KSH-CDE(1)

📦

version 2.5.3 · 2 versions

2.5.3 · nixos 26.05 (latest)
2.5.3 · nixpkgs unstable

👑 `ksh` — KornShell, a standard/restricted command and programming language 💅

🌸 NAME 🌸

Bestie, this is the ksh(user cmd) page. ksh is the KornShell, a standard/restricted command and programming language. Periodt. ✨

✨ SYNOPSIS ✨

ksh [±abcefhikmnoprstuvxCP]  [-I  | file ]  [±o  | option ]  [...]  [-]
[arg ...]  [rksh]  [±abcefhikmnoprstuvxCP]  [-I  | file ]  [±o  | option
]  [...]  [-]  [arg ...]

💖 DESCRIPTION 💖

Okay so ksh is a command and programming language that executes commands read from a terminal or a file. rksh is a restricted version of the command interpreter ksh; it is used to set up login names and execution environments whose capabilities are more controlled than those of the standard shell. See Invocation below for the meaning of arguments to the shell. 🫶

🦋 Definitions 🦋

A metacharacter is one of the following characters:

; & ( ) | < > new-line space tab

A blank is a tab or a space.
An identifier is a sequence of letters, digits, or underscores starting with a letter or underscore. Identifiers are used as components of variable names.
A vname is a sequence of one or more identifiers separated by a . and optionally preceded by a .. Vnames are used as function and variable names.
A word is a sequence of characters excluding non-quoted metacharacters.
A command is a sequence of characters in the syntax of the shell language. The shell reads each command and carries out the desired action either directly or by invoking separate utilities.
A built-in command is a command that is carried out by the shell itself without creating a separate process. Some commands are built-in purely for convenience and are not documented here. Built-ins that cause side effects in the shell environment and built-ins that are found before performing a path search (see Execution below) are documented here. For historical reasons, some of these built-ins behave differently than other built-ins and are called special built-ins.

🛠️ Commands 🛠️

A simple-command is a list of variable assignments (see Variable Assignments below) or a sequence of blank separated words which may be preceded by a list of variable assignments (see Environment below). The first word specifies the name of the command to be executed. Except as specified below, the remaining words are passed as arguments to the invoked command. The command name is passed as argument 0 (see exec(2)). The value of a simple-command is its exit status; 0-255 if it terminates normally; 256+signum if it terminates abnormally (the name of the signal corresponding to the exit status can be obtained via the -l $? option of the kill built-in utility). 😭

An arithmetic command begins with ((, and consists of an arithmetic expression formed by all the characters until a matching )). (See Arithmetic evaluation below.) The exit status of an arithmetic command is 0 when the arithmetic expression evaluates to a non-zero value and is 1 when the arithmetic expression evaluates to 0.

A pipeline is a sequence of one or more commands separated by |. The standard output of each command but the last is connected by a pipe(2) to the standard input of the next command. Each command, except possibly the last, is run as a separate process; the shell waits for the last command to terminate. The exit status of a pipeline is the exit status of the last command. Each pipeline can be preceded by the reserved word ! which negates the exit status of the pipeline. 🔥

A list is a sequence of one or more pipelines separated by ;, &, |&, &&, or ||, and optionally terminated by ;, &, or |&. Of these five symbols, ;, &, and |& have equal precedence, which is lower than that of && and ||. The symbols && and || also have equal precedence. A semicolon (;) causes sequential execution of the preceding pipeline; an ampersand (&) causes asynchronous execution of the preceding pipeline (i.e., the shell does not wait for that pipeline to finish). The symbol |& causes asynchronous execution of the preceding pipeline with a two-way pipe established to the parent shell; the standard input and output of the spawned pipeline can be written to and read from by the parent shell by applying the redirection operators <& and >& with arg p to commands and by using -p option of the built-in commands read and print described later. The symbol && (||) causes the list following it to be executed only if the preceding pipeline returns a zero (non-zero) value. An arbitrary number of new-lines may appear in a list instead of a semicolon, to delimit a command. 💀

A command is either a simple-command or one of the following. Unless otherwise stated, the value returned by a command is that of the last simple-command executed in the command.

for vname in word ... ;do list ;done — Each time a for command is executed, vname (the name) is set to the next word taken from the in word list. If in word ... is omitted, then the for command executes the do list once for each positional parameter that is set (see Parameter Expansion below). Execution ends when there are no more words in the list.
for (( expr1; expr2; expr3 )) ;do list ;done — The arithmetic expression expr1 is evaluated first. (See Arithmetic evaluation below.) The arithmetic expression expr2 is repeatedly evaluated until it evaluates to zero and when non-zero, list is executed and the arithmetic expression expr3 evaluated. If any expression is omitted, then it behaves as if it evaluated to 1.
select vname in word ... ;do list ;done — A select command prints on standard error (file descriptor 2) the set of words, each preceded by a number. If in word ... is omitted, then the positional parameters are used instead (see Parameter Expansion below). The PS3 prompt is printed and a line is read from the standard input. If this line consists of the number of one of the listed words, then the value of the variable vname is set to the word corresponding to this number. If this line is empty the selection list is printed again. Otherwise the value of the variable vname is set to null. The contents of the line read from standard input is saved in the variable REPLY. The list is executed for each selection until a break or end-of-file is encountered. If the REPLY variable is set to null by the execution of list, then the selection list is printed before displaying the PS3 prompt for the next selection. 🎀
case word in (pattern | pattern ...) list ;; ... esac — A case command executes the list associated with the first pattern that matches word. The form of the patterns is the same as that used for file-name generation (see File Name Generation below). The ;; operator causes execution of case to terminate. If ;& is used in place of ;; the next subsequent list, if any, is executed.
if list ;then list elif list ;then list ] ... [ ;else list ] ;fi — The list following if is executed and, if it returns a zero exit status, the list following the first then is executed. Otherwise, the list following elif is executed and, if its value is zero, the list following the next then is executed. Failing that, the else list is executed. If the if list has non-zero exit status and there is no else list, then the if command returns a zero exit status.
while list ;do list ;done and until list ;do list ;done — A while command repeatedly executes the while list and, if the exit status of the last command in the list is zero, executes the do list; otherwise the loop terminates. If no commands in the do list are executed, then the while command returns a zero exit status; until may be used in place of while to negate the loop termination test.
(list) — Execute list in a separate environment. Note, that if two adjacent open parentheses are needed for nesting, a space must be inserted to avoid evaluation as an arithmetic command as described above.
{ list;} — list is simply executed. Note that unlike the metacharacters ( and ), { and } are reserved words and must occur at the beginning of a line or after a ; in order to be recognized.
[[ expression ]] — Evaluates expression and returns a zero exit status when expression is true. See Conditional Expressions below, for a description of expression.
function varname { list ;} and varname () { list ;} — Define a function which is referenced by varname. A function whose varname contains a . is called a discipline function and the portion of the varname preceding the last . must refer to an existing variable. The body of the function is the list of commands between { and }. A function defined with the function varname syntax can also be used as an argument to the . special built-in command to get the equivalent behavior as if the varname() syntax were used to define it. (See Functions below.)
time pipeline — If pipeline is omitted the user and system time for the current shell and completed child processes is printed on standard error. Otherwise, pipeline is executed and the elapsed time as well as the user and system time are printed on standard error. ⏳

The following reserved words are only recognized as such when they are the first word of a command and are not quoted:

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

🎀 Variable Assignments 🎀

One or more variable assignments can start a simple command or can be arguments to the typeset, export, or readonly special built-in commands. The syntax for an assignment is of the form:

varname=word

varname[word]=word

No space is permitted between varname and the = or between = and word.

varname=(assign_list)

No space is permitted between varname and the =. An assign_list can be one of the following:

word ... — Indexed array assignment. 🌈
[word]=word ... — Associative array assignment.
assignment ... — Nested variable assignment.
typeset options assignment ... — Nested variable assignment. Multiple assignments can be specified by separating each of them with a ;. 💜

💬 Comments 💬

Bestie, if a word starts with #, that whole word plus literally everything after it up to the newline gets ignored. Poof. Gone. Not read, not executed, not your problem. ✨

🎀 Aliasing 🎀

Okay so this is the glow-up mechanic: the first word of every command gets swapped out for the text of an alias, if one's been defined for that word. Simple as that.

An alias name can be literally any number of characters, except these party-poopers:

metacharacters
quoting characters
file expansion characters
parameter expansion and command substitution characters
=

The replacement string, though? That can be a full valid shell script, metacharacters and all. No cap. 🔥

Now the recursive bit: the first word of each command inside the replaced text also gets tested for aliases — except any words that are already in the process of being replaced (no infinite loops, we're not doing that today 🙈).

And here's a sneaky little detail: if the last character of the alias value is a blank, then the word following the alias also gets checked for alias substitution. Yes, that trailing space is doing real work. Respect it. 💅

Aliases can be used to redefine built-in commands, but they cannot redefine the reserved words listed above. Sorry babe, those are off-limits.

Make and list them with the alias command, remove them with the unalias command. Easy.

⚠️ Big one: aliasing happens when scripts are read, not while they're executed. So for an alias to take effect, the alias definition command has to be executed before the command that references it is even read. Order matters, bestie. 😭

The following exported aliases are compiled into the shell, but you can totally unset or redefine them — they're not locked in:

autoload='typeset -fu'
command='command '
fc=hist
float='typeset -E'
functions='typeset -f'
hash='alias -t --'
history='hist -l'
integer='typeset -i'
nameref='typeset -n'
nohup='nohup '
r='hist -s'
redirect='command exec'
stop='kill -s STOP'
times='{ {time;} 2>&1;}'
type='whence -v'

🦋 Tilde Substitution 🦋

After alias substitution does its thing, every word gets checked: does it begin with an unquoted ~? 👀

And for the record, "word" here also means the word portion of a parameter expansion — see Parameter Expansion below, iykyk.

If it does start with ~, then the part of the word up to a / gets checked against the user names in the password database (often the /etc/passwd file, the classic). Match found? The ~ and the matched login name get replaced by that user's login directory. 💖

No match found? Original text stays exactly as it was. No changes, no notes.

The specials:

A ~ by itself, or one sitting in front of a /, gets replaced by $HOME.
A ~ followed by a + or - gets replaced by the value of $PWD and $OLDPWD respectively. Cute, right?

Also ✨bonus round✨: when you're expanding a variable assignment, tilde substitution is attempted when the value of the assignment begins with a ~, and also when a ~ appears after a :. And heads up — that : also terminates a ~ login name.

🐚 Command Substitution 🐚

You can take the standard output of a command and use it as part or all of a word. Two ways to wrap it:

wrapped in parentheses preceded by a dollar sign — $()
wrapped in a pair of grave accents — ``

Trailing newlines get removed, naturally. 🧹

In that second (obsolete 🫠) form, the string between the quotes gets processed for special quoting characters before the command is executed — check Quoting below for that whole situation.

Little flex for you: this command substitution

$(cat file)

can be replaced by the equivalent but faster

$(<file)

Same energy, less work. Slay. ⭐

🧮 Arithmetic Substitution 🧮

An arithmetic expression wrapped in double parentheses with a dollar sign out front — $(( )) — gets replaced by the value of the arithmetic expression inside those double parentheses. Math, but make it shell. 🔥

🌈 Process Substitution 🌈

💀 Heads up: this feature is only available on versions of the UNIX operating system that support the /dev/fd directory for naming open files. Check before you commit, bestie.

Every command argument shaped like <(list) or >(list) will run the process list asynchronously, connected to some file in /dev/fd. The name of that file then becomes the argument to your command. Iconic behaviour.

Pick the > form and writing on that file provides input for list.
Use < and the file passed as an argument will contain the output of the list process.

Example time, look at this beauty:

paste <(cut -f1 file1) <(cut -f3 file2) | tee >(process1) >(process2)

What that does: cuts field 1 out of file1 and field 3 out of file2, pastes the results together, and sends it to the processes process1 and process2 — as well as putting it onto standard output. Multitasking queen. 👑

⚠️ Important vibe check: that file being passed as an argument to the command is a UNIX pipe(2), so programs that expect to lseek(2) on the file will not work. No cap, don't try it.

💜 Parameter Expansion 💜

A parameter is a variable, one or more digits, or any of these characters: *, @, #, ?, -, $, and \.

A variable is denoted by a vname. Now, to create a variable whose vname contains a ., a variable whose vname consists of everything before the last . must already exist. That's the rule, I don't make 'em.

A variable has a value and zero or more attributes. You assign values and attributes with the typeset special built-in command. All the attributes the shell supports are described later with the typeset special built-in command. Exported variables pass values and attributes to the environment. 🫶

Arrays, both kinds 🍓

The shell supports both indexed and associative arrays.

An element of an array variable is referenced by a subscript.

A subscript for an indexed array is an arithmetic expression (see Arithmetic evaluation below) sitting between a [ and a ].
To assign values to an indexed array, use set -A vname value ...
The value of all subscripts must be in the range of 0 through 4095. That's the whole range, don't push it.
Indexed arrays don't need to be declared. Any reference to a variable with a valid subscript is legal, and an array will be created if necessary. She'll manifest herself. ✨

An associative array is created with the -A option to typeset. A subscript for an associative array is a string enclosed between [ and ].

Referencing any array without a subscript is equivalent to referencing the array with subscript 0.

Assigning values 💅

The value of a variable may be assigned by writing:

vname=value [ vname=value ] ...

vname[subscript]=value [ vname[subscript]=value ] ...

😭 Note that no space is allowed before or after the =. Not one. This is the number one cause of heartbreak in shell scripting.

Namerefs 👑

A nameref is a variable that is a reference to another variable. Created with the -n attribute of typeset.

The value of the variable at the time of the typeset command becomes the variable that will be referenced whenever the nameref variable is used.

Rules, bestie:

The name of a nameref variable cannot contain a .
When a variable or function name contains a ., and the portion of the name up to the first . matches the name of a nameref, the variable referred to is obtained by replacing the nameref portion with the name of the variable referenced by the nameref.

A nameref is a genuinely convenient way to refer to the variable inside a function whose name is passed as an argument to a function. For example, if the name of a variable is passed as the first argument to a function, this command:

typeset -n var=$1

...inside the function causes references and assignments to var to be references and assignments to the variable whose name has been passed to the function. Chef's kiss. 🤌

Numbers and positionals 🧚

If either of the floating point attributes, -E or -F, or the integer attribute -i, is set for vname, then the value is subject to arithmetic evaluation as described below.

Positional parameters — parameters denoted by a number — may be assigned values with the set special built-in command. Parameter $0 is set from argument zero when the shell is invoked.

The character $ is used to introduce substitutable parameters. 🎀

${parameter} — The shell reads all the characters from ${ to the matching } as part of the same word, even if it contains braces or metacharacters. The value of the parameter, if any, is substituted. The braces are required when parameter is followed by a letter, digit, or underscore that is not to be interpreted as part of its name, when the variable name contains a ., or when a variable is subscripted. If parameter is one or more digits, it's a positional parameter. A positional parameter of more than one digit must be enclosed in braces. If parameter is * or @, then all the positional parameters starting with $1 are substituted (separated by a field separator character). If an array vname with subscript * or @ is used, then the value for each of the elements is substituted (separated by a field separator character).
${#parameter} — If parameter is * or @, the number of positional parameters is substituted. Otherwise, the length of the value of the parameter is substituted.
${#vname[*]} — The number of elements in the array vname is substituted.
${!vname} — Expands to the name of the variable referred to by vname. This will be vname except when vname is a name reference.
${!vname[subscript]} — Expands to the name of the subscript, unless subscript is * or @. When subscript is *, the list of array subscripts for vname is generated. For a variable that is not an array, the value is 0 if the variable is set, otherwise it's null. When subscript is @, same as above, except that when used in double quotes, each array subscript yields a separate argument. 🌸
${!prefix*} — Expands to the names of the variables whose names begin with prefix.
${parameter:-word} — If parameter is set and is non-null, substitute its value; otherwise substitute word.
${parameter:=word} — If parameter is not set or is null, set it to word; the value of the parameter is then substituted. > 🙈 Positional parameters may not be assigned to this way. Hard no.
${parameter:?word} — If parameter is set and is non-null, substitute its value; otherwise, print word and exit from the shell. 💀 If word is omitted, a standard message is printed.
${parameter:+word} — If parameter is set and is non-null, substitute word; otherwise substitute nothing.
${parameter:offset:length}
${parameter:offset} — Expands to the portion of the value of parameter starting at the character determined by expanding offset as an arithmetic expression, and consisting of the number of characters determined by the arithmetic expression defined by length. In the second form, the remainder of the value is used. If parameter is * or @, or is an array name indexed by * or @, then offset and length refer to the array index and number of elements respectively.
${parameter#pattern} — okay bestie this one's description is coming right up, keep scrolling 👀
${parameter##pattern} — okay so if the shell pattern matches the beginning of the value of parameter, then what you get back is the value of parameter with that matched chunk deleted, periodt. If it doesn't match? You just get the value of parameter substituted right back at you, no drama. And bestie, listen: in the first form (${parameter#pattern}) the smallest matching pattern gets chopped, and in the second form (${parameter##pattern}) the largest matching pattern gets chopped. When parameter is @, *, or an array variable with subscript @ or *, the substring operation is applied to each element in turn. slay 💅
${parameter%pattern}
${parameter%%pattern} — same vibe but from the other end, iconic honestly. If the shell pattern matches the end of the value of parameter, then the value of this expansion is the value of parameter with the matched part deleted; otherwise you just substitute the value of parameter. In the first form the smallest matching pattern is deleted and in the second form the largest matching pattern is deleted. And again — when parameter is @, *, or an array variable with subscript @, or *, the substring operation is applied to each element in turn. 💖
${parameter/pattern/string}
${parameter//pattern/string} — this one expands parameter and replaces pattern with the given string. In the first form, only the first occurrence of pattern gets replaced. In the second form, each match for pattern is replaced by the given string — every single one, no cap, she ate. 🎀 When string is null, the pattern will be deleted and the / in front of string may be omitted. When parameter is @, *, or an array variable with subscript @, or *, the substitution operation is applied to each element in turn.

In all of the above, word is not evaluated unless it's actually going to be used as the substituted string — so in this little example right here, pwd is executed only if d is not set or is null:

print ${d:-$(pwd)}

Lowkey genius, right? 🧠✨ And if the colon ( : ) is omitted from the above expressions, then the shell only checks whether parameter is set or not — it stops caring about null. Important distinction, iykyk.

🌸 The parameters the shell sets for you (automatic besties) 🌸

The following parameters are automatically set by the shell:

# — The number of positional parameters in decimal. 🍓
- — Options supplied to the shell on invocation or by the set command.
? — The decimal value returned by the last executed command.
$ — The process number of this shell.
_ — Okay so initially, the value of _ is an absolute pathname of the shell or script being executed as passed in the environment. Subsequently it is assigned the last argument of the previous command. This parameter is not set for commands which are asynchronous. This parameter is also used to hold the name of the matching MAIL file when checking for mail. multitasking icon 💅
! — The process number of the last background command invoked.
.sh.edchar — This variable contains the value of the keyboard character (or sequence of characters if the first character is an ESC, ascii 033) that has been entered when processing a KEYBD trap. If the value is changed as part of the trap action, then the new value replaces the key (or key sequence) that caused the trap. 🔥
.sh.edcol — The character position of the cursor at the time of the most recent KEYBD trap.
.sh.edmode — The value is set to ESC when processing a KEYBD trap while in vi insert mode. (See Vi Editing Mode below.) Otherwise, .sh.edmode is null when processing a KEYBD trap.
.sh.edtext — The characters in the input buffer at the time of the most recent KEYBD trap. The value is null when not processing a KEYBD trap.
.sh.name — Set to the name of the variable at the time a set or get discipline is invoked.
.sh.subscript — Set to the name subscript of the variable at the time a set or get discipline is invoked.
.sh.value — Set to the value of the variable at the time a set discipline.
.sh.version — Set to a value that identifies the version of this shell. 👑
LINENO — The line number of the current line within the script or function being executed.
OLDPWD — The previous working directory set by the cd command.
OPTARG — The value of the last option argument processed by the getopts built-in command.
OPTIND — The index of the last option argument processed by the getopts built-in command.
PPID — The process number of the parent of the shell.
PWD — The present working directory set by the cd command.
RANDOM — Each time this variable is referenced, a random integer, uniformly distributed between 0 and 32767, is generated. ✨ The sequence of random numbers can be initialized by assigning a numeric value to RANDOM.
REPLY — This variable is set by the select statement and by the read built-in command when no arguments are supplied.
SECONDS — Each time this variable is referenced, the number of seconds since shell invocation is returned. If this variable is assigned a value, then the value returned upon reference will be the value that was assigned plus the number of seconds since the assignment. time math, we love her ⏰

🦋 The variables the shell actually uses 🦋

The following variables are used by the shell:

CDPATH — The search path for the cd command.
COLUMNS — If this variable is set, the value is used to define the width of the edit window for the shell edit modes and for printing select lists.
EDITOR — If the value of this variable ends in emacs, gmacs, or vi and the VISUAL variable is not set, then the corresponding option (see Special Command set below) will be turned on. 👀
ENV — If this variable is set, then parameter expansion, command substitution, and arithmetic substitution are performed on the value to generate the pathname of the script that will be executed when the shell is invoked. (See Invocation below.) This file is typically used for alias and function definitions.
FCEDIT — Obsolete name for the default editor name for the hist command. FCEDIT is not used when HISTEDIT is set. (retired but still hanging around, we respect the elder 💀)
FIGNORE — A pattern that defines the set of filenames that will be ignored when performing filename matching.
FPATH — The search path for function definitions. This path is searched when a function with the -u attribute is referenced and when a command is not found. If an executable file is found, then it is read and executed in the current environment.
IFS — Internal field separators, normally space, tab, and new-line, that are used to separate the results of command substitution or parameter expansion and to separate fields with the built-in command read. The first character of the IFS variable is used to separate arguments for the "$*" substitution. (See Quoting below.) Each single occurrence of an IFS character in the string to be split — except space, tab, and new-line — separates a field. One or more space, tab, or new-line characters separate a field. 🌈
HISTEDIT — Name for the default editor name for the hist command.
HISTFILE — If this variable is set when the shell is invoked, then the value is the pathname of the file that will be used to store the command history. (See Command re-entry below.)
HISTSIZE — If this variable is set when the shell is invoked, then the number of previously entered commands that are accessible by this shell will be greater than or equal to this number. The default is 128.
HOME — The default argument (home directory) for the cd command. 🏠
LINES — If this variable is set, the value is used to determine the column length for printing select lists. select lists will print vertically until about two-thirds of LINES lines are filled.
MAIL — If this variable is set to the name of a mail file and the MAILPATH variable is not set, then the shell informs the user of arrival of mail in the specified file.
MAILCHECK — This variable specifies how often (in seconds) the shell will check for changes in the modification time of any of the files specified by the MAILPATH or MAIL variables. The default value is 600 seconds. When the time has elapsed the shell will check before issuing the next prompt.
MAILPATH — A colon ( : ) separated list of file names. If this variable is set then the shell informs the user of any modifications to the specified files that have occurred within the last MAILCHECK seconds. Each file name can be followed by a ? and a message that will be printed. The message will undergo parameter expansion, command substitution, and arithmetic substitution with the variable $_ defined as the name of the file that has changed. The default message is you have mail in $_. 📬
PATH — The search path for commands (see Execution below). The user may not change PATH if executing under rksh (except in .profile). ⚠️ like, actually not allowed, don't try it
PS1 — The value of this variable is expanded for parameter expansion, command substitution, and arithmetic substitution to define the primary prompt string which by default is $. The character ! in the primary prompt string is replaced by the command number (see Command Re-entry below). Two successive occurrences of ! will produce a single ! when the prompt string is printed. the prompt is literally the main character ✨
PS2 — Secondary prompt string, by default > .
PS3 — Selection prompt string used within a select loop, by default #? .
PS4 — The value of this variable is expanded for parameter evaluation, command substitution, and arithmetic substitution and precedes each line of an execution trace. If omitted, the execution trace prompt is + . 🔍
SHELL — The pathname of the shell is kept in the environment. At invocation, if the basename of this variable is rsh, rksh, or krsh, then the shell becomes restricted. 💜
TMOUT — if you set this to anything greater than zero, the read built-in dips out after TMOUT seconds whenever it's reading from a terminal. And if you're not in read? Then the whole shell just 💀 terminates if you don't enter a line within that many seconds while reading from a terminal. (Heads up: the shell can be compiled with a maximum bound for this value, and you literally cannot exceed it — it's giving hard cap energy.)
VISUAL — if the value of this variable ends in emacs, gmacs, or vi, then the corresponding option gets turned on (peep the Special Command set below for the deets).

The shell hands out default values to PATH, PS1, PS2, PS3, PS4, MAILCHECK, HISTEDIT, TMOUT and IFS — but HOME, SHELL, ENV, and MAIL are not set at all by the shell (though HOME gets set by login(1)). On some systems, MAIL and SHELL are also set by login(1).

🧩 Field Splitting

After parameter expansion and command substitution do their thing, the results of those substitutions get scanned for the field separator characters — aka the ones chilling in IFS — and split into distinct fields wherever those characters show up. Explicit null fields ("" or '') are kept. Implicit null fields (the ones that come from parameters with no values, or command substitutions with no output) get yeeted. 🙈

🌸 File Name Generation

Next up, after splitting, each field gets scanned for the characters *, ?, (, and [ — unless the -f option has been set. If one of those characters shows up, the word is treated as a pattern. Every file name component that contains any pattern character gets replaced with a lexicographically sorted set of names that match the pattern from that directory. And if no file name matches the pattern? That component of the filename just stays exactly as it was, no cap. If FIGNORE is set, then each file name component that matches the pattern defined by the value of FIGNORE is ignored while generating the matching filenames. The names . and .. are also ignored. If FIGNORE is not set, the character . at the start of each file name component gets ignored unless the first character of the pattern corresponding to that component is the character . itself. Note (and this is important bestie): for other uses of pattern matching, the / and . are not treated specially.

* — matches any string, including the null string.
? — matches any single character.
[...] — matches any one of the enclosed characters. A pair of characters separated by - matches any character lexically between the pair, inclusive. If the first character following the opening [ is a ! then any character not enclosed is matched. A - can be included in the character set by putting it as the first or last character. Within [ and ], character classes can be specified with the syntax [:class:] where class is one of the following:

alnum alpha cntrl digit graph lower print punct space upper xdigit

A pattern-list is a list of one or more patterns separated from each other with a |. Composite patterns can be formed with one or more of the following:

?(pattern-list) — optionally matches any one 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 exactly one of the given patterns.
!(pattern-list) — matches anything except one of the given patterns.

🎀 Quoting

Every one of the metacharacters listed earlier (see Definitions above) has a special meaning to the shell and causes termination of a word unless you quote it. A character can be quoted (i.e., made to stand for itself) by preceding it with a \. The pair \ new-line is removed. All characters enclosed between a pair of single quote marks (´´) that is not preceded by a $ are quoted. A single quote cannot appear within the single quotes — like, at all, that's the rule. A single quoted string preceded by an unquoted $ is processed as an ANSI-C string, except that a \ within the string causes the remainder of the string to be ignored, and \fP is equivalent to the escape character (ascii 033). Inside double quote marks (""), parameter and command substitution occur, and \fP quotes the characters \fP, `, ", and $. The meaning of $* and $@ is identical when not quoted, or when used as a variable assignment value, or as a file name. However — when used as a command argument — "$*" is equivalent to "$1d$2d...", where d is the first character of the IFS variable, whereas "$@" is equivalent to "$1" "$2" .... Inside grave quote marks ( ), `\fP` quotes the characters `\fP`, ``, and$. If the grave quotes occur within double quotes, then \fPalso quotes the character"`.

The special meaning of reserved words or aliases can be removed by quoting any character of the reserved word. But the recognition of function names or built-in command names listed below cannot be altered by quoting them. Periodt. 💅

🔢 Arithmetic Evaluation

The shell does arithmetic evaluation for arithmetic substitution, to evaluate an arithmetic command, to evaluate an indexed array subscript, and to evaluate arguments to the built-in commands shift and let. Evaluations are performed using double precision floating point arithmetic. Floating point constants follow the ANSI-C programming language conventions. Integer constants are of the form [base#]n where base is a decimal number between two and sixty-four representing the arithmetic base, and n is a number in that base. The digits above 9 are represented by the lower case characters, the upper case characters, @, and _ respectively. For bases less than 36, upper and lower case characters can be used interchangeably. If base is omitted then base 10 is used. 🍓

An arithmetic expression uses the same syntax, precedence, and associativity of expression as the C language. All the C language operators that apply to floating point quantities can be used. In addition, when the value of an arithmetic variable or sub-expression can be represented as a long integer, all C language integer arithmetic operations can be performed. Variables can be referenced by name within an arithmetic expression without using the parameter expansion syntax. When a variable is referenced, its value is evaluated as an arithmetic expression. The following math library functions can be used with an arithmetic expression:

abs acos asin atan cos cosh exp int log sin sinh sqrt tan tanh

An internal representation of a variable as a double precision floating point can be specified with the -E [n] or -F [n] option of the typeset special built-in command. The -E option causes the expansion of the value to be represented using scientific notation when it is expanded — and the optional option argument n defines the number of significant figures. The -F option causes the expansion to be represented as a floating decimal number when it is expanded, and the optional option argument n defines the number of places after the decimal point in this case. ✨

An internal integer representation of a variable can be specified with the -i [n] option of the typeset special built-in command. The optional option argument n specifies an arithmetic base to be used when expanding the variable. If you do not specify an arithmetic base, the first assignment to the variable determines the arithmetic base.

Arithmetic evaluation is performed on the value of each assignment to a variable with the -E, -F, or -i attribute. And assigning a floating point number to a variable whose type is an integer causes the fractional part to be truncated — yes, it just gets chopped, sorry babes. 😭

💬 Prompting

When used interactively, the shell prompts with the value of PS1 after expanding it for parameter expansion, command substitution, and arithmetic substitution, before reading a command. In addition, each single ! in the prompt is replaced by the command number. A !! is required to place ! in the prompt. If at any time a new-line is typed and further input is needed to complete a command, then the secondary prompt (i.e., the value of PS2) is issued. 🫶

🦋 Conditional Expressions

A conditional expression is used with the [[ compound command to test attributes of files and to compare strings. Field splitting and file name generation are not performed on the words between [[ and ]]. Each expression can be constructed from one or more of the following unary or binary expressions:

string — true, if string is not null.
-a file — same is -e below. This is obsolete. ⚠️
-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 an ordinary file.
-g file — true, if file exists and is has its setgid bit set.
-k file — true, if file exists and is has its sticky bit set.
-n string — true, if length of string is non-zero.
-o option — true, if option named option is on.
-p file — true, if file exists and is a fifo special file or a pipe.
-r file — true, if file exists and is readable by current process.
-s file — true, if file exists and has size greater than zero.
-t fildes — true, if file descriptor number fildes is open and associated with a terminal device.
-u file — true, if file exists and is has its setuid bit set.
-w file — true, if file exists and is writable by current process.
-x file — true, if file exists and is executable by current process. If file exists and is a directory, then true if the current process has permission to search in the directory.
-z string — true, if length of string is zero.
-L file — true, if file exists and is a symbolic link.
-O file — true, if file exists and is owned by the effective user id of this process.
-G file — true, if file exists and its group matches the effective group id of this process.
-S file — true, if file exists and is a socket.
file1 -nt file2 — true, if file1 exists and is newer than file2.
file1 -ot file2 — true, if file1 exists and is older than file2.
file1 -ef file2 — true, if file1 and file2 exist and refer to the same file.
string == pattern — true, if string matches pattern. Any part of pattern can be quoted to cause it to be matched as a string.
string = pattern — same as == above, but is obsolete. ⚠️
string != pattern — true, if string does not match pattern.
string1 &< string2 — true, if string1 comes before string2 based on ASCII value of their characters.
string1 > string2 — true, if string1 comes after string2 based on ASCII value of their characters.

okay so real talk, there's also a whole little family of obsolete arithmetic comparisons that are still, like, allowed to exist (they're giving legacy but make them iconic):

exp1 -eq exp2 — true if exp1 is equal to exp2. periodt.
exp1 -ne exp2 — true if exp1 is not equal to exp2.
exp1 -lt exp2 — true if exp1 is less than exp2.
exp1 -gt exp2 — true if exp1 is greater than exp2.
exp1 -le exp2 — true if exp1 is less than or equal to exp2.
exp1 -ge exp2 — true if exp1 is greater than or equal to exp2.

okay and this part is lowkey important bestie: in each of the expressions above, if file is of the form /dev/fd/n, where n is an integer, then the test is applied to the open file whose descriptor number is n. no cap, that's the whole vibe. 🫶

You can also build a compound expression out of these primitives using any of the following, and they're listed in decreasing order of precedence (so the top ones bind tighter, iykyk):

(expression) — true if expression is true. basically just here to group expressions together ✨
! expression — true if expression is false. the hater of the group.
expression1 && expression2 — true if expression1 and expression2 are both true.
expression1 || expression2 — true if either expression1 or expression2 is true.

💾 Input/Output 💾

So before a command even gets executed, its input and output can be redirected using a special notation that the shell interprets itself. The following can appear anywhere in a simple-command, or before/after a command, and they are not passed on to the invoked command. Command substitution, parameter expansion, and arithmetic substitution all happen before word or digit is used, except as noted below. File name generation only happens if the shell is interactive and the pattern matches a single file, and field splitting is not performed. slay, let's go through them 👇

< word — use file word as standard input (file descriptor 0).
> word — use file word as standard output (file descriptor 1). If the file does not exist then it's created. If the file exists and the noclobber option is on, this causes an error; otherwise, it gets truncated to zero length. (yes, RIP to your file's contents, be careful 💀)
>\| word — same as >, except that it overrides the noclobber option. main character energy, ignores the rules.
>> word — use file word as standard output. If the file exists then output is appended to it (by first seeking to the end-of-file); otherwise, the file is created.
<> word — open file word for reading and writing as standard input.
<<[-]word — the shell input is read up to a line that is the same as word after any quoting has been removed, or to an end-of-file. No parameter substitution, command substitution, arithmetic substitution or file name generation is performed on word. The resulting document, called a here-document, becomes the standard input. If any character of word is quoted, then no interpretation is placed upon the characters of the document; otherwise, parameter expansion, command substitution, and arithmetic substitution occur, a \newline is ignored, and \ must be used to quote the characters \, $, and `. If - is appended to <<, then all leading tabs are stripped from word and from the document. (here-docs are literally the bestie of every shell script, obsessed 🎀)
<& digit — the standard input is duplicated from file descriptor digit (see dup(2)). Similarly for the standard output using >&digit.
<&- — the standard input is closed. Similarly for the standard output using >&-.
<&p — the input from the co-process is moved to standard input.
>&p — the output to the co-process is moved to standard output.

And if one of the above is preceded by a digit, then the file descriptor number referred to is the one that digit specifies (instead of the default 0 or 1). For example:

... 2>&1

means file descriptor 2 is to be opened for writing as a duplicate of file descriptor 1. okay that one's iconic, you've definitely seen it in a Stack Overflow answer before 💅

The order in which redirections are specified is significant, bestie — the shell evaluates each redirection in terms of the (file descriptor, file) association at the time of evaluation. For example:

... 1>fname 2>&1

first associates file descriptor 1 with file fname. It then associates file descriptor 2 with the file associated with file descriptor 1 (i.e. fname). If the order of redirections were reversed, file descriptor 2 would be associated with the terminal (assuming file descriptor 1 had been) and then file descriptor 1 would be associated with file fname. order matters, always has, always will 🔥

If a command is followed by & and job control is not active, then the default standard input for the command is the empty file /dev/null. Otherwise, the environment for the execution of a command contains the file descriptors of the invoking shell as modified by input/output specifications.

🌍 Environment 🌍

The environment (see environ(7)) is a list of name-value pairs that gets passed to an executed program in the same way as a normal argument list. The names must be identifiers and the values are character strings. The shell interacts with the environment in several ways. On invocation, the shell scans the environment and creates a variable for each name found, giving it the corresponding value and attributes and marking it export. Executed commands inherit the environment. If you modify the values of these variables or create new ones using the export or typeset -x commands, they become part of the environment. The environment seen by any executed command is thus composed of any name-value pairs originally inherited by the shell, whose values may be modified by the current shell, plus any additions which must be noted in export or typeset -x commands. it's giving inheritance hierarchy but make it shell 💖

The environment for any simple-command or function may be augmented by prefixing it with one or more variable assignments. A variable assignment argument is a word of the form identifier=value. So:

TERM=450 cmd args

and

(export TERM; TERM=450; cmd args)

are equivalent (as far as the above execution of cmd is concerned, except for special built-in commands listed below — those that are preceded with a dagger 🗡️).

If the obsolete -k option is set, all variable assignment arguments are placed in the environment, even if they occur after the command name. The following first prints a=b c and then c:

echo a=b c
set -k
echo a=b c

⚠️ bestie this feature is intended for use with scripts written for early versions of the shell, and its use in new scripts is strongly discouraged. It is likely to disappear someday. like, don't build your whole personality on it. 😭

🧩 Functions 🧩

For historical reasons, there are two ways to define functions: the name() syntax and the function name syntax, both described in the Commands section above. Shell functions are read in and stored internally. Alias names are resolved when the function is read. Functions are executed like commands with the arguments passed as positional parameters. (See Execution below.)

Functions defined by the function name syntax and called by name execute in the same process as the caller and share all files and present working directory with the caller. Traps caught by the caller are reset to their default action inside the function. A trap condition that is not caught or ignored by the function causes the function to terminate and the condition to be passed on to the caller. A trap on EXIT set inside a function is executed after the function completes, in the environment of the caller. Ordinarily, variables are shared between the calling program and the function. However, the typeset special built-in command used within a function defines local variables whose scope includes the current function and all functions it calls. Errors within functions return control to the caller. it's giving shared apartment but with your own locked room 🚪✨

Functions defined with the name() syntax and functions defined with the function name syntax that are invoked with the . special built-in are executed in the caller's environment and share all variables and traps with the caller. Errors within these function executions cause the script that contains them to abort. no soft landings, we die like scripts 💀

The special built-in command return is used to return from function calls.

Function names can be listed with the -f or +f option of the typeset special built-in command. The text of functions, when available, will also be listed with -f. Functions can be undefined with the -f option of the unset special built-in command.

Ordinarily, functions are unset when the shell executes a shell script. Functions that need to be defined across separate invocations of the shell should be placed in a directory and the FPATH variable should contain the name of this directory. They may also be specified in the ENV file. (yes, FPATH is the autoload plug, she's iconic 👑)

🎛️ Jobs 🎛️

If the monitor option of the set command is turned on, an interactive shell associates a job with each pipeline. It keeps a table of current jobs, printed by the jobs command, and assigns them small integer numbers. When a job is started asynchronously with &, the shell prints a line which looks like:

[1] 1234

indicating that the job which was started asynchronously was job number 1 and had one (top-level) process, whose process id was 1234. cute little receipt for your background job 🧾

heads up bestie: this paragraph and the next require features that are not in all versions of UNIX and may not apply. so like, vibe check your OS first 🦋

If you are running a job and wish to do something else, you may hit the key ^Z (Ctrl+Z) which sends a STOP signal to the current job. The shell will then normally indicate that the job has been Stopped, and print another prompt. You can then manipulate the state of this job, putting it in the background with the bg command, or run some other commands and then eventually bring the job back into the foreground with the foreground command fg. A ^Z takes effect immediately and is like an interrupt in that pending output and unread input are discarded when it is typed. gone, reduced to atoms 😭

A job being run in the background will stop if it tries to read from the terminal. Background jobs are normally allowed to produce output, but this can be disabled by giving the command stty tostop. If you set this tty option, then background jobs will stop when they try to produce output like they do when they try to read input. rude? maybe. effective? absolutely. 💅

There are several ways to refer to jobs in the shell. A job can be referred to by the process id of any process of the job or by one of the following:

% number — the job with the given number.
% string — any job whose command line begins with string.
%? string — any job whose command line contains string.
%% — current job.
%+ — equivalent to %%.
%- — previous job.

The shell picks up on job state changes immediately — like, the second something shifts, she knows. Normally she'll tell you when a job gets blocked so no further progress is possible, but only right before she prints a prompt, so she doesn't interrupt your whole flow. That's the whole point. But if the notify option is on, she prints those job change messages the instant they happen instead. Instant updates, no waiting. 💅

When the monitor option is on, every background job that completes triggers any trap you've set for CHLD.

Now, the dramatic part: if you try to dip out of the shell while jobs are still running or stopped, she will warn you with You have stopped(running) jobs. Use the jobs command to see what's out there. If you immediately try to exit again, she won't warn you twice — the stopped jobs just get terminated. 😭 And when a login shell catches a HUP signal, it sends a HUP signal to every job that hasn't been disowned with the disown built-in command described below.

🚦 Signals. 🚦

The INT and QUIT signals for an invoked command get ignored if the command is followed by && and the monitor option is not active. Otherwise? Signals just have the values inherited by the shell from its parent — but also peep the trap built-in command below. ✨

⚙️ Execution. ⚙️

Every single time a command is read, all those substitutions up above get carried out. Then, if the command name matches one of the Special built-in Commands listed below, it runs inside the current shell process. Next up: the command name gets checked against user defined functions. If it matches, the positional parameters are saved and then reset to the arguments of the function call. When the function completes or issues a return, the positional parameter list gets restored. For functions defined with the function name syntax, any trap set on EXIT within the function is executed. The value of a function is the value of the last command executed. A function also runs in the current shell process. And if a command name isn't a special built-in command or a user defined function, but it is one of the built-in commands listed below, it runs in the current shell process too. 🫶

The shell variable PATH defines the search path for the directory containing the command. Alternative directory names are separated by a colon (:). The default path is /bin:/usr/bin: — which specifies /bin, /usr/bin, and the current directory, in that order. The current directory can be specified by two or more adjacent colons, or by a colon at the beginning or end of the path list. If the command name contains a / then the search path isn't used at all. Otherwise, each directory in the path is searched for an executable file that is not a directory. If the shell figures out there's a built-in version of a command corresponding to a given pathname, that built-in is invoked in the current process. A process is created and an attempt is made to execute the command via exec(2). If the file has execute permission but is not an a.out file, it's assumed to be a file containing shell commands — a separate shell is spawned to read it. All non-exported variables are removed in this case. If the shell command file doesn't have read permission, or if the setuid and/or setgid bits are set on the file, then the shell executes an agent whose job it is to set up the permissions and execute the shell with the shell command file passed down as an open file. A parenthesized command is executed in a sub-shell without removing non-exported variables.

🕰️ Command Re-entry. 🕰️

The text of the last HISTSIZE commands entered from a terminal device is saved in a history file. (HISTSIZE defaults to 128, bestie.) The file $HOME/.sh_history is used if the HISTFILE variable isn't set, or if the file it names isn't writable. A shell can access the commands of all interactive shells that use the same named HISTFILE. The built-in command hist is used to list or edit a portion of this file. The portion to be edited or listed can be selected by number, or by giving the first character or characters of the command. A single command or range of commands can be specified. If you don't specify an editor program as an argument to hist, then the value of the variable HISTEDIT is used. If HISTEDIT is unset, the obsolete variable FCEDIT is used. If FCEDIT isn't defined either, then /bin/ed is used. The edited command(s) is printed and re-executed upon leaving the editor — unless you quit without writing, sneaky. 😏

The -s option (and, in obsolete versions, the editor name -) is used to skip the editing phase and just re-execute the command. In this case a substitution parameter of the form old=new can be used to modify the command before execution. For example, with the preset alias r, which is aliased to hist -s, typing r bad=good c will re-execute the most recent command which starts with the letter c, replacing the first occurrence of the string bad with the string good. Obsessed with this trick, ngl. 🔥

✏️ In-line Editing Options ✏️

Normally, each command line entered from a terminal device is simply typed followed by a new-line (RETURN or LINE FEED). But if either the emacs, gmacs, or vi option is active, you can edit the command line, bestie. To be in any of these edit modes, just set the corresponding option. An editing option is automatically selected each time the VISUAL or EDITOR variable is assigned a value ending in either of these option names. ✨

Heads up! The editing features require that your terminal accept RETURN as carriage return without line feed, and that a space ( ) must overwrite the current character on the screen.

The editing modes implement a concept where you're looking through a window at the current line. The window width is the value of COLUMNS if it's defined, otherwise 80. If the window width is too small to display the prompt and leave at least 8 columns to enter input, the prompt gets truncated from the left. If the line is longer than the window width minus two, a mark is displayed at the end of the window to notify you. As the cursor moves and reaches the window boundaries, the window gets centered about the cursor. That mark is a > (<, *) if the line extends on the right (left, both) side(s) of the window.

The search commands in each edit mode give you access to the history file. Only strings are matched, not patterns — although a leading ^ in the string restricts the match to begin at the first character in the line. 👑

Each of the edit modes has an operation to list the files or commands that match a partially entered word. When applied to the first word on the line, or the first word after a ;, |, &&, or (, and the word doesn't begin with ~ or contain a /, the list of aliases, functions, and executable commands defined by the PATH variable that could match the partial word is displayed. Otherwise, the list of files that match the given word is displayed. If the partially entered word doesn't contain any file expansion characters, a * is appended before generating these lists. After displaying the generated list, the input line is redrawn. These operations are called command name listing and file name listing, respectively. 🎀

There are also additional operations, referred to as command name completion and file name completion, which compute the list of matching commands or files — but instead of printing the list, they replace the current word with a complete or partial match. For file name completion, if the match is unique, a / is appended if the file is a directory, and a space is appended if the file is not a directory. Otherwise, the longest common prefix for all the matching files replaces the word. For command name completion, only the portion of the file names after the last / are used to find the longest command prefix. If only a single name matches this prefix, then the word is replaced with the command name followed by a space. It's giving efficiency. 💅

⌨️ Key Bindings. ⌨️

The KEYBD trap can be used to intercept keys as they're typed and change the characters that are actually seen by the shell. This trap is executed after each character (or sequence of characters when the first character is ESC) is entered while reading from a terminal. The variable .sh.edchar contains the character or character sequence which generated the trap. Changing the value of .sh.edchar in the trap action causes the shell to behave as if the new value were entered from the keyboard rather than the original value. 🪄

The variable .sh.edcol is set to the input column number of the cursor at the time of the input. The variable .sh.edmode is set to ESC when in vi insert mode (see below) and is null otherwise. By prepending ${.sh.editmode} to a value assigned to .sh.edchar it will cause the shell to change to control mode if it isn't already in this mode.

Real talk: this trap is not invoked for characters entered as arguments to editing directives, or while reading input for a character search. 🙈

💅 Emacs Editing Mode. 💅

This mode is entered by enabling either the emacs or gmacs option. The only difference between these two modes is the way they handle ^T. To edit, you move the cursor to the point needing correction and then insert or delete characters or words as needed. All the editing commands are control characters or escape sequences. The notation for control characters is caret (^) followed by the character. For example, ^F is the notation for control F. This is entered by depressing f while holding down the CTRL (control) key. The SHIFT key is not depressed. (The notation ^? indicates the DEL (delete) key.)

The notation for escape sequences is M- followed by a character. For example, M-f (pronounced Meta f) is entered by depressing ESC (ascii 033) followed by f. (M-F would be the notation for ESC followed by SHIFT (capital) F.)

All edit commands operate from any place on the line (not just at the beginning). Neither the RETURN nor the LINE FEED key is entered after edit commands except when noted. ⭐

^F — Move cursor forward (right) one character.
M-f — Move cursor forward one word. (The emacs editor's idea of a word is a string of characters consisting of only letters, digits and underscores.)
^B — Move cursor backward (left) one character.
M-b — Move cursor backward one word.
^A — Move cursor to start of line.
^E — Move cursor to end of line.
^] char — Move cursor forward to character char on current line.
M-^] char — Move cursor backward to character char on current line.
^X^X — Interchange the cursor and mark.
erase — (User defined erase character as defined by the stty(1) command, usually ^H or #.) Delete previous character.
^D — Delete current character.
M-d — Delete current word.
M-^H — (Meta-backspace) Delete previous word.
M-h — Delete previous word.
M-⁁? — (Meta-DEL) deletes the previous word. Heads up, bestie: if your interrupt character is ⁁? (DEL, the default), then this command will not work 😭.
⁁T — Transposes the current character with the next character in emacs mode. In gmacs mode it transposes the two previous characters ✨.
⁁C — Capitalizes the current character 👑.
M-c — Capitalizes the current word 👑.
M-l — Changes the current word to lower case 🔡.
⁁K — Deletes from the cursor to the end of the line. If it's preceded by a numerical parameter whose value is less than the current cursor position, then it deletes from the given position up to the cursor. If it's preceded by a numerical parameter whose value is greater than the current cursor position, then it deletes from the cursor up to the given cursor position 💅.
⁁W — Kills from the cursor to the mark 💀.
M-p — Pushes the region from the cursor to the mark on the stack 🫶.
kill — (User defined kill character as defined by the stty command, usually ⁁G or @.) Kills the entire current line. If two kill characters are entered in succession, all kill characters from then on cause a line feed — useful when using paper terminals 🌸.
⁁Y — Restores the last item removed from the line. (Yank item back to the line.) 🪄
⁁L — Line feed and print current line 🖨️.
⁁@ — (Null character) Sets mark 📍.
M- space — (Meta space) Sets mark 📍.
⁁J — (New line) Executes the current line ▶️.
⁁M — (Return) Executes the current line ▶️.
eof — End-of-file character, normally ⁁D, is processed as an End-of-file only if the current line is null 🫠.
⁁P — Fetches the previous command. Each time ⁁P is entered, the previous command back in time is accessed. Moves back one line when not on the first line of a multi-line command ⏪.
M-&< — Fetches the least recent (oldest) history line ⏮️.
M-> — Fetches the most recent (youngest) history line ⏭️.
⁁N — Fetches the next command line. Each time ⁁N is entered, the next command line forward in time is accessed ⏩.
⁁R string — Reverse searches history for a previous command line containing string. If a parameter of zero is given, the search is forward. String is terminated by a "RETURN" or "NEW LINE". If string is preceded by a ⁁, the matched line must begin with string. If string is omitted, then the next command line containing the most recent string is accessed. In this case a parameter of zero reverses the direction of the search 🔎.
⁁O — Operate — executes the current line and fetches the next line relative to the current line from the history file 🎬.
M- digits — (Escape) Defines a numeric parameter: the digits are taken as a parameter to the next command. The commands that accept a parameter are ⁁F, ⁁B, erase, ⁁C, ⁁D, ⁁K, ⁁R, ⁁P, ⁁N, ⁁], M-., M-⁁], M-_, M-b, M-c, M-d, M-f, M-h, M-l and M-⁁H 🔢.
M- letter — Soft-key — your alias list is searched for an alias by the name _letter, and if an alias of this name is defined, its value will be inserted on the input queue. The letter must not be one of the above meta-functions ✨.
M-[ letter — Soft-key — your alias list is searched for an alias by the name __letter, and if an alias of this name is defined, its value will be inserted on the input queue. This can be used to program function keys on many terminals 🎹.
M-. — The last word of the previous command is inserted on the line. If preceded by a numeric parameter, the value of this parameter determines which word to insert rather than the last word 🪄.
M-_ — Same as M-. 💫.
M-* — Attempts file name generation on the current word. An asterisk is appended if the word doesn't match any file or contain any special pattern characters 🌟.
Command or file name completion — as described above ✨.
M-= — Command or file name listing as described above 📋.
⁁U — Multiplies the parameter of the next command by 4 ✖️.
Escape next character. — Editing characters, the user's erase, kill and interrupt (normally ⁁?) characters may be entered in a command line or in a search string if preceded by a \fP. The \fP removes the next character's editing features (if any) 🫶.
⁁V — Displays the version of the shell 🏷️.
M-# — If the line does not begin with a #, a # is inserted at the beginning of the line and after each new-line, and the line is entered. This causes a comment to be inserted in the history file. If the line begins with a #, the # is deleted and one # after each new-line is also deleted 💬.

🌀 Vi Editing Mode 💅

There are two typing modes. Initially, when you enter a command you are in the input mode. To edit, the user enters control mode by typing ESC (033) and moves the cursor to the point needing correction, then inserts or deletes characters or words as needed. Most control commands accept an optional repeat count prior to the command 😌.

When in vi mode on most systems, canonical processing is initially enabled and the command will be echoed again if the speed is 1200 baud or greater and it contains any control characters or less than one second has elapsed since the prompt was printed. The ESC character terminates canonical processing for the remainder of the command and the user can then modify the command line. This scheme has the advantages of canonical processing with the type-ahead echoing of raw mode 🦋.

If the option viraw is also set, the terminal will always have canonical processing disabled. This mode is implicit for systems that do not support two alternate end of line delimiters, and may be helpful for certain terminals ⚙️.

⌨️ Input Edit Commands ✨

By default the editor is in input mode 💖.

erase — (User defined erase character as defined by the stty command, usually ⁁H or #.) Deletes the previous character ✂️.
⁁W — Deletes the previous blank separated word. On some systems the viraw option may be required for this to work 🧹.
eof — As the first character of the line, causes the shell to terminate unless the ignoreeof option is set. Otherwise this character is ignored 🫠.
⁁V — Escapes the next character. Editing characters and the user's erase or kill characters may be entered in a command line or in a search string if preceded by a ⁁V. The ⁁V removes the next character's editing features (if any). On some systems the viraw option may be required for this to work 🛡️.
Escape the next erase or kill character. — exactly that vibe 🫶.

🧭 Motion Edit Commands 🔥

These commands will move the cursor 🕹️.

[count]l — Cursor forward (right) one character ➡️.
[count]w — Cursor forward one alpha-numeric word 🚶‍♀️.
[count]W — Cursor to the beginning of the next word that follows a blank ⏭️.
[count]e — Cursor to end of word 🏁.
[count]E — Cursor to end of the current blank delimited word 🏁.
[count]h — Cursor backward (left) one character ⬅️.
[count]b — Cursor backward one word 🚶‍♀️.
[count]B — Cursor to preceding blank separated word ⏮️.
[count]| — Cursor to column count 📐.
[count]fc — Find the next character c in the current line 🔎.
[count]Fc — Find the previous character c in the current line 🔍.
[count]tc — Equivalent to f followed by h 💫.
[count]Tc — Equivalent to F followed by l 💫.
[count]; — Repeats count times, the last single character find command: f, F, t, or T 🔁.
[count], — Reverses the last single character find command count times 🔄.
0 — Cursor to start of line 🏠.
⁁ — Cursor to first non-blank character in line ✨.
$ — Cursor to end of line 🏁.
% — Moves to balancing (, ), {, }, [, or ]. If the cursor is not on one of the above characters, the remainder of the line is searched for the first occurrence of one of the above characters first 🧩.

🔎 Search Edit Commands 🌈

These commands access your command history 📚.

[count]k — Fetches the previous command. Each time k is entered the previous command back in time is accessed ⏪.
[count]- — Equivalent to k 💫.
[count]j — Fetches the next command. Each time j is entered the next command forward in time is accessed ⏩.
[count]+ — Equivalent to j 💫.
[count]G — The command number count is fetched. The default is the least recent history command ⏮️.
/ string — Searches backward through history for a previous command containing string. String is terminated by a "RETURN" or "NEW LINE". If string is preceded by a ⁁, the matched line must begin with string. If string is null the previous string will be used 🔍.
? string — Same as / except that the search will be in the forward direction 🔎.
n — Searches for the next match of the last pattern to / or ? commands ⏭️.
N — Searches for the next match of the last pattern to / or ?, but in the reverse direction 🔄.

✂️ Text Modification Edit Commands 💅

These commands will modify the line 🛠️.

a — Enters input mode and enters text after the current character ✍️.
A — Appends text to the end of the line. Equivalent to $a 💫.
[count]cmotion / c[count]motion — Deletes the current character through the character that motion would move the cursor to and enters input mode. If motion is c, the entire line will be deleted and input mode entered 💀.
C — Deletes the current character through the end of line and enters input mode. Equivalent to c$ ✂️.
S — Equivalent to cc 💫.
D — Deletes the current character through the end of line. Equivalent to d$ ✂️.
[count]dmotion / d[count]motion — Deletes the current character through the character that motion would move to. If motion is d, the entire line will be deleted 💀.
i — Enters input mode and inserts text before the current character ✍️.
I — Inserts text before the beginning of the line. Equivalent to 0i 💫.
[count]P — Places the previous text modification before the cursor ⬅️.
[count]p — Places the previous text modification after the cursor ➡️.
R — Enters input mode and replaces characters on the screen with characters you type, overlay fashion 🎨.
[count]rc — Replaces the count character(s) starting at the current cursor position with c, and advances the cursor 🔁.
[count]x — Deletes current character ❌.
[count]X — Deletes preceding character ❌.
[count]. — Repeats the previous text modification command 🔂.
[count]≈ — Inverts the case of the count character(s) starting at the current cursor position and advances the cursor 🔠.
[count]_ — appends the count-th word of the previous command and drops you into input mode 💫 if you leave out count, it just grabs the last word, iykyk.
* — sticks an * onto the current word and tries file name generation ✨ if nothing matches it rings the bell (rude). otherwise the word gets replaced by the matching pattern and you're in input mode.
- Command or file name completion as described above.

🎀 Other Edit Commands 🎀

Miscellaneous commands, aka the little extras that live rent-free in your muscle memory 💅

[count]ymotion
y[count]motion — yanks from the current character through the character that motion would move the cursor to, and puts them into the delete buffer 🫶 the text and the cursor stay exactly where they were, no cap.
Y — yanks from the current position to the end of the line. literally equivalent to y$ 👑
u — undo the last text modifying command 😭 bless.
U — undo all the text modifying commands performed on the line 💀 the full factory reset, bestie.
[count]v — returns the command hist -e ${VISUAL:-${EDITOR:-vi}} count in the input buffer 🦋 if count is omitted, then the current line is used.
⁁L — line feed and print current line. only does anything in control mode, so don't come for it if nothing happens 🙈
⁁J — (New line) execute the current line, regardless of mode. no thoughts, just send it 🔥
⁁M — (Return) execute the current line, regardless of mode. same energy, different key ⭐
# — if the first character of the command is a #, then this command deletes this # and each # that follows a newline. otherwise, it sends the line after inserting a # in front of each line in the command 💅 super useful for causing the current line to be inserted in the history as a comment, and for uncommenting previously commented commands in the history file.
= — command or file name listing as described above ✨
@ letter — your alias list is searched for an alias by the name _letter and if an alias of this name is defined, its value will be inserted on the input queue for processing 🌸 cute little indirection moment.

👑 Built-in Commands. 👑

The following simple-commands are executed in the shell process 🐚 Input/Output redirection is permitted. Unless otherwise indicated, the output is written on file descriptor 1 and the exit status, when there is no syntax error, is zero.

Except for :, true, false, echo, command, newgrp, and login, all built-in commands accept -- to indicate end of options. They also interpret the option -? as a help request and print a usage message on standard error 😌 very demure, very mindful.

Commands that are preceded by one or two - are special built-in commands and are treated specially in the following ways 🚨:

Variable assignment lists preceding the command remain in effect when the command completes.
I/O redirections are processed after variable assignments.
Errors cause a script that contains them to abort.
They are not valid function names.
Words, following a command preceded by -- that are in the format of a variable assignment, are expanded with the same rules as a variable assignment. This means that tilde substitution is performed after the = sign and field splitting and file name generation are not performed.

-: arg ... — the command only expands parameters. that's it, that's the whole job 💫
- . name arg ... — if name is a function defined with the function name reserved word syntax, the function is executed in the current environment (as if it had been defined with the name() syntax 🙈). Otherwise if name refers to a file, the file is read in its entirety and the commands are executed in the current shell environment.

The search path specified by PATH is used to find the directory containing file. If any arguments arg are given, they become the positional parameters while processing the . command and are restored upon completion 🎀 Otherwise the positional parameters are unchanged. The exit status is the exit status of the last command executed, periodt.

-- alias -ptx name=value ... — alias with no arguments prints the list of aliases in the form name=value on standard output 💅 The -p option causes the word alias to be inserted before each one. When one or more arguments are given an alias is defined for each name whose value is given.

A trailing space in value causes the next word to be checked for alias substitution 👀 The obsolete -t option is used to set and list tracked aliases. The value of a tracked alias is the full pathname corresponding to the given name. The value becomes undefined when the value of PATH is reset but the alias remains tracked.

Without the -t option, for each name in the argument list for which no value is given, the name and value of the alias is printed. The obsolete -x option has no effect 💀 The exit status is non-zero if a name is given, but no value, and no alias has been defined for the name.

bg job... — this command is only on systems that support job control 🧚 Puts each specified job into the background. The current job is put in the background if job is not specified. See Jobs for a description of the format of job.
- break n — exit from the enclosing for, while, until, or select loop, if any. If n is specified then break n levels 🔥 go touch grass, loop.
builtin -ds -f file name ... — if name is not specified, the built-ins are printed on standard output. The -s option prints only the special built-ins ✨ Otherwise, each name represents the pathname whose basename is the name of the built-in.

The entry point function name is determined by prepending b_ to the built-in name. Special built-ins cannot be bound to a pathname or deleted 🚫 The -d option deletes each of the given built-ins.

On systems that support dynamic loading, the -f option names a shared library containing the code for built-ins 🦋 Once a library is loaded, its symbols become available for subsequent invocations of builtin. Multiple libraries can be specified with separate invocations of the builtin command. Libraries are searched in the reverse order in which they are specified 👀

Each command that is to be built-in must be written as a C function whose name is of the form b_funcName, where funcName is the name of the built-in to be added. This function has the same argument calling convention as main (i.e. argc and argv — a list of strings). The b_funcName parses and error-checks the incoming arguments and then calls the real funcName function 😌 very structured, we love a wrapper.

cd -LP arg
cd -LP old new — this command can be in either of two forms 🎀 In the first form it changes the current directory to arg. If arg is - the directory is changed to the previous directory. The shell variable HOME is the default arg. The variable PWD is set to the current directory.

The shell variable CDPATH defines the search path for the directory containing arg. Alternative directory names are separated by a colon (:). The default path is &<null> (specifying the current directory). Note that the current directory is specified by a null path name, which can appear immediately after the equal sign or between the colon delimiters anywhere else in the path list 🫶 If arg begins with a / then the search path is not used. Otherwise, each directory in the path is searched for arg.

The obsolete second form of cd substitutes the string new for the string old in the current directory name, PWD, and tries to change to this new directory 👑

By default, symbolic links are not followed when finding the directory name. This is equivalent to the -L option. The -P option causes symbolic links to be followed when determining the directory. The last instance of -L or -P determines which method is used 💅

The cd command may not be executed by rksh 🚫

command -pvV name arg ... — without the -v or -V options, command executes name with the arguments given by arg. The -p option causes a default path to be searched rather than the one defined by the value of PATH. Functions will not be searched for when finding name 🙈

In addition, if name refers to a special built-in, none of the special properties associated with the leading daggers will be honored. (For example, the predefined alias redirect='command exec' prevents a script from terminating when an invalid redirection is given 🔥.) With the -v option, command is equivalent to the built-in whence command described below. The -V option causes command to whence -v.

- continue n — resume the next iteration of the enclosing for, while, until, or select loop 🌀 If n is specified then resume at the n-th enclosing loop.
disown job... — causes the shell not to send a HUP signal to each given job, or all active jobs if job is omitted, when a login shell terminates 💖 protecting your babies from the hangup, we stan.
echo arg ... — when the first arg does not begin with a -, and none of the arguments contain a \ then echo prints each of its arguments separated by a space and terminated by a new-line 🌸 Otherwise, the behavior of echo is system dependent and print or printf described below should be used. See echo(1) for usage and description 👀
- eval arg ... — the arguments are read as input to the shell and the resulting command(s) executed 😭 the "I'm going to make it make sense at runtime" of it all.

🌟 `exec`

exec -c -a name arg ...

okay so if arg is given, the command built from those arguments runs in place of this shell — no new process gets created, it just straight-up replaces you, main character takeover energy 👑

-c — clears the environment first, before applying any variable assignments that came along with the exec invocation.
-a — makes name (instead of the first arg) become argv[0] for the new process.

Input/output arguments can appear here too, and they affect the current process, not some baby new one.

If arg is not given, the whole flex of this command is to modify file descriptors exactly as prescribed by the input/output redirection list. But bestie, listen: in that case, any file descriptor number greater than 2 that gets opened this way is closed when you invoke another program. 😭

🚪 `exit`

exit n

Makes the shell exit with the exit status n. The value that actually gets used is the least significant 8 bits of the status you specified. If n is omitted, the exit status is whatever the last command executed gave you.

An end-of-file will also make the shell exit — except for a shell that has the ignoreeof option turned on (see set below). 🫶

📤 `export`

export -p name=value ...

If name isn't given, the names and values of every variable carrying the export attribute get printed, with the values quoted in a manner that lets them be re-inputed later (iykyk, round-trip safe ✨).

-p — sticks the word export in front of each one.

Otherwise, the names you hand it get marked for automatic export to the environment of commands executed later. 💅

🎬 `fg`

fg job...

This command only exists on systems that support job control, so vibe check your OS first.

Each job you specify is brought to the foreground and waited for, in the order you specified them. If you don't specify any, the current job gets brought into the foreground. See Jobs for a description of the format of job. 🌸

🔍 `getconf`

getconf name pathname

Prints the current value of the configuration parameter given by name. That's it, that's the whole thing. 🎀

🎯 `getopts`

getopts optstring vname arg ...

Checks arg for legal options. If arg is omitted, the positional parameters get used instead.

An option argument begins with a + or a -. An option that doesn't begin with + or -, or the argument --, ends the options. optstring holds the letters getopts recognizes — and if a letter is followed by a :, that option is expected to have an argument. The options can be separated from the argument by blanks. 🦋

getopts drops the next option letter it finds into the variable vname every time it's invoked, with a + prepended when arg begins with a +. The index of the next arg lives in OPTIND. The option argument, if there is one, gets stored in OPTARG. 👑

A leading : in optstring makes getopts store the letter of an invalid option in OPTARG, and set vname to ? for an unknown option and to : when a required option is missing. Otherwise, getopts prints an error message instead. The exit status is non-zero when there are no more options.

Heads up, there is no way to specify any of the options :, +, -, ?, [, and ]. The option # can only be specified as the first option. 🙈

📜 `hist`

hist -e ename -nlr first last

hist -s old=new command

In the first form, a range of commands from first to last is selected from the last HISTSIZE commands that were typed at the terminal.

first and last may be given as a number or as a string. A string is used to locate the most recent command starting with that string. A negative number is used as an offset to the current command number.

If the -l option is selected, the commands get listed on standard output.
Otherwise, the editor program ename is invoked on a file containing these keyboard commands. If ename isn't supplied, the value of the variable HISTEDIT is used. If HISTEDIT isn't set, then FCEDIT (default /bin/ed) is used as the editor. ✨

When editing is complete, the edited command(s) is executed — but only if the changes have been saved.

If last isn't specified it gets set to first. If first isn't specified, the default is the previous command for editing and -16 for listing. The -r option reverses the order of the commands, and the -n option suppresses command numbers when listing.

In the second form, the command is re-executed after the substitution old=new is performed. 🎀

💼 `jobs`

jobs -lnp job ...

Lists info about each given job — or all active jobs if job is omitted.

-l — lists process ids in addition to the normal information.
-n — only displays jobs that have stopped or exited since you were last notified.
-p — causes only the process group to be listed.

See Jobs for a description of the format of job. 🌈

💀 `kill`

kill -s signame job ...

kill -n signum job ...

kill -l sig ...

Sends either the TERM (terminate) signal or the specified signal to the specified jobs or processes.

Signals are either given by number with the -n option, or by name with the -s option (as given in <signal.h>, stripped of the prefix SIG, with the exception that SIGCLD is named CHLD). For backward compatibility, the n and s can be omitted and the number or name placed immediately after the -.

If the signal being sent is TERM (terminate) or HUP (hangup), then the job or process gets sent a CONT (continue) signal if it's stopped. 🫶

The argument job can be the process id of a process that is not a member of one of the active jobs. See Jobs for a description of the format of job.

In the third form, kill -l: if sig isn't specified, the signal names are listed. Otherwise, for each sig that is a name, the corresponding signal number is listed; and for each sig that is a number, the signal name corresponding to the least significant 8 bits of sig is listed. 🦋

🧮 `let`

let arg ...

Each arg is a separate arithmetic expression to be evaluated. See Arithmetic Evaluation above for a description of arithmetic expression evaluation. ✨

The exit status is 0 if the value of the last expression is non-zero, and 1 otherwise. no cap, that's the whole rule 💅

👥 `newgrp`

newgrp arg ...

Equivalent to exec /bin/newgrp arg .... literally just that. 🎀

🖨️ `print`

print -Rnprs -u unit -f format arg ...

With no options, or with option - or --, each arg is printed on standard output.

The -f option causes the arguments to be printed as described by printf. In that case any n, r, R options are ignored. Otherwise, unless -R or -r are specified, the following escape conventions get applied:

\a — the alert character (ascii 07)
\b — the backspace character (ascii 010)
\c — causes print to end without processing more arguments and not adding a new-line
\f — the formfeed character (ascii 014)
\n — the new-line character (ascii 012)
\r — the carriage return character (ascii 015)
\t — the tab character (ascii 011)
\v — the vertical tab character (ascii 013)
\e — the escape character (ascii 033)
\\ — the backslash character

Now the actual flags:

-R — prints all subsequent arguments and options other than -n.
-p — causes the arguments to be written onto the pipe of the process spawned with |&& instead of standard output.
-s — causes the arguments to be written onto the history file instead of standard output.
-u — lets you specify a one digit file descriptor unit number unit on which the output will be placed. The default is 1.
-n — no new-line gets added to the output.

💖

🎨 `printf`

printf format arg ...

The arguments arg are printed on standard output in accordance with the ANSI-C formatting rules associated with the format string format. The following extensions can also be used:

A %b format can be used instead of %s to cause escape sequences in the corresponding arg to be expanded as described in print.
A %P format can be used instead of %s to cause arg to be interpreted as an extended regular expression and be printed as a shell pattern.
A %q format can be used instead of %s to cause the resulting string to be quoted in a manner than can be reinput to the shell.
The precision field of the %d format can be followed by a . and the output base.

obsessed with %q tbh, it's giving safety 🧚

📍 `pwd`

pwd -LP

Outputs the value of the current working directory.

-P — all symbolic links are resolved from the name.

⭐

🌸 `read` — the "bestie, tell me what you want" command

read -Aprs -d delim -t timeout -u unit vname?prompt vname ...

Okay so this is the shell input mechanism, and it's giving main character energy. It reads one line, then breaks that line up into fields using the characters in IFS as separators. The escape character (\) removes any special meaning for the next character and handles line continuation. Periodt.

-d delim — makes the read keep going until it hits the first character of delim instead of a new-line. Iconic little redefinition of "line", honestly.
-r — raw mode, bestie. The \ character is not treated specially here. No cap.
-A — the variable vname gets unset, and then each field that's read gets stored in successive elements of the indexed array vname. Slay.
-p — the input line gets taken from the input pipe of a process spawned by the shell using |&. Cute.
-s — the input gets saved as a command in the history file. Little memory-keeper. 🎀
-u unit — specify a one digit file descriptor unit to read from. The file descriptor can be opened with the exec special built-in command. The default value of unit n is 0. (Zero, queen.)
-t timeout — timeout in decimal seconds, used when reading from a terminal or pipe. Don't wait forever, know your worth. 💅

Field assignment goes: first field → first vname, second field → second vname, etc., and any leftover fields all get dumped onto the last vname. If vname is omitted entirely, REPLY is used as the default vname — she's the default for a reason.

An end-of-file with the -p option causes cleanup for this process so that another can be spawned. Very responsible.

Also, if the first argument contains a ?, the remainder of this word is used as a prompt on standard error when the shell is interactive. So read name?what's ur name literally asks. Obsessed. 🦋

The exit status is 0 unless an end-of-file is encountered or read has timed out.

🔒 `readonly`

readonly -p vname=value ...

If vname is not given, it prints the names and values of every variable with the readonly attribute, with the values quoted in a manner that lets them be re-inputed. The -p option causes the word readonly to be inserted before each one. Otherwise, the given vnames are marked readonly, and those names cannot be changed by any subsequent assignment. Marked. Locked. No take-backs. 🔥

↩️ `return`

return n

Causes a shell function or a . script to return to the invoking script with the return status specified by n. The value will be the least significant 8 bits of the specified status — so yes, it's doing the bit thing, that's just how it is.

If n is omitted, then the return status is whatever the last command executed was. And if return is invoked while you're not in a function or a . script, it behaves the same as exit. It's giving "same energy, different context". 💀

⚙️ `set` — the big one

set ±CPabefhkmnopstuvx ±o option ... ±A vname arg ...

The options for this command have meaning as follows, and no, we are not skipping any, bestie, there are like a million of them and I love that for us:

-A — Array assignment. Unset the variable vname and assign values sequentially from the list arg. If +A is used, the variable vname is not unset first.
-C — Prevents redirection > from truncating existing files. Files that are created are opened with the O_EXCL mode. You need >| to truncate a file when this is turned on. (Very "don't touch my stuff" of it.)
-P — Causes the cd and pwd built-in commands to default to physical mode.
-a — All subsequent variables that are defined are automatically exported. Automatic. Effortless. ✨
-b — Prints job completion messages as soon as a background job changes state, rather than waiting for the next prompt. She's impatient, we love her.
-e — If a command has a non-zero exit status, execute the ERR trap, if set, and exit. This mode is disabled while reading profiles.
-f — Disables file name generation. Bye globbing. 🦋
-h — Each command becomes a tracked alias when first encountered.
-k — (Obsolete). All variable assignment arguments are placed in the environment for a command, not just those that precede the command name. It's giving legacy, handle with care.
-m — Background jobs will run in a separate process group and a line will print upon completion. The exit status of background jobs is reported in a completion message. On systems with job control, this option is turned on automatically for interactive shells.
-n — Read commands and check them for syntax errors, but don't execute them. Ignored for interactive shells. All vibes, no action. 🙈
-o — The following argument can be one of the following option names (okay this list is long, bestie, sit down):
- allexport — Same as -a.
- errexit — Same as -e.
- bgnice — All background jobs are run at a lower priority. This is the default mode.
- emacs — Puts you in an emacs style in-line editor for command entry.
- gmacs — Puts you in a gmacs style in-line editor for command entry.
- ignoreeof — The shell will not exit on end-of-file. The command exit must be used.
- keyword — Same as -k.
- markdirs — All directory names resulting from file name generation have a trailing / appended.
- monitor — Same as -m.
- noclobber — Same as -C.
- noexec — Same as -n.
- noglob — Same as -f.
- nolog — Do not save function definitions in history file.
- notify — Same as -b.
- nounset — Same as -u.
- physical — Same as -P.
- privileged — Same as -p.
- verbose — Same as -v.
- trackall — Same as -h.
- vi — Puts you in insert mode of a vi style in-line editor until you hit escape character 033. This puts you in control mode. A return sends the line.
- viraw — Each character is processed as it is typed in vi mode.
- xtrace — Same as -x.

If no option name is supplied, then the current option settings are printed. Peek at the settings, why not. 👑

-p — Disables processing of the $HOME/.profile file and uses the file /etc/suid_profile instead of the ENV file. This mode is on whenever the effective uid (gid) is not equal to the real uid (gid). Turning this off causes the effective uid and gid to be set to the real uid and gid. (Security-flavoured, pay attention.)
-s — Sort the positional parameters lexicographically. Alphabetical queen. 🔤
-t — (Obsolete). Exit after reading and executing one command.
-u — Treat unset parameters as an error when substituting.
-v — Print shell input lines as they are read.
-x — Print commands and their arguments as they are executed. The classic "why is this happening" debugging slay. 🔥
-- — Do not change any of the options; useful in setting $1 to a value beginning with -. If no arguments follow this option, then the positional parameters are unset.

As an obsolete feature, if the first arg is - then the -x and -v options are turned off and the next arg is treated as the first argument. Using + rather than - causes these options to be turned off. These options can also be used upon invocation of the shell. The current set of options may be found in $-. Unless -A is specified, the remaining arguments are positional parameters and are assigned, in order, to $1 $2 .... If no arguments are given, then the names and values of all variables are printed on the standard output. No cap.

🔀 `shift`

shift n

The positional parameters from $n+1 ... are renamed $1 ..., and the default n is 1. The parameter n can be any arithmetic expression that evaluates to a non-negative number less than or equal to $#. Shuffle the lineup, bestie. 🎀

😴 `sleep`

sleep seconds

Suspends execution for the number of decimal seconds (or fractions of a second) given by seconds. Rest era. Self-care era. 💜

🪤 `trap`

trap -p action sig ...

The -p option causes the trap action associated with each trap as specified by the arguments to be printed with appropriate quoting. Otherwise, action will be processed as if it were an argument to eval when the shell receives signal(s) sig. Each sig can be given as a number or as the name of the signal. Trap commands are executed in order of signal number. Very orderly. Iconic.

Any attempt to set a trap on a signal that was ignored on entry to the current shell is ineffective — that one's just not gonna work, sorry bestie. 😭

If action is omitted and the first sig is a number, or if action is -, then the trap(s) for each sig are reset to their original values. If action is the null string, then this signal is ignored by the shell and by the commands it invokes.

Special sigs, and these matter so pay attention:

If sig is ERR, then action will be executed whenever a command has a non-zero exit status.
If sig is DEBUG, then action will be executed before each command.
If sig is 0 or EXIT and the trap statement is executed inside the body of a function, then the command action is executed after the function completes.
If sig is 0 or EXIT for a trap set outside any function, then the command action is executed on exit from the shell.
If sig is KEYBD, then action will be executed whenever a key is read while in emacs, gmacs, or vi mode.

And the trap command with no arguments prints a list of commands associated with each signal number. Receipts, basically. 🧚

typeset ±AHflnprtux ±EFLRZin vname=value ... — okay bestie, this is the variable-and-function glow-up station ✨. It sets attributes and values for shell variables and functions. When invoked inside a function, a new instance of the variables vname is created. The variables' value and type are restored when the function completes. The following list of attributes may be specified:
- -A — declares vname to be an associate array. Subscripts are strings rather than arithmetic expressions. 🎀
- -E — declares vname to be a double precision floating point number. If n is non-zero it defines the number of significant figures that are used when expanding vname. Otherwise ten significant figures will be used.
- -F — declares vname to be a double precision floating point number. If n is non-zero it defines the number of places after the decimal point that are used when expanding vname. Otherwise ten places after the decimal point will be used.
- -H — this option provides UNIX to host-name file mapping on non-UNIX machines.
- -L — left justify and remove leading blanks from value. If n is non-zero it defines the width of the field, otherwise it is determined by the width of the value of first assignment. When the variable is assigned to, it is filled on the right with blanks or truncated, if necessary, to fit into the field. The -R option is turned off.
- -R — right justify and fill with leading blanks. If n is non-zero it defines the width of the field, otherwise it is determined by the width of the value of first assignment. The field is left filled with blanks or truncated from the end if the variable is reassigned. The -L option is turned off.
- -Z — right justify and fill with leading zeros if the first non-blank character is a digit and the -L option has not been set. Remove leading zeros if the -L option is also set. If n is non-zero it defines the width of the field, otherwise it is determined by the width of the value of first assignment.
- -f — the names refer to function names rather than variable names. No assignments can be made and the only other valid options are -t, -u and -x. The option -t turns on execution tracing for this function. The option -u causes this function to be marked undefined. The FPATH variable will be searched to find the function definition when the function is referenced. 🫶
- -i — declares vname to be represented internally as integer. The right hand side of an assignment is evaluated as an arithmetic expression when assigning to an integer. If n is non-zero it defines the output arithmetic base, otherwise the output base will be ten.
- -l — all upper-case characters are converted to lower-case. The upper-case option, -u is turned off.
- -n — declares vname to be a reference to the variable whose name is defined by the value of variable vname. This is usually used to reference a variable inside a function whose name has been passed as an argument.
- -r — the given vnames are marked readonly and these names cannot be changed by subsequent assignment.
- -t — tags the variables. Tags are user definable and have no special meaning to the shell.
- -u — all lower-case characters are converted to upper-case characters. The lower-case option, -l, is turned off.
- -x — the given vnames are marked for automatic export to the environment of subsequently-executed commands. Variables whose names contain a . can not be exported.

Heads up, queen: the -i attribute can not be specified along with -R, -L, -Z, or -f.

Using + rather than - causes these options to be turned off. If no vname arguments are given a list of vnames (and optionally the values) of the variables is printed. (Using + rather than - keeps the values from being printed.) The -p option causes typeset followed by the option letters to be printed before each name rather than the names of the options. If any option other than -p is given, only those variables which have all of the given options are printed. Otherwise, the vnames and attributes of all variables are printed.

ulimit -HSacdfmnpstv limit — set or display a resource limit. The available resource limits are listed below. Many systems do not contain one or more of these limits. The limit for a specified resource is set when limit is specified. The value of limit can be a number in the unit specified below with each resource, or the value unlimited. The -H and -S options specify whether the hard limit or the soft limit for the given resource is set. A hard limit cannot be increased once it is set. A soft limit can be increased up to the value of the hard limit. If neither the H or S options is specified, the limit applies to both. The current resource limit is printed when limit is omitted. In this case the soft limit is printed unless H is specified. When more than one resource is specified, then the limit name and unit is printed before the value. 💅
- -a — lists all of the current resource limits.
- -c — the number of 512-byte blocks on the size of core dumps.
- -d — the number of K-bytes on the size of the data area.
- -f — the number of 512-byte blocks on files that can be written by the current process or by child processes (files of any size may be read).
- -m — the number of K-bytes on the size of physical memory.
- -n — the number of file descriptors plus 1.
- -p — the number of 512-byte blocks for pipe buffering.
- -s — the number of K-bytes on the size of the stack area.
- -t — the number of seconds to be used by each process.
- -v — the number of K-bytes for virtual memory.

If no option is given, -f is assumed.

umask -S mask — the user file-creation mask is set to mask (see umask(2)). mask can either be an octal number or a symbolic value as described in chmod(1). If a symbolic value is given, the new umask value is the complement of the result of applying mask to the complement of the previous umask value. If mask is omitted, the current value of the mask is printed. The -S option causes the mode to be printed as a symbolic value. Otherwise, the mask is printed in octal. 🌸
unalias -a name ... — the aliases given by the list of names are removed from the alias list. The -a option causes all the aliases to be unset. 🦋
-unset -fv vname ... — this is the unset builtin, and yes, the synopsis carries that exact -unset spelling. The variables given by the list of vnames are unassigned, i.e., their values and attributes are erased. Readonly variables cannot be unset. If the -f option is set, then the names refer to function names. If the -v option is set, then the names refer to variable names. The default is equivalent to -v. Unsetting ERRNO, LINENO, MAILCHECK, OPTARG, OPTIND, RANDOM, SECONDS, TMOUT, and _ removes their special meaning even if they are subsequently assigned to. 💀
wait job ... — wait for the specified job and report its termination status. If job is not given then all currently active child processes are waited for. The exit status from this command is that of the last process waited for. See Jobs for a description of the format of job. ⭐
whence -apv name ... — for each name, indicate how it would be interpreted if used as a command name. The -v option produces a more verbose report. The -p option does a path search for name even if name is an alias, a function, or a reserved word. The -a is similar to the -v option but causes all interpretations of the given name to be reported. 🔥

🌀 Invocation 🌀

If the shell is invoked by exec(2), and the first character of argument zero ($0) is -, then the shell is assumed to be a login shell and commands are read from /etc/profile and then from either .profile in the current directory or $HOME/.profile, if either file exists. Next, commands are read from the file named by performing parameter expansion, command substitution, and arithmetic substitution on the value of the environment variable ENV if the file exists. If the -s option is not present and arg is, then a path search is performed on the first arg to determine the name of the script to execute. The script arg must have read permission and any setuid and getgid settings will be ignored. If the script is not found on the path, arg is processed as if it named a built-in command or function. Commands are then read as described below; the following options are interpreted by the shell when it is invoked:

-c — if the -c option is present then commands are read from the first arg. Any remaining arguments become position parameters starting at 0. 👑
-s — okay so if this flag is present or if no arguments remain, then commands are read from standard input. Shell output — except for the output of the Special Commands listed above — is written to file descriptor 2. 💅
-i — if this flag is present, or if the shell input and output are attached to a terminal (as told by ioctl(2)), then this shell is interactive. In this case TERM is ignored (so that kill 0 does not kill an interactive shell) and INTR is caught and ignored (so that wait is interruptible). In all cases, QUIT is ignored by the shell. Bestie, that's a lot of signal drama, but it's giving guarded. 🫶
-r — if this flag is present, the shell is a restricted shell. 🔒
-I filename — used to generate a cross reference database that can be used by a separate utility to find definitions and references for variables and commands. No cap, very main-character utility energy. 🎀

The remaining options and arguments are described under the set command above. An optional - as the first argument is ignored.

💅 Rksh Only

Rksh is used to set up login names and execution environments whose capabilities are more controlled than those of the standard shell. The actions of rksh are identical to those of ksh, except that the following are disallowed:

changing directory (see cd(1))
setting or unsetting the value or attributes of SHELL, ENV, or PATH
specifying path or command names containing /
redirecting output (>, >|, &<>, and >>)

The restrictions above are enforced after .profile and the ENV files are interpreted.

When a command to be executed is found to be a shell procedure, rksh invokes ksh to execute it. Thus, it is possible to provide to the end-user shell procedures that have access to the full power of the standard shell, while imposing a limited menu of commands; this scheme assumes that the end-user does not have write and execute permissions in the same directory.

The net effect of these rules is that the writer of the .profile has complete control over user actions, by performing guaranteed setup actions and leaving the user in an appropriate directory (probably not the login directory).

The system administrator often sets up a directory of commands (i.e., /usr/rbin) that can be safely invoked by rksh. 💖

🚪 EXIT

Errors detected by the shell, such as syntax errors, cause the shell to return a non-zero exit status. Otherwise, the shell returns the exit status of the last command executed (see also the exit command above). If the shell is being used non-interactively then execution of the shell file is abandoned.

Run time errors detected by the shell are reported by printing the command or function name and the error condition. If the line number that the error occurred on is greater than one, then the line number is also printed in square brackets ([]) after the command or function name. 😭

📁 FILES

/etc/passwd
/etc/profile
/etc/suid_profile
$HOME/.profile
/tmp/sh*
/dev/null

🔎 SEE

Go read these, bestie: cat(1), cd(1), chmod(1), cut(1), echo(1), emacs(1), env(1), gmacs(1), newgrp(1), stty(1), test(1), umask(1), vi(1), dup(2), exec(2), fork(2), ioctl(2), lseek(2), paste(1), pipe(2), umask(2), ulimit(2), wait(2), rand(3), a.out(5), profile(5), environ(7).

Also: Morris I. Bolsky and David G. Korn, The KornShell Command and Programming Language, Prentice Hall, 1989, ISBN 0-13-516972-0. Obsessed with this citation, ngl. 📚✨

💀 CAVEATS

If a command is executed, and then a command with the same name is installed in a directory in the search path before the directory where the original command was found, the shell will continue to exec the original command. Use the -t option of the alias command to correct this situation. Not me being haunted by stale paths. 🙈
Some very old shell scripts contain a ⁁ as a synonym for the pipe character |. Ancient lore, fr fr. 🧚
Using the hist built-in command within a compound command will cause the whole command to disappear from the history file. History said bye. 💀
The built-in command . file reads the whole file before any commands are executed. Therefore, alias and unalias commands in the file will not apply to any commands defined in the file. That's the twist. 😭
Traps are not processed while a job is waiting for a foreground process. Thus, a trap on CHLD won't be executed until the foreground job terminates. It's giving patience, bestie. 🫶

That's the whole manual, bestie! Footer check: source/version ksh(user cmd), and the date is right there in the footer too — both noted with love. 💖✨👑

📦 where this page comes from

nixos 26.05: cde 2.5.3
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on 2026-11-04: a language model's rewrite of the original, under its terms