment word on the line, or after a command delimiter such as ; or |, which is usually command name completion. If multiple options are supplied, the -D option takes precedence over -E, and both take precedence over -I. If any of -D, -E, or -I are supplied, any other name arguments are ignored; these completions only apply to the case specified by the option.

The process of applying these completion specifications when attempting word completion is described above under Programmable Completion.

Other options, if specified, have the following meanings. The arguments to the -G, -W, and -X options (and, if necessary, the -P and -S options) should be quoted to protect them from expansion before the complete builtin is invoked.

-o comp-option — the comp-option controls several aspects of the compspec's behavior beyond the simple generation of completions. comp-option may be one of:
- bashdefault — perform the rest of the default bash completions if the compspec generates no matches.
- default — use readline's default filename completion if the compspec generates no matches.
- dirnames — perform directory name completion if the compspec generates no matches.
- filenames — tell readline that the compspec generates filenames, so it can perform any filename-specific processing (such as adding a slash to directory names, quoting special characters, or suppressing trailing spaces). This is intended to be used with shell functions.
- fullquote — tell readline to quote all the completed words even if they are not filenames.
- noquote — tell readline not to quote the completed words if they are filenames (quoting filenames is the default).
- nosort — tell readline not to sort the list of possible completions alphabetically.
- nospace — tell readline not to append a space (the default) to words completed at the end of the line.
- plusdirs — after generating any matches defined by the compspec, attempt directory name completion and add any matches to the results of the other actions.
-A action — the action may be one of the following to generate a list of possible completions:
- alias — alias names. May also be specified as -a.
- arrayvar — array variable names.
- binding — readline key binding names.
- builtin — names of shell builtin commands. May also be specified as -b.
- command — command names. May also be specified as -c.
- directory — directory names. May also be specified as -d.
- disabled — names of disabled shell builtins.
- enabled — names of enabled shell builtins.
- export — names of exported shell variables. May also be specified as -e.
- file — file and directory names, similar to readline's filename completion. May also be specified as -f.
- function — names of shell functions.
- group — group names. May also be specified as -g.
- helptopic — help topics as accepted by the help builtin.
- hostname — hostnames, as taken from the file specified by the HOSTFILE shell variable.
- job — job names, if job control is active. May also be specified as -j.
- keyword — shell reserved words. May also be specified as -k.
- running — names of running jobs, if job control is active.
- service — service names. May also be specified as -s.
- setopt — valid arguments for the -o option to the set builtin.
- shopt — shell option names as accepted by the shopt builtin.
- signal — signal names.
- stopped — names of stopped jobs, if job control is active.
- user — user names. May also be specified as -u.
- variable — names of all shell variables. May also be specified as -v.
-C command — command is executed in a subshell environment, and its output is used as the possible completions. Arguments are passed as with the -F option.
-F function — the shell function function is executed in the current shell environment. When the function is executed, the first argument ($1) is the name of the command whose arguments are being completed, the second argument ($2) is the word being completed, and the third argument ($3) is the word preceding the word being completed on the current command line. When function finishes, programmable completion retrieves the possible completions from the value of the COMPREPLY array variable. She's the blueprint, bestie. 🫶
-G globpat — expand the pathname expansion pattern globpat to generate the possible completions.
-P prefix — add prefix to the beginning of each possible completion after all other options have been applied.
-S suffix — append suffix to each possible completion after all other options have been applied.
-W wordlist — split the wordlist using the characters in the IFS special variable as delimiters, and expand each resulting word. Shell quoting is honored within wordlist, in order to provide a mechanism for the words to contain shell metacharacters or characters in the value of IFS. The possible completions are the members of the resultant list which match a prefix of the word being completed.
-X filterpat — filterpat is a pattern as used for pathname expansion. It is applied to the list of possible completions generated by the preceding options and arguments, and each completion matching filterpat is removed from the list. A leading ! in filterpat negates the pattern; in this case, any completion not matching filterpat is removed.

The return value is true unless an invalid option got supplied, an option that isn't -p, -r, -D, -E, or -I got supplied with no name argument, someone tried to remove a completion specification for a name that straight-up has no specification, or an error pops off while adding a completion specification. So basically: true is the vibe, unless you did something cursed 💀.

🌸 `compopt` 🌸

compopt [-o option] [-DEI] [+o option] [name]

Bestie, this one modifies the completion options for each name according to the options you pass, or — if you don't supply any names — for the completion that's currently executing. And if you supply no options at all, it just shows you the completion options for each name, or for the current completion. No notes, just vibes. ✨

The possible values of option are exactly the ones that are valid for the complete builtin described above 👆.

-D — says the other options you supplied should apply to the "default" command completion.
-E — says the other options you supplied should apply to "empty" command completion.
-I — says the other options you supplied should apply to completion on the initial word on the line.

These get figured out the exact same way as in the complete builtin. iykyk 💅

If you supply multiple options, the -D option takes precedence over -E, and both of those take precedence over -I. Ranked. Iconic behavior, honestly.

The return value is true unless an invalid option is supplied, an attempt is made to modify the options for a name that has no completion specification, or an output error occurs. That's it, that's the whole thing 🫶.

🦋 `continue` 🦋

continue [n]

continue resumes the next iteration of the enclosing for, while, until, or select loop. Like, you're in the loop and you're like "okay skip to the next round bestie, we're not doing this one." 🙈

If n is specified, bash resumes the nth enclosing loop. And n must be ≥ 1 — no zero, no negatives, don't even try it.

If n is greater than the number of enclosing loops, the shell just resumes the last enclosing loop — the "top-level" loop. It clamps, basically.

The return value is 0 unless n is not greater than or equal to 1.

👑 `declare` / `typeset` 👑

declare [-aAfFgiIlnrtux] [-p] [name[=value] ...]
typeset [-aAfFgiIlnrtux] [-p] [name[=value] ...]

Declare variables and/or give them attributes. Two names, same energy — typeset is literally just declare's twin 🫶.

If you give it no names, it displays the values of variables or functions. The -p option displays the attributes and values of each name. And when -p is used with name arguments, any additional options other than -f and -F get ignored. Ignored! Not even acknowledged 😭.

When -p is supplied without name arguments, declare displays the attributes and values of all variables that have the attributes specified by the additional options. If no other options come along with -p, declare displays the attributes and values of all shell variables. The -f option restricts the display to shell functions.

The -F option inhibits the display of function definitions — only the function name and attributes get printed. And if the extdebug shell option is enabled using shopt, then the source file name and line number where each name is defined get displayed too. -F implies -f, so it's a package deal.

The -g option forces variables to be created or modified at the global scope, even when declare is executed inside a shell function. It's ignored when declare is not executed in a shell function — so it just sits there, doing nothing, being ignored 🎀.

The -I option causes local variables to inherit the attributes (except the nameref attribute) and the value of any existing variable with the same name at a surrounding scope. And if there's no existing variable? The local variable starts out unset. Fresh start, no drama ✨.

The following options can be used to restrict output to variables with the specified attribute, or to give variables attributes:

-a — each name is an indexed array variable (see Arrays above).
-A — each name is an associative array variable (see Arrays above).
-f — each name refers to a shell function.
-i — the variable is treated as an integer; arithmetic evaluation (see ARITHMETIC EVALUATION above) is performed when the variable is assigned a value.
-l — when the variable is assigned a value, all upper-case characters get converted to lower-case. The upper-case attribute is disabled.
-n — give each name the nameref attribute, making it a name reference to another variable. That other variable is defined by the value of name. All references, assignments, and attribute modifications to name — except those using or changing the -n attribute itself — are performed on the variable referenced by name's value. The nameref attribute cannot be applied to array variables. 🚫
-r — make names readonly. These names can't be assigned values by subsequent assignment statements or unset after that. Locked. Periodt.
-t — give each name the trace attribute. Traced functions inherit the DEBUG and RETURN traps from the calling shell. The trace attribute has no special meaning for variables.
-u — when the variable is assigned a value, all lower-case characters get converted to upper-case. The lower-case attribute is disabled.
-x — mark each name for export to subsequent commands via the environment.

Using "+" instead of "-" turns the specified attribute off instead — with the exceptions that +a and +A may not be used to destroy array variables, and +r will not remove the readonly attribute. Some doors you just can't un-open, bestie 💅.

When used in a function, declare and typeset make each name local, exactly like the local command — unless the -g option is supplied. If a variable name is followed by =value, the value of the variable is set to value. And when using -a or -A together with the compound assignment syntax to create array variables, additional attributes don't take effect until subsequent assignments. Patience, babe 🧚.

The return value is 0 unless:

an invalid option is encountered,
an attempt is made to define a function using -f foo=bar,
an attempt is made to assign a value to a readonly variable,
an attempt is made to assign a value to an array variable without using the compound assignment syntax (see Arrays above),
one of the names is not a valid shell variable name,
an attempt is made to turn off readonly status for a readonly variable,
an attempt is made to turn off array status for an array variable,
or an attempt is made to display a non-existent function with -f.

That's a lot of ways to fail, but honestly? Every single one is a skill issue on your end 🙈.

💖 `dirs` 💖

dirs [-clpv] [+n] [-n]

Without options, it displays the list of currently remembered directories. The default display is on a single line with directory names separated by spaces. Directories get added to the list with the pushd command, and the popd command removes entries from the list. The current directory is always the first directory in the stack. Always. Main character position, as it should be 👑.

Options, if supplied, mean the following:

-c — clears the directory stack by deleting all of the entries. Nuke it. 💀
-l — produces a listing using full pathnames; the default listing format uses a tilde to denote the home directory.
-p — print the directory stack with one entry per line.
-v — print the directory stack with one entry per line, prefixing each entry with its index in the stack.
+n — displays the *n*th entry counting from the left of the list shown by dirs when invoked without options, starting with zero.
-n — displays the *n*th entry counting from the right of the list shown by dirs when invoked without options, starting with zero.

The return value is 0 unless an invalid option is supplied or n indexes beyond the end of the directory stack. Don't go past the end, babe, there's nothing there 🌈.

🎀 `disown` 🎀

disown [-ar] [-h] [id ...]

Without options, remove each id from the table of active jobs. Each id may be a job specification jobspec or a process ID pid; and if id is a pid, disown uses the job containing pid as jobspec. Sneaky but valid 💅.

If the -h option is supplied, disown does not remove the jobs corresponding to each id from the jobs table — instead it marks them so the shell doesn't send SIGHUP to the job if the shell receives a SIGHUP. So it's like... letting them stay but making them immune. Protective queen behavior 🫶.

If no id is supplied, the -a option means to remove or mark all jobs; the -r option without an id argument removes or marks running jobs. And if no id is supplied and neither -a nor -r is supplied, disown removes or marks the current job.

The return value is 0 unless an id doesn't specify a valid job.

⭐ `echo` ⭐

echo [-neE] [arg ...]

Outputs the args, separated by spaces, followed by a newline. The return status is 0 unless a write error occurs. If -n is specified, the trailing newline is not printed. (Yes, that's the classic "why is my prompt eating itself" option. You know the one 😭.)

If the -e option is given, echo interprets the following backslash-escaped characters. The -E option disables interpretation of these escape characters — even on systems where they're interpreted by default. The xpg_echo shell option determines whether or not echo interprets any options and expands these escape characters. And heads up: echo does not interpret -- to mean the end of options. Nope. It just doesn't. 🙈

echo interprets the following escape sequences:

\a — alert (bell)
\b — backspace
\c — suppress further output
\e, \E — an escape character
\f — form feed
\n — new line
\r — carriage return
\t — horizontal tab
\v — vertical tab
\\ — backslash
\0nnn — the eight-bit character whose value is the octal value nnn (zero to three octal digits).
\xHH — the eight-bit character whose value is the hexadecimal value HH (one or two hex digits).
\uHHHH — the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHH (one to four hex digits).
\UHHHHHHHH — the Unicode (ISO/IEC 10646) character whose value is the hexadecimal value HHHHHHHH (one to eight hex digits).

echo writes any unrecognized backslash-escaped characters unchanged. So if you made it up, it goes through as-is, no judgment 🌸.

🌈 `enable` 🌈

enable [-a] [-dnps] [-f filename] [name ...]

Enable and disable builtin shell commands. Disabling a builtin allows an executable file which has the same name as a shell builtin to be executed without specifying a full pathname — even though the shell normally searches for builtins before files. So it's the loophole that lets the file version get its turn 👑.

If -n is supplied, every name gets disabled — otherwise, bestie, those names get enabled. 💅 For example, to use the test binary found using PATH instead of the shell builtin version, run enable -n test.

If no name arguments are supplied, or if the -p option is supplied, it prints a list of shell builtins. With no other option arguments, the list consists of all enabled shell builtins. If -n is supplied, print only the disabled builtins. If -a is supplied, the list printed includes all builtins, with an indication of whether or not each is enabled. The -s option means to restrict the output to the POSIX special builtins. 🎀

The -f option means to load the new builtin command name from shared object filename, on systems that support dynamic loading. If filename does not contain a slash, Bash will use the value of the BASH_LOADABLES_PATH variable as a colon-separated list of directories in which to search for filename. The default for BASH_LOADABLES_PATH is system-dependent, and may include . to force a search of the current directory. The -d option will delete a builtin previously loaded with -f. If -s is used with -f, the new builtin becomes a POSIX special builtin. ✨

If no options are supplied and a name is not a shell builtin, enable will attempt to load name from a shared object named name, as if the command were enable -f name name.

The return value is 0 unless a name is not a shell builtin or there is an error loading a new builtin from a shared object. 🫶

`eval` 🧚

eval [arg ...]

Concatenate the args together into a single command, separating them with spaces. Bash then reads and execute this command, and returns its exit status as the return status of eval. If there are no args, or only null arguments, eval returns 0. no cap, it's that simple 💖

`exec` 🔥

exec [-cl] [-a name] [command [arguments]]

If command is specified, it replaces the shell without creating a new process. command cannot be a shell builtin or function. The arguments become the arguments to command. If the -l option is supplied, the shell places a dash at the beginning of the zeroth argument passed to command. This is what login(1) does. The -c option causes command to be executed with an empty environment. If -a is supplied, the shell passes name as the zeroth argument to the executed command.

If command cannot be executed for some reason, a non-interactive shell exits, unless the execfail shell option is enabled. In that case, it returns a non-zero status. An interactive shell returns a non-zero status if the file cannot be executed. A subshell exits unconditionally if exec fails. 😭

If command is not specified, any redirections take effect in the current shell, and the return status is 0. If there is a redirection error, the return status is 1.

`exit` 👑

exit [n]

Cause the shell to exit with a status of n. If n is omitted, the exit status is that of the last command executed. Any trap on EXIT is executed before the shell terminates. periodt 💅

`export` 🌈

export [-fn] [name[=value]] ...
export -p [-f]

The supplied names are marked for automatic export to the environment of subsequently executed commands. If the -f option is given, the names refer to functions. 🦋

The -n option unexports, or removes the export attribute, from each name. If no names are given, or if only the -p option is supplied, export displays a list of names of all exported variables on the standard output. Using -p and -f together displays exported functions. The -p option displays output in a form that may be reused as input. ✨

export allows the value of a variable to be set when it is exported or unexported by following the variable name with =value. This sets the value of the variable to value while modifying the export attribute. export returns an exit status of 0 unless an invalid option is encountered, one of the names is not a valid shell variable name, or -f is supplied with a name that is not a function.

`false` 💀

Does nothing; returns a non-zero status. iconic, honestly. not me using this as a personality trait 🙈

`fc` 🎀

fc [-e ename] [-lnr] [first] [last]
fc -s [pat=rep] [cmd]

The first form selects a range of commands from first to last from the history list and displays or edits and re-executes them. First and last may be specified as a string (to locate the last command beginning with that string) or as a number (an index into the history list, where a negative number is used as an offset from the current command number).

When listing, a first or last of 0 is equivalent to -1 and -0 is equivalent to the current command (usually the fc command); otherwise 0 is equivalent to -1 and -0 is invalid. If last is not specified, it is set to the current command for listing (so that fc -l -10 prints the last 10 commands) and to first otherwise. If first is not specified, it is set to the previous command for editing and -16 for listing.

If the -l option is supplied, the commands are listed on the standard output. The -n option suppresses the command numbers when listing. The -r option reverses the order of the commands.

Otherwise, fc invokes the editor named by ename on a file containing those commands. If ename is not supplied, fc uses the value of the FCEDIT variable, and the value of EDITOR if FCEDIT is not set. If neither variable is set, fc uses vi. When editing is complete, fc reads the file containing the edited commands and echoes and executes them. ✨

In the second form, fc re-executes command after replacing each instance of pat with rep. Command is interpreted the same as first above.

A useful alias to use with fc is r="fc -s", so that typing r cc runs the last command beginning with cc and typing r re-executes the last command. bestie this alias is lowkey essential 🫶

If the first form is used, the return value is zero unless an invalid option is encountered or first or last specify history lines out of range. When editing and re-executing a file of commands, the return value is the value of the last command executed or failure if an error occurs with the temporary file. If the second form is used, the return status is that of the re-executed command, unless cmd does not specify a valid history entry, in which case fc returns a non-zero status.

`fg` 🔥

fg [jobspec]

Resume jobspec in the foreground, and make it the current job. If jobspec is not present, fg uses the shell's notion of the current job. The return value is that of the command placed into the foreground, or failure if run when job control is disabled or, when run with job control enabled, if jobspec does not specify a valid job or jobspec specifies a job that was started without job control. 😭

`getopts` 🌸

getopts optstring name [arg ...]

getopts is used by shell scripts and functions to parse positional parameters and obtain options and their arguments. optstring contains the option characters to be recognized; if a character is followed by a colon, the option is expected to have an argument, which should be separated from it by white space. The colon and question mark characters may not be used as option characters. 💅

Each time it is invoked, getopts places the next option in the shell variable name, initializing name if it does not exist, and the index of the next argument to be processed into the variable OPTIND. OPTIND is initialized to 1 each time the shell or a shell script is invoked. When an option requires an argument, getopts places that argument into the variable OPTARG.

⚠️ The shell does not reset OPTIND automatically; it must be manually reset between multiple calls to getopts within the same shell invocation to use a new set of parameters. fr fr, this one bites everybody 🫠

When it reaches the end of options, getopts exits with a return value greater than zero. OPTIND is set to the index of the first non-option argument, and name is set to ?.

getopts normally parses the positional parameters, but if more arguments are supplied as arg values, getopts parses those instead.

getopts can report errors in two ways. If the first character of optstring is a colon, getopts uses silent error reporting. In normal operation, getopts prints diagnostic messages when it encounters invalid options or missing option arguments. If the variable OPTERR is set to 0, getopts does not display any error messages, even if the first character of optstring is not a colon.

If getopts detects an invalid option, it places ? into name and, if not silent, prints an error message and unsets OPTARG. If getopts is silent, it assigns the option character found to OPTARG and does not print a diagnostic message.

If a required argument is not found, and getopts is not silent, it sets the value of name to a question mark (?), unsets OPTARG, and prints a diagnostic message. If getopts is silent, it sets the value of name to a colon (:) and sets OPTARG to the option character found.

getopts returns true if an option, specified or unspecified, is found. It returns false if the end of options is encountered or an error occurs.

`hash` 🍓

hash [-lr] [-p filename] [-dt] [name]

Each time hash is invoked, it remembers the full pathname of the command name as determined by searching the directories in $PATH. Any previously-remembered pathname associated with name is discarded. If the -p option is supplied, hash uses filename as the full pathname of the command. ✨

The -r option causes the shell to forget all remembered locations. Assigning to the PATH variable also clears all hashed filenames. The -d option causes the shell to forget the remembered location of each name.

If the -t option is supplied, hash prints the full pathname corresponding to each name. If multiple name arguments are supplied with -t, hash prints the name before the corresponding hashed full pathname. The -l option displays output in a format that may be reused as input. 💖

Okay so if you just run it with no arguments at all, or hand it literally only -l, hash spills everything it remembers about your commands 💭✨.

Also, important: the -t, -d, and -p options — the ones that actually act on the name arguments — are mutually exclusive. Only one of them gets to be active. And if you're being chaotic and supply more than one, here's the ranking: -t has higher priority than -p, and both of them outrank -d. The hierarchy is real, no cap 👑.

The return status is zero unless a name isn't found or you fed it an invalid option. Rude, but fair.

🎀 `help`

help [-dms] [pattern]

This one shows you helpful info about builtin commands — it's basically the builtin that explains the other builtins, which is very main character of it 💅.

If you give it a pattern, help gives detailed help on all commands matching pattern, as described below. If you don't, it just dumps a list of all the builtins and shell compound commands. Like a little menu 🍓.

The options, if you supply them, mean the following:

-d — display a short description of each pattern
-m — display the description of each pattern in a manpage-like format
-s — display only a short usage synopsis for each pattern

Now here's the matching lore:

If pattern contains pattern matching characters (see Pattern Matching above), it's treated as a shell pattern, and help prints the description of each help topic matching pattern.
If it doesn't, and pattern exactly matches the name of a help topic, help prints the description associated with that topic.
Otherwise, help performs prefix matching and prints the descriptions of all matching help topics.

The return status is 0 unless no command matches pattern. So if it comes back with nothing, that's a fail, bestie 😭.

🦋 `history`

history [n]
history -c
history -d offset
history -d start-end
history -anrw [filename]
history -p arg [arg ...]
history -s arg [arg ...]

With no options, it displays the command history list, with numbers. Entries prefixed with a * have been modified — so if you see a lil star, that entry got a glow-up ✨.

An argument of n lists only the last n entries.

If the shell variable HISTTIMEFORMAT is set and not null, it gets used as a format string for strftime(3) to display the time stamp associated with each displayed history entry. And here's the detail that matters: if history uses HISTTIMEFORMAT, it does not print an intervening space between the formatted time stamp and the history entry. Tight. No gaps 🫶.

If filename is supplied, history uses it as the name of the history file. If not, it uses the value of HISTFILE. And if filename is not supplied and HISTFILE is unset or null, then the -a, -n, -r, and -w options have no effect. Nothing. Zero. Nada 💀.

Options, if supplied, have the following meanings:

-c — clear the history list by deleting all the entries. This can be used with the other options to replace the history list.
-d offset — delete the history entry at position offset. If offset is negative, it's interpreted as relative to one greater than the last history position, so negative indices count back from the end of the history, and an index of -1 refers to the current history -d command. Sneaky lil self-reference, we love her 🙈.
-d start-end — delete the range of history entries between positions start and end, inclusive. Positive and negative values for start and end are interpreted as described above.
-a — append the “new” history lines to the history file. These are history lines entered since the beginning of the current bash session, but not already appended to the history file.
-n — read the history lines not already read from the history file and add them to the current history list. These are lines appended to the history file since the beginning of the current bash session.
-r — read the history file and append its contents to the current history list.
-w — write the current history list to the history file, overwriting the history file. Bye to whatever was there before 😳.
-p — perform history substitution on the following args and display the result on the standard output, without storing the results in the history list. Each arg must be quoted to disable normal history expansion. Quote it or it gets weird, fr fr.
-s — store the args in the history list as a single entry. The last command in the history list is removed before adding the args.

If the HISTTIMEFORMAT variable is set, history writes the time stamp information associated with each history entry to the history file, marked with the history comment character as described above. And when the history file is read, lines beginning with the history comment character followed immediately by a digit are interpreted as timestamps for the following history entry. The lore runs deep 📜.

The return value is 0 unless an invalid option is encountered, an error occurs while reading or writing the history file, an invalid offset or range is supplied as an argument to -d, or the history expansion supplied as an argument to -p fails.

💜 `jobs`

jobs [-lnprs] [ jobspec ... ]
jobs -x command [ args ... ]

The first form lists the active jobs 🫡. The options mean:

-l — list process IDs in addition to the normal information.
-n — display information only about jobs that have changed status since the user was last notified of their status.
-p — list only the process ID of the job's process group leader.
-r — display only running jobs.
-s — display only stopped jobs.

If jobspec is supplied, jobs restricts output to information about that job. The return status is 0 unless an invalid option is encountered or an invalid jobspec is supplied.

If the -x option is supplied, jobs replaces any jobspec found in command or args with the corresponding process group ID, and executes command, passing it args, returning its exit status. Glow-up swap, iconic ✨.

🔥 `kill`

kill [-s sigspec | -n signum | -sigspec] id [ ... ]
kill -l|-L [sigspec | exit_status]

Sends the signal specified by sigspec or signum to the processes named by each id. Each id may be a job specification jobspec or a process ID pid.

sigspec is either a case-insensitive signal name such as SIGKILL (with or without the SIG prefix) or a signal number. signum is a signal number. And if sigspec is not supplied, then kill sends SIGTERM. Default behavior, noted 💅.

The -l option lists the signal names. If any arguments are supplied when -l is given, kill lists the names of the signals corresponding to the arguments, and the return status is 0.

The exit_status argument to -l is a number specifying either a signal number or the exit status of a process terminated by a signal. If it's supplied, kill prints the name of the signal that caused the process to terminate. And listen — kill assumes that process exit statuses are greater than 128; anything less than that is a signal number. Remember that rule, it'll save you 🧠✨.

The -L option is equivalent to -l.

kill returns true if at least one signal was successfully sent, or false if an error occurs or an invalid option is encountered.

🌸 `let`

let arg [arg ...]

Each arg is evaluated as an arithmetic expression (see ARITHMETIC EVALUATION above).

If the last arg evaluates to 0, let returns 1; otherwise let returns 0. Yes, it's backwards from what your brain wants. That's just how she is 😌.

🎀 `local`

local [option] [name[=value] ... | - ]

For each argument, create a local variable named name and assign it value. The option can be any of the options accepted by declare.

When local is used within a function, it causes the variable name to have a visible scope restricted to that function and its children. And it is an error to use local when not within a function. Don't try it, bestie, it will simply not work.

If name is -, it makes the set of shell options local to the function in which local is invoked: any shell options changed using the set builtin inside the function after the call to local are restored to their original values when the function returns. The restore is performed as if a series of set commands were executed to restore the values that were in place before the function. Time travel but for shell options, obsessed 🕰️.

With no operands, local writes a list of local variables to the standard output.

The return status is 0 unless local is used outside a function, an invalid name is supplied, or name is a readonly variable.

🌈 `logout`

logout [n]

Exit a login shell, returning a status of n to the shell's parent. That's it, that's the whole thing. Short queen 👑.

🧚 `mapfile` / `readarray`

mapfile [-d delim] [-n count] [-O origin] [-s count] [-t] [-u fd] [-C
callback] [-c quantum] [array]
readarray [-d delim] [-n count] [-O origin] [-s count] [-t] [-u fd] [-C
callback] [-c quantum] [array]

Read lines from the standard input — or from file descriptor fd if the -u option is supplied — into the indexed array variable array. The variable MAPFILE is the default array. Yes, they're the same command wearing different outfits 💅.

Options, if supplied, have the following meanings:

-d — use the first character of delim to terminate each input line, rather than newline. If delim is the empty string, mapfile will terminate a line when it reads a NUL character.
-n — copy at most count lines. If count is 0, copy all lines.
-O — begin assigning to array at index origin. The default index is 0.
-s — discard the first count lines read.
-t — remove a trailing delim (default newline) from each line read.
-u — read lines from file descriptor fd instead of the standard input.
-C — evaluate callback each time quantum lines are read. The -c option specifies quantum.
-c — specify the number of lines read between each call to callback.

If -C is specified without -c, the default quantum is 5000. When callback is evaluated, it is supplied the index of the next array element to be assigned and the line to be assigned to that element as additional arguments. And callback is evaluated after the line is read but before the array element is assigned. Order matters, don't forget it ⭐.

If not supplied with an explicit origin, mapfile will clear array before assigning to it. Heads up 🙈.

mapfile returns zero unless an invalid option or option argument is supplied, array is invalid or unassignable, or if array is not an indexed array.

💅 `popd`

popd [-n] [+n] [-n]

okay so popd is literally the ✨yeet✨ button for your directory stack, bestie. It removes entries from the directory stack. The elements are numbered from 0 starting at the first directory listed by dirs, so popd is equivalent to popd +0. 🫶

With no arguments at all, popd removes the top directory from the stack, and changes to the new top directory. If you do hand it arguments, here's what they mean:

-n — Suppress the normal change of directory when removing directories from the stack, only manipulate the stack. (Just vibes, no actual cd. 🎀)
+n — Remove the *n*th entry counting from the left of the list shown by dirs, starting with zero, from the stack. For example: popd +0 removes the first directory, popd +1 the second. 💖
-n — Remove the nth entry counting from the right of the list shown by dirs, starting with zero. For example: popd -0 removes the last directory, popd -1 the next to last. (yes bestie this is a different -n than the first one, the shell said "two -ns, one letter, deal with it" 😭)

If the top element of the directory stack is modified, and the -n option was not supplied, popd uses the cd builtin to change to the directory at the top of the stack. If the cd fails, popd returns a non-zero value. 😔

Otherwise, popd returns false if an invalid option is supplied, the directory stack is empty, or n specifies a non-existent directory stack entry. 🚫

If the popd command is successful, bash runs dirs to show the final contents of the directory stack, and the return status is 0. slay. ✨

🖨️ `printf`

printf [-v var] format [arguments]

Write the formatted arguments to the standard output under the control of the format. The -v option assigns the output to the variable var rather than printing it to the standard output. 💅

The format is a character string which contains three types of objects: plain characters, which are simply copied to standard output, character escape sequences, which are converted and copied to the standard output, and format specifications, each of which causes printing of the next successive argument. In addition to the standard printf(3) format characters cCsSndiouxXeEfFgGaA, printf interprets the following additional format specifiers:

%b — causes printf to expand backslash escape sequences in the corresponding argument in the same way as echo -e. 🌈
%q — causes printf to output the corresponding argument in a format that can be reused as shell input. %q and %Q use the $'' quoting style if any characters in the argument string require it, and backslash quoting otherwise. If the format string uses the printf alternate form, these two formats quote the argument string using single quotes.
%Q — like %q, but applies any supplied precision to the argument before quoting it.
%(datefmt)T — causes printf to output the date-time string resulting from using datefmt as a format string for strftime(3). The corresponding argument is an integer representing the number of seconds since the epoch. This format specifier recognizes two special argument values: -1 represents the current time, and -2 represents the time the shell was invoked. If no argument is specified, conversion behaves as if -1 had been supplied. This is an exception to the usual printf behavior. ⏰

The %b, %q, and %T format specifiers all use the field width and precision arguments from the format specification and write that many bytes from (or use that wide a field for) the expanded argument, which usually contains more characters than the original. 👑

The %n format specifier accepts a corresponding argument that is treated as a shell variable name.

The %s and %c format specifiers accept an l (long) modifier, which forces them to convert the argument string to a wide-character string and apply any supplied field width and precision in terms of characters, not bytes. The %S and %C format specifiers are equivalent to %ls and %lc, respectively. 💖

Arguments to non-string format specifiers are treated as C constants, except that a leading plus or minus sign is allowed, and if the leading character is a single or double quote, the value is the numeric value of the following character, using the current locale.

The format is reused as necessary to consume all of the arguments. If the format requires more arguments than are supplied, the extra format specifications behave as if a zero value or null string, as appropriate, had been supplied. The return value is zero on success, non-zero if an invalid option is supplied or a write or assignment error occurs. 💀

📚 `pushd`

pushd [-n] [+n] [-n]
pushd [-n] [dir]

Add a directory to the top of the directory stack, or rotate the stack, making the new top of the stack the current working directory. 🌸

With no arguments, pushd exchanges the top two elements of the directory stack. Arguments, if supplied, have the following meanings:

-n — Suppress the normal change of directory when rotating or adding directories to the stack, only manipulate the stack. (again: no cd, just stack drama 🎀)
+n — Rotate the stack so that the *n*th directory (counting from the left of the list shown by dirs, starting with zero) is at the top.
-n — Rotates the stack so that the *n*th directory (counting from the right of the list shown by dirs, starting with zero) is at the top.
dir — Adds dir to the directory stack at the top. 💅

After the stack has been modified, if the -n option was not supplied, pushd uses the cd builtin to change to the directory at the top of the stack. If the cd fails, pushd returns a non-zero value. 😭

Otherwise, if no arguments are supplied, pushd returns zero unless the directory stack is empty. When rotating the directory stack, pushd returns zero unless the directory stack is empty or n specifies a non-existent directory stack element.

If the pushd command is successful, bash runs dirs to show the final contents of the directory stack. ✨

📍 `pwd`

pwd [-LP]

Print the absolute pathname of the current working directory. The pathname printed contains no symbolic links if the -P option is supplied or the -o physical option to the set builtin command is enabled. If the -L option is used, the pathname printed may contain symbolic links. The return status is 0 unless an error occurs while reading the name of the current directory or an invalid option is supplied. 🦋

📖 `read`

read [-Eers] [-a aname] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]

Read one line from the standard input, or from the file descriptor fd supplied as an argument to the -u option, split it into words as described above under Word Splitting, and assign the first word to the first name, the second word to the second name, and so on. 🌸

If there are more words than names, the remaining words and their intervening delimiters are assigned to the last name. If there are fewer words read from the input stream than names, the remaining names are assigned empty values. 💖

The characters in the value of the IFS variable are used to split the line into words using the same rules the shell uses for expansion (described above under Word Splitting). The backslash character (\) removes any special meaning for the next character read and is used for line continuation. 🌈

Okay so, options? Supply them, and here's what they do:

-a aname — the words get handed out to sequential indices of the array variable aname, starting at 0. aname is unset before any new values are assigned, so it's a clean slate, no leftovers. Any other name arguments just... don't get a turn. Ignored. 💅
-d delim — the first character of delim ends the input line instead of newline. And if delim is the empty string, read will terminate a line when it reads a NUL character. Iconic behaviour change, honestly.
-e — if standard input is coming from a terminal, read uses readline (peep READLINE above 👆) to grab the line. Readline uses the current editing settings (or the default ones, if line editing wasn't previously active), but it uses readline's default filename completion.
-E — same deal, if standard input is a terminal, read uses readline (again, READLINE above). Readline uses the current (or default, if line editing wasn't previously active) editing settings, but this time it uses bash's default completion, programmable completion included. So like, the glow-up version of -e. ✨
-i text — if readline is being used to read the line, read plops text into the editing buffer before editing even begins. Pre-filled, bestie. 🎀
-n nchars — read comes back after reading nchars characters instead of waiting around for a whole line of input, unless it hits EOF or read times out. It does still honor a delimiter if it reads fewer than nchars characters before that delimiter shows up.
-N nchars — read returns after reading exactly nchars characters, not waiting for a complete line, unless it encounters EOF or read times out. Any delimiter characters in the input are not treated specially and do not cause read to return until it has read nchars characters. The result is not split on the characters in IFS — the whole intent is that the variable gets assigned exactly the characters read (the one exception being backslash, see the -r option below).
-p prompt — display prompt on standard error, with no trailing newline, before it tries to read any input. But only if input is coming from a terminal. Otherwise it's giving nothing. 💀
-r — backslash stops acting as an escape character. The backslash is just part of the line now. And in particular, a backslash-newline pair may not be used as a line continuation when this is on.
-s — silent mode. If input is coming from a terminal, characters are not echoed. Sneaky. 🙈
-t timeout — makes read time out and return failure if it doesn't read a complete line of input (or the specified number of characters) within timeout seconds. timeout can be a decimal number with a fractional portion after the decimal point. This option is only effective if read is reading input from a terminal, pipe, or other special file — it does absolutely nothing when reading from regular files. If read times out, it saves any partial input it read into the specified variable name, and the exit status is greater than 128. If timeout is 0, read returns immediately without even trying to read any data. In that case, the exit status is 0 if input is available on the specified file descriptor, or the read will return EOF, non-zero otherwise.
-u fd — read input from file descriptor fd instead of standard input. Pick your own adventure. 🦋

Other than the case where delim is the empty string, read ignores any NUL characters in the input. Just straight up ignores them. Not its problem.

If no names are supplied, read assigns the line it read — without the ending delimiter, but otherwise unmodified — to the variable REPLY. That's your default girlie. 💖

The exit status is zero, unless end-of-file is encountered, read times out (in which case the status is greater than 128), a variable assignment error happens (like assigning to a readonly variable 😭), or an invalid file descriptor is supplied as the argument to -u.

🩷 `readonly`

readonly [-aAf] [-p] [name[=word] ...]

The given names are marked readonly, meaning the values of those names may not be changed by any subsequent assignment or unset. Locked in. Periodt.

If -f is supplied, each name refers to a shell function.
-a restricts the variables to indexed arrays.
-A restricts the variables to associative arrays.
If both -a and -A are supplied, -A takes precedence. Sorry -a, you're the side character here.
If no name arguments are supplied, or if -p is supplied, it prints a list of all readonly names. The other options can be used to narrow the output down to a subset of the readonly names.
-p displays output in a format that may be reused as input. Copy-paste friendly, we love to see it.

readonly also lets you set the value of a variable at the same time as you flip on the readonly attribute, by following the variable name with =value. So that sets the value of the variable to value while it's modifying the readonly attribute. Two birds, one stone. 🔥

The return status is 0 unless an invalid option is encountered, one of the names is not a valid shell variable name, or -f is supplied with a name that is not a function.

🫶 `return`

return [n]

Stops executing a shell function or a sourced file, and returns the value specified by n back to its caller. If n is omitted, the return status is that of the last command executed.

If return is executed by a trap handler, the last command used to determine the status is the last command executed before the trap handler. And if return is executed during a DEBUG trap, the last command used to determine the status is the last command executed by the trap handler before return was invoked. Very specific. Very important. Don't mix them up bestie. 💀

When return is used to terminate execution of a script being executed by the . (source) command, it causes the shell to stop executing that script and return either n or the exit status of the last command executed within the script, as the exit status of the script. If n is supplied, the return value is its least significant 8 bits. Yep, just 8. That's the whole budget.

Any command associated with the RETURN trap is executed before execution resumes after the function or script.

The return status is non-zero if return is supplied a non-numeric argument, or if it's used outside a function and not during execution of a script by . or source.

✨ `set`

set [-abefhkmnptuvxBCEHPT] [-o option-name] [--] [-] [arg ...]
set [+abefhkmnptuvxBCEHPT] [+o option-name] [--] [-] [arg ...]
set -o
set +o

Without options, set displays the name and value of each shell variable in a format that can be reused as input for setting or resetting the currently-set variables. Read-only variables cannot be reset, obviously, they said what they said. In posix mode, only shell variables are listed. The output is sorted according to the current locale. 🌈

When options are specified, they set or unset shell attributes. Any arguments remaining after option processing are treated as values for the positional parameters and are assigned, in order, to $1, $2, ..., $n.

Options, if specified, have the following meanings:

-a — each variable or function that is created or modified is given the export attribute and marked for export to the environment of subsequent commands. Everything's going public now. 📣
-b — report the status of terminated background jobs immediately, rather than before the next primary prompt or after a foreground command terminates. This is effective only when job control is enabled.
-e — exit immediately if a pipeline (which may consist of a single simple command), a list, or a compound command (peep SHELL GRAMMAR above) exits with a non-zero status. But okay, hold on, there are so many exceptions, so listen up:

The shell does not exit if the command that fails is:

part of the command list immediately following a while or until reserved word
part of the test following the if or elif reserved words
part of any command executed in a && or || list except the command following the final && or ||
any command in a pipeline but the last (subject to the state of the pipefail shell option)
or if the command's return value is being inverted with !

If a compound command other than a subshell returns a non-zero status because a command failed while -e was being ignored, the shell does not exit. A trap on ERR, if set, is executed before the shell exits.

This option applies to the shell environment and each subshell environment separately (see COMMAND EXECUTION ENVIRONMENT above), and it may cause subshells to exit before executing all the commands in the subshell. Yeah. It's a lot. Take notes. 📝

and okay, more -e lore before we move on, because this bit matters: If a compound command or shell function executes in a context where -e is being ignored, none of the commands executed within the compound command or function body will be affected by the -e setting, even if -e is set and a command returns a failure status. And if a compound command or shell function sets -e while executing in a context where -e is ignored, that setting will not have any effect until the compound command or the command containing the function call completes. yes really, it just sits there doing nothing, so rude 😭

-f — Disable pathname expansion. no more globbing, bestie 💀
-h — Remember the location of commands as they are looked up for execution. This is enabled by default. ✨
-k — All arguments in the form of assignment statements are placed in the environment for a command, not just those that precede the command name.
-m — Monitor mode. Job control is enabled. This option is on by default for interactive shells on systems that support it (see JOB CONTROL above). All processes run in a separate process group. When a background job completes, the shell prints a line containing its exit status. background job said "i'm done" and the shell tells you fr fr
-n — Read commands but do not execute them. This may be used to check a shell script for syntax errors. This is ignored by interactive shells. vibe check your script before you actually run it 🫶
-o option-name — and option-name can be one of the following, so buckle up:
- allexport — Same as -a.
- braceexpand — Same as -B.
- emacs — Use an emacs-style command line editing interface. This is enabled by default when the shell is interactive, unless the shell is started with the --noediting option. This also affects the editing interface used for read -e.
- errexit — Same as -e.
- errtrace — Same as -E.
- functrace — Same as -T.
- hashall — Same as -h.
- histexpand — Same as -H.
- history — Enable command history, as described above under HISTORY. This option is on by default in interactive shells.
- ignoreeof — The effect is as if the shell command IGNOREEOF=10 had been executed (see Shell Variables above).
- keyword — Same as -k.
- monitor — Same as -m.
- noclobber — Same as -C.
- noexec — Same as -n.
- noglob — Same as -f.
- nolog — Currently ignored. lol, it's just there, hanging out 🙈
- notify — Same as -b.
- nounset — Same as -u.
- onecmd — Same as -t.
- physical — Same as -P.
- pipefail — If set, the return value of a pipeline is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands in the pipeline exit successfully. This option is disabled by default. the icon of pipeline options, no cap 👑
- posix — Enable posix mode; change the behavior of bash where the default operation differs from the POSIX standard to match the standard. See SEE ALSO below for a reference to a document that details how posix mode affects bash's behavior.
- privileged — Same as -p.
- verbose — Same as -v.
- vi — Use a vi-style command line editing interface. This also affects the editing interface used for read -e.
- xtrace — Same as -x.

If -o is supplied with no option-name, set prints the current shell option settings. If +o is supplied with no option-name, set prints a series of set commands to recreate the current option settings on the standard output. handy for saving your whole setup, obsessed 💖

-p — Turn on privileged mode. In this mode, the shell does not read the $ENV and $BASH_ENV files, shell functions are not inherited from the environment, and the SHELLOPTS, BASHOPTS, CDPATH, and GLOBIGNORE variables, if they appear in the environment, are ignored. If the shell is started with the effective user (group) id not equal to the real user (group) id, and the -p option is not supplied, these actions are taken and the effective user id is set to the real user id. If the -p option is supplied at startup, the effective user id is not reset. Turning this option off causes the effective user and group ids to be set to the real user and group ids. this one is doing the MOST, read it twice 💀
-r — Enable restricted shell mode. This option cannot be unset once it has been set. like, cannot. unset. ever. no takebacks 🚫
-t — Exit after reading and executing one command.
-u — Treat unset variables and parameters other than the special parameters “@” and “*”, or array variables subscripted with “@” or “*”, as an error when performing parameter expansion. If expansion is attempted on an unset variable or parameter, the shell prints an error message, and, if not interactive, exits with a non-zero status.
-v — Print shell input lines as they are read.
-x — After expanding each simple command, for command, case command, select command, or arithmetic for command, display the expanded value of PS4, followed by the command and its expanded arguments or associated word list, to the standard error. the debug-trace bestie, your literal best friend when everything is on fire 🔥
-B — The shell performs brace expansion (see Brace Expansion above). This is on by default.
-C — If set, bash does not overwrite an existing file with the >, >&, and <> redirection operators. Using the redirection operator >| instead of > will override this and force the creation of an output file.
-E — If set, any trap on ERR is inherited by shell functions, command substitutions, and commands executed in a subshell environment. The ERR trap is normally not inherited in such cases.
-H — Enable ! style history substitution. This option is on by default when the shell is interactive.
-P — If set, the shell does not resolve symbolic links when executing commands such as cd that change the current working directory. It uses the physical directory structure instead. By default, bash follows the logical chain of directories when performing commands which change the current directory.
-T — If set, any traps on DEBUG and RETURN are inherited by shell functions, command substitutions, and commands executed in a subshell environment. The DEBUG and RETURN traps are normally not inherited in such cases.
-- — If no arguments follow this option, unset the positional parameters. Otherwise, set the positional parameters to the args, even if some of them begin with a -. the "okay actually stop parsing flags now" move ✋
- — Signal the end of options, and assign all remaining args to the positional parameters. The -x and -v options are turned off. If there are no args, the positional parameters remain unchanged.

The options are off by default unless otherwise noted. Using + rather than - causes these options to be turned off. The options can also be specified as arguments to an invocation of the shell. The current set of options may be found in $-. The return status is always zero unless an invalid option is encountered.

🌸 `shift [n]` 🌸

Rename positional parameters from n+1 ... to $1 .... Parameters represented by the numbers $# down to $#-n+1 are unset. n must be a non-negative number less than or equal to $#. If n is 0, no parameters are changed. If n is not given, it is assumed to be 1. If n is greater than $#, the positional parameters are not changed. The return status is greater than zero if n is greater than $# or less than zero; otherwise 0. sliding the whole lineup down like a group chat roster, periodt 💅

✨ `shopt [-pqsu] [-o] [optname ...]` ✨

Toggle the values of settings controlling optional shell behavior. The settings can be either those listed below, or, if the -o option is used, those available with the -o option to the set builtin command.

With no options, or with the -p option, display a list of all settable options, with an indication of whether or not each is set; if any optnames are supplied, the output is restricted to those options. The -p option displays output in a form that may be reused as input. reusable output?? we love a copy-paste-able queen 👑

Other options have the following meanings:

-s — Enable (set) each optname.
-u — Disable (unset) each optname.
-q — Suppresses normal output (quiet mode); the return status indicates whether the optname is set or unset. If multiple optname arguments are supplied with -q, the return status is zero if all optnames are enabled; non-zero otherwise.
-o — Restricts the values of optname to be those defined for the -o option to the set builtin.

If either -s or -u is used with no optname arguments, shopt shows only those options which are set or unset, respectively. Unless otherwise noted, the shopt options are disabled (unset) by default.

The return status when listing options is zero if all optnames are enabled, non-zero otherwise. When setting or unsetting options, the return status is zero unless an optname is not a valid shell option.

The list of shopt options is: 🎀

array_expand_once — bestie, when this is on, the shell suppresses multiple evaluation of associative and indexed array subscripts while it's doing arithmetic expression evaluation, while it's running builtins that can do variable assignments, and while it's running builtins that do array dereferencing. Translation: it stops the shell from re-chewing on your subscripts like five times. 💅
assoc_expand_once — deprecated bestie 🫶 it's literally just a synonym for array_expand_once. She's retired but still showing up to the group chat.
autocd — if this is set, a command name that happens to be the name of a directory gets executed as if it were the argument to the cd command. So you just type the folder and you're in the folder. ✨ Heads up though: this option is only used by interactive shells.
bash_source_fullpath — if set, filenames added to the BASH_SOURCE array variable get converted to full pathnames (peep the Shell Variables section above). No more vague vibes, full addresses only. 📍
cdable_vars — if set, an argument to the cd builtin command that is not a directory gets assumed to be the name of a variable whose value is the directory you actually want to change to. So cd myvar where myvar holds a path? Slay, it just works. 🎀
cdspell — if set, the cd command tries to correct minor errors in the spelling of a directory component. Minor errors include transposed characters, a missing character, and one extra character. And if cd fixes the name, it prints the corrected filename and the command proceeds. Iconic autocorrect behavior. This option is only used by interactive shells.
checkhash — if set, bash double-checks that a command found in the hash table actually exists before trying to execute it. If a hashed command no longer exists, bash performs a normal path search instead of yelling into the void. 🔍
checkjobs — if set, bash lists the status of any stopped and running jobs before exiting an interactive shell. If any jobs are still running, bash defers the exit until you attempt a second exit without an intervening command (see JOB CONTROL above). And the shell always postpones exiting if any jobs are stopped. She is not letting you ghost your background processes, no cap. 💀
checkwinsize — if set, bash checks the window size after each external (non-builtin) command and, if necessary, updates the values of LINES and COLUMNS, using the file descriptor associated with the standard error if it's a terminal. This option is enabled by default. ✨
cmdhist — if set, bash tries to save all lines of a multiple-line command in the same history entry, which makes re-editing multi-line commands so much easier. This option is enabled by default, but it only has an effect if command history is enabled, as described above under HISTORY. 👑
compat31, compat32, compat40, compat41, compat42, compat43, compat44 — these control aspects of the shell's compatibility mode (see SHELL COMPATIBILITY MODE below). A whole little time capsule lineup fr fr. 🕰️
complete_fullquote — if set, bash quotes all shell metacharacters in filenames and directory names when performing completion. If not set, bash removes metacharacters such as the dollar sign from the set of characters that will be quoted in completed filenames when these metacharacters appear in shell variable references in words to be completed. What that means: dollar signs in variable names that expand to directories will not be quoted — however, any dollar signs appearing in filenames will not be quoted either. This is active only when bash is using backslashes to quote completed filenames. This variable is set by default, which is the default bash behavior in versions through 4.2. 💅
direxpand — if set, bash replaces directory names with the results of word expansion when performing filename completion. Fair warning: this changes the contents of the readline editing buffer. If not set, bash tries to preserve exactly what you typed. 🦋
dirspell — if set, bash attempts spelling correction on directory names during word completion, but only if the directory name you initially supplied doesn't exist. Helpful when your fingers are typing faster than your brain. 😭
dotglob — if set, bash includes filenames beginning with a “.” in the results of pathname expansion. But plot twist: the filenames . and .. must always be matched explicitly, even if dotglob is set. 💜
execfail — if set, a non-interactive shell will not exit if it can't execute the file specified as an argument to the exec builtin. (An interactive shell doesn't exit if exec fails anyway, she's built different.) 🫶
expand_aliases — if set, aliases are expanded as described above under ALIASES. This option is enabled by default for interactive shells. Your lil shortcuts get to live their truth. 🌈
extdebug — okay this one is a whole situation. If set at shell invocation, or in a shell startup file, it arranges to execute the debugger profile before the shell starts, identical to the --debugger option. If set after invocation, behavior intended for use by debuggers gets enabled:
1. The -F option to the declare builtin displays the source file name and line number corresponding to each function name supplied as an argument.
2. If the command run by the DEBUG trap returns a non-zero value, the next command is skipped and not executed.
3. If the command run by the DEBUG trap returns a value of 2, and the shell is executing in a subroutine (a shell function or a shell script executed by the . or source builtins), the shell simulates a call to return.
4. BASH_ARGC and BASH_ARGV are updated as described in their descriptions above).
5. Function tracing is enabled: command substitution, shell functions, and subshells invoked with ( command ) inherit the DEBUG and RETURN traps.
6. Error tracing is enabled: command substitution, shell functions, and subshells invoked with ( command ) inherit the ERR trap.

Not me listing all six like they're eras. 🔥

extglob — if set, enable the extended pattern matching features described above under Pathname Expansion. Glow up for your globs. ✨
extquote — if set, $'string' and $"string" quoting is performed within ${parameter} expansions enclosed in double quotes. This option is enabled by default. 🎀
failglob — if set, patterns which fail to match filenames during pathname expansion result in an expansion error. No silent nothing-burger, it's gonna tell you. 😭
force_fignore — if set, the suffixes specified by the FIGNORE shell variable cause words to be ignored when performing word completion, even if the ignored words are the only possible completions. See Shell Variables above for a description of FIGNORE. This option is enabled by default. 💅
globasciiranges — if set, range expressions used in pattern matching bracket expressions (see Pattern Matching above) behave as if in the traditional C locale when performing comparisons. That is, pattern matching does not take the current locale's collating sequence into account, so b will not collate between A and B, and upper-case and lower-case ASCII characters will collate together. Locale drama? Declined. 🚫
globskipdots — if set, pathname expansion will never match the filenames . and .., even if the pattern begins with a “.”. This option is enabled by default. 🦋
globstar — if set, the pattern ** used in a pathname expansion context will match all files and zero or more directories and subdirectories. And if the pattern is followed by a /, only directories and subdirectories match. The ** girlies know. iykyk. 👑
gnu_errfmt — if set, shell error messages are written in the standard GNU error message format. Very demure, very formatted. 💜
histappend — if set, the history list is appended to the file named by the value of the HISTFILE variable when the shell exits, rather than overwriting the file. Protect your history bestie, don't let it get deleted. 🫶
histreedit — if set, and readline is being used, you get the opportunity to re-edit a failed history substitution. Second chances, we love to see it. 💖
histverify — if set, and readline is being used, the results of history substitution are not immediately passed to the shell parser. Instead, the resulting line is loaded into the readline editing buffer, allowing further modification. Preview before you commit, periodt. ✨
hostcomplete — if set, and readline is being used, bash will attempt to perform hostname completion when a word containing a @ is being completed (see Completing under READLINE above). This is enabled by default. 📨
huponexit — if set, bash will send SIGHUP to all jobs when an interactive login shell exits. Everybody's leaving together. 🚪💀
inherit_errexit — if set, command substitution inherits the value of the errexit option, instead of unsetting it in the subshell environment. This option is enabled when posix mode is enabled. 🌸
interactive_comments — in an interactive shell, a word beginning with # causes that word and all remaining characters on that line to be ignored, just like in a non-interactive shell (see COMMENTS above). This option is enabled by default. Comment your code, bestie. 🎀
lastpipe — if set, and job control is not active, the shell runs the last command of a pipeline not executed in the background in the current shell environment. Sneaky but iconic. 💅
lithist — if set, and the cmdhist option is enabled, multi-line

okay picking up right where we left off on that last one — commands are saved to the history with embedded newlines rather than using semicolon separators where possible. 💅

localvar_inherit — if this is set, local variables inherit the value and the attributes of a variable of the same name that already exists at a previous scope, before any new value is assigned. the nameref attribute is not inherited. 🙅‍♀️
localvar_unset — if set, calling unset on local variables in previous function scopes marks them so that subsequent lookups find them unset until that function returns. literally identical to the behavior of unsetting local variables at the current function scope. 🫶
login_shell — the shell sets this one itself if it's started as a login shell (see INVOCATION above). the value may not be changed. 🔒
mailwarn — if set, and a file that bash is checking for mail has been accessed since the last time it was checked, bash displays the message “The mail in mailfile has been read”. 💌
no_empty_cmd_completion — if set, and readline is being used, bash does not search PATH for possible completions when completion is attempted on an empty line. 🙈
nocaseglob — if set, bash matches filenames in a case-insensitive fashion when performing pathname expansion (see Pathname Expansion above). 🔤
nocasematch — if set, bash matches patterns in a case-insensitive fashion when performing matching while executing case or [[ conditional commands, when performing pattern substitution word expansions, or when filtering possible completions as part of programmable completion. 🌸
noexpand_translation — if set, bash encloses the translated results of $"..." quoting in single quotes instead of double quotes. if the string is not translated, this has no effect. 💬
nullglob — if set, pathname expansion patterns which match no files (see Pathname Expansion above) expand to nothing and are removed, rather than expanding to themselves. 🕳️
patsub_replacement — if set, bash expands occurrences of & in the replacement string of pattern substitution to the text matched by the pattern, as described under Parameter Expansion above. this option is enabled by default. ✨
progcomp — if set, enable the programmable completion facilities (see Programmable Completion above). this option is enabled by default. 🎀
progcomp_alias — if set, and programmable completion is enabled, bash treats a command name that doesn't have any completions as a possible alias and attempts alias expansion. if it has an alias, bash attempts programmable completion using the command word resulting from the expanded alias. 👑
promptvars — if set, prompt strings undergo parameter expansion, command substitution, arithmetic expansion, and quote removal after being expanded as described in PROMPTING above. this option is enabled by default. 💖
restricted_shell — the shell sets this option if it is started in restricted mode (see RESTRICTED SHELL below). the value may not be changed. this is not reset when the startup files are executed, allowing the startup files to discover whether or not a shell is restricted. 🔐
shift_verbose — if set, the shift builtin prints an error message when the shift count exceeds the number of positional parameters. 😭
sourcepath — if set, the . (source) builtin uses the value of PATH to find the directory containing the file supplied as an argument when the -p option is not supplied. this option is enabled by default. 🦋
varredir_close — if set, the shell automatically closes file descriptors assigned using the {varname} redirection syntax (see REDIRECTION above) instead of leaving them open when the command completes. 🚪
syslog_history — if set, command history is logged to syslog. 📜
xpg_echo — if set, the echo builtin expands backslash-escape sequences by default. if the posix shell option is also enabled, echo does not interpret any options. ⚡

💤 `suspend` `[-f]`

Suspend the execution of this shell until it receives a SIGCONT signal. bestie, a login shell — or a shell without job control enabled — cannot be suspended; the -f option will override this and force the suspension anyway. 😤

The return status is 0 unless the shell is a login shell or job control is not enabled and -f is not supplied.

🧪 `test` expr

🧪 `[` expr `]`

Return a status of 0 (true) or 1 (false) depending on the evaluation of the conditional expression expr. every operator and operand must be a separate argument. expressions are composed of the primaries described above under CONDITIONAL EXPRESSIONS. 🎯

heads up: test does not accept any options, nor does it accept and ignore an argument of -- as signifying the end of options. no cap, it just doesn't. 🙅‍♀️

Expressions may be combined using the following operators, listed in decreasing order of precedence. the evaluation depends on the number of arguments; see below. test uses operator precedence when there are five or more arguments.

! expr — true if expr is false. 🙃
( expr ) — returns the value of expr. this may be used to override normal operator precedence. 👑
expr1 -a expr2 — true if both expr1 and expr2 are true. 🤝
expr1 -o expr2 — true if either expr1 or expr2 is true. 🌈

🧮 the rules by argument count

test and [ evaluate conditional expressions using a set of rules based on the number of arguments.

0 arguments — the expression is false. 💀
1 argument — the expression is true if and only if the argument is not null.
2 arguments — if the first argument is !, the expression is true if and only if the second argument is null. if the first argument is one of the unary conditional operators listed above under CONDITIONAL EXPRESSIONS, the expression is true if the unary test is true. if the first argument is not a valid unary conditional operator, the expression is false.
3 arguments — the following conditions are applied in the order listed. if the second argument is one of the binary conditional operators listed above under CONDITIONAL EXPRESSIONS, the result of the expression is the result of the binary test using the first and third arguments as operands. the -a and -o operators are considered binary operators when there are three arguments. if the first argument is !, the value is the negation of the two-argument test using the second and third arguments. if the first argument is exactly ( and the third argument is exactly ), the result is the one-argument test of the second argument. otherwise, the expression is false.
4 arguments — the following conditions are applied in the order listed. if the first argument is !, the result is the negation of the three-argument expression composed of the remaining arguments. if the first argument is exactly ( and the fourth argument is exactly ), the result is the two-argument test of the second and third arguments. otherwise, the expression is parsed and evaluated according to precedence using the rules listed above.
5 or more arguments — the expression is parsed and evaluated according to precedence using the rules listed above.

When the shell is in posix mode, or if the expression is part of the [[ command, the < and > operators sort using the current locale. 🌍 if the shell is not in posix mode, the test and [ commands sort lexicographically using ASCII ordering. 🔡

⚠️ okay this part actually matters, listen up: the historical operator-precedence parsing with 4 or more arguments can lead to ambiguities when it encounters strings that look like primaries. the POSIX standard has deprecated the -a and -o primaries and enclosing expressions within parentheses. scripts should no longer use them. it's much more reliable to restrict test invocations to a single primary, and to replace uses of -a and -o with the shell's && and || list operators. do this, your future self will thank you. 🫶

⏱️ `times`

Print the accumulated user and system times for the shell and for processes run from the shell. the return status is 0. 📊

🪤 `trap` `[-lpP]` `[[action] sigspec ...]`

The action is a command that is read and executed when the shell receives any of the signals sigspec. 🎣

If action is absent (and there is a single sigspec) or -, each specified sigspec is reset to the value it had when the shell was started. 🔄

If action is the null string the signal specified by each sigspec is ignored by the shell and by the commands it invokes. 🤫

Okay so continuing the trap tea, bestie 💅

If you run trap with no arguments at all, it shows you the actions attached to every trapped signal — formatted as a set of trap commands you can literally reuse as shell input to restore the current signal dispositions. Iconic, right? It hands you the receipts in a re-usable form. ✨

If -p is given, and action is not present, then trap displays the actions associated with each sigspec — or, if none are supplied, for all trapped signals — again as a set of trap commands reusable as shell input to restore the current signal dispositions. The -P option behaves similarly, but displays only the actions associated with each sigspec argument. And -P requires at least one sigspec argument — no argument, no vibe. 🫶

The -P or -p options may be used in a subshell environment (e.g., command substitution) and, as long as they are used before trap is used to change a signal's handling, will display the state of its parent's traps. Sneaky little window into your parent's business, lowkey. 👀

The -l option prints a list of signal names and their corresponding numbers. Each sigspec is either a signal name defined in <signal.h>, or a signal number. Signal names are case insensitive and the SIG prefix is optional. If -l is supplied with no sigspec arguments, it prints a list of valid signal names. Slay. 📋

Now the special ones, and they matter:

If a sigspec is EXIT (0), action is executed on exit from the shell.
If a sigspec is DEBUG, action is executed before every simple command, for command, case command, select command, (( arithmetic command, [[ conditional command, arithmetic for command, and before the first command executes in a shell function (see SHELL GRAMMAR above). Refer to the description of the extdebug shell option (see shopt above) for details of its effect on the DEBUG trap.
If a sigspec is RETURN, action is executed each time a shell function or a script executed with the . or source builtins finishes executing.

If a sigspec is ERR, action is executed whenever a pipeline (which may consist of a single simple command), a list, or a compound command returns a non-zero exit status — subject to the following conditions. The ERR trap is not executed if:

the failed command is part of the command list immediately following a while or until reserved word,
part of the test in an if statement,
part of a command executed in a && or || list except the command following the final && or ||,
any command in a pipeline but the last (subject to the state of the pipefail shell option),
or if the command's return value is being inverted using !.

These are the same conditions obeyed by the errexit (-e) option. Same energy, same rules. 💀

When the shell is not interactive, signals ignored upon entry to the shell cannot be trapped, reset or listed. Interactive shells permit trapping signals ignored on entry. Trapped signals that are not being ignored are reset to their original values in a subshell or subshell environment when one is created. The return status is false if any sigspec is invalid; otherwise trap returns true.

`true` 🧘

Does nothing, returns a 0 status. Literally just vibes. The chillest builtin in the whole shell.

`type` 🔍

type [-aftpP] name [name ...]

Indicate how each name would be interpreted if used as a command name. It's the "who even are you" builtin, but politely. 🎀

If the -t option is used, type prints a string which is one of alias, keyword, function, builtin, or file if name is an alias, shell reserved word, function, builtin, or executable file, respectively. If the name is not found, type prints nothing and returns a non-zero exit status. Ghosted fr. 👻

If the -p option is used, type either returns the pathname of the executable file that would be found by searching $PATH for name or nothing if type -t name would not return file. The -P option forces a PATH search for each name, even if type -t name would not return file. If name is present in the table of hashed commands, -p and -P print the hashed value, which is not necessarily the file that appears first in PATH. Hashed values said "I was here first" and honestly? They were. 💅

If the -a option is used, type prints all of the places that contain a command named name. This includes aliases, reserved words, functions, and builtins, but the path search options (-p and -P) can be supplied to restrict the output to executable files. type does not consult the table of hashed commands when using -a with -p, and only performs a PATH search for name. Main character behavior: it's showing you every appearance. 🌈

The -f option suppresses shell function lookup, as with the command builtin. type returns true if all of the arguments are found, false if any are not found. No partial credit, sorry bestie. 😭

`ulimit` 🚦

ulimit [-HS] -a
ulimit [-HS] [-bcdefiklmnpqrstuvxPRT [limit]]

Provides control over the resources available to the shell and to processes it starts, on systems that allow such control. It's the builtin that tells your processes "know your limits, bestie." ✋

The -H and -S options specify whether the hard or soft limit is set for the given resource. A hard limit cannot be increased by a non-root user once it is set; a soft limit may be increased up to the value of the hard limit. If neither -H nor -S is specified, ulimit sets both the soft and hard limits.

The value of limit can be a number in the unit specified for the resource or one of the special values hard, soft, or unlimited, which stand for the current hard limit, the current soft limit, and no limit, respectively. If limit is omitted, ulimit prints the current value of the soft limit of the resource, unless the -H option is given. When more than one resource is specified, the limit name and unit, if appropriate, are printed before the value.

Other options are interpreted as follows:

-a — Report all current limits; no limits are set.
-b — The maximum socket buffer size.
-c — The maximum size of core files created.
-d — The maximum size of a process's data segment.
-e — The maximum scheduling priority ("nice").
-f — The maximum size of files written by the shell and its children.
-i — The maximum number of pending signals.
-k — The maximum number of kqueues that may be allocated.
-l — The maximum size that may be locked into memory.
-m — The maximum resident set size (many systems do not honor this limit).
-n — The maximum number of open file descriptors (most systems do not allow this value to be set).
-p — The pipe size in 512-byte blocks (this may not be set).
-q — The maximum number of bytes in POSIX message queues.
-r — The maximum real-time scheduling priority.
-s — The maximum stack size.
-t — The maximum amount of cpu time in seconds.
-u — The maximum number of processes available to a single user.
-v — The maximum amount of virtual memory available to the shell and, on some systems, to its children.
-x — The maximum number of file locks.
-P — The maximum number of pseudoterminals.
-R — The maximum time a real-time process can run before blocking, in microseconds.
-T — The maximum number of threads.

If limit is supplied, and the -a option is not used, limit is the new value of the specified resource. If no option is supplied, then -f is assumed. Default behavior said "file size, I guess." 🍓

Values are in 1024-byte increments, except for:

-t, which is in seconds;
-R, which is in microseconds;
-p, which is in units of 512-byte blocks;
-P, -T, -b, -k, -n, and -u, which are unscaled values;
and, when in posix mode, -c and -f, which are in 512-byte increments.

The return status is 0 unless an invalid option or argument is supplied, or an error occurs while setting a new limit.

In POSIX Mode 512-byte blocks are used for the `-c' and `-f' options.

`umask` 🎭

umask [-p] [-S] [mode]

Set the user file-creation mask to mode. If mode begins with a digit, it is interpreted as an octal number; otherwise it is interpreted as a symbolic mode mask similar to that accepted by chmod(1). If mode is omitted, umask prints the current value of the mask. The -S option without a mode argument prints the mask in a symbolic format; the default output is an octal number. If the -p option is supplied, and mode is omitted, the output is in a form that may be reused as input. Copy-paste friendly, we love to see it. ✨

The return status is zero if the mode was successfully changed or if no mode argument was supplied, and non-zero otherwise.

`unalias` 💔

unalias [-a] [name ...]

Remove each name from the list of defined aliases. If -a is supplied, remove all alias definitions. It's the "we're not doing this anymore" builtin. The return value is true unless a supplied name is not a defined alias — if you try to break up with an alias that never existed, it clocks you. 🙈

`unset` 🫥

unset [-fv] [-n] [name ...]

For each name, remove the corresponding variable or function. If the -v option is given, each name refers to a shell variable, and that variable is removed. If -f is specified, each name refers to a shell function, and the function definition is removed. If the -n option is supplied, and name is a variable with the nameref attribute, name will be unset rather than the variable it references. -n has no effect if the -f option is supplied.

Read-only variables and functions may not be unset. They said "no take-backs," and they meant it. 🚫

When variables or functions are removed, they are also removed from the environment passed to subsequent commands. If no options are supplied, each name refers to a variable; if there is no variable by that name, a function with that name, if any, is unset. Some shell variables may not be unset.

If any of BASH_ALIASES, BASH_ARGV0, BASH_CMDS, BASH_COMMAND, BASH_SUBSHELL, BASHPID, COMP_WORDBREAKS, DIRSTACK, EPOCHREALTIME, EPOCHSECONDS, FUNCNAME, GROUPS, HISTCMD, LINENO, RANDOM, SECONDS, or SRANDOM are unset, they lose their special properties, even if they are subsequently reset. No cap, you can't undo that. 😭

The exit status is true unless a name is readonly or may not be unset.

`wait` ⏳

wait [-fn] [-p varname] [id ...]

Wait for each specified child process id and return the termination status of the last id. Each id may be a process ID pid or a job specification jobspec; if a jobspec is supplied, wait waits for all processes in the job. It's the "let me sit here until you're done, I'm not mad" builtin. 🧘‍♀️

If no options or ids are supplied, wait waits for all running background jobs and the last-executed process substitution, if its process id is the same as $!, and the return status is zero.

okay so back to wait — she has layers 💅

If the -n option is supplied, wait waits for any one of the given ids or, if no ids are supplied, any job or process substitution, to complete and returns its exit status. If none of the supplied ids is a child of the shell, or if no ids are supplied and the shell has no unwaited-for children, the exit status is 127.
If the -p option is supplied, wait assigns the process or job identifier of the job for which the exit status is returned to the variable varname named by the option argument. The variable, which cannot be readonly, will be unset initially, before any assignment. This is useful only when used with the -n option.
Supplying the -f option, when job control is enabled, forces wait to wait for each id to terminate before returning its status, instead of returning when it changes status.
If none of the ids specify one of the shell's active child processes, the return status is 127. If wait is interrupted by a signal, any varname will remain unset, and the return status will be greater than 128, as described under SIGNALS above. Otherwise, the return status is the exit status of the last id.

✨ SHELL COMPATIBILITY MODE ✨

Bash-4.0 introduced the concept of a shell compatibility level, specified as a set of options to the shopt builtin (compat31, compat32, compat40, compat41, and so on). There is only one current compatibility level — each option is mutually exclusive. The compatibility level is intended to allow users to select behavior from previous versions that is incompatible with newer versions while they migrate scripts to use current features and behavior. It's intended to be a temporary solution. 🩹

This section does not mention behavior that is standard for a particular version (e.g., setting compat32 means that quoting the right hand side of the regexp matching operator quotes special regexp characters in the word, which is default behavior in bash-3.2 and subsequent versions).

If a user enables, say, compat32, it may affect the behavior of other compatibility levels up to and including the current compatibility level. The idea is that each compatibility level controls behavior that changed in that version of bash, but that behavior may have been present in earlier versions. For instance, the change to use locale-based comparisons with the [[ command came in bash-4.1, and earlier versions used ASCII-based comparisons, so enabling compat32 will enable ASCII-based comparisons as well. That granularity may not be sufficient for all uses, and as a result users should employ compatibility levels carefully. Read the documentation for a particular feature to find out the current behavior.

Bash-4.3 introduced a new shell variable: BASH_COMPAT. The value assigned to this variable (a decimal version number like 4.2, or an integer corresponding to the compatNN option, like 42) determines the compatibility level.

Starting with bash-4.4, bash began deprecating older compatibility levels. Eventually, the options will be removed in favor of BASH_COMPAT.

Bash-5.0 was the final version for which there was an individual shopt option for the previous version. BASH_COMPAT is the only mechanism to control the compatibility level in versions newer than bash-5.0.

The following table describes the behavior changes controlled by each compatibility level setting. The compatNN tag is used as shorthand for setting the compatibility level to NN using one of the following mechanisms. For versions prior to bash-5.0, the compatibility level may be set using the corresponding compatNN shopt option. For bash-4.3 and later versions, the BASH_COMPAT variable is preferred, and it is required for bash-5.1 and later versions.

compat31
- Quoting the rhs of the [[ command's regexp matching operator (=~) has no special effect.
compat32
- The < and > operators to the [[ command do not consider the current locale when comparing strings; they use ASCII ordering.
compat40
- The < and > operators to the [[ command do not consider the current locale when comparing strings; they use ASCII ordering. Bash versions prior to bash-4.1 use ASCII collation and strcmp(3); bash-4.1 and later use the current locale's collation sequence and strcoll(3).
compat41
- In posix mode, time may be followed by options and still be recognized as a reserved word (this is POSIX interpretation 267).
- In posix mode, the parser requires that an even number of single quotes occur in the word portion of a double-quoted parameter expansion and treats them specially, so that characters within the single quotes are considered quoted (this is POSIX interpretation 221).
compat42
- The replacement string in double-quoted pattern substitution does not undergo quote removal, as it does in versions after bash-4.2.
- In posix mode, single quotes are considered special when expanding the word portion of a double-quoted parameter expansion and can be used to quote a closing brace or other special character (this is part of POSIX interpretation 221); in later versions, single quotes are not special within double-quoted word expansions.
compat43
- Word expansion errors are considered non-fatal errors that cause the current command to fail, even in posix mode (the default behavior is to make them fatal errors that cause the shell to exit).
- When executing a shell function, the loop state (while/until/etc.) is not reset, so break or continue in that function will break or continue loops in the calling context. Bash-4.4 and later reset the loop state to prevent this.
compat44
- The shell sets up the values used by BASH_ARGV and BASH_ARGC so they can expand to the shell's positional parameters even if extended debugging mode is not enabled.
- A subshell inherits loops from its parent context, so break or continue will cause the subshell to exit. Bash-5.0 and later reset the loop state to prevent the exit
- Variable assignments preceding builtins like export and readonly that set attributes continue to affect variables with the same name in the calling environment even if the shell is not in posix mode.
compat50
- Bash-5.1 changed the way $RANDOM is generated to introduce slightly more randomness. If the shell compatibility level is set to 50 or lower, it reverts to the method from bash-5.0 and previous versions, so seeding the random number generator by assigning a value to RANDOM will produce the same sequence as in bash-5.0.
- If the command hash table is empty, bash versions prior to bash-5.1 printed an informational message to that effect, even when producing output that can be reused as input. Bash-5.1 suppresses that message when the -l option is supplied.
compat51
- The unset builtin treats attempts to unset array subscripts @ and * differently depending on whether the array is indexed or associative, and differently than in previous versions.
- Arithmetic commands ( ((...)) ) and the expressions in an arithmetic for statement can be expanded more than once.
- Expressions used as arguments to arithmetic operators in the [[ conditional command can be expanded more than once.
- The expressions in substring parameter brace expansion can be expanded more than once.
- The expressions in the $((...)) word expansion can be expanded more than once.
- Arithmetic expressions used as indexed array subscripts can be expanded more than once.
- test -v, when given an argument of A[@], where A is an existing associative array, will return true if the array has any set elements. Bash-5.2 will look for and report on a key named @.
- The ${parameter[:]=value} word expansion will return value, before any variable-specific transformations have been performed (e.g., converting to lowercase). Bash-5.2 will return the final value assigned to the variable.
- Parsing command substitutions will behave as if extended globbing (see the description of the shopt builtin above) is enabled, so that parsing a command substitution containing an extglob pattern (say, as part of a shell function) will not fail. This assumes the intent is to enable extglob before the command is executed and word expansions are performed. It will fail at word expansion time if extglob hasn't been enabled by the time the command is executed.
compat52
- The test builtin uses its historical algorithm to parse parenthesized subexpressions when given five or more arguments.
- If the -p or -P option is supplied to the bind builtin, bind treats any arguments remaining after option processing as bindable command names, and displays any key sequences bound to those commands, instead of treating the arguments as key sequences to bind.

🔒 RESTRICTED SHELL 🔒

okay so this is the ✨locked down✨ mode and it's giving strict-parent energy, but like, in a good way? So if bash gets started under the name rbash, or you hand it the -r option at invocation time, the shell becomes restricted. A restricted shell is what you use when you want an environment that's more controlled than the regular shell, bestie. It behaves identically to bash except for these things, which are either disallowed or just straight-up not performed:

Changing directories with cd. 🚫
Setting or unsetting the values of SHELL, PATH, HISTFILE, ENV, or BASH_ENV.
Specifying command names containing /.
Specifying a filename containing a / as an argument to the . builtin command.
Using the -p option to the . builtin command to specify a search path.
Specifying a filename containing a slash as an argument to the history builtin command.
Specifying a filename containing a slash as an argument to the -p option to the hash builtin command.
Importing function definitions from the shell environment at startup.
Parsing the values of BASHOPTS and SHELLOPTS from the shell environment at startup.
Redirecting output using the >, >|, <>, >&, &>, and >> redirection operators. 😭 no redirects for u
Using the exec builtin command to replace the shell with another command.
Adding or deleting builtin commands with the -f and -d options to the enable builtin command.
Using the enable builtin command to enable disabled shell builtins.
Specifying the -p option to the command builtin command.
Turning off restricted mode with set +r or shopt -u restricted_shell.

bestie these restrictions get enforced after any startup files are read. like, the door locks behind you, not in front of you. 🚪🔐

And here's a lil plot twist: when a command that turns out to be a shell script gets executed (peep COMMAND EXECUTION above, iykyk), rbash turns off any restrictions in the shell that gets spawned to run that script. so the child shell is lowkey free. 💅

👀 SEE ALSO 👀

like go read these too, they're iconic:

Bash Reference Manual, Brian Fox and Chet Ramey
The Gnu Readline Library, Brian Fox and Chet Ramey
The Gnu History Library, Brian Fox and Chet Ramey
Portable Operating System Interface (POSIX) Part 2: Shell and Utilities, IEEE — http://pubs.opengroup.org/onlinepubs/9799919799/
http://tiswww.case.edu/~chet/bash/POSIX — a description of posix mode
sh(1), ksh(1), csh(1)
emacs(1), vi(1)
readline(3)

📁 FILES 📁

the whole friend group of paths, fr fr:

/bin/bash — The bash executable 🫶
/etc/profile — The systemwide initialization file, executed for login shells
/etc/bash.bash_logout — The systemwide login shell cleanup file, executed when a login shell exits
~/.bash_profile — The personal initialization file, executed for login shells
~/.bashrc — The individual per-interactive-shell startup file
~/.bash_logout — The individual login shell cleanup file, executed when a login shell exits
~/.bash_history — The default value of HISTFILE, the file in which bash saves the command history
~/.inputrc — Individual readline initialization file

🌟 AUTHORS 🌟

the literal main characters who built this whole thing:

Brian Fox, Free Software Foundation — bfox@gnu.org
Chet Ramey, Case Western Reserve University — chet.ramey@case.edu

🐛 BUG REPORTS 🐛

okay so if you find a bug in bash, you should report it. periodt. But like — first, make sure it's actually a bug, and that it shows up in the latest version of bash. The latest version is always available from ftp://ftp.gnu.org/pub/gnu/bash/ and http://git.savannah.gnu.org/cgit/bash.git/snapshot/bash-master.tar.gz. go grab it, bestie. ⬇️

Once you've determined that a bug actually exists, use the bashbug command to submit a bug report. And if you have a fix?? you are encouraged to mail that too! 💌 You may send suggestions and “philosophical” bug reports to bug-bash@gnu.org or post them to the Usenet newsgroup gnu.bash.bug.

ALL bug reports should include:

The version number of bash
The hardware and operating system
The compiler used to compile
A description of the bug behavior
A short script or “recipe” which exercises the bug

bashbug inserts the first three items automatically into the template it provides for filing a bug report. so convenient, honestly slay 🎀

Comments and bug reports concerning this manual page should be directed to chet.ramey@case.edu.

💀 BUGS 💀

the known-issues section, aka the group chat receipts:

It's too big and too slow.
There are some subtle differences between bash and traditional versions of sh, mostly because of the POSIX specification.
Aliases are confusing in some uses.
Shell builtin commands and functions are not stoppable/restartable.
Compound commands and command lists of the form “a ; b ; c” are not handled gracefully when combined with process suspension. When a process is stopped, the shell immediately executes the next command in the list or breaks out of any existing loops. It suffices to enclose the command in parentheses to force it into a subshell, which may be stopped as a unit, or to start the command in the background and immediately bring it into the foreground.
Array variables may not (yet) be exported.

and that's a wrap on the whole entire page, bestie!! 🎉👑 this was GNU Bash 5.3, dated 2025 April 7 — like, the version and the date at the bottom of the manpage, and honestly? iconic. now go forth and shell responsibly, no cap. 💖✨🦋

📦 where this page comes from

fedora 43: bash 5.3.0-2.fc43 · GPL-3.0-or-later
fedora 44: bash 5.3.9-3.fc44 · GPL-3.0-or-later
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-10-18: a language model's rewrite of the original, under its terms

BASH(1)

🎀 NAME 🎀

✨ SYNOPSIS ✨

💖 COPYRIGHT 💖

🦋 DESCRIPTION 🦋

👑 OPTIONS 👑

🌸 ARGUMENTS 🌸

💫 INVOCATION 💫

🌈 DEFINITIONS 🌈

👑 RESERVED WORDS 👑

🎀 SHELL GRAMMAR 🎀

Simple Commands

Pipelines

Lists

Compound Commands

{ list; }

((expression))

[[ expression ]]

for name [ [ in word ... ] ; ] do list ; done

for (( expr1 ; expr2 ; expr3 )) [;] do list ; done

select name [ in word ] ; do list ; done

case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac

if list; then list; [ elif list; then list; ] ... [ else list; ] fi

while list-1; do list-2; done

until list-1; do list-2; done

🧚 Coprocesses 🧚

🧚 Shell Function Definitions

💬 COMMENTS

🎀 QUOTING

🌸 Translating Strings

🦋 PARAMETERS

🌸 Positional Parameters 🌸

👑 Special Parameters 👑

🦋 Shell Variables 🦋

BASH_COMPAT 💅

BASH_ENV ✨

BASH_XTRACEFD 🔥

CDPATH 🦋

CHILD_MAX 🌸

COLUMNS 🫶

COMPREPLY 😭

EMACS 💀

ENV 🍓

EXECIGNORE 🧚

FCEDIT 💜

FIGNORE ⭐

FUNCNEST 🌈

GLOBIGNORE 💖

GLOBSORT 👑

🍓 Arrays 🍓

✨ EXPANSION ✨

🎀 Brace Expansion 🎀

🌸 Tilde Expansion 🌸

💖 Parameter Expansion 💖

✨ Command Substitution ✨

🌈 Arithmetic Expansion 🌈

🦋 Process Substitution 🦋

🎀 Word Splitting 🎀

⭐ Pathname Expansion ⭐

✨ Pattern Matching ✨

🌸 Quote Removal 🌸

💅 REDIRECTION 💅

🎀 the order matters, bestie 🎀

🌸 special filenames 🌸

✨ Redirecting Input ✨

✨ Redirecting Output ✨

✨ Appending Redirected Output ✨

✨ Redirecting Standard Output and Standard Error ✨

✨ Appending Standard Output and Standard Error ✨

✨ Here Documents ✨

✨ Here Strings ✨

✨ Duplicating File Descriptors ✨

✨ Moving File Descriptors ✨

✨ Opening File Descriptors for Reading and Writing ✨

✨ ALIASES ✨

🫶 FUNCTIONS 🫶

🧮 ARITHMETIC EVALUATION 🧮

🦋 CONDITIONAL EXPRESSIONS 🦋

🎀 The file primaries 🎀

🌸 The variable primaries 🌸

`{ list; }`

`((expression))`

`[[ expression ]]`

`for name [ [ in word ... ] ; ] do list ; done`

`for (( expr1 ; expr2 ; expr3 )) [;] do list ; done`

`select name [ in word ] ; do list ; done`

`case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac`

`if list; then list; [ elif list; then list; ] ... [ else list; ] fi`

`while list-1; do list-2; done`

`until list-1; do list-2; done`

`BASH_COMPAT` 💅

`BASH_ENV` ✨

`BASH_XTRACEFD` 🔥

`CDPATH` 🦋

`CHILD_MAX` 🌸

`COLUMNS` 🫶

`COMPREPLY` 😭

`EMACS` 💀

`ENV` 🍓

`EXECIGNORE` 🧚

`FCEDIT` 💜

`FIGNORE` ⭐

`FUNCNEST` 🌈

`GLOBIGNORE` 💖

`GLOBSORT` 👑

`:` [arguments]

`.` [-p path] filename [arguments] / `source` [-p path] filename [arguments]

`alias` ✨

`bg` 💤

`bind` 🎀

`break` 💥

`builtin` 🧱

`caller` 📞

`cd` 📁💖

🌸 `compopt` 🌸

🦋 `continue` 🦋

👑 `declare` / `typeset` 👑

💖 `dirs` 💖

🎀 `disown` 🎀

⭐ `echo` ⭐

🌈 `enable` 🌈

`eval` 🧚

`exec` 🔥

`exit` 👑

`export` 🌈

`false` 💀

`fc` 🎀

`fg` 🔥

`getopts` 🌸

`hash` 🍓

🎀 `help`

🦋 `history`

💜 `jobs`

🔥 `kill`

🌸 `let`

🎀 `local`

🌈 `logout`

🧚 `mapfile` / `readarray`

💅 `popd`