PERLFUNC(1perl)
✨ NAME ✨
okay bestie, welcome to perlfunc (aka the PERLFUNC(1perl) page, Perl Programmers Reference Guide energy 👑) — it's literally just Perl builtin functions, all of them, in one place 💅
🎀 DESCRIPTION 🎀
okay so. every function down here can be a term in an expression, like it's just out here living its life inside your math. and they come in two big flavors: list operators and named unary operators. the difference between them? how they vibe with a comma that shows up right after them (the precedence table lives in perlop, go peek 🫶).
list operators take more than one argument. unary operators can never take more than one argument, periodt. so a comma terminates the argument of a unary operator, but with a list operator it's just out here separating the arguments, minding its own business. a unary operator generally hands scalar context to its argument, while a list operator can hand out either scalar or list contexts to its arguments. and if it does both? scalar arguments come first, list argument follows, and there can only ever be one such list argument. for instance, splice has three scalar arguments followed by a list, whereas gethostbyname has four scalar arguments. see? it's giving very specific ✨
in the syntax descriptions that follow, list operators that expect a list (and provide list context for the elements of that list) get shown with LIST as an argument. and that list can be any combo of scalar arguments or list values — the list values get included in the list as if each individual element were interpolated right there at that point, forming one longer single-dimensional list value. commas should separate the literal elements of the LIST, no cap 🙈
any function in the list below can be used either with or without parentheses around its arguments (the syntax descriptions just omit the parens, lowkey lazy of them). and if you do use parentheses, here's the simple but occasionally surprising rule: it looks like a function, therefore it is a function, and precedence doesn't matter. otherwise it's a list operator or unary operator, and then precedence does matter. whitespace between the function and the left parenthesis doesn't count, so sometimes you have to be careful, bestie:
print 1+2+4; # Prints 7.
print(1+2) + 4; # Prints 3.
print (1+2)+4; # Also prints 3!
print +(1+2)+4; # Prints 7.
print ((1+2)+4); # Prints 7.if you run Perl with the use warnings pragma, it can warn you about this 💀 like, the third line up there produces:
print (...) interpreted as function at - line 1.
Useless use of integer addition in void context at - line 1.a few functions take no arguments at all, and therefore work as neither unary nor list operators. iconic. these include such functions as time and endpwent. for example, time+86_400 always means time() + 86_400. told you. 💅
for functions that can be used in either a scalar or list context, nonabortive failure is generally indicated in scalar context by returning the undefined value, and in list context by returning the empty list. so like, it didn't work, that's all.
remember this important rule, it's giving main character: there is no rule that relates the behavior of an expression in list context to its behavior in scalar context, or vice versa. it might do two totally different things. each operator and function decides for itself which sort of value would be most appropriate to return in scalar context. some operators return the length of the list that would have been returned in list context. some return the first value in the list. some return the last value in the list. some return a count of successful operations. in general, they do what you want — unless you want consistency 😭
a named array in scalar context is quite different from what would at first glance appear to be a list in scalar context. you can't get a list like (1,2,3) into being in scalar context, because the compiler knows the context at compile time. it would generate the scalar comma operator there, not the list concatenation version of the comma. which means... it was never a list to start with. plot twist 🦋
in general, functions in Perl that serve as wrappers for system calls (syscalls) of the same name (like chown(2), fork(2), closedir(2), etc.) return true when they succeed and undef otherwise, as is usually mentioned in the descriptions below. this is different from the C interfaces, which return -1 on failure. the exceptions to this rule include wait, waitpid, and syscall. system calls also set the special $! variable on failure. other functions do not, except accidentally. so like, don't count on it 🙈
extension modules can also hook into the Perl parser to define new kinds of keyword-headed expression. these may look like functions, but may also look completely different — the syntax following the keyword is defined entirely by the extension. if you are an implementor, see "PL_keyword_plugin" in perlapi for the mechanism. if you are using such a module, see the module's documentation for details of the syntax that it defines. iykyk 💜
🌸 Perl Functions by Category 🌸
here are Perl's functions (including things that look like functions, like some keywords and named operators) arranged by category. some functions appear in more than one place, because they contain multitudes 💖 any warnings, including those produced by keywords, are described in perldiag and warnings.
🍓 Functions for SCALARs or strings
chomp, chop, chr, crypt, fc, hex, index, lc, lcfirst, length, oct, ord, pack, q//, qq//, reverse, rindex, sprintf, substr, tr///, uc, ucfirst, y///
fc is available only if the fc feature is enabled or if it is prefixed with CORE::. the fc feature is enabled automatically with a use v5.16 (or higher) declaration in the current scope ✨
🦋 Regular expressions and pattern matching
m//, pos, qr//, quotemeta, s///, split, study
🌈 Numeric functions
abs, atan2, cos, exp, hex, int, log, oct, rand, sin, sqrt, srand
🎀 Functions for real @ARRAYs
each, keys, pop, push, shift, splice, unshift, values
💜 Functions for list data
all, any, grep, join, map, qw//, reverse, sort, unpack
💅 Functions for real %HASHes
delete, each, exists, keys, values
🫶 Input and output functions
binmode, close, closedir, dbmclose, dbmopen, die, eof, fileno, flock, format, getc, print, printf, read, readdir, readline, rewinddir, say, seek, seekdir, select, syscall, sysread, sysseek, syswrite, tell, telldir, truncate, warn, write
say is available only if the say feature is enabled or if it is prefixed with CORE::. the say feature is enabled automatically with a use v5.10 (or higher) declaration in the current scope. slay 💅
⭐ Functions for fixed-length data or records
pack, read, syscall, sysread, sysseek, syswrite, unpack, vec
🧚 Functions for filehandles, files, or directories
-X, chdir, chmod, chown, chroot, fcntl, glob, ioctl, link, lstat, mkdir, open, opendir, readlink, rename, rmdir, select, stat, symlink, sysopen, umask, unlink, utime
👑 Keywords related to the control flow of your Perl program
break, caller, continue, die, do, dump, eval, evalbytes, exit, __FILE__, goto, last, __LINE__, method, next, __PACKAGE__, redo, return, sub, __SUB__, wantarray
break is available only if you enable the experimental switch feature or use the CORE:: prefix. the switch feature also enables the default, given and when statements, which are documented in "Switch Statements" in perlsyn. the switch feature is enabled automatically with a use v5.10 (or higher) declaration in the current scope. in Perl v5.14 and earlier, continue required the switch feature, like the other keywords. nostalgia era 😭
evalbytes is only available with the evalbytes feature (see feature) or if prefixed with CORE::. __SUB__ is only available with the current_sub feature or if prefixed with CORE::. both the evalbytes and current_sub features are enabled automatically with a use v5.16 (or higher) declaration in the current scope.
🌸 Keywords related to scoping
caller, class, field, import, local, my, our, package, state, use
state is available only if the state feature is enabled or if it is prefixed with CORE::. the state feature is enabled automatically with a use v5.10 (or higher) declaration in the current scope ✨
💖 Miscellaneous functions
defined, formline, lock, prototype, reset, scalar, undef
🔥 Functions for processes and process groups
alarm, exec, fork, getpgrp, getppid, getpriority, kill, pipe, qx//, readpipe, setpgrp, setpriority, sleep, system, times, wait, waitpid
💜 Keywords related to Perl modules
do, import, no, package, require, use
🌈 Keywords related to classes and object-orientation
bless, class, __CLASS__, dbmclose, dbmopen, field, method, package, ref, tie, tied, untie, use
💀 Low-level socket functions
accept, bind, connect, getpeername, getsockname, getsockopt, listen, recv, send, setsockopt, shutdown, socket, socketpair
🎀 System V interprocess communication functions
msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop, shmctl, shmget, shmread, shmwrite
🍓 Fetching user and group info
endgrent, endhostent, endnetent, endpwent, getgrent, getgrgid, getgrnam, getlogin, getpwent, getpwnam, getpwuid, setgrent, setpwent
🦋 Fetching network info
endprotoent, endservent, gethostbyaddr, gethostbyname, gethostent, getnetbyaddr, getnetbyname, getnetent, getprotobyname, getprotobynumber, getprotoent, getservbyname, getservbyport, getservent, sethostent, setnetent, setprotoent, setservent
⭐ Time-related functions
gmtime, localtime, time, times
🫶 Non-function keywords
ADJUST, and, AUTOLOAD, BEGIN, catch, CHECK, cmp, CORE, __DATA__, default, defer, DESTROY, else, elseif, elsif, END, __END__, eq, finally, for, foreach, ge, given, gt, if, INIT, isa, le, lt, ne, not, or, try, UNITCHECK, unless, until, when, while, x, xor
🌍 Portability 🌍
Perl was born in Unix 🍼 and can therefore access all common Unix system calls. in non-Unix environments, the functionality of some Unix system calls may not be available, or details of the available functionality may differ slightly. the Perl functions affected by this are:
"-X", "binmode", "chmod", "chown", "chroot", "crypt", "dbmclose",
"dbmopen", "dump", "endgrent", "endhostent", "endnetent", "endprotoent",
"endpwent", "endservent", "exec", "fcntl", "flock", "fork", "getgrent",
"getgrgid", "gethostbyname", "gethostent", "getlogin", "getnetbyaddr",
"getnetbyname", "getnetent", "getppid", "getpgrp", "getpriority",
"getprotobynumber", "getprotoent", "getpwent", "getpwnam", "getpwuid",
"getservbyport", "getservent", "getsockopt", "glob", "ioctl", "kill",
"link", "lstat", "msgctl", "msgget", "msgrcv", "msgsnd", "open", "pipe",
"readlink", "rename", "select", "semctl", "semget", "semop", "setgrent",
"sethostent", "setnetent", "setpgrp", "setpriority", "setprotoent",
"setpwent", "setservent", "setsockopt", "shmctl", "shmget", "shmread",
"shmwrite", "socket", "socketpair", "stat", "symlink", "syscall",
"sysopen", "system", "times", "truncate", "umask", "unlink", "utime",
"wait", "waitpid"okay bestie, if you wanna know how portable any of those cuties actually are, go read perlport and whatever other platform-specific docs you've got lying around. iykyk 💅
✨ Alphabetical Listing of Perl Functions ✨
🦋 -X FILEHANDLE
🦋 -X EXPR
🦋 -X DIRHANDLE
🦋 -X
A file test, where X is one of the letters we're about to list. This unary operator takes exactly one argument, and that argument can be a filename, a filehandle, or a dirhandle, and it tests the associated file to see if something is true about it. If you don't give it an argument, it tests $_ — except for -t, which tests STDIN. Unless it says otherwise somewhere in here, it returns 1 for true and '' for false. And if the file doesn't exist or can't be examined? It returns undef and sets $! (errno), bestie. 😭
Heads up: with the exception of the -l test, they all follow symbolic links because they use stat() and not lstat() — which means dangling symlinks can't be examined and will therefore report failure. 💀
Despite the funny names, precedence is the same as any other named unary operator. The operator may be any of:
-r File is readable by effective uid/gid.
-w File is writable by effective uid/gid.
-x File is executable by effective uid/gid.
-o File is owned by effective uid.
-R File is readable by real uid/gid.
-W File is writable by real uid/gid.
-X File is executable by real uid/gid.
-O File is owned by real uid.
-e File exists.
-z File has zero size (is empty).
-s File has nonzero size (returns size in bytes).
-f File is a plain file.
-d File is a directory.
-l File is a symbolic link (false if symlinks aren't
supported by the file system).
-p File is a named pipe (FIFO), or Filehandle is a pipe.
-S File is a socket.
-b File is a block special file.
-c File is a character special file.
-t Filehandle is opened to a tty.
-u File has setuid bit set.
-g File has setgid bit set.
-k File has sticky bit set.
-T File is an ASCII or UTF-8 text file (heuristic guess).
-B File is a "binary" file (opposite of -T).
-M Script start time minus file modification time, in days.
-A Same for access time.
-C Same for inode change time (Unix, may differ for other
platforms)Example, because obviously:
while (<>) {
chomp;
next unless -f $_; # ignore specials
#...
}🚨 Note that -s/a/b/ does not do a negated substitution. Saying -exp($foo) still works as expected though, don't stress — only single letters following a minus are interpreted as file tests. slay
These operators are exempt from the "looks like a function rule" we talked about earlier. So an opening parenthesis after the operator does not affect how much of the following code constitutes the argument. Wanna separate it from the code that follows? Put that opening paren before the operator (and this only applies to operators with higher precedence than unary operators, obviously):
-s($file) + 1024 # probably wrong; same as -s($file + 1024)
(-s $file) + 1024 # correctThe interpretation of the file permission operators -r, -R, -w, -W, -x, and -X is by default based solely on the mode of the file and the uids and gids of the user. But bestie, there may be other reasons you can't actually read, write, or execute the file — think network filesystem access controls, ACLs (access control lists), read-only filesystems, and unrecognized executable formats. 🙈 Note that using these six specific operators to verify whether some operation is possible is usually a mistake, because it may be open to race conditions.
Also note that, for the superuser on the local filesystems, the -r, -R, -w, and -W tests always return 1, and -x and -X return 1 if any execute bit is set in the mode. So scripts run by the superuser may need to do a stat to determine the actual mode of the file, or temporarily set their effective uid to something else. the way that root just vibes past your permission bits 😭
If you're using ACLs, there's a pragma called filetest that may produce more accurate results than the bare stat mode bits. When you're under use filetest 'access', those filetests up there test whether the permission can(not) be granted using the access(2) family of system calls. Also note that the -x and -X tests may, under this pragma, return true even if there are no execute permission bits set (nor any extra execute permission ACLs) — yeah, that's weird, it's just how the underlying system calls are defined. 🙃 And also: because of how use filetest 'access' is implemented, the _ special filehandle won't cache the results of the file tests when this pragma is in effect. Go read the filetest pragma docs for more info.
The -T and -B tests work like this. The first block or so of the file gets examined to see if it's valid UTF-8 that includes non-ASCII characters. If so, it's a -T file. Otherwise that same portion of the file gets examined for odd characters like strange control codes or characters with the high bit set. If more than a third of the characters are strange, it's a -B file; otherwise it's a -T file. Also, any file containing a zero byte in the examined portion is considered a binary file. (If this is executed within the scope of a use locale that includes LC_CTYPE, then "odd characters" means anything that isn't a printable nor a space in the current locale. 🌍) If -T or -B is used on a filehandle, the current IO buffer is examined rather than the first block. Both -T and -B return true on an empty file, or on a file at EOF when testing a filehandle. And because you have to read a file to do the -T test, on most occasions you want to use a -f against the file first, like in next unless -f $file && -T $file. fr fr, do the cheap check first
If any of the file tests (or either the stat or lstat operator) is given the special filehandle consisting of a solitary underline, then the stat structure of the previous file test (or stat operator) is used, saving a system call 💅 (This doesn't work with -t, and you need to remember that lstat and -l leave values in the stat structure for the symbolic link, not the real file.) (Also, if the stat buffer was filled by an lstat call, -T and -B will reset it with the results of stat _.) Example:
print "Can do.\n" if -r $x || -w _ || -x _;
stat($filename);
print "Readable\n" if -r _;
print "Writable\n" if -w _;
print "Executable\n" if -x _;
print "Setuid\n" if -u _;
print "Setgid\n" if -g _;
print "Sticky\n" if -k _;
print "Text\n" if -T _;
print "Binary\n" if -B _;As of Perl 5.10.0, as a form of purely syntactic sugar, you can stack file test operators, in a way that -f -w -x $file is equivalent to -x $file && -w _ && -f _. (This is only fancy syntax though: if you use the return value of -f $file as an argument to another filetest operator, no special magic will happen.) ✨
Portability issues: -X in perlport.
And to avoid confusing would-be users of your code with mysterious syntax errors, put something like this at the top of your script:
use v5.10; # so filetest ops can stack🌸 abs VALUE
🌸 abs
Returns the absolute value of its argument. If VALUE is omitted, it uses $_. 🫶
🎀 accept NEWSOCKET,GENERICSOCKET
Accepts an incoming socket connect, just like accept(2) does. Returns the packed address if it succeeded, false otherwise. See the example in "Sockets: Client/Server Communication" in perlipc. 🔌
On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor, as determined by the value of $^F. See $^F in perlvar.
💜 alarm SECONDS
💜 alarm
Arranges to have a SIGALRM delivered to this process after the specified number of wallclock seconds has elapsed. If SECONDS is not specified, the value stored in $_ is used. (On some machines, unfortunately, the elapsed time may be up to one second less or more than you specified because of how seconds are counted, and process scheduling may delay the delivery of the signal even further. 😭)
Only one timer may be counting at once. Each call disables the previous timer, and an argument of 0 may be supplied to cancel the previous timer without starting a new one. The returned value is the amount of time remaining on the previous timer. ⏰
For delays of finer granularity than one second, the Time::HiRes module (from CPAN, and starting from Perl 5.8 part of the standard distribution) provides ualarm. You may also use Perl's four-argument version of select leaving the first three arguments undefined, or you might be able to use the syscall interface to access setitimer(2) if your system supports it. See perlfaq8 for details. 🧚
It is usually a mistake to intermixalarmandsleepcalls, becausesleepmay be internally implemented on your system withalarm. 💀
If you want to use alarm to time out a system call you need to use an eval/die pair. You can't rely on the alarm causing the system call to fail with $! set to EINTR, because Perl sets up signal handlers to restart system calls on some systems. Using eval/die always works, modulo the caveats given in "Signals" in perlipc. this is the way, bestie 🌈
eval {
local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
alarm $timeout;
my $nread = sysread $socket, $buffer, $size;
alarm 0;
};
if ($@) {
die unless $@ eq "alarm\n"; # propagate unexpected errors
# timed out
}
else {
# didn't
}wanna go deeper? bestie, go read perlipc 💖
Portability tea ☕: alarm in perlport.
🌈 all BLOCK LIST 🌈
okay so all runs the BLOCK for every single element of LIST (and yeah, it locally sets $_ to each element while it does that), then vibe-checks whether the result of that block is true.
Returns true if every element makes the block come out true, and returns false if even one element makes the block false.
The very second some element makes the block yield false, the whole result is locked in — it short-circuits right there and does not consider any further elements. Queen of not doing extra work 💅
Used as a condition, this is similar to using grep to count that every value satisfies the condition, except for that short-circuit behaviour.
if( all { length $_ } @strings ) {
say "Every string is non-empty";
}is roughly equivalent to
if( @strings == grep { length $_ } @strings ) ...⚠️ This operator is only available if the keyword_all feature is enabled.
It is currently considered experimental, and will issue a compile-time warning in the category experimental::all unless that category is silenced. so like. be aware bestie 😭
⭐ any BLOCK LIST ⭐
same energy, different question: any evaluates the BLOCK for each element of the LIST (locally setting $_ to each element, you know the drill) and checks the truth of the result of that block.
Returns true if at least one element makes the block yield true, or returns false if no element is found to make it true.
And again: as soon as any element makes the block yield true, the result of this operator is determined. It short-circuits in that case and doesn't consider any further elements. Obsessed with the efficiency honestly ✨
When used as a condition, this is similar to grep, except for this short-circuit behaviour.
if( any { length $_ } @strings ) {
say "At least one string is non-empty";
}is roughly equivalent to
if( grep { length $_ } @strings ) ...⚠️ This operator is only available if the keyword_any feature is enabled.
It is currently considered experimental, and will issue a compile-time warning in the category experimental::any unless that category is silenced. fr fr.
🦋 atan2 Y,X 🦋
Returns the arctangent of Y/X, in the range -PI to PI. math girlies rise 🫶
For the tangent operation, you may use the Math::Trig::tan function, or use the familiar relation:
sub tan { sin($_[0]) / cos($_[0]) }The return value for atan2(0,0) is implementation-defined — so go consult your atan2(3) manpage for more info, no cap.
Portability tea ☕: atan2 in perlport.
🎀 bind SOCKET,NAME 🎀
Binds a network address to a socket, just as bind(2) does. Returns true if it slayed, false otherwise. NAME should be a packed address of the appropriate type for the socket. Scroll the examples in "Sockets: Client/Server Communication" in perlipc.
💜 binmode FILEHANDLE, LAYER 💜
💜 binmode FILEHANDLE 💜
Arranges for FILEHANDLE to be read or written in "binary" or "text" mode on systems where the run-time libraries actually distinguish between binary and text files. If FILEHANDLE is an expression, the value is taken as the name of the filehandle. Returns true on success, otherwise it returns undef and sets $! (errno).
On some systems (in general, DOS- and Windows-based systems) binmode is necessary when you're not working with a text file. For the sake of portability it is a good idea always to use it when appropriate, and never to use it when it isn't appropriate. Also, people can set their I/O to be by default UTF8-encoded Unicode, not bytes.
In other words: regardless of platform, use binmode on binary data, like images, for example. 🖼️
If LAYER is present it is a single string, but may contain multiple directives. The directives alter the behaviour of the filehandle. When LAYER is present, using binmode on a text file makes sense.
If LAYER is omitted or specified as ":raw" the filehandle is made suitable for passing binary data. This includes turning off possible CRLF translation and marking it as bytes (as opposed to Unicode characters). Note that, despite what may be implied in "Programming Perl" (the Camel, 3rd edition) or elsewhere, ":raw" is not simply the inverse of ":crlf". Other layers that would affect the binary nature of the stream are also disabled. See perlopen and the discussion about the PERLIO environment variable in perlrun.
The ":bytes", ":crlf", ":utf8", and any other directives of the form ":...", are called I/O layers. The open pragma can be used to establish default I/O layers.
The LAYER parameter of the binmode function is described as "DISCIPLINE" in "Programming Perl, 3rd Edition". However, since the publishing of this book, by many known as "Camel III", the consensus of the naming of this functionality has moved from "discipline" to "layer". All documentation of this version of Perl therefore refers to "layers" rather than to "disciplines". Now back to the regularly scheduled documentation... 📺
To mark FILEHANDLE as UTF-8, use ":utf8" or :encoding(UTF-8). ":utf8" just marks the data as UTF-8 without further checking, while :encoding(UTF-8) checks the data for actually being valid UTF-8. More details can be found in PerlIO::encoding.
In general, binmode should be called after open but before any I/O is done on the filehandle. Calling binmode normally flushes any pending buffered output data (and perhaps pending input data) on the handle. An exception to this is the ":encoding" layer that changes the default character encoding of the handle. The ":encoding" layer sometimes needs to be called in mid-stream, and it doesn't flush the stream. ":encoding" also implicitly pushes on top of itself the ":utf8" layer because internally Perl operates on UTF8-encoded Unicode characters.
The operating system, device drivers, C libraries, and Perl run-time system all conspire to let the programmer treat a single character ("\n") as the line terminator, irrespective of external representation. On many operating systems, the native text file representation matches the internal representation, but on some platforms the external representation of "\n" is made up of more than one character. 😭
All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use a single character to end each line in the external representation of text (even though that single character is CARRIAGE RETURN on old, pre-Darwin flavors of Mac OS, and is LINE FEED on Unix and most VMS files). In other systems like OS/2, DOS, and the various flavors of MS-Windows, your program sees a "\n" as a simple "\cJ", but what's stored in text files are the two characters "\cM\cJ". That means that if you don't use binmode on these systems, "\cM\cJ" sequences on disk will be converted to "\n" on input, and any "\n" in your program will be converted back to "\cM\cJ" on output. This is what you want for text files, but it can be disastrous for binary files. 💀
Another consequence of using binmode (on some systems) is that special end-of-file markers will be seen as part of the data stream. For systems from the Microsoft family this means that, if your binary data contain "\cZ", the I/O subsystem will regard it as the end of the file, unless you use binmode.
binmode is important not only for readline and print operations, but also when using read, seek, sysread, syswrite and tell (see perlport for more details). See the $/ and "$\" variables in perlvar for how to manually set your input and output line-termination sequences.
Portability tea ☕: binmode in perlport.
👑 bless REF,CLASSNAME 👑
👑 bless REF 👑
bless tells Perl to mark the item referred to by REF as an object in a package. The two-argument version of bless is always preferable unless there is a specific reason to not use it.
- Bless the referred-to item into a specific package (the recommended form):
bless $ref, $package;The two-argument form adds the object to the package specified as the second argument.
- Bless the referred-to item into package
main:
bless $ref, "";If the second argument is an empty string, bless adds the object to package main.
- Bless the referred-to item into the current package (not inheritable):
bless $ref;If bless is used without its second argument, the object is created in the current package. The second argument should always be supplied if a derived class might inherit a method executing bless. Because it is a potential source of bugs, one-argument bless is discouraged. 🙈
See perlobj for more about the blessing (and blessings) of objects.
bless returns its first argument, the supplied reference, as the value of the function; since bless is commonly the last thing executed in constructors, this means that the reference to the object is returned as the constructor's value and allows the caller to immediately use this returned object in method calls. main character energy 🫶
CLASSNAME should always be a mixed-case name, as all-uppercase and all-lowercase names are meant to be used only for Perl builtin types and pragmas, respectively. Avoid creating all-uppercase or all-lowercase package names to prevent confusion.
Also avoid blessing things into the class name 0; this will cause code which (erroneously) checks the result of ref to see if a reference is blessed to fail, as 0, a false value, is returned. 💀
See "Perl Modules" in perlmod for more details.
🧚 break 🧚
Break out of a given block.
break is available only if the switch feature is enabled or if it is prefixed with CORE::. The switch feature is enabled automatically with a use v5.10 (or higher) declaration in the current scope.
🌟 caller — okay bestie, who's calling?? 💅
caller EXPR
callerSo this one returns the context of the current pure perl subroutine call. In scalar context it hands you back the caller's package name if there even is a caller — meaning we're inside a subroutine, or an eval, or a require — and the undefined value otherwise. ✨
Big thing: caller never returns XS subs, it skips right over them. The next pure perl sub is what shows up instead of the XS sub in caller's return values. No cap.
In list context, caller gives you:
# 0 1 2
my ($package, $filename, $line) = caller;Just like __FILE__ and __LINE__, that filename and line number can get altered by the mechanism described at "Plain Old Comments (Not!)" in perlsyn. 🙈
Now with EXPR? It dumps extra info that the debugger uses to print a stack trace. The value of EXPR tells you how many call frames to go back before the current one.
# 0 1 2 3 4
my ($package, $filename, $line, $subroutine, $hasargs,
# 5 6 7 8 9 10
$wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
= caller($i);Here's the tea on all of those:
$subroutineis the function that the caller called (not the function that contains the caller, okay?). Heads up:$subroutinecan be"(eval)"if that frame isn't a subroutine call at all but aneval.- When it is an eval situation, the extra elements
$evaltextand$is_requireget set:$is_requireis true if the frame was created by arequireorusestatement, and$evaltextholds the text of theeval EXPRstatement. Specifically for aneval BLOCKstatement,$subroutineis"(eval)"but$evaltextis undefined. (Also note that everyusestatement creates arequireframe inside aneval EXPRframe.) $subroutinecan also be"(unknown)"if that particular subroutine happened to get deleted from the symbol table. 💀$hasargsis true if a new instance of@_was set up for the frame.$hintsand$bitmaskhold pragmatic hints that the caller was compiled with.$hintscorresponds to$^H, and$bitmaskcorresponds to${^WARNING_BITS}. Both of these are subject to change between versions of Perl and are not meant for external use. so like… don't.$hinthashis a reference to a hash containing the value of%^Hwhen the caller was compiled, orundefif%^Hwas empty. Do not modify the values of this hash — they're the actual values stored in the optree. 👑
Also, the only types of call frames that are visible are subroutine calls and eval. Other forms of context, like while or foreach loops or try blocks, just aren't interesting to caller, because they don't change the behaviour of the return expression.
And when it's called from inside the DB package in list context, with an argument? caller returns more detailed info: it sets the list variable @DB::args to be the arguments the subroutine was invoked with. 🎀
⚠️ Warning time, bestie: the optimizer might have optimized call frames away beforecallereven had a chance to grab the info. Which meanscaller(N)might not return info about the call frame you expect it to, forN > 1. In particular,@DB::argsmight hold info from the previous timecallerwas called. 😭
⚠️ Also: setting@DB::argsis best effort, meant for debugging or generating backtraces, and should not be relied upon. Because@_holds aliases to the caller's arguments, Perl doesn't take a copy of@_, so@DB::argswill contain the modifications the subroutine makes to@_or its contents — not the original values at call time. And@DB::args, like@_, doesn't hold explicit references to its elements, so in certain cases its elements may have become freed and reallocated for other variables or temporary values. Finally, a side effect of the current implementation is that the effects ofshift @_can normally be undone (but notpop @_or other splicing, and not if a reference to@_has been taken, and subject to the caveat about reallocated elements) — so@DB::argsis actually a hybrid of the current state and the initial state of@_. Buyer beware. 🫶
📁 chdir — move the whole vibe to a new folder
chdir EXPR
chdir FILEHANDLE
chdir DIRHANDLE
chdirChanges the working directory to EXPR, if possible. If EXPR is omitted, it changes to the directory specified by $ENV{HOME} if that's set; if it's not, it changes to the directory specified by $ENV{LOGDIR}. (Under VMS, the variable $ENV{'SYS$LOGIN'} gets checked too, and used if it's set.) If neither is set, chdir does absolutely nothing and fails. Returns true on success, false otherwise. There's an example under "die" btw. 🌸
On systems that support fchdir(2), you can pass a filehandle or directory handle as the argument. On systems that don't support fchdir(2), passing handles raises an exception. periodt.
🔐 chmod LIST — permission glow-up
Changes the permissions of a list of files. The first element of the list has to be the numeric mode, which should probably be an octal number, and which definitely should not be a string of octal digits: 0644 is okay, but "0644" is not. Sigh. Returns the number of files successfully changed. See also "oct" if all you've got is a string.
my $cnt = chmod 0755, "foo", "bar";
chmod 0755, @executables;
my $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
# --w----r-T
my $mode = "0644"; chmod oct($mode), "foo"; # this is better
my $mode = 0644; chmod $mode, "foo"; # this is bestOn systems that support fchmod(2), you can pass filehandles among the files. On systems that don't support fchmod(2), passing filehandles raises an exception. Filehandles must be passed as globs or glob references to be recognized — barewords are treated as filenames.
open(my $fh, "<", "foo");
my $perm = (stat $fh)[2] & 07777;
chmod($perm | 0600, $fh);You can also import the symbolic S_I* constants from the Fcntl module:
use Fcntl qw( :mode );
chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
# Identical to the chmod 0755 of the example above.Portability issues: "chmod" in perlport.
✂️ chomp — the safe little snip
chomp VARIABLE
chomp( LIST )
chompThis safer version of chop removes any trailing string that corresponds to the current value of $/ (also known as $INPUT_RECORD_SEPARATOR in the English module). It returns the total number of characters removed from all its arguments. It's often used to remove the newline from the end of an input record when you're worried the final record might be missing its newline. When in paragraph mode ($/ = ''), it removes all trailing newlines from the string. When in slurp mode ($/ = undef) or fixed-length record mode ($/ is a reference to an integer or the like; see perlvar), chomp won't remove anything. 🦋
If VARIABLE is omitted, it chomps $_. Example:
while (<>) {
chomp; # avoid \n on last field
my @array = split(/:/);
# ...
}If VARIABLE is a hash, it chomps the hash's values, but not its keys, resetting the each iterator in the process.
You can actually chomp anything that's an lvalue, including an assignment:
chomp(my $cwd = `pwd`);
chomp(my $answer = <STDIN>);If you chomp a list, every element gets chomped, and the total number of characters removed is returned.
⚠️ Note that parens are necessary when you're chomping anything that isn't a simple variable. Why? Becausechomp $cwd = \pwd`;gets interpreted as(chomp $cwd) = `pwd`;rather than aschomp( $cwd = `pwd` )which you might expect. Similarly,chomp $x, $yis interpreted aschomp($x), $yrather than aschomp($x, $y)`. 🙈
🔪 chop — the feral version
chop VARIABLE
chop( LIST )
chopChops off the last character of a string and returns that character. It's way more efficient than s/.$//s because it neither scans nor copies the string. If VARIABLE is omitted, it chops $_. If VARIABLE is a hash, it chops the hash's values, but not its keys, resetting the each iterator in the process.
You can actually chop anything that's an lvalue, including an assignment.
If you chop a list, each element is chopped. Only the value of the last chop is returned. 💀
Note that chop returns the last character. To return all but the last character, use substr($string, 0, -1).
See also "chomp".
👑 chown LIST — changing who literally owns the file
Changes the owner (and group) of a list of files. The first two elements of the list must be the numeric uid and gid, in that order. A value of -1 in either position is interpreted by most systems to leave that value unchanged. Returns the number of files successfully changed.
my $cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;On systems that support fchown(2), you can pass filehandles among the files. On systems that don't support fchown(2), passing filehandles raises an exception. Filehandles must be passed as globs or glob references to be recognized — barewords are considered filenames.
Here's an example that looks up nonnumeric uids in the passwd file:
print "User: ";
chomp(my $user = <STDIN>);
print "Files: ";
chomp(my $pattern = <STDIN>);
my ($login,$pass,$uid,$gid) = getpwnam($user)
or die "$user not in passwd file";
my @ary = glob($pattern); # expand filenames
chown $uid, $gid, @ary;On most systems you are not allowed to change the ownership of a file unless you're the superuser, although you should be able to change the group to any of your secondary groups. On insecure systems these restrictions may be relaxed, but that is not a portable assumption. On POSIX systems, you can detect this condition like this:
use POSIX qw(pathconf _PC_CHOWN_RESTRICTED);
my $can_chown_giveaway =
! pathconf($path_of_interest, _PC_CHOWN_RESTRICTED);Portability issues: "chown" in perlport.
🧚 chr — number to character, iconic behaviour
chr NUMBER
chrReturns the character represented by that NUMBER in the character set. For example, chr(65) is "A" in either ASCII or Unicode, and chr(0x263a) is a Unicode smiley face. 😊✨
Negative values give you the Unicode replacement character, chr(0xfffd) — unless you're under the bytes pragma, in which case it's just the low eight bits of the value (truncated to an integer) that get used. 💀
If NUMBER is left out, it uses $_. Bestie.
Want the reverse? That's ord. 🔁
Also real talk: characters from 128 to 255 (inclusive) are, by default, internally not encoded as UTF-8, and that's on purpose — backward compatibility reasons, iykyk. If you wanna know more about Unicode, go read perlunicode. 🌈
✨ chroot FILENAME
✨ chroot
Does exactly what the system call of the same name does: it makes that named directory the new root directory for every single pathname that starts with a "/" from your process and all its children. 🏠 (It does not change your current working directory, ok — that one's unaffected.)
For security reasons this call is restricted to the superuser. If FILENAME is omitted, it does a chroot to $_.
NOTE: it is mandatory for security tochdir("/")(chdirto the root directory) immediately after achroot— otherwise your current working directory can end up outside of the new root. Not the vibe you want. 🚨
Portability tea: "chroot" in perlport.
✨ class NAMESPACE
✨ class NAMESPACE VERSION
✨ class NAMESPACE BLOCK
✨ class NAMESPACE VERSION BLOCK
Declares the BLOCK — or the rest of the compilation unit — as living in the given namespace, which implements an object class (peep perlclass). It behaves similarly to package, except the newly-created package behaves as a class. Slay, classes. 👑
✨ close FILEHANDLE
✨ close
Closes the file or pipe attached to the filehandle, flushes the IO buffers, and closes the system file descriptor. It returns true if those operations succeed, and if no error was reported by any PerlIO layer, and there was no existing error on the filehandle. Triple-check energy. ✅
If there was an existing error on the filehandle, close returns false and $! gets set to the error from the failing operation — so you can safely use its value when you're reporting that error. Nice.
If you leave the argument out, it closes the currently selected filehandle.
You don't have to close FILEHANDLE if you're immediately going to do another open on it, because open closes it for you (see open). BUT — and this is the plot twist — an explicit close on an input file resets the line counter ($.), while the implicit close done by open does not. Lowkey important.
If the filehandle came from a piped open, close returns false if one of the other syscalls involved fails, or if its program exits with non-zero status. If the only problem was that the program exited non-zero, $! will be set to 0. Closing a pipe also waits for the process executing on the pipe to exit — in case you wanna look at the output of the pipe afterwards — and implicitly drops the exit status value of that command into $? and "${^CHILD_ERROR_NATIVE}". 🫡
If there are multiple threads running, close on a filehandle from a piped open returns true without waiting for the child process to terminate, if the filehandle is still open in another thread.
Closing the read end of a pipe before the process writing to it at the other end is done writing means the writer receives a SIGPIPE. If the other end can't handle that, be sure to read all the data before closing the pipe. 💅
Example:
open(OUTPUT, '|sort >foo') # pipe to sort
or die "Can't start sort: $!";
#... # print stuff to output
close OUTPUT # wait for sort to finish
or warn $! ? "Error closing sort pipe: $!"
: "Exit status $? from sort";
open(INPUT, 'foo') # get sort's results
or die "Can't open 'foo' for input: $!";FILEHANDLE may be an expression whose value can be used as an indirect filehandle — usually the real filehandle name or an autovivified handle. ✨
If an error occurs when perl implicitly closes a handle, perl will produce a warning. Explicitly calling close on the handle prevents that warning. You're welcome. 🎀
✨ closedir DIRHANDLE
Closes a directory that was opened by opendir, and returns the success of that system call. Simple, clean, iconic. 🦋
✨ connect SOCKET,NAME
Attempts to connect to a remote socket, just like connect(2). Returns true if it worked, false if it didn't. NAME should be a packed address of the appropriate type for the socket. The examples live in "Sockets: Client/Server Communication" in perlipc. 🌐
✨ continue BLOCK
✨ continue
When it's followed by a BLOCK, continue is actually a flow control statement, not a function. If there's a continue BLOCK attached to a BLOCK (typically inside a while or foreach), it is always executed right before the conditional is about to be evaluated again — just like the third part of a for loop in C. So you can use it to increment a loop variable, even when the loop has been continued via the next statement (which is the thing similar to C's continue statement).
last, next, or redo may appear inside a continue block; last and redo behave as if they'd been executed inside the main block. So does next, but since it'll execute a continue block, it may be more... entertaining. 🎢
while (EXPR) {
### redo always comes here
do_something;
} continue {
### next always comes here
do_something_else;
# then back to the top to re-check EXPR
}
### last always comes hereOmitting the continue section is equivalent to using an empty one, logically enough — so next just goes straight back to check the condition at the top of the loop.
When there's no BLOCK, continue is a function that falls through the current when or default block instead of iterating a dynamically enclosing foreach or exiting a lexically enclosing given. In Perl 5.14 and earlier, this form of continue was only available when the switch feature was enabled. See feature and "Switch Statements" in perlsyn for the full lore. 📜
✨ cos EXPR
✨ cos
Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, it takes the cosine of $_. 📐
For the inverse cosine operation, you can use the Math::Trig::acos function, or just use this relation:
sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }✨ crypt PLAINTEXT,SALT
Creates a digest string exactly like the crypt(3) function in the C library (assuming you actually have a version of it there that hasn't been extirpated as a potential munition 💀).
crypt is a one-way hash function. The PLAINTEXT and SALT get turned into a short string called a digest, and that's what's returned. The same PLAINTEXT and SALT will always return the same string, but there is no (known) way to get the original PLAINTEXT back out of the hash. Even tiny changes in the PLAINTEXT or SALT result in big changes in the digest. ✨
There is no decrypt function. This one isn't actually all that useful for cryptography (for that, go look for Crypt modules on your nearby CPAN mirror 🌸) and the name "crypt" is a bit of a misnomer, ngl. Instead it's mainly used to check whether two pieces of text are the same without having to transmit or store the text itself. Classic example: checking if a correct password was given. You store the digest of the password, not the password itself. The user types in a password, that gets crypt'd with the same salt as the stored digest, and if the two digests match — correct password, periodt. 🔐
When you're verifying an existing digest string you should use the digest as the salt, like crypt($plain, $digest) eq $digest. The SALT used to create the digest is visible as part of the digest. This makes sure crypt will hash the new string with the same salt as the digest, which lets your code work with the standard crypt and with more exotic implementations. In other words: assume nothing about the returned string itself, and assume nothing about how many bytes of SALT may matter. 🙈
Traditionally the result is a string of 13 bytes: two first bytes of the salt, followed by 11 bytes from the set "[./0-9A-Za-z]", and only the first eight bytes of PLAINTEXT mattered. But alternative hashing schemes (like MD5), higher level security schemes (like C2), and implementations on non-Unix platforms may produce different strings.
When you're choosing a new salt, create a random two character string whose characters come from the set "[./0-9A-Za-z]" (like join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]). That set of characters is just a recommendation though — the characters allowed in the salt depend solely on your system's crypt library, and Perl can't restrict what salts crypt accepts. 🤷♀️
Here's an example that makes sure whoever runs this program knows their password:
my $pwd = (getpwuid($<))[1];
system "stty -echo";
print "Password: ";
chomp(my $word = <STDIN>);
print "\n";
system "stty echo";
if (crypt($word, $pwd) ne $pwd) {
die "Sorry...\n";
} else {
print "ok\n";
}Of course, typing in your own password to whoever asks you for it is unwise. 🚩
The crypt function is unsuitable for hashing large quantities of data, not least of all because you can't get the information back. Look at the Digest module for more robust algorithms. 💪
If you're using crypt on a Unicode string (which potentially has characters with codepoints above 255), Perl tries to make sense of the whole situation by trying to downgrade (a copy of) the string back to an eight-bit byte string before calling crypt (on that copy). If that works — great. If it doesn't, crypt dies with Wide character in crypt. 😭
Portability issues: "crypt" in perlport.
✨ dbmclose HASH
[This function has been largely superseded by the untie function.]
Breaks the binding between a DBM file and a hash. 💔
Portability issues: "dbmclose" in perlport.
✨ dbmopen HASH,DBNAME,MASK
[This function has been largely superseded by the tie function.]
Okay so this is the rest of the dbmopen tea — it ties a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a hash. HASH is the name of the hash — and bestie, heads up, unlike normal open, the first argument is not a filehandle, even though it absolutely looks like one. It's giving filehandle but it's not. 🙈 DBNAME is the name of the database (without the .dir or .pag extension if any). If the database doesn't exist, it gets created with protection specified by MASK (as modified by the umask). Wanna prevent creation if it's not already there? Totally valid — you can specify a MASK of 0, and the function will return a false value if it can't find an existing database. And if your system only supports the older DBM functions, you may make only one dbmopen call in your program. Only one! In older versions of Perl, if your system had neither DBM nor ndbm, calling dbmopen produced a fatal error; it now falls back to sdbm(3). Glow up fr. ✨
If you don't have write access to the DBM file, you can only read hash variables, not set them. If you wanna test whether you can write, either use file tests or try setting a dummy hash entry inside an eval to trap the error. Sneaky but effective. 💅
Note that functions such askeysandvaluesmay return huge lists when used on large DBM files. Like, enormous. You may prefer to use theeachfunction to iterate over large DBM files. Main character energy: iterate, don't dump.
Example, bestie:
# print out history file offsets
dbmopen(%HIST,'/usr/lib/news/history',0666);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
dbmclose(%HIST);See also AnyDBM_File for a more general description of the pros and cons of the various dbm approaches, as well as DB_File for a particularly rich implementation.
You can control which DBM library you use by loading that library before you call dbmopen:
use DB_File;
dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
or die "Can't open netscape history file: $!";Portability issues: "dbmopen" in perlport.
defined EXPR
defined
Returns a Boolean value telling whether EXPR has a value other than the undefined value undef. If EXPR is not present, $_ is checked. Simple as that, no cap. 💖
So many operations return undef to indicate failure, end of file, system error, uninitialized variable, and other exceptional conditions. This function is how you distinguish undef from other values. (A simple Boolean test will not distinguish among undef, zero, the empty string, and "0", which are all equally false.) Note that since undef is a valid scalar, its presence doesn't necessarily indicate an exceptional condition: pop returns undef when its argument is an empty array, or when the element to return happens to be undef. It's giving "nothing personal, just no value." 🫶
You may also use defined(&func) to check whether subroutine func has ever been defined. The return value is unaffected by any forward declarations of func. A subroutine that is not defined may still be callable: its package may have an AUTOLOAD method that makes it spring into existence the first time that it is called; see perlsub. Iconic behavior honestly. ✨
Use of defined on aggregates (hashes and arrays) is no longer supported. It used to report whether memory for that aggregate had ever been allocated. You should instead use a simple test for size:
if (@an_array) { print "has array elements\n" }
if (%a_hash) { print "has hash members\n" }When used on a hash element, it tells you whether the value is defined, not whether the key exists in the hash. Use exists for the latter purpose. Two different questions, bestie, don't mix them. 💅
Examples:
print if defined $switch{D};
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
sub foo { defined &$bar ? $bar->(@_) : die "No bar"; }
$debugging = 0 unless defined $debugging;Note: Many folks tend to overusedefinedand are then surprised to discover that the number0and""(the zero-length string) are, in fact, defined values. For example, if you say
"ab" =~ /a(.*)b/;The pattern match succeeds and $1 is defined, although it matched "nothing". It didn't really fail to match anything. Rather, it matched something that happened to be zero characters long. This is all very above-board and honest. 🤝 When a function returns an undefined value, it's an admission that it couldn't give you an honest answer. So you should use defined only when questioning the integrity of what you're trying to do. At other times, a simple comparison to 0 or "" is what you want.
See also "undef", "exists", "ref".
delete EXPR
Given an expression that specifies an element or slice of a hash, delete deletes the specified elements from that hash so that exists on that element no longer returns true. Setting a hash element to the undefined value does not remove its key, but deleting it does; see "exists". Ripping the key out vs just making it sad and empty — very different. 😭
In list context, usually returns the value or values deleted, or the last such element in scalar context. The return list's length corresponds to that of the argument list: deleting non-existent elements returns the undefined value in their corresponding positions. Since Perl 5.28, a key/value hash slice can be passed to delete, and the return value is a list of key/value pairs (two elements for each item deleted from the hash). Slay. 🔥
delete may also be used on arrays and array slices, but its behavior is less straightforward. Although exists will return false for deleted entries, deleting array elements never changes indices of existing values; use shift or splice for that. However, if any deleted elements fall at the end of an array, the array's size shrinks to the position of the highest element that still tests true for exists, or to 0 if none do. In other words, an array won't have trailing nonexistent elements after a delete.
WARNING: Calling delete on array values is strongly discouraged. The notion of deleting or checking the existence of Perl array elements is not conceptually coherent, and can lead to surprising behavior. 💀 Like… don't. Just don't.
Deleting from %ENV modifies the environment. Deleting from a hash tied to a DBM file deletes the entry from the DBM file. Deleting from a "tied" hash or array may not necessarily return anything; it depends on the implementation of the "tied" package's DELETE method, which may do whatever it pleases. It's giving chaos, iykyk. 🙈
The delete local EXPR construct localizes the deletion to the current block at run time. Until the block exits, elements locally deleted temporarily no longer exist. See "Localized deletion of elements of composite types" in perlsub.
my %hash = (foo => 11, bar => 22, baz => 33);
my $scalar = delete $hash{foo}; # $scalar is 11
$scalar = delete @hash{qw(foo bar)}; # $scalar is 22
my @array = delete @hash{qw(foo baz)}; # @array is (undef,33)The following (inefficiently) deletes all the values of %HASH and @ARRAY:
foreach my $key (keys %HASH) {
delete $HASH{$key};
}
foreach my $index (0 .. $#ARRAY) {
delete $ARRAY[$index];
}And so do these:
delete @HASH{keys %HASH};
delete @ARRAY[0 .. $#ARRAY];But both are slower than assigning the empty list or undefining %HASH or @ARRAY, which is the customary way to empty out an aggregate:
%HASH = (); # completely empty %HASH
undef %HASH; # forget %HASH ever existed
@ARRAY = (); # completely empty @ARRAY
undef @ARRAY; # forget @ARRAY ever existedThe EXPR can be arbitrarily complicated provided its final operation is an element or slice of an aggregate:
delete $ref->[$x][$y]{$key};
delete $ref->[$x][$y]->@{$key1, $key2, @morekeys};
delete $ref->[$x][$y][$index];
delete $ref->[$x][$y]->@[$index1, $index2, @moreindices];die LIST
die raises an exception. Inside an eval the exception is stuffed into $@ and the eval is terminated with the undefined value. If the exception is outside of all enclosing evals, then the uncaught exception is printed to STDERR and perl exits with an exit code indicating failure. If you need to exit the process with a specific exit code, see "exit". Not die's job, bestie — pick the right tool. 💅
Equivalent examples:
die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"Most of the time, die is called with a string to use as the exception. You may either give a single non-reference operand to serve as the exception, or a list of two or more items, which will be stringified and concatenated to make the exception.
If the string exception does not end in a newline, the current script line number and input line number (if any) and a newline are appended to it. Note that the "input line number" (also known as "chunk") is subject to whatever notion of "line" happens to be currently in effect, and is also available as the special variable $.. See "$/" in perlvar and "$." in perlvar.
Hint: sometimes appending ", stopped" to your message will cause it to make better sense when the string "at foo line 123" is appended. Suppose you are running script "canasta".
die "/etc/games is no good";
die "/etc/games is no good, stopped";produce, respectively
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.If LIST was empty or made an empty string, and $@ already contains an exception value (typically from a previous eval), then that value is reused after appending "\t...propagated". This is useful for propagating exceptions:
eval { ... };
die unless $@ =~ /Expected exception/;If LIST was empty or made an empty string, and $@ contains an object reference that has a PROPAGATE method, that method will be called with additional file and line number parameters. The return value replaces the value in $@; i.e., as if "$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };" were called.
If LIST was empty or made an empty string, and $@ is also empty, then the string "Died" is used. Rip. 🪦
Okay so bestie, plot twist: you can also die with a reference as your argument, and if that die gets trapped inside an eval, then $@ holds that reference. Iconic. This unlocks way fancier exception handling — you can build exception objects that carry around whatever arbitrary state you want about what went wrong. Sometimes that's just plain better than sitting there matching particular string values of $@ with regexes like a medieval peasant. 💅
Since Perl stringifies uncaught exception messages before it shows them to anybody, you'll probably want to overload the stringification operations on your exception objects. Go read overload for the whole scoop on that. Two little rules for the stringified message though: it should be non-empty, and it should end in a newline, so it plays nice with how string exceptions get treated. 🎀 And heads up — because an exception object reference cannot be stringified without destroying it, Perl does not try to append location or other info to a reference exception. Want location info with a complex exception object? You gotta arrange to stick that location info into the object yourself, babes. ✨
🫶 Watch out:$@is a global variable. So be careful that analyzing an exception you caught withevaldoesn't clobber the reference sitting in that global. Easiest fix: make a local copy of the reference before you do any manipulations.
Here's the example, exactly as it lives:
use Scalar::Util "blessed";
eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
if (my $ev_err = $@) {
if (blessed($ev_err)
&& $ev_err->isa("Some::Module::Exception")) {
# handle Some::Module::Exception
}
else {
# handle all other possible exceptions
}
}Now, if an uncaught exception results in the interpreter exiting, the exit code gets determined from $! and $? using this pseudocode:
exit $! if $!; # errno
exit $? >> 8 if $? >> 8; # child exit status
exit 255; # last resortSame deal as with exit: $? is set prior to unwinding the call stack, so any DESTROY or END handlers can come along after and alter that value — and therefore alter Perl's exit code. 🙈
The whole intent here is to squeeze as much possible info about the likely cause as you can into the tiny limited space of a system exit code. But plot twist: since $! is the value of C's errno, which can be set by any system call, that means the exit code value used by die can be non-predictable. So do not rely on it — other than trusting that it's non-zero. No cap. 💀
You can also arrange for a callback to run right before die does its deed by setting the $SIG{__DIE__} hook. That handler gets called with the exception as an argument, and it can change the exception if it feels like it, by calling die again. See "%SIG" in perlvar for the details on setting %SIG entries, and "eval" for some examples.
😭 BUT. Even though this feature was supposed to only run right before your program exits... that is not currently so. The$SIG{__DIE__}hook is currently called even insideevaled blocks/strings! If you want the hook to do absolutely nothing in those situations, put this
die @_ if $^S;as the first line of the handler (see "$^S" in perlvar). Because this promotes strange action at a distance, this counterintuitive behavior may be fixed in a future release. 🦋
See also "exit", "warn", and the Carp module.
🌸 do BLOCK 🌸
Not really a function, bestie, it's just vibing. It returns the value of the last command in the sequence of commands indicated by BLOCK. 💖
When it's modified by the "while" or "until" loop modifier, it executes the BLOCK once before testing the loop condition. (On other statements, the loop modifiers test the conditional first — so yeah, different behavior, iykyk.)
do BLOCK does not count as a loop, so the loop control statements "next", "last", or "redo" cannot be used to leave or restart the block. See "Statement Modifiers" in perlsyn for alternative strategies. ✨
🎀 do EXPR 🎀
Takes the value of EXPR as a filename, and executes the contents of that file as a Perl script. Like this:
# load the exact specified file (./ and ../ special-cased)
do '/foo/stat.pl';
do './stat.pl';
do '../foo/stat.pl';
# search for the named file within @INC
do 'stat.pl';
do 'foo/stat.pl';do './stat.pl' is largely like
eval `cat stat.pl`;...except that it's more concise, it runs no external processes, and it keeps track of the current filename for error messages. 💅 It also differs in that code evaluated with do FILE cannot see lexicals in the enclosing scope — but eval STRING can. It's the same, however, in that it does reparse the file every time you call it, so you probably don't want to do this inside a loop. 🙈
Using do with a relative path (except for ./ and ../), like
do 'foo/stat.pl';will search the @INC directories, and update %INC if the file is found. See "@INC" in perlvar and "%INC" in perlvar for these variables. In particular — note that whilst historically @INC contained '.' (the current directory), making those two cases equivalent, that is no longer necessarily the case, because '.' is not included in @INC by default in perl versions 5.26.0 onwards. Instead, perl will now warn:
do "stat.pl" failed, '.' is no longer in @INC;
did you mean do "./stat.pl"?If do can read the file but cannot compile it, it returns undef and sets an error message in $@. If do cannot read the file, it returns undef and sets $! to the error. Always check $@ first, because compilation could fail in a way that also sets $!. And if the file is successfully compiled, do returns the value of the last expression evaluated. ⭐
🫶 Including library modules is better done with the"use"and"require"operators, which also do automatic error checking and raise an exception if there's a problem. Periodt.
You might like to use do to read in a program configuration file. Manual error checking can be done this way:
# Read in config files: system first, then user.
# Beware of using relative pathnames here.
for $file ("/share/prog/defaults.rc",
"$ENV{HOME}/.someprogrc")
{
unless ($return = do $file) {
warn "couldn't parse $file: $@" if $@;
warn "couldn't do $file: $!" unless defined $return;
warn "couldn't run $file" unless $return;
}
}💀 dump LABEL 💀
💀 dump EXPR 💀
💀 dump 💀
This function causes an immediate core dump. 💥 See also the -u command-line switch in perlrun, which does the same thing. The main point of this is so you can use the undump program (not supplied) to turn your core dump into an executable binary, after you've initialized all your variables at the beginning of the program. When that new binary is executed, it begins by executing a goto LABEL (with all the restrictions that "goto" suffers). Think of it as a goto with an intervening core dump and reincarnation. 🧚 If "LABEL" is omitted, it restarts the program from the top. The dump EXPR form, available starting in Perl 5.18.0, allows a name to be computed at run time, being otherwise identical to dump LABEL.
WARNING: Any files opened at the time of the dump will not be open any more when the program is reincarnated, with possible resulting confusion by Perl. 😭
This function is now largely obsolete, mostly because it's very hard to convert a core file into an executable. As of Perl 5.30, it must be invoked as CORE::dump().
Unlike most named operators, this has the same precedence as assignment. It's also exempt from the looks-like-a-function rule, so dump ("foo")."bar" will cause "bar" to be part of the argument to dump. 👑
Portability issues: "dump" in perlport.
💖 each HASH 💖
💖 each ARRAY 💖
When called on a hash in list context, returns a 2-element list consisting of the key and value for the next element of a hash. When called in scalar context, returns only the key (not the value). Slay. ✨
When called on an array in list context, in Perl 5.12 and later, it returns a 2-element list consisting of the index and value for the next element of the array, so you can iterate over it — older Perls consider this a syntax error. 😬 When called in scalar context, returns only the index in the array.
Hash entries are returned in an apparently random order. The actual random order is specific to a given hash; the exact same series of operations on two hashes may result in a different order for each hash. Any insertion into the hash may change the order, as will any deletion — with the exception that the most recent key returned by "each" or "keys" may be deleted without changing the order. So long as a given hash is unmodified, you may rely on "keys", "values" and "each" to repeatedly return the same order as each other. See "Algorithmic Complexity Attacks" in perlsec for the details on why hash order is randomized. Aside from the guarantees provided here, the exact details of Perl's hash algorithm and the hash traversal order are subject to change in any release of Perl. 💀
Array entries are returned lowest index first. 🌈
my @colors = (qw(red, green, blue));
while (my ($index, $value) = each @colors) {
print "[$index] = $value\n";
} [0] = red
[1] = green
[2] = blueAfter "each" has returned all entries from the hash or array, the next call to "each" returns the empty list in list context and undef in scalar context; the next call following that one restarts iteration. 🔁 Each hash or array has its own internal iterator, accessed by "each", "keys", and "values". That iterator is implicitly reset when "each" has reached the end as just described; it can be explicitly reset by calling "keys" or "values" on the hash or array, or by referencing the hash (but not array) in list context. 🎀
😭 If you add or delete a hash's elements while iterating over it, the effect on the iterator is unspecified — for example, entries may be skipped or duplicated. So don't do that. 💅
Exception: It is always safe to delete the item most recently returned by "each", so the following code works properly:
while (my ($key, $value) = each %hash) {
print $key, "\n";
delete $hash{$key}; # This is safe
}tied hashes may have a different ordering behaviour to perl's hash implementation. just so you know 💅
okay so the iterator that each uses? it's attached to the hash or array itself, and it is shared between all iteration operations applied to that same hash or array 😳 so every single use of each on a particular hash or array advances the same iterator location. and every use of each is also subject to having the iterator reset by any use of keys or values on the same hash or array, or by the hash (but not the array) being referenced in list context. which makes each-based loops kind of fragile, ngl: it's easy to arrive at such a loop with the iterator already part way through the object, or to accidentally clobber the iterator state during execution of the loop body 😭 it's easy enough to explicitly reset the iterator before starting a loop, but there is no way to insulate the iterator state used by a loop from the iterator state used by anything else that might execute during the loop body. fr fr.
and this extends to using each on the result of an anonymous hash or array constructor. a new underlying array or hash is created each time, so each will always start iterating from scratch, eg:
# loops forever
while (my ($key, $value) = each %{ +{ a => 1 } }) {
print "$key=$value\n";
}to avoid these problems resulting from the hash-embedded iterator, use a foreach loop rather than while-each. as of Perl 5.36, you can iterate over both keys and values directly with a multiple-value foreach loop 👑
# retrieves the keys one time for iteration
# iteration is unaffected by any operations on %hash within
foreach my $key (keys %hash) {
my $value = $hash{$key};
$hash{$key} = {keys => scalar keys %hash, outer => [%hash]};
some_function_that_may_mess_with(\%hash, $key, $value);
$hash{"new$key"} = delete $hash{$key};
}# Perl 5.36+
foreach my ($key, $value) (%{ +{ a => 1 } }) {
print "$key=$value\n";
}this one prints out your environment like the printenv(1) program, but in a different order:
while (my ($key,$value) = each %ENV) {
print "$key=$value\n";
}starting with Perl 5.14, an experimental feature allowed each to take a scalar expression. this experiment has been deemed unsuccessful, and was removed as of Perl 5.24 🙈 rip bozo.
as of Perl 5.18 you can use a bare each in a while loop, which will set $_ on every iteration. if either an each expression or an explicit assignment of an each expression to a scalar is used as a while/for condition, then the condition actually tests for definedness of the expression's value, not for its regular truth value. sneaky little detail:
while (each %ENV) {
print "$_=$ENV{$_}\n";
}to avoid confusing would-be users of your code who are running earlier versions of Perl with mysterious syntax errors, put this sort of thing at the top of your file to signal that your code will work only on Perls of a recent vintage:
use v5.12; # so keys/values/each work on arrays
use v5.18; # so each assigns to $_ in a lone while testsee also keys, values, and sort. 🫶
eof FILEHANDLE
eof FILEHANDLE
eof ()
eofreturns 1 if the next read on FILEHANDLE will return end of file, or if FILEHANDLE is not open. FILEHANDLE may be an expression whose value gives the real filehandle. (note that this function actually reads a character and then ungetcs it, so it isn't useful in an interactive context 🙈) do not read from a terminal file (or call eof(FILEHANDLE) on it) after end-of-file is reached. file types such as terminals may lose the end-of-file condition if you do 😬 that's a real one.
an eof without an argument uses the last file read. but using eof() with empty parentheses is different. it refers to the pseudo file formed from the files listed on the command line and accessed via the <> operator. since <> isn't explicitly opened, as a normal filehandle is, an eof() before <> has been used will cause @ARGV to be examined to determine if input is available. similarly, an eof() after <> has returned end-of-file will assume you are processing another @ARGV list, and if you haven't set @ARGV, will read input from STDIN; see "I/O Operators" in perlop.
in a while (<>)" loop, eoforeof(ARGV)can be used to detect the end of each file, whereaseof()` will detect the end of the very last file only. examples:
# reset line numbering on each input file
while (<>) {
next if /^\s*#/; # skip comments
print "$.\t$_";
} continue {
close ARGV if eof; # Not eof()!
}# insert dashes just before last line of last file
while (<>) {
if (eof()) { # check for end of last file
print "--------------\n";
}
print;
last if eof(); # needed if we're reading from a terminal
}practical hint bestie: you almost never need to use eof in Perl, because the input operators typically return undef when they run out of data or encounter an error. ✨
eval EXPR
eval EXPR
eval BLOCK
evaleval in all its forms is used to execute a little Perl program, trapping any errors encountered so they don't crash the calling program. it's giving protective girlfriend energy 🫶
plain eval with no argument is just eval EXPR, where the expression is understood to be contained in $_. so there are only two real eval forms; the one with an EXPR is often called "string eval". in a string eval, the value of the expression (which is itself determined within scalar context) is first parsed, and if there were no errors, executed as a block within the lexical context of the current Perl program. this form is typically used to delay parsing and subsequent execution of the text of EXPR until run time. note that the value is parsed every time the eval executes 👀
the other form is called "block eval". it is less general than string eval, but the code within the BLOCK is parsed only once (at the same time the code surrounding the eval itself was parsed) and executed within the context of the current Perl program. this form is typically used to trap exceptions more efficiently than the first, while also providing the benefit of checking the code within BLOCK at compile time. BLOCK is parsed and compiled just once. since errors are trapped, it often is used to check if a given feature is available. iconic behaviour honestly.
in both forms, the value returned is the value of the last expression evaluated inside the mini-program; a return statement may also be used, just as with subroutines. the expression providing the return value is evaluated in void, scalar, or list context, depending on the context of the eval itself. see "wantarray" for more on how the evaluation context can be determined.
if there is a syntax error or runtime error, or a die statement is executed, eval returns undef in scalar context, or an empty list in list context, and $@ is set to the error message 😭 (prior to 5.16, a bug caused undef to be returned in list context for syntax errors, but not for runtime errors.) if there was no error, $@ is set to the empty string. a control flow operator like last or goto can bypass the setting of $@. beware that using eval neither silences Perl from printing warnings to STDERR, nor does it stuff the text of warning messages into $@. to do either of those, you have to use the $SIG{__WARN__} facility, or turn off warnings inside the BLOCK or EXPR using no warnings 'all'. see "warn", perlvar, and warnings. big caveat, don't skip it 💀
note that, because eval traps otherwise-fatal errors, it is useful for determining whether a particular feature (such as socket or symlink) is implemented. it is also Perl's exception-trapping mechanism, where the die operator is used to raise exceptions. the way that die and eval are literally a matched set 💖
before Perl 5.14, the assignment to $@ occurred before restoration of localized variables, which means that for your code to run on older versions, a temporary is required if you want to mask some, but not all errors:
# alter $@ on nefarious repugnancy only
{
my $e;
{
local $@; # protect existing $@
eval { test_repugnancy() };
# $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
$@ =~ /nefarious/ and $e = $@;
}
die $e if defined $e
}there are some different considerations for each form:
String eval
since the return value of EXPR is executed as a block within the lexical context of the current Perl program, any outer lexical variables are visible to it, and any package variable settings or subroutine and format definitions remain afterwards.
note that when BEGIN {} blocks are embedded inside of an eval block the contents of the block will be executed immediately and before the rest of the eval code is executed. you can disable this entirely by
local ${^MAX_NESTED_EVAL_BEGIN_BLOCKS} = 0;
eval $string;which will cause any embedded BEGIN blocks in $string to throw an exception.
Under the "unicode_eval" feature
if this feature is enabled (which is the default under a use 5.16 or higher declaration), Perl assumes that EXPR is a character string. any use utf8 or no utf8 declarations within the string thus have no effect. source filters are forbidden as well. (unicode_strings, however, can appear within the string.)
see also the evalbytes operator, which works properly with source filters.
Outside the "unicode_eval" feature
in this case, the behavior is problematic and is not so easily described. here are two bugs that cannot easily be fixed without breaking existing programs:
- Perl's internal storage of EXPR affects the behavior of the executed code. for example:
my $v = eval "use utf8; '$expr'";- If
$expris"\xc4\x80"(U+0100 in UTF-8), then the value stored in$vwill depend on whether Perl stores$expr"upgraded" (cf. utf8) or not:- If upgraded,
$vwill be"\xc4\x80"(i.e., theuse utf8has no effect.) - If non-upgraded,
$vwill be"\x{100}".
- If upgraded,
- This is undesirable, bestie, because whether a string is upgraded or not should not affect a string's behavior. Like, at all. 💀
- Source filters activated within
evalleak out into whichever file scope is currently being compiled. Wanna see it with the CPAN moduleSemi::Semicolons?
BEGIN { eval "use Semi::Semicolons; # not filtered" }
# filtered here!evalbytes fixes that to work the way one would expect:
use feature "evalbytes";
BEGIN { evalbytes "use Semi::Semicolons; # filtered" }
# not filteredPeriodt. ✨
Problems can arise if the string expands a scalar containing a floating point number. That scalar can expand to letters, such as "NaN" or "Infinity"; or, within the scope of a use locale, the decimal point character may be something other than a dot (such as a comma). None of these are likely to parse as you are likely expecting. 🙈
You should be especially careful to remember what's being looked at when:
eval $x; # CASE 1
eval "$x"; # CASE 2
eval '$x'; # CASE 3
eval { $x }; # CASE 4
eval "\$$x++"; # CASE 5
$$x++; # CASE 6Cases 1 and 2 above behave identically: they run the code contained in the variable $x. (Although case 2 has misleading double quotes making the reader wonder what else might be happening (nothing is).) Cases 3 and 4 likewise behave in the same way: they run the code '$x', which does nothing but return the value of $x. (Case 4 is preferred for purely visual reasons, but it also has the advantage of compiling at compile-time instead of at run-time.) Case 5 is a place where normally you would like to use double quotes, except that in this particular situation, you can just use symbolic references instead, as in case 6. 👑
An eval '' executed within a subroutine defined in the DB package doesn't see the usual surrounding lexical scope, but rather the scope of the first non-DB piece of code that called it. You don't normally need to worry about this unless you are writing a Perl debugger. 🧚
The final semicolon, if any, may be omitted from the value of EXPR.
🎀 Block eval 🎀
If the code to be executed doesn't vary, you may use the eval-BLOCK form to trap run-time errors without incurring the penalty of recompiling each time. The error, if any, is still returned in $@. Examples, bestie:
# make divide-by-zero nonfatal
eval { $answer = $x / $y; }; warn $@ if $@;
# same thing, but less efficient
eval '$answer = $x / $y'; warn $@ if $@;
# a compile-time error
eval { $answer = }; # WRONG
# a run-time error
eval '$answer ='; # sets $@If you want to trap errors when loading an XS module, some problems with the binary interface (such as Perl version skew) may be fatal even with eval unless $ENV{PERL_DL_NONLAZY} is set. See perlrun.
Using the eval {} form as an exception trap in libraries does have some issues. Due to the current arguably broken state of __DIE__ hooks, you may wish not to trigger any __DIE__ hooks that user code may have installed. You can use the local $SIG{__DIE__} construct for this purpose, as this example shows:
# a private exception trap for divide-by-zero
eval { local $SIG{'__DIE__'}; $answer = $x / $y; };
warn $@ if $@;This is especially significant, given that __DIE__ hooks can call die again, which has the effect of changing their error messages:
# __DIE__ hooks may modify error messages
{
local $SIG{'__DIE__'} =
sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
eval { die "foo lives here" };
print $@ if $@; # prints "bar lives here"
}Because this promotes action at a distance, this counterintuitive behavior may be fixed in a future release. 👀
eval BLOCK does not count as a loop, so the loop control statements next, last, or redo cannot be used to leave or restart the block. No sneaky loop vibes here, babes.
The final semicolon, if any, may be omitted from within the BLOCK.
🦋 evalbytes EXPR 🦋
evalbytes
This function is similar to a string eval, except it always parses its argument (or $_ if EXPR is omitted) as a byte string. If the string contains any code points above 255, then it cannot be a byte string, and the evalbytes will fail with the error stored in $@.
use utf8 and no utf8 within the string have their usual effect.
Source filters activated within the evaluated code apply to the code itself.
evalbytes is available starting in Perl v5.16. To access it, you must say CORE::evalbytes, but you can omit the CORE:: if the evalbytes feature is enabled. This is enabled automatically with a use v5.16 (or higher) declaration in the current scope. Iconic vintage behavior, fr fr. 🌈
💅 exec LIST 💅
exec PROGRAM LIST
The exec function executes a system command and never returns; use system instead of exec if you want it to return. It fails and returns false only if the command does not exist and it is executed directly instead of via your system's command shell (see below).
Since it's a common mistake to use exec instead of system, Perl warns you if exec is called in void context and if there is a following statement that isn't die, warn, or exit (if warnings are enabled--but you always do that, right?). If you really want to follow an exec with some other statement, you can use one of these styles to avoid the warning:
exec ('foo') or print STDERR "couldn't exec foo: $!";
{ exec ('foo') }; print STDERR "couldn't exec foo: $!";If there is more than one argument in LIST, this calls execvp(3) with the arguments in LIST. If there is only one element in LIST, the argument is checked for shell metacharacters, and if there are any, the entire argument is passed to the system's command shell for parsing (this is /bin/sh -c on Unix platforms, but varies on other platforms). If there are no shell metacharacters in the argument, it is split into words and passed directly to execvp, which is more efficient. Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";If you don't really want to execute the first argument, but want to lie to the program you are executing about its own name, you can specify the program you actually want to run as an "indirect object" (without a comma) in front of the LIST, as in exec PROGRAM LIST. (This always forces interpretation of the LIST as a multivalued list, even if there is only a single scalar in the list.) Example:
my $shell = '/bin/csh';
exec $shell '-sh'; # pretend it's a login shellor, more directly,
exec {'/bin/csh'} '-sh'; # pretend it's a login shellWhen the arguments get executed via the system shell, results are subject to its quirks and capabilities. See "STRING" in perlop for details.
Using an indirect object with exec or system is also more secure. This usage (which also works fine with system) forces interpretation of the arguments as a multivalued list, even if the list had just one argument. That way you're safe from the shell expanding wildcards or splitting up words with whitespace in them.
my @args = ( "echo surprise" );
exec @args; # subject to shell escapes
# if @args == 1
exec { $args[0] } @args; # safe even with one-arg listThe first version, the one without the indirect object, ran the echo program, passing it "surprise" as an argument. The second version didn't; it tried to run a program named "echo surprise", didn't find it, and set $? to a non-zero value indicating failure. 💀
On Windows, only the exec PROGRAM LIST indirect object syntax will reliably avoid using the shell; exec LIST, even with more than one element, will fall back to the shell if the first spawn fails.
Perl attempts to flush all files opened for output before the exec, but this may not be supported on some platforms (see perlport). To be safe, you may need to set $| ($AUTOFLUSH in English) or call the autoflush method of IO::Handle on any open handles to avoid lost output. Don't lose your output, bestie, that's tragic. 😭
Note that exec will not call your END blocks, nor will it invoke DESTROY methods on your objects.
Portability issues: "exec" in perlport.
✨ exists EXPR ✨
Given an expression that specifies an element of a hash, returns true if the specified element in the hash has ever been initialized, even if the corresponding value is undefined.
print "Exists\n" if exists $hash{$key};
print "Defined\n" if defined $hash{$key};
print "True\n" if $hash{$key};exists may also be called on array elements, but its behavior is much less obvious and is strongly tied to the use of delete on arrays.
WARNING: Calling exists on array values is strongly discouraged. The notion of deleting or checking the existence of Perl array elements is not conceptually coherent, and can lead to surprising behavior. 🙈
print "Exists\n" if exists $array[$index];
print "Defined\n" if defined $array[$index];
print "True\n" if $array[$index];A hash or array element can be true only if it's defined and defined only if it exists, but the reverse doesn't necessarily hold true. That's the vibe check: exists → maybe defined → maybe true, but never assume backwards, iykyk. 🦋
Okay so when you slide exists a subroutine name instead of a variable? 🫶 It returns true if that subroutine has ever been declared, even if it's undefined. Sneaky lil caveat: just mentioning a sub name for exists or defined doesn't count as declaring it. And heads up — a sub that doesn't exist can still be callable! Its package might have an AUTOLOAD method that makes it just ✨spring into existence✨ the first time somebody calls it. (see perlsub for that drama.)
print "Exists\n" if exists &subroutine;
print "Defined\n" if defined &subroutine;Not me pointing out that EXPR can be arbitrarily complicated 💅 as long as the final operation is a hash or array key lookup, or a subroutine name:
if (exists $ref->{A}->{B}->{$key}) { }
if (exists $hash{A}{B}{$key}) { }
if (exists $ref->{A}->{B}->[$ix]) { }
if (exists $hash{A}{B}[$ix]) { }
if (exists &{$ref->{A}{B}{$key}}) { }Okay so the way that only the deepest nested array or hash element refuses to spring into existence just because you tested it — any intervening ones totally will. So $ref->{"A"} and $ref->{"A"}->{"B"} will auto-pop into existence from the existence test for the $key element up there. This happens literally anywhere the arrow operator shows up, including even here:
undef $ref;
if (exists $ref->{"Some key"}) { }
print $ref; # prints HASH(0x80d3d5c)🚨 Using a subroutine call instead of a subroutine name as an argument to exists is an error. Like, not even a soft launch fail. A real error.
exists ⊂ # OK
exists &sub(); # Error💅 exit EXPR / exit
Evaluates EXPR and peaces out immediately with that value. Main character exit. 🌈 Example:
my $ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;See also die. If EXPR is omitted, it exits with 0 status.
The only universally recognized values for EXPR are 0 for success and 1 for error; any other value is subject to interpretation depending on the environment the Perl program is running in. Like, exiting 69 (EX_UNAVAILABLE) from a sendmail incoming-mail filter will cause the mailer to return the item undelivered — but that's not true everywhere, bestie. Context is everything fr fr.
🚫 Don't useexitto abort a subroutine if there's any chance someone might want to trap whatever error happened. Usedieinstead — it can be trapped by aneval. Slay responsibly.
Also lowkey important: exit does not always exit immediately. It calls any defined END routines first, but these END routines may not themselves abort the exit. Likewise, any object destructors that need to be called get called before the real exit. END routines and destructors can change the exit status by modifying $?. If that's a problem for you, call POSIX::_exit($status) to avoid END and destructor processing. See perlmod for details.
Portability tea: exit in perlport.
⭐ exp EXPR / exp
Returns e (the natural logarithm base) to the power of EXPR. If EXPR is omitted, gives exp($_).
🦋 fc EXPR / fc
Returns the casefolded version of EXPR. This is the internal function implementing the \F escape in double-quoted strings. It's giving "the engine behind the magic".
Casefolding is the process of mapping strings to a form where case differences are erased; comparing two strings in their casefolded form is effectively a way of asking if two strings are equal, regardless of case. Iconic, honestly.
Roughly, if you ever found yourself writing this:
lc($this) eq lc($that) # Wrong!
# or
uc($this) eq uc($that) # Also wrong!
# or
$this =~ /^\Q$that\E\z/i # Right!Now you can write:
fc($this) eq fc($that)And get the correct results. Periodt. 💅
Perl only implements the full form of casefolding, but you can access the simple folds using casefold() in Unicode::UCD and prop_invmap() in Unicode::UCD. For further information on casefolding, refer to the Unicode Standard, specifically sections 3.13 "Default Case Operations", 4.2 "Case-Normative", and 5.18 "Case Mappings", available at <https://www.unicode.org/versions/latest/>, as well as the Case Charts available at <https://www.unicode.org/charts/case/>. (Yes bestie, homework links. It's giving thorough.)
If EXPR is omitted, uses $_.
This function behaves the same way under various pragmas, such as within use feature 'unicode_strings', as lc does, with the single exception of fc of LATIN CAPITAL LETTER SHARP S (U+1E9E) within the scope of use locale. The foldcase of this character would normally be ss, but as explained in the lc section, case changes that cross the 255/256 boundary are problematic under locales, and are hence prohibited. Therefore, this function under locale returns instead the string \x{17F}\x{17F}, which is the LATIN SMALL LETTER LONG S. Since that character itself folds to s, the string of two of them together should be equivalent to a single U+1E9E when foldcased. (Not me explaining a Unicode edge case at 2am. 😭)
While the Unicode Standard defines two additional forms of casefolding — one for Turkic languages and one that never maps one character into multiple characters — these are not provided by the Perl core. However, the CPAN module Unicode::Casing may be used to provide an implementation. 🎀
fc is available only if the fc feature is enabled or if it is prefixed with CORE::. The fc feature is enabled automatically with a use v5.16 (or higher) declaration in the current scope.
🔥 fcntl FILEHANDLE,FUNCTION,SCALAR
Implements the fcntl(2) function. You'll probably have to say:
use Fcntl;first to get the correct constant definitions. Argument processing and value returned work just like ioctl below. For example:
use Fcntl;
my $flags = fcntl($filehandle, F_GETFL, 0)
or die "Can't fcntl F_GETFL: $!";You don't have to check for defined on the return from fcntl. Like ioctl, it maps a 0 return from the system call into "0 but true" in Perl. This string is true in boolean context and 0 in numeric context. It is also exempt from the normal Argument "..." isn't numeric warnings on improper numeric conversions. So it's a little liar but a well-behaved one. 🙈
⚠️ Note thatfcntlraises an exception if used on a machine that doesn't implement fcntl(2). See theFcntlmodule or your fcntl(2) manpage to learn what functions are available on your system.
Here's an example of setting a filehandle named $REMOTE to be non-blocking at the system level. You'll have to negotiate $| on your own, though.
use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
my $flags = fcntl($REMOTE, F_GETFL, 0)
or die "Can't get flags for the socket: $!\n";
fcntl($REMOTE, F_SETFL, $flags | O_NONBLOCK)
or die "Can't set flags for the socket: $!\n";Portability issues: fcntl in perlport.
👑 __FILE__
A special token that returns the name of the file in which it occurs. It can be altered by the mechanism described at "Plain Old Comments (Not!)" in perlsyn. Yes it's got lore.
🌸 field VARNAME
Declares a new field variable within the current class. Methods and ADJUST blocks of the class will have access to this variable as if it was a lexical in scope at that point. It's giving object-oriented but make it cozy. ✨
💖 fileno FILEHANDLE / fileno DIRHANDLE
Returns the file descriptor for a filehandle or directory handle, or undefined if the filehandle is not open. If there is no real file descriptor at the OS level — as can happen with filehandles connected to memory objects via open with a reference for the third argument — -1 is returned.
This is mainly useful for constructing bitmaps for select and low-level POSIX tty-handling operations. If FILEHANDLE is an expression, the value is taken as an indirect filehandle, generally its name.
You can use this to find out whether two handles refer to the same underlying descriptor:
if (fileno($this) != -1 && fileno($this) == fileno($that)) {
print "\$this and \$that are dups\n";
} elsif (fileno($this) != -1 && fileno($that) != -1) {
print "\$this and \$that have different " .
"underlying file descriptors\n";
} else {
print "At least one of \$this and \$that does " .
"not have a real file descriptor\n";
}The behavior of fileno on a directory handle depends on the operating system. On a system with dirfd(3) or similar, fileno on a directory handle returns the underlying file descriptor associated with the handle; on systems with no such support, it returns the undefined value, and sets $! (errno). OS-dependent behavior, classic. 💀
🧚 flock FILEHANDLE,OPERATION
Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true for success, false on failure. Produces a fatal error if used on a machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3). flock is Perl's portable file-locking interface, although it locks entire files only, not records. Whole file or nothing, bestie.
Two potentially non-obvious but traditional flock semantics are that it waits indefinitely until the lock is granted, and that its locks are merely advisory. Such discretionary locks are more flexible, but offer fewer guarantees. This means that programs that do not also use flock may modify files locked with flock. See perlport, your port's specific documentation, and your system-specific local manpages for details. It's best to assume traditional behavior if you're writing portable programs. (But if you're not, you should as always feel perfectly free to write for your own system's idiosyncrasies (sometimes called "features"). Slavish adherence to portability concerns shouldn't get in the way of your getting your job done.) 💅
OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but you can use the symbolic names if you import them from the Fcntl module, either individually, or as a group using the :flock tag. LOCK_SH requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN releases a previously requested lock. If LOCK_NB is bitwise-or'ed with LOCK_SH or LOCK_EX, then flock returns immediately rather than blocking waiting for the lock; check the return status to see if you got it. No cap.
To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE before locking or unlocking it. 🌈
Okay so continuing the flock tea ☕ — read this part carefully because the semantics lowkey matter:
- The emulation built with lockf(3)? It does not give you shared locks, and it requires that FILEHANDLE be open with write intent. That's just the semantics lockf(3) implements, not a Perl thing. BUT — most if not all systems implement lockf(3) in terms of fcntl(2) locking, so those differing semantics shouldn't bite too many people. ✨
- And the fcntl(2) emulation of flock(3)? It requires that FILEHANDLE be open with read intent to use
LOCK_SH, and open with write intent to useLOCK_EX. No exceptions bestie. - Also note: some versions of
flockliterally cannot lock things over the network 😭. For that you'd need the more system-specificfcntl. If you want, you can force Perl to ignore your system's flock(2) function and so provide its own fcntl(2)-based emulation, by passing the switch-Ud_flockto theConfigureprogram when you configure and build a new Perl.
Here's a mailbox appender for BSD systems, iconic behavior honestly:
# import LOCK_* and SEEK_END constants
use Fcntl qw(:flock SEEK_END);
sub lock {
my ($fh) = @_;
flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!\n";
# and, in case we're running on a very old UNIX
# variant without the modern O_APPEND semantics...
seek($fh, 0, SEEK_END) or die "Cannot seek - $!\n";
}
sub unlock {
my ($fh) = @_;
flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!\n";
}
open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
or die "Can't open mailbox: $!";
lock($mbox);
print $mbox $msg,"\n\n";
unlock($mbox);On systems that support a real flock(2), locks are inherited across fork calls 🫶 — whereas the ones that have to resort to the more capricious fcntl(2) function lose their locks, making it seriously harder to write servers. Not me crying about it.
See also DB_File for other flock examples.
Portability issues: "flock" in perlport.
🍴 fork
Does a fork(2) system call to create a new process running the same program at the same point. The way it returns is:
- the child pid to the parent process,
- 0 to the child process,
- or
undefif the fork is unsuccessful. 💀
File descriptors (and sometimes locks on those descriptors) are shared, while everything else is copied. On most systems supporting fork(2), great care has gone into making it extremely efficient (for example, using copy-on-write technology on data pages), which is why it's been the dominant paradigm for multitasking over the last few decades. Main character energy fr.
Perl attempts to flush all files opened for output before forking the child process, but this may not be supported on some platforms (see perlport). To be safe, you may need to set $| ($AUTOFLUSH in English) or call the autoflush method of IO::Handle on any open handles to avoid duplicate output. Don't skip this!! 🙈
If you fork without ever waiting on your children, you will accumulate zombies 🧟. On some systems, you can avoid this by setting $SIG{CHLD} to "IGNORE". See also perlipc for more examples of forking and reaping moribund children.
Heads up: if your forked child inherits system file descriptors like STDIN and STDOUT that are actually connected by a pipe or socket, even if you exit, then the remote server (such as, say, a CGI script or a backgrounded job launched from a remote shell) won't think you're done. You should reopen those to /dev/null if it's any issue. 🫠
On some platforms such as Windows, where the fork(2) system call is not available, Perl can be built to emulate fork in the Perl interpreter. The emulation is designed, at the level of the Perl program, to be as compatible as possible with the Unix fork(2). However it has limitations that have to be considered in code intended to be portable. See perlfork for more details.
Portability issues: "fork" in perlport.
🎀 format
Declare a picture format for use by the write function. For example:
format Something =
Test: @<<<<<<<< @||||| @>>>>>
$str, $%, '$' . int($num)
.
$str = "widget";
$num = $cost/$quantity;
$~ = 'Something';
write;See perlform for many details and examples. 💖
✨ formline PICTURE,LIST
This is an internal function used by formats, though you may call it too — no gatekeeping. It formats (see perlform) a list of values according to the contents of PICTURE, placing the output into the format output accumulator, $^A (or $ACCUMULATOR in English). Eventually, when a write is done, the contents of $^A are written to some filehandle. You could also read $^A and then set $^A back to "".
Note that a format typically does one formline per line of form, but the formline function itself doesn't care how many newlines are embedded in the PICTURE. Which means the ~ and ~~ tokens treat the entire PICTURE as a single line. So you may therefore need to use multiple formlines to implement a single record format, just like the format compiler does. 🧠
Be careful if you put double quotes around the picture, because an @ character may be taken to mean the beginning of an array name. 😭
formline always returns true. See perlform for other examples.
If you are trying to use this instead of write to capture the output, you may find it easier to open a filehandle to a scalar (open my $fh, ">", \$output) and write to that instead. Pro tip from me to you 💅
⌨️ getc FILEHANDLE / getc
Returns the next character from the input file attached to FILEHANDLE, or the undefined value at end of file or if there was an error (in the latter case $! is set). If FILEHANDLE is omitted, it reads from STDIN.
This is not particularly efficient. And lowkey the bigger issue: it cannot be used by itself to fetch single characters without waiting for the user to hit enter. For that, try something more like this:
if ($BSD_STYLE) {
system "stty cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system "stty", '-icanon', 'eol', "\001";
}
my $key = getc(STDIN);
if ($BSD_STYLE) {
system "stty -cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL
}
print "\n";Determination of whether $BSD_STYLE should be set is left as an exercise to the reader. 😌 (you got this)
The POSIX::getattr function can do this more portably on systems purporting POSIX compliance. See also the Term::ReadKey module on CPAN.
🪪 getlogin
This implements the C library function of the same name, which on most systems returns the current login from /etc/utmp, if any. If it returns the empty string, use getpwuid.
my $login = getlogin || getpwuid($<) || "Kilroy";Do not considergetloginfor authentication: it is not as secure asgetpwuid. Periodt. 🔒
Portability issues: "getlogin" in perlport.
🔌 getpeername SOCKET
Returns the packed sockaddr address of the other end of the SOCKET connection. Look how cute:
use Socket;
my $hersockaddr = getpeername($sock);
my ($port, $iaddr) = sockaddr_in($hersockaddr);
my $herhostname = gethostbyaddr($iaddr, AF_INET);
my $herstraddr = inet_ntoa($iaddr);👯 getpgrp PID
Returns the current process group for the specified PID. Use a PID of 0 to get the current process group for the current process. Will raise an exception if used on a machine that doesn't implement getpgrp(2). If PID is omitted, returns the process group of the current process.
Some very old machines may not support PID != 0 and will throw an exception if PID != 0. 🙃
Portability issues: "getpgrp" in perlport.
👩👧 getppid
Returns the process id of the parent process.
Note for Linux users 🐧: Between v5.8.1 and v5.16.0 Perl would work around non-POSIX thread semantics the minority of Linux systems (and Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation has since been removed. See the documentation for $$ for details.
Portability issues: "getppid" in perlport.
🎚️ getpriority WHICH,WHO
Returns the current priority for a process, a process group, or a user. (See getpriority(2).) Will raise a fatal exception if used on a machine that doesn't implement getpriority(2).
WHICH can be any of PRIO_PROCESS, PRIO_PGRP or PRIO_USER imported from "RESOURCE CONSTANTS" in POSIX.
Portability issues: "getpriority" in perlport.
🌈 the whole get* / set* / end* squad 🌈
getpwnam NAME, getgrnam NAME, gethostbyname NAME, getnetbyname NAME, getprotobyname NAME, getpwuid UID, getgrgid GID, getservbyname NAME,PROTO, gethostbyaddr ADDR,ADDRTYPE, getnetbyaddr ADDR,ADDRTYPE, getprotobynumber NUMBER, getservbyport PORT,PROTO, getpwent, getgrent, gethostent, getnetent, getprotoent, getservent, setpwent, setgrent, sethostent STAYOPEN, setnetent STAYOPEN, setprotoent STAYOPEN, setservent STAYOPEN, endpwent, endgrent, endhostent, endnetent, endprotoent, endservent
These routines are the same as their counterparts in the system C library. (This means that platform-specific quirks may be encountered, please refer to perlport for examples.) 🫶
In list context, the return values from the various get routines are as follows:
# 0 1 2 3 4
my ( $name, $passwd, $gid, $members ) = getgr*
my ( $name, $aliases, $addrtype, $net ) = getnet*
my ( $name, $aliases, $port, $proto ) = getserv*
my ( $name, $aliases, $proto ) = getproto*
my ( $name, $aliases, $addrtype, $length, @addrs ) = gethost*
my ( $name, $passwd, $uid, $gid, $quota,
$comment, $gcos, $dir, $shell, $expire ) = getpw*
# 5 6 7 8 9(If the entry doesn't exist, the return value is a single meaningless true value.) 💀 not a vibe, but okay.
The exact meaning of the $gcos field is like, it depends, but usually it's got the user's real name (y'know, the actual human name, not the login name) plus some other info about them. ⚠️ But bestie, beware: on a lot of systems users can just... change this themselves?? So it's not trustworthy at all, which means $gcos is tainted (go read perlsec, fr). Same deal with $passwd and $shell — the encrypted password and the login shell — those are tainted too, for exactly the same reason.
In scalar context you get the name, unless the function was a lookup by name, in which case you get the other thing instead, whatever that other thing happens to be. (And if the entry straight up doesn't exist, you get the undefined value, RIP.) Like:
my $uid = getpwnam($name);
my $name = getpwuid($num);
my $name = getpwent();
my $gid = getgrnam($name);
my $name = getgrgid($num);
my $name = getgrent();
# etc.Now, in getpw*() the fields $quota, $comment, and $expire are special little guys because they're just... not supported on many systems. 😭 If $quota isn't supported, it's an empty scalar. If it is supported, it usually encodes the disk quota. If $comment isn't supported, empty scalar. If it is, it usually encodes some admin comment about the user. On some systems the $quota field might actually be $change or $age — fields that are all about password aging. On some systems $comment might be $class instead. The $expire field, if it's even there, encodes the expiration period of the account or the password. For the availability and the exact meaning of these fields on your system, go consult getpwnam(3) and your system's pwd.h file. You can also find out from inside Perl what your $quota and $comment fields mean, and whether you even have the $expire field, by using the Config module and the values d_pwquota, d_pwage, d_pwchange, d_pwcomment, and d_pwexpire. Shadow password files are supported only if your vendor implemented them in the intuitive fashion where calling the regular C library routines gets you the shadow versions when you're running under privilege, or if the shadow(3) functions exist like in System V (which includes Solaris and Linux, bestie 💅). Those systems that implemented some proprietary shadow password facility? Yeah, unlikely to be supported, sorry not sorry.
The $members value returned by getgr*() is a space-separated list of the login names of the members of the group. Simple as that. ✨
For the gethost*() functions, if the h_errno variable is supported in C, it'll come back to you via $? if the function call fails. The @addrs value returned by a successful call is a list of raw addresses returned by the corresponding library call. In the Internet domain, each address is four bytes long, and you can unpack it by saying something like:
my ($w,$x,$y,$z) = unpack('W4',$addr[0]);The Socket library makes this a little easier:
use Socket;
my $iaddr = inet_aton("127.1"); # or whatever address
my $name = gethostbyaddr($iaddr, AF_INET);
# or going the other way
my $straddr = inet_ntoa($iaddr);Going the opposite direction — resolving a hostname to the IP address — you can write this:
use Socket;
my $packed_ip = gethostbyname("www.perl.org");
my $ip_address;
if (defined $packed_ip) {
$ip_address = inet_ntoa($packed_ip);
}Make sure gethostbyname is called in SCALAR context, and make sure you check its return value for definedness. No skipping that check, bestie. 🙈
The getprotobynumber function, even though it only takes one argument, has the precedence of a list operator, so beware:
getprotobynumber $number eq 'icmp' # WRONG
getprotobynumber($number eq 'icmp') # actually means this
getprotobynumber($number) eq 'icmp' # better this wayIf you get tired of remembering which element of the return list contains which return value (and honestly, who wouldn't 😭), by-name interfaces are provided in standard modules: File::stat, Net::hostent, Net::netent, Net::protoent, Net::servent, Time::gmtime, Time::localtime, and User::grent. These override the normal built-ins, supplying versions that return objects with the appropriate names for each field. For example:
use File::stat;
use User::pwent;
my $is_theirs = (stat($filename)->uid == getpwnam($whoever)->uid);And even though it looks like they're the same method calls (uid), they are not, because a File::stat object is a totally different creature from a User::pwent object. Two different vibes entirely.
Many of these functions are not safe in a multi-threaded environment where more than one thread can be using them. In particular, functions like getpwent() iterate per-process, not per-thread, so if two threads are iterating at the same time, neither of them will get all the records. 💀
Some systems have thread-safe versions of some of these functions, like getpwnam_r() instead of getpwnam(). There, Perl automatically and invisibly substitutes the thread-safe version, without notice. Which means code that safely runs on some systems can just... fail on others that lack the thread-safe versions. Not me finding out the hard way. 😭
Portability issues: getpwnam in perlport to endservent in perlport.
💫 getsockname SOCKET
Returns the packed sockaddr address of this end of the SOCKET connection — which is super handy in case you don't know the address because you've got several different IPs that the connection might have come in on.
use Socket;
my $mysockaddr = getsockname($sock);
my ($port, $myaddr) = sockaddr_in($mysockaddr);
printf "Connect to %s [%s]\n",
scalar gethostbyaddr($myaddr, AF_INET),
inet_ntoa($myaddr);👑 getsockopt SOCKET,LEVEL,OPTNAME
Queries the option named OPTNAME associated with SOCKET at a given LEVEL. Options may exist at multiple protocol levels depending on the socket type, but at least the uppermost socket level SOL_SOCKET (defined in the Socket module) will be there, bestie. To query options at another level you gotta supply the protocol number of the appropriate protocol controlling the option. Like, to indicate that an option is to be interpreted by the TCP protocol, LEVEL should be set to the protocol number of TCP, which you can get using getprotobyname.
The function returns a packed string representing the requested socket option, or undef on error, with the reason for the error placed in $!. Just what's in that packed string depends on LEVEL and OPTNAME, so consult getsockopt(2) for the deets. A common case is that the option is an integer, in which case the result is a packed integer, which you can decode using unpack with the i (or I) format.
Here's an example to test whether Nagle's algorithm is enabled on a socket:
use Socket qw(:all);
defined(my $tcp = getprotobyname("tcp"))
or die "Could not determine the protocol number for tcp";
# my $tcp = IPPROTO_TCP; # Alternative
my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
or die "getsockopt TCP_NODELAY: $!";
my $nodelay = unpack("I", $packed);
print "Nagle's algorithm is turned ",
$nodelay ? "off\n" : "on\n";Portability issues: getsockopt in perlport.
🎀 glob EXPR
🎀 glob
In list context, returns a (possibly empty) list of filename expansions on the value of EXPR, like the Unix shell Bash would do. In scalar context, glob iterates through those filename expansions, returning undef when the list is exhausted. If EXPR is omitted, $_ is used.
# List context
my @txt_files = glob("*.txt");
my @perl_files = glob("*.pl *.pm");
# Scalar context
while (my $file = glob("*.mp3")) {
# Do stuff
}Glob also supports an alternate syntax using < > as delimiters. While this syntax is supported, it's recommended that you use glob instead, because it's more readable and searchable. iykyk
my @txt_files = <"*.txt">;If you need case insensitive file globbing, that can be achieved using the :nocase parameter of the bsd_glob module.
use File::Glob qw(:globally :nocase);
my @txt = glob("readme*"); # README readme.txt Readme.mdNote that glob splits its arguments on whitespace and treats each segment as a separate pattern. So glob("*.c *.h") matches all files with a .c or .h extension. The expression glob(".* *") matches all files in the current working directory. If you want to glob filenames that might contain whitespace, you'll have to use extra quotes around the spacey filename to protect it. For example, to glob filenames that have an e followed by a space followed by an f, use one of:
my @spacies = <"*e f*">;
my @spacies = glob('"*e f*"');
my @spacies = glob(q("*e f*"));And if you had to get a variable through, you could do this:
my @spacies = glob("'*${var}e f*'");
my @spacies = glob(qq("*${var}e f*"));If non-empty braces are the only wildcard characters used in the glob, no filenames are matched — but potentially many strings are returned. Like, this produces nine strings, one for each pairing of fruits and colors:
my @many = glob("{apple,tomato,cherry}={green,yellow,red}");This operator is implemented using the standard File::Glob extension. See bsd_glob for details, including bsd_glob, which does not treat whitespace as a pattern separator. 🦋
If a glob expression is used as the condition of a while or for loop, then it'll be implicitly assigned to $_. And if either a glob expression or an explicit assignment of a glob expression to a scalar is used as a while/for condition, then the condition actually tests for definedness of the expression's value, not for its regular truth value. Sneaky, right?
Internal implementation details:
This is the internal function implementing the <*.c> operator, but you can use it directly. The <*.c> operator is discussed in more detail in "I/O Operators" in perlop.
Portability issues: glob in perlport.
🌸 gmtime EXPR
🌸 gmtime
Works just like localtime, but the returned values are localized for the standard Greenwich time zone. That's it, that's the whole vibe. 💖
📝 Note: when it's called in list context,$isdst(that's the last valuegmtimehands back) is always0. GMT has no Daylight Saving Time, bestie. It's giving permanently unbothered. 🕰️
Portability issues: "gmtime" in perlport.
goto LABEL / goto EXPR / goto &NAME
So goto yeets execution over to a specified point in the program. But here's the thing — unlike a function call, execution does not come back to goto. No returns, no take-backsies. 💀
The goto LABEL form hunts down the statement labeled with LABEL and resumes execution right there. It can't be used to escape a block or subroutine that was handed to sort. It can be used to go almost anywhere else within the dynamic scope, including booking it out of subroutines — but honestly, it's usually better to reach for some other construct like last or die. The author of Perl has literally never felt the need to use this form of goto (in Perl, that is; C is another matter entirely). The diff being that C doesn't offer named loops combined with loop control. Perl does, and that replaces most of the structured uses of goto you'd find in other languages. iconic behavior tbh 👑
The goto EXPR form expects to evaluate EXPR into either a code reference or a label name. If it comes back as a code reference, it gets handled just like goto &NAME down below. This is especially useful for implementing tail recursion via goto __SUB__. ✨
If the expression evaluates to a label name, its scope gets resolved dynamically. This lets you do computed gotos FORTRAN-style, but ngl it isn't necessarily recommended if you're optimizing for maintainability:
goto ("FOO", "BAR", "GLARCH")[$i];As shown up there, goto EXPR is exempt from the "looks like a function" rule. A pair of parentheses following it does not (necessarily) delimit its argument. goto("NE")."XT" is equivalent to goto NEXT. Also — unlike most named operators — this one has the same precedence as assignment. 🤯
⚠️ BIG ONE, bestie: use ofgoto LABELorgoto EXPRto jump into a construct becomes an exception in Perl 5.44; a fatal error notification is emitted. It may not be used to go into any construct that requires initialization, like a subroutine, aforeachloop, or agivenblock. In general, it may not be used to jump into the parameter of a binary or list operator — but it may be used to jump into the first parameter of a binary operator. (The=assignment operator's "first" operand is its right-hand operand.) It also can't be used to go into a construct that's been optimized away. so don't even try it 🚫
The goto &NAME form is a whole different creature from the other forms of goto. Fr fr, it isn't a goto in the normal sense at all, and it doesn't carry the stigma the other gotos have. Instead, it exits the current subroutine (losing any changes set by local) and immediately calls in its place the named subroutine using the current value of @_. This is what AUTOLOAD subroutines use when they wanna load another subroutine and then pretend that the other subroutine had been called in the first place (except that any modifications to @_ in the current subroutine are propagated to the other subroutine.) After the goto, not even caller will be able to tell that this routine was called first. sneaky! 🫣
NAME doesn't have to be the name of a subroutine — it can be a scalar variable holding a code reference, or a block that evaluates to a code reference. flexible queen 💅
grep BLOCK LIST / grep EXPR,LIST
This is similar in spirit to grep(1) and its relatives, but it is not the same. In particular, it is not limited to using regular expressions. 🌈
It evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each element) and returns the list value made up of those elements for which the expression evaluated to true. In scalar context, it returns the number of times the expression was true. no cap.
my @foo = grep(!/^#/, @bar); # weed out commentsor equivalently,
my @foo = grep {!/^#/} @bar; # weed out commentsNote that $_ is an alias to the list value, so you can use it to modify the elements of the LIST. While that's useful and supported, it can cause absolutely unhinged results if the elements of LIST aren't variables. Similarly, grep returns aliases into the original list, much like how a for loop's index variable aliases the list elements. Which means: modifying an element of a list returned by grep (for example, inside a foreach, map, or another grep) actually modifies the element in the original list. This is usually something to be avoided when you're writing clear code. 🙈
See also map for a list composed of the results of the BLOCK or EXPR.
hex EXPR / hex
Interprets EXPR as a hex string and returns the corresponding numeric value. If EXPR is omitted, it uses $_.
print hex '0xAf'; # prints '175'
print hex 'aF'; # same
$valid_input =~ /\A(?:0?[xX])?(?:_?[0-9a-fA-F])*\z/A hex string is made of hex digits plus an optional 0x or x prefix. Each hex digit may be preceded by a single underscore, and that underscore gets ignored. Any other character triggers a warning and causes the rest of the string to be ignored (even leading whitespace — unlike oct, so watch out for that one 🍓). Only integers can be represented, and integer overflow triggers a warning.
If you need to convert strings that might start with any of 0, 0x, or 0b, go see oct. And if you want to present something as hex, look into printf, sprintf, and unpack. 💖
import LIST
Plot twist: there is no builtin import function. It's just an ordinary method (subroutine) defined (or inherited) by modules that wish to export names to another module. The use function calls the import method for the package used. See also use, perlmod, and Exporter. 🎀
index STR,SUBSTR,POSITION / index STR,SUBSTR
The index function searches for one string inside another — but without the wildcard-like behavior of a full regular-expression pattern match. It returns the position of the first occurrence of SUBSTR in STR at or after POSITION. If POSITION is omitted, it starts searching from the beginning of the string. A POSITION before the beginning of the string or after its end is treated as if it were the beginning or the end, respectively. POSITION and the return value are based at zero. And if the substring isn't found, index returns -1. 😭
Finding characters or strings:
index("Perl is great", "P"); # Returns 0
index("Perl is great", "g"); # Returns 8
index("Perl is great", "great"); # Also returns 8Attempting to find something not there:
index("Perl is great", "Z"); # Returns -1 (not found)Using an offset to find the second occurrence:
index("Perl is great", "e", 5); # Returns 10int EXPR / int
Returns the integer portion of EXPR. If EXPR is omitted, it uses $_.
⚠️ You should not use this function for rounding. reason one: it truncates towards 0. reason two: machine representations of floating-point numbers can sometimes produce counterintuitive results. for example,int(-6.725/0.025)produces-268rather than the correct-269; that's because it's really more like-268.99999999999994315658instead. 💀
Usually, the sprintf, printf, or the POSIX::floor and POSIX::ceil functions will serve you better than int will. Moreover, int 'Inf' produces Inf (see "Special floating point: infinity (Inf) and not-a-number (NaN)" in perldata for the explanation), but so do all the alternative methods mentioned above. so it's giving consistent, at least 🔥
ioctl FILEHANDLE,FUNCTION,SCALAR
Implements the ioctl(2) function. You'll probably first have to say
require "sys/ioctl.ph"; # probably in
# $Config{archlib}/sys/ioctl.phto get the correct function definitions. If sys/ioctl.ph doesn't exist or doesn't have the correct definitions, you'll have to roll your own, based on your C header files such as <sys/ioctl.h>. (There's a Perl script called h2ph that ships with the Perl kit that may help you with this, but ngl it's nontrivial. 🙈)
SCALAR will be read and/or written depending on the FUNCTION; a C pointer to the string value of SCALAR will be passed as the third argument of the actual ioctl call. (If SCALAR has no string value but does have a numeric value, that value will be passed rather than a pointer to the string value. To guarantee this to be true, add a 0 to the scalar before using it.) The pack and unpack functions may be needed to manipulate the values of structures used by ioctl.
The return value of ioctl (and fcntl) goes like this:
if OS returns: then Perl returns:
-1 undefined value
0 string "0 but true"
anything else that numberSo Perl returns true on success and false on failure, yet you can still easily determine the actual value returned by the operating system:
my $retval = ioctl(...) || -1;
printf "System returned %d\n", $retval;The special string "0 but true" is exempt from Argument "..." isn't numeric warnings on improper numeric conversions. blessed ✨
Portability issues: "ioctl" in perlport.
join EXPR,LIST
Joins the separate strings of LIST into one single string, with fields separated by the value of EXPR, and returns that new string. Example:
my $rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);🚨 Beware bestie: unlikesplit,joindoes not take a pattern as its first argument. do not get it twisted. 😤
Compare split.
keys HASH / keys ARRAY
Called in list context, this returns a list consisting of all the keys of the named hash — or, in Perl 5.12 or later, the indices of an array. Perl releases prior to 5.12 will produce a syntax error if you try to use an array argument. In scalar context, it returns the number of keys or indices. 🔢
Array entries are returned lowest index first. tidy and organized, we love to see it 💅
Hash entries come back in an apparently random order, bestie. Key word: apparently. The actual random order is specific to a given hash — so the exact same series of operations run on two different hashes may totally give you a different order for each one 😭. Any insertion into the hash may change the order, and so will any deletion, with one lil exception: the most recent key returned by each or keys may be deleted without changing the order. And as long as a given hash is unmodified, you can rely on keys, values and each to repeatedly return the same order as each other. Periodt.
Why is hash order randomized at all? Girl, it's a whole security thing — go read "Algorithmic Complexity Attacks" in perlsec for the details 🛡️. And aside from the guarantees listed right here, the exact details of Perl's hash algorithm and the hash traversal order are subject to change in any release of Perl. Also: tied hashes may behave differently from Perl's hashes with respect to changes in order on insertion and deletion of items. Noted 📝.
As a side effect, calling keys resets the internal iterator of the HASH or ARRAY (see "each") before yielding the keys. And here's the slay part: calling keys in void context resets the iterator with no other overhead 💅.
Here is yet another way to print your environment:
my @keys = keys %ENV;
my @values = values %ENV;
while (@keys) {
print pop(@keys), '=', pop(@values), "\n";
}or how about sorted by key:
foreach my $key (sort(keys %ENV)) {
print $key, '=', $ENV{$key}, "\n";
}The returned values are copies of the original keys in the hash, so modifying them will not affect the original hash. Compare "values" 👀.
To sort a hash by value, you'll need to use a sort function. Here's a descending numeric sort of a hash by its values:
foreach my $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
printf "%4d %s\n", $hash{$key}, $key;
}Used as an lvalue on a hash, keys allows you to increase the number of hash buckets allocated for the given hash 🔥. This can gain you a measure of efficiency if you know the hash is going to get big. (This is similar to pre-extending an array by assigning a larger number to $#array.) If you say
keys %hash = 200;then %hash will have at least 200 buckets allocated for it — 256 of them, in fact, since it rounds up to the next power of two ✨. These buckets will be retained even if you do %hash = (), so use undef %hash if you want to free the storage while %hash is still in scope. You can't shrink the number of buckets allocated for the hash using keys in this way (but you needn't worry about doing this by accident, as trying has no effect 🙈).
keys @array in an lvalue context is a syntax error 🚫.
Starting with Perl 5.14, an experimental feature allowed keys to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24. Rip bozo 💀.
To avoid confusing would-be users of your code who are running earlier versions of Perl with mysterious syntax errors, put this sort of thing at the top of your file to signal that your code will work only on Perls of a recent vintage:
use v5.12; # so keys/values/each work on arraysSee also "each", "values", and "sort".
💀 kill SIGNAL, LIST / kill SIGNAL
Sends a signal to a list of processes. Returns the number of arguments that were successfully used to signal (which is not necessarily the same as the number of processes actually killed, e.g. where a process group is killed 👀).
my $cnt = kill 'HUP', $child1, $child2;
kill 'KILL', @goners;SIGNAL may be either a signal name (a string) or a signal number. A signal name may start with a SIG prefix, so FOO and SIGFOO refer to the same signal 💫. The string form of SIGNAL is recommended for portability because the same signal may have different numbers in different operating systems — so like, use the name, bestie.
A list of signal names supported by the current platform can be found in $Config{sig_name}, which is provided by the Config module. See Config for more details 📋.
A negative signal name is the same as a negative signal number, killing process groups instead of processes. For example, kill '-KILL', $pgrp and kill -9, $pgrp will send SIGKILL to the entire process group specified. That means you usually want to use positive not negative signals 🙏.
If SIGNAL is either the number 0 or the string ZERO (or SIGZERO), no signal is sent to the process, but kill checks whether it's possible to send a signal to it (that means, to be brief, that the process is owned by the same user, or we are the super-user). This is useful to check that a child process is still alive (even if only as a zombie 🧟) and hasn't changed its UID. See perlport for notes on the portability of this construct.
The behavior of kill when a PROCESS number is zero or negative depends on the operating system. For example, on POSIX-conforming systems, zero will signal the current process group, -1 will signal all processes, and any other negative PROCESS number will act as a negative signal number and kill the entire process group specified.
If both the SIGNAL and the PROCESS are negative, the results are undefined 😵. A warning may be produced in a future version.
See "Signals" in perlipc for more details.
On some platforms such as Windows where the fork(2) system call is not available, Perl can be built to emulate fork at the interpreter level. This emulation has limitations related to kill that have to be considered, for code running on Windows and in code intended to be portable 🪟.
See perlfork for more details.
If there is no LIST of processes, no signal is sent, and the return value is 0. This form is sometimes used, however, because it causes tainting checks to be run, if your perl supports taint checks. But see "Laundering and Detecting Tainted Data" in perlsec 🧼.
Portability issues: "kill" in perlport.
🌸 last LABEL / last EXPR / last
The last command is like the break statement in C (as used in loops); it immediately exits the loop in question 🚪. If the LABEL is omitted, the command refers to the innermost enclosing loop. The last EXPR form, available starting in Perl 5.18.0, allows a label name to be computed at run time, and is otherwise identical to last LABEL. The continue block, if any, is not executed:
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
#...
}last cannot return a value from a block that typically returns a value, such as eval {}, sub {}, or do {}. (For do, see "Statement Modifiers" in perlsyn for workarounds.) It will perform its flow control behavior, which precludes any return value. It should not be used to exit a grep or map operation 👎.
Note that a block by itself is semantically identical to a loop that executes once. Thus last can be used to effect an early exit out of such a block ✨.
See also "continue" for an illustration of how last, next, and redo work.
Unlike most named operators, this has the same precedence as assignment. It is also exempt from the looks-like-a-function rule, so last ("foo")."bar" will cause bar to be part of the argument to last 😌.
🔤 lc EXPR / lc
Returns a lowercased version of EXPR. If EXPR is omitted, uses $_.
my $str = lc("Perl is GREAT"); # "perl is great"What gets returned depends on several factors 🧐:
- If
use bytesis in effect: — The results follow ASCII rules. Only the charactersA-Zchange, toa-zrespectively. - Otherwise, if
use localeforLC_CTYPEis in effect: — Respects currentLC_CTYPElocale for code points < 256; and uses Unicode rules for the remaining code points (this last can only happen if the UTF8 flag is also set). See perllocale. Starting in v5.20, Perl uses full Unicode rules if the locale is UTF-8. Otherwise, there is a deficiency in this scheme, which is that case changes that cross the 255/256 boundary are not well-defined 😬. For example, the lower case of LATIN CAPITAL LETTER SHARP S (U+1E9E) in Unicode rules is U+00DF (on ASCII platforms). But underuse locale(prior to v5.20 or not a UTF-8 locale), the lower case of U+1E9E is itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the current locale, and Perl has no way of knowing if that character even exists in the locale, much less what code point it is. Perl returns a result that is above 255 (almost always the input character unchanged), for all instances (and there aren't many) where the 255/256 boundary would otherwise be crossed; and starting in v5.22, it raises a locale warning ⚠️. - Otherwise, If EXPR has the UTF8 flag set: — Unicode rules are used for the case change.
- Otherwise, if
use feature 'unicode_strings'oruse locale ':not_characters'is in effect: — Unicode rules are used for the case change. - Otherwise: — ASCII rules are used for the case change. The lowercase of any character outside the ASCII range is the character itself.
Note: This is the internal function implementing the \L escape in double-quoted strings 💅.
my $str = "Perl is \LGREAT\E"; # "Perl is great"🎀 lcfirst EXPR / lcfirst
Returns the value of EXPR with the first character lowercased. This is the internal function implementing the \l escape in double-quoted strings ✨.
If EXPR is omitted, uses $_.
This function behaves the same way under various pragmas, such as in a locale, as lc does. Same energy, different escape 🫶.
📏 length EXPR / length
Returns the length in characters of the value of EXPR. If EXPR is omitted, returns the length of $_. If EXPR is undefined, returns undef 🫠.
This function cannot be used on an entire array or hash to find out how many elements these have. For that, use scalar @array and scalar keys %hash, respectively. Don't do it, bestie, it will not work the way you want 💀.
Like all Perl character operations, length normally deals in logical characters, not physical bytes 🦋. For how many bytes a string encoded as UTF-8 would take up, use length(Encode::encode('UTF-8', EXPR)) (you'll have to use Encode first). See Encode and perlunicode.
✨ __LINE__ ✨
A special token that compiles to the current line number. Bestie, it can be altered by the mechanism described at "Plain Old Comments (Not!)" in perlsyn. (Yes, really. Comments doing crimes. 💅)
🔗 link OLDFILE,NEWFILE
Creates a new filename linked to the old filename. Returns true for success, false otherwise. That's it, that's the whole function, no notes. 🫶
Portability issues: "link" in perlport.
👂 listen SOCKET,QUEUESIZE
Does the same thing that the listen(2) system call does. Returns true if it succeeded, false otherwise. See the example in "Sockets: Client/Server Communication" in perlipc. Networking era 💖
📍 local EXPR
Okay so real talk? You really probably want to be using my instead, because local isn't what most people think of as "local". It's giving temporary localized value, not brand new variable, and those are two deeply different vibes. See "Private Variables via my()" in perlsub for the details.
A local modifies the listed variables to be local to the enclosing block, file, or eval. If more than one value is listed, the list must be placed in parentheses. See "Temporary Values via local()" in perlsub for details, including issues with tied arrays and hashes.
Like my, state, and our, local can operate on a variable anywhere it appears in an expression (aside from interpolation in strings). Unlike the other declarations, the effect of local happens at runtime, and so it will apply to additional uses of the same variable executed after the declaration, even within the same statement. Note that this does not include uses within an expression assigned to the variable when it is localized, because the assigned expression is evaluated before the localization. The order of operations is not playing around. 😭
package main;
our $x = 2;
{
foo($x, local $x = $x + 1, $x); # foo() receives (2, 3, 3)
# $main::x is 3 within the call to foo()
}
foo($x); # foo() receives (2) and $main::x is 2The delete local EXPR construct can also be used to localize the deletion of array/hash elements to the current block. See "Localized deletion of elements of composite types" in perlsub. Sneaky and iconic. 🎀
🕰️ localtime EXPR
🕰️ localtime
Converts a time as returned by the time function to a 9-element list, with the time analyzed for the local time zone. If EXPR is omitted, localtime uses the current time (as returned by time). No args? No problem. ✨
Typically used as follows:
# 0 1 2 3 4 5 6 7 8
my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
localtime(time);All list elements are numeric and come straight out of the C struct tm. $sec, $min, and $hour are the seconds, minutes, and hours of the specified time.
$mday is the day of the month in the range 1..31 (i.e. 1-based). $mon is the month in the range 0..11 (i.e. 0-based), with 0 indicating January and 11 indicating December. This makes it easy to get a month name from a list:
my @abbr = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec);
print "$abbr[$mon] $mday";
# $mon=9, $mday=18 gives "Oct 18"$year contains the number of years since 1900 (e.g. 129 for 2029). Yes bestie, 1900. The boomers won that one. 💀
$wday is the day of the week, with 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of the year, in the range 0..364 (or 0..365 in leap years.)
$isdst is true if the specified time occurs when Daylight Saving Time is in effect, false otherwise.
To get a human-readable date/time string, use "strftime" in POSIX:
use POSIX qw(strftime);
my @now = localtime;
my $now_string = strftime "%Y-%m-%d %H:%M:%S", @now;
# e.g. "2025-11-29 15:19:02"To get just the year, you can use either "strftime" in POSIX:
use POSIX qw(strftime);
# full year:
my $year = strftime "%Y", localtime;
# just the last two digits of the year:
my $ar = strftime "%y", localtime;... or manual arithmetic:
my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
localtime;
# full year:
$year += 1900;
# just the last two digits of the year:
my $ar = sprintf("%02d", $year % 100);In scalar context, localtime returns the ctime(3) value:
my $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"This scalar value is always in English, and is not locale-dependent. To get similar but locale-dependent date strings, try for example:
use POSIX qw(strftime);
my $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
# or for GMT formatted appropriately for your locale:
my $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;$now_string will be formatted according to the current LC_TIME locale the program or thread is running in. See perllocale for how to set up and change that locale. Note that %a and %b, the short forms of the day of the week and the month of the year, may not necessarily be three characters wide. Do not assume. Do not come for me. 🙈
The Time::gmtime and Time::localtime modules provide a convenient, by-name access mechanism to the gmtime and localtime functions, respectively.
For a comprehensive date and time representation look at the DateTime module on CPAN.
For GMT instead of local time use the gmtime builtin.
See also the Time::Local module (for converting seconds, minutes, hours, and such back to the integer value returned by time), and the POSIX module's mktime function.
Portability issues: "localtime" in perlport.
🔒 lock THING
This function places an advisory lock on a shared variable or referenced object contained in THING until the lock goes out of scope. Keep it safe in here, bestie. 🫶
The value returned is the scalar itself, if the argument is a scalar, or a reference, if the argument is a hash, array or subroutine.
lock is a "weak keyword"; this means that if you've defined a function by this name (before any calls to it), that function will be called instead. If you are not under use threads::shared this does nothing. See threads::shared.
📈 log EXPR
📈 log
Returns the natural logarithm (base e) of EXPR. If EXPR is omitted, returns the log of $_. To get the log of another base, use basic algebra: The base-N log of a number is equal to the natural log of that number divided by the natural log of N. Math class said you'd need this and honestly? She was right. 🧚
For example:
sub log10 {
my $n = shift;
return log($n)/log(10);
}See also "exp" for the inverse operation. They're a matched set, your honor. 💖
🔍 lstat FILEHANDLE
🔍 lstat EXPR
🔍 lstat DIRHANDLE
🔍 lstat
Does the same thing as the "stat" function (including setting the special _ filehandle) but stats a symbolic link instead of the file the symbolic link points to. So you get the link's tea, not the target's tea. 💅 If symbolic links are unimplemented on your system, a normal stat is done. For much more detailed information, please see the documentation for "stat".
If EXPR is omitted, stats $_.
Portability issues: "lstat" in perlport.
🎯 m//
The match operator. See "Regexp Quote-Like Operators" in perlop.
🗺️ map BLOCK LIST
🗺️ map EXPR,LIST
Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each element) and composes a list of the results of each such evaluation. Each element of LIST may produce zero, one, or more elements in the generated list, so the number of elements in the generated list may differ from that in LIST. In scalar context, returns the total number of elements so generated. In list context, returns the generated list. Transformative era ✨
my @chars = map(chr, @numbers);translates a list of numbers to the corresponding characters.
my @squares = map { $_ * $_ } @numbers;translates a list of numbers to their squared values.
my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;shows that number of returned elements can differ from the number of input elements. To omit an element, return an empty list (). This could also be achieved by writing
my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;which makes the intention more clear. (Iconic move honestly — let grep do the filtering, let map do the transforming. Division of labor. 👑)
Map always returns a list, which can be assigned to a hash such that the elements become key/value pairs. See perldata for more details.
my %hash = map { get_a_key_for($_) => $_ } @array;is just a funny way to write
my %hash;
foreach (@array) {
$hash{get_a_key_for($_)} = $_;
}Note that $_ is an alias to the list value, so it can be used to modify the elements of the LIST. While this is useful and supported, it can cause bizarre results if the elements of LIST are not variables. Using a regular "foreach" loop for this purpose would be clearer in most cases. See also "grep" for a list composed of those items of the original list for which the BLOCK or EXPR evaluates to true.
{ starts both hash references and blocks, so map { ... could be either the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look ahead for the closing } it has to take a guess at which it's dealing with based on what it finds just after the {. Usually it gets it right, but if it doesn't it won't realize something is wrong until it gets to the } and encounters the missing (or unexpected) comma. The syntax error will be reported close to the }, but you'll need to change something near the { such as using a unary + or semicolon to give Perl some help. She's doing her best, help her out 💀
my %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong
my %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right
my %hash = map {; "\L$_" => 1 } @array # this also works
my %hash = map { ("\L$_" => 1) } @array # as does this
my %hash = map { lc($_) => 1 } @array # and this.
my %hash = map +( lc($_) => 1 ), @array # this is EXPR and works!
my %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)or to force an anon hash constructor use +{:
my @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs
# comma at endto get a list of anonymous hashes each with only one entry apiece. So many hashes. So little time. 🦋
🌟 method NAME BLOCK
🌟 method NAME : ATTRS BLOCK
Creates a new named method in the scope of the class that it appears within. This is only valid inside a "class" declaration. Very main character. Very OOP-coded. 👑
🌸 mkdir — make a whole directory, periodt
mkdir FILENAME,MODE
mkdir FILENAME
mkdirOkay so mkdir makes you a directory called FILENAME, with permissions given by MODE — and yes, MODE gets modified by your umask, as it should 💅. If it succeeds it returns true ✨. If it flops it returns false AND sets $! (that's errno, bestie). Skip MODE and it defaults to 0777. Skip FILENAME too and it defaults to $_.
Real talk though: in general it is better to create directories with a permissive MODE and let the user dial it back with their ownumask, than to supply a restrictive MODE and give the user no way to be more permissive. The exceptions to this rule are when the file or directory should be kept private (mail files, for instance). The documentation forumaskdiscusses the choice of MODE in more detail. Also heads up 👀: if bits in MODE other than the permission bits are set, the result may be implementation defined, per POSIX 1003.1-2008.
Fun little history moment: according to the POSIX 1003.1-1996, FILENAME may have any number of trailing slashes. Some operating and filesystems do not get this right 😭, so Perl automatically removes all trailing slashes to keep everyone happy 🫶.
Want to recursively create a directory structure? Go look at the make_path function of the File::Path module. Iconic behavior. 🌈
📬 msgctl — System V IPC, but make it sysadmin
msgctl ID,CMD,ARGCalls the System V IPC function msgctl(2). You'll probably have to say
use IPC::SysV;first to get the correct constant definitions. If CMD is IPC_STAT, then ARG must be a variable that will hold the returned msqid_ds structure. Returns like ioctl: the undefined value for error, 0 but true for zero, or the actual return value otherwise. See also "SysV IPC" in perlipc and the documentation for IPC::SysV and IPC::Semaphore.
Portability issues: msgctl in perlport. 🦋
📨 msgget — get you a message queue id
msgget KEY,FLAGSCalls the System V IPC function msgget(2). Returns the message queue id, or undef on error 😭. See also "SysV IPC" in perlipc and the documentation for IPC::SysV and IPC::Msg.
Portability issues: msgget in perlport.
📥 msgrcv — receive the message, bestie
msgrcv ID,VAR,SIZE,TYPE,FLAGSCalls the System V IPC function msgrcv to receive a message from message queue ID into variable VAR with a maximum message size of SIZE. Note that when a message is received, the message type as a native long integer will be the first thing in VAR, followed by the actual message. This packing may be opened with unpack("l! a*"). Taints the variable. Returns true if successful, false on error. See also "SysV IPC" in perlipc and the documentation for IPC::SysV and IPC::Msg.
Portability issues: msgrcv in perlport.
📤 msgsnd — send it, no cap
msgsnd ID,MSG,FLAGSCalls the System V IPC function msgsnd to send the message MSG to the message queue ID. MSG must begin with the native long integer message type, followed by the message itself. This kind of packing can be achieved with pack("l! a*", $type, $message). Returns true if successful, false on error. See also "SysV IPC" in perlipc and the documentation for IPC::SysV and IPC::Msg.
Portability issues: msgsnd in perlport.
🎀 my — your variables, your rules, lexically scoped
my VARLIST
my TYPE VARLIST
my VARLIST : ATTRS
my TYPE VARLIST : ATTRSA my declares the listed variables to be local (lexically) to the enclosing block, file, or eval. If more than one variable is listed, the list must be placed in parentheses. 🫶
Note that with a parenthesised list, undef can be used as a dummy placeholder, for example to skip assignment of initial values:
my ( undef, $min, $hour ) = localtime;Like state, local, and our, my can operate on a variable anywhere it appears in an expression (aside from interpolation in strings). The declaration will not apply to additional uses of the same variable until the next statement. This means additional uses of that variable within the same statement will act as they would have before that declaration occurred, or result in a strict vars error, as appropriate. 😵💫
package main;
our $x = 2;
foo($x, my $x = $x + 1, $x); # foo() receives (2, 3, 2)
foo($x, $main::x); # foo() receives (3, 2)Redeclaring a variable in the same scope or statement will "shadow" the previous declaration, creating a new instance and preventing access to the previous one. This is usually undesired and, if warnings are enabled, will result in a warning in the shadow category. 💀
The exact semantics and interface of TYPE and ATTRS are still evolving. TYPE may be a bareword, a constant declared with use constant, or __PACKAGE__. It is currently bound to the use of the fields pragma, and attributes are handled using the attributes pragma, or starting from Perl 5.8.0 also via the Attribute::Handlers module. See "Private Variables via my()" in perlsub for details. ✨
⏭️ next — skip to the next iteration, queen
next LABEL
next EXPR
nextThe next command is like the continue statement in C; it starts the next iteration of the loop:
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
#...
}Note that if there were a continue block on the above, it would get executed even on discarded lines. If LABEL is omitted, the command refers to the innermost enclosing loop. The next EXPR form, available as of Perl 5.18.0, allows a label name to be computed at run time, being otherwise identical to next LABEL.
next cannot return a value from a block that typically returns a value, such as eval {}, sub {}, or do {}. It will perform its flow control behavior, which precludes any return value. It should not be used to exit a grep or map operation. 🙈
Note that a block by itself is semantically identical to a loop that executes once. Thus next will exit such a block early.
See also "continue" for an illustration of how last, next, and redo work. 🌸
Unlike most named operators, this has the same precedence as assignment. It is also exempt from the looks-like-a-function rule, so next ("foo")."bar" will cause bar to be part of the argument to next.
🚫 no — the anti-use, and that's the whole vibe
no MODULE VERSION LIST
no MODULE VERSION
no MODULE LIST
no MODULE
no VERSIONSee the use function, of which no is the opposite. 💅
🐙 oct — octal strings, decoded
oct EXPR
octInterprets EXPR as an octal string and returns the corresponding value. An octal string consists of octal digits and, as of Perl 5.33.5, an optional 0o or o prefix. Each octal digit may be preceded by a single underscore, which will be ignored. (If EXPR happens to start off with 0x or x, interprets it as a hex string. If EXPR starts off with 0b or b, it is interpreted as a binary string. Leading whitespace is ignored in all three cases.) 😌 The following will handle decimal, binary, octal, and hex in standard Perl notation:
$val = oct($val) if $val =~ /^0/;If EXPR is omitted, uses $_. To go the other way (produce a number in octal), use sprintf or printf:
my $dec_perms = (stat("filename"))[2] & 07777;
my $oct_perm_str = sprintf "%o", $perms;The oct function is commonly used when a string such as 644 needs to be converted into a file mode, for example. Although Perl automatically converts strings into numbers as needed, this automatic conversion assumes base 10. 🍓
Leading white space is ignored without warning, as too are any trailing non-digits, such as a decimal point (oct only handles non-negative integers, not negative integers or floating point).
🔓 open — the main character of filehandles
open FILEHANDLE,MODE,EXPR
open FILEHANDLE,MODE,EXPR,LIST
open FILEHANDLE,MODE,REFERENCE
open FILEHANDLE,EXPR
open FILEHANDLEAssociates an internal FILEHANDLE with the external file specified by EXPR. That filehandle will subsequently allow you to perform I/O operations on that file, such as reading from it or writing to it. 👑
Instead of a filename, you may specify an external command (plus an optional argument list) or a scalar reference, in order to open filehandles on commands or in-memory scalars, respectively. 🌈
A thorough reference to open follows. For a gentler introduction to the basics of open, see also the perlopentut manual page. 🫶
💖 Working with files
Most often, open gets invoked with three arguments: the required FILEHANDLE (usually an empty scalar variable), followed by MODE (usually a literal describing the I/O mode the filehandle will use), and then the filename that the new filehandle will refer to.
✨ Simple examples
Reading from a file:
open(my $fh, "<", "input.txt")
or die "Can't open < input.txt: $!";
# Process every line in input.txt
while (my $line = readline($fh)) {
#
# ... do something interesting with $line here ...
#
}or writing to one:
open(my $fh, ">", "output.txt")
or die "Can't open > output.txt: $!";
print $fh "This line gets printed into output.txt.\n";For a summary of common filehandle operations such as these, see "Files and I/O" in perlintro. 🌸
✨ About filehandles
The first argument to open, labeled FILEHANDLE in this reference, is usually a scalar variable. (Exceptions exist, described in "Other considerations", below.) If the call to open succeeds, then the expression provided as FILEHANDLE will get assigned an open filehandle. That filehandle provides an internal reference to the specified external file, conveniently stored in a Perl variable, and ready for I/O operations such as reading and writing. 💅
✨ About modes
When calling open with three or more arguments, the second argument — labeled MODE here — defines the open mode. MODE is usually a literal string comprising special characters that define the intended I/O role of the filehandle being created: whether it's read-only, or read-and-write, and so on.
If MODE is "<", bestie, we're opening that file for input (read-only, no touchy). If MODE is ">", we're opening for output, which means any file that already exists gets truncated ("clobbered" 💀) first and any that doesn't gets freshly created. And if MODE is ">>", we're appending, creating the file if it's not there yet.
You can slap a "+" in front of that ">" or "<" when you want both read and write access, so like: "+<" is almost always the move for read/write updates, because the "+>" mode would clobber the file first 😭. Real talk though — you can't usually use either read-write mode for updating textfiles, since they have variable-length records. See the -i switch in perlrun for a better approach. The file gets created with permissions of 0666 modified by the process's umask value ✨.
These various prefixes line up with the fopen(3) modes of "r", "r+", "w", "w+", "a", and "a+".
More examples of different modes in action:
# Open a file for concatenation
open(my $log, ">>", "/usr/spool/news/twitlog")
or warn "Couldn't open log file; discarding input";
# Open a file for reading and writing
open(my $dbase, "+<", "dbase.mine")
or die "Can't open 'dbase.mine' for update: $!";💖 Checking the return value 💖
open returns nonzero on success, and the undefined value otherwise. If the open involved a pipe, the return value just so happens to be the pid of the subprocess.
When you're opening a file, continuing after a failed request is seldom a good idea, so open is like constantly used with die. And even if you wanna do something other than die on a failed open, bestie, you should still always check the return value from opening a file. No cap.
🌸 Specifying I/O layers in MODE 🌸
You can use the three-argument form of open to specify I/O layers (sometimes called "disciplines") to apply to the new filehandle. These affect how input and output get processed (peep open and PerlIO for more deets). For example:
# loads PerlIO::encoding automatically
open(my $fh, "<:encoding(UTF-8)", $filename)
|| die "Can't open UTF-8 encoded $filename: $!";This opens the UTF8-encoded file containing Unicode characters; see perluniintro. Note that if layers are specified in the three-argument form, then default layers stored in ${^OPEN} (usually set by the open pragma or the switch -CioD) are ignored. Those layers will also be ignored if you specify a colon with no name following it. In that case the default layer for the operating system (:raw on Unix, :crlf on Windows) is used.
On some systems (in general, DOS- and Windows-based systems) binmode is necessary when you're not working with a text file. For the sake of portability it's a good idea to always use it when appropriate, and never to use it when it isn't appropriate. Also, people can set their I/O to be by default UTF8-encoded Unicode, not bytes. 👑
🦋 Using undef for temporary files 🦋
As a special case, the three-argument form with a read/write mode and the third argument being undef:
open(my $tmp, "+>", undef) or die ...opens a filehandle to a newly created empty anonymous temporary file. (This happens under any mode, which makes "+>" the only useful and sensible mode to use.) You will need to seek to do the reading.
🎀 Opening a filehandle into an in-memory scalar 🎀
You can open filehandles directly to Perl scalars instead of a file or other resource external to the program. To do so, just hand over a reference to that scalar as the third argument to open, like so:
open(my $memory, ">", \$var)
or die "Can't open memory file: $!";
print $memory "foo!\n"; # output will appear in $varTo (re)open STDOUT or STDERR as an in-memory file, close it first:
close STDOUT;
open(STDOUT, ">", \$variable)
or die "Can't open STDOUT: $!";The scalars for in-memory files are treated as octet strings: unless the file is being opened with truncation, the scalar may not contain any code points over 0xFF.
Opening in-memory files can fail for a variety of reasons. As with any other open, check the return value for success 🙏.
Technical note: This feature works only when Perl is built with PerlIO — the default, except with older (pre-5.16) Perl installations that were configured to not include it (e.g. viaConfigure -Uuseperlio). You can see whether your Perl was built with PerlIO by runningperl -V:useperlio. If it says'define', you have PerlIO; otherwise you don't.
See perliol for detailed info on PerlIO.
💅 Opening a filehandle into a command 💅
If MODE is "|-", then the filename gets interpreted as a command to which output is piped, and if MODE is "-|", the filename is interpreted as a command that pipes output to us. In the two-argument (and one-argument) form, one should replace dash ("-") with the command. See "Using open() for IPC" in perlipc for more examples of this. (You are not allowed to open to a command that pipes both in and out, but see IPC::Open2, IPC::Open3, and "Bidirectional Communication with Another Process" in perlipc for alternatives.)
open(my $article_fh, "-|", "caesar <$article") # decrypt
# article
or die "Can't start caesar: $!";
open(my $article_fh, "caesar <$article |") # ditto
or die "Can't start caesar: $!";
open(my $out_fh, "|-", "sort >Tmp$$") # $$ is our process id
or die "Can't start sort: $!";In the form of pipe opens taking three or more arguments, if LIST is specified (extra arguments after the command name) then LIST becomes arguments to the command invoked if the platform supports it. The meaning of open with more than three arguments for non-pipe modes is not yet defined, but experimental "layers" may give extra LIST arguments meaning.
If you open a pipe on the command "-" (that is, specify either "|-" or "-|" with the one- or two-argument forms of open), an implicit fork is done, so open returns twice: in the parent process it returns the pid of the child process, and in the child process it returns (a defined) 0. Use defined($pid) or "//" to determine whether the open was successful.
For example, use either
my $child_pid = open(my $from_kid, "-|")
// die "Can't fork: $!";or
my $child_pid = open(my $to_kid, "|-")
// die "Can't fork: $!";followed by
if ($child_pid) {
# am the parent:
# either write $to_kid or else read $from_kid
...
waitpid $child_pid, 0;
} else {
# am the child; use STDIN/STDOUT normally
...
exit;
}The filehandle behaves normally for the parent, but I/O to that filehandle is piped from/to the STDOUT/STDIN of the child process. In the child process, the filehandle isn't opened — I/O happens from/to the new STDOUT/STDIN. Typically this is used like the normal piped open when you want to exercise more control over just how the pipe command gets executed, like when running setuid and you don't wanna have to scan shell commands for metacharacters 🔥.
The following blocks are more or less equivalent:
open(my $fh, "|tr '[a-z]' '[A-Z]'");
open(my $fh, "|-", "tr '[a-z]' '[A-Z]'");
open(my $fh, "|-") || exec 'tr', '[a-z]', '[A-Z]';
open(my $fh, "|-", "tr", '[a-z]', '[A-Z]');
open(my $fh, "cat -n '$file'|");
open(my $fh, "-|", "cat -n '$file'");
open(my $fh, "-|") || exec "cat", "-n", $file;
open(my $fh, "-|", "cat", "-n", $file);The last two examples in each block show the pipe as "list form", which is not yet supported on all platforms. (If your platform has a real fork, such as Linux and macOS, you can use the list form; it also works on Windows with Perl 5.22 or later.) You would want to use the list form of the pipe so you can pass literal arguments to the command without risk of the shell interpreting any shell metacharacters in them. However, this also bars you from opening pipes to commands that intentionally contain shell metacharacters, such as:
open(my $fh, "|cat -n | expand -4 | lpr")
|| die "Can't open pipeline to lpr: $!";See "Safe Pipe Opens" in perlipc for more examples of this.
🫶 Duping filehandles 🫶
You may also, in the Bourne shell tradition, specify an EXPR beginning with ">&", in which case the rest of the string gets interpreted as the name of a filehandle (or file descriptor, if numeric) to be duped (as in dup(2)) and opened. You may use "&" after ">", ">>", "<", "+>", "+>>", and "+<". The mode you specify should match the mode of the original filehandle. (Duping a filehandle does not take into account any existing contents of IO buffers.) If you use the three-argument form, then you can pass either a number, the name of a filehandle, or the normal "reference to a glob".
Here's a script that saves, redirects, and restores STDOUT and STDERR using various methods:
#!/usr/bin/perl
open(my $oldout, ">&STDOUT")
or die "Can't dup STDOUT: $!";
open(OLDERR, ">&", \*STDERR)
or die "Can't dup STDERR: $!";
open(STDOUT, '>', "foo.out")
or die "Can't redirect STDOUT: $!";
open(STDERR, ">&STDOUT")
or die "Can't dup STDOUT: $!";select STDERR; $| = 1; # make unbuffered
select STDOUT; $| = 1; # make unbuffered
print STDOUT "stdout 1\n"; # this works for
print STDERR "stderr 1\n"; # subprocesses too
open(STDOUT, ">&", $oldout)
or die "Can't dup \$oldout: $!";
open(STDERR, ">&OLDERR")
or die "Can't dup OLDERR: $!";
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";periodt, that's the mad scientist version of swapping your output streams around 💅
So if you specify '<&=X', where X is a file descriptor number or a filehandle, then Perl will do an equivalent of C's fdopen(3) of that file descriptor (and not call dup(2), bestie — no duplicating today). This is more parsimonious of file descriptors. For example:
# open for input, reusing the fileno of $fd
open(my $fh, "<&=", $fd)or
open(my $fh, "<&=$fd")or
# open for append, using the fileno of $oldfh
open(my $fh, ">>&=", $oldfh)Being parsimonious on filehandles is also useful (besides being parsimonious, which is its own whole vibe) for example when something is dependent on file descriptors — like for example locking using flock. If you do just open(my $A, ">>&", $B), the filehandle $A will not have the same file descriptor as $B, and therefore flock($A) will not flock($B) nor vice versa. But with open(my $A, ">>&=", $B), the filehandles will share the same underlying system file descriptor. 🫶
Heads up bestie: under Perls older than 5.8.0, Perl uses the standard C library'sfdopen(3)to implement the=functionality. On many Unix systems,fdopen(3)fails when file descriptors exceed a certain value, typically255. For Perls 5.8.0 and later, PerlIO is (most often) the default.
🌈 Legacy usage 🌈
This section describes ways to call open outside of best practices; you may encounter these uses in older code. Perl does not consider their use deprecated, exactly, but neither is it recommended in new code, for the sake of clarity and readability. It's giving "vintage, not vintage-chic" 🎀
Specifying mode and filename as a single argument
In the one- and two-argument forms of the call, the mode and filename should be concatenated (in that order), preferably separated by white space. You can — but shouldn't — omit the mode in these forms when that mode is <. It is safe to use the two-argument form of open if the filename argument is a known literal.
open(my $dbase, "+<dbase.mine") # ditto
or die "Can't open 'dbase.mine' for update: $!";In the two-argument (and one-argument) form, opening <- or - opens STDIN and opening >- opens STDOUT.
New code should favor the three-argument form of open over this older form. Declaring the mode and the filename as two distinct arguments avoids any confusion between the two. fr fr, separate them, they're not a couple 💀
Assigning a filehandle to a bareword
An older style is to use a bareword as the filehandle, as
open(FH, "<", "input.txt")
or die "Can't open < input.txt: $!";Then you can use FH as the filehandle, in close FH and <FH> and so on. Note that it's a global variable, so this form is not recommended when dealing with filehandles other than Perl's built-in ones (e.g. STDOUT and STDIN). In fact, using a bareword for the filehandle is an error when the bareword_filehandles feature has been disabled. This feature is disabled automatically when in the scope of use v5.36.0 or later. so yeah, that ship has lowkey sailed ⛵
Calling open with one argument via global variables
As a shortcut, a one-argument call takes the filename from the global scalar variable of the same name as the bareword filehandle:
$ARTICLE = 100;
open(ARTICLE)
or die "Can't find article $ARTICLE: $!\n";Here $ARTICLE must be a global scalar variable in the same package as the filehandle — not one declared with my or state. no lexical sneaking allowed 🙈
✨ Other considerations ✨
Automatic filehandle closure
The filehandle will be closed when its reference count reaches zero. If it is a lexically scoped variable declared with my, that usually means the end of the enclosing scope. However, this automatic close does not check for errors, so it is better to explicitly close filehandles, especially those used for writing:
close($handle)
|| warn "close failed: $!";Automatic pipe flushing
Perl will attempt to flush all files opened for output before any operation that may do a fork, but this may not be supported on some platforms (see perlport). To be safe, you may need to set $| ($AUTOFLUSH in English) or call the autoflush method of IO::Handle on any open handles.
On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor as determined by the value of $^F. See "$^F" in perlvar.
Closing any piped filehandle causes the parent process to wait for the child to finish, then returns the status value in $? and ${^CHILD_ERROR_NATIVE}.
Direct versus by-reference assignment of filehandles
If FILEHANDLE — the first argument in a call to open — is an undefined scalar variable (or array or hash element), a new filehandle is autovivified, meaning that the variable is assigned a reference to a newly allocated anonymous filehandle. Otherwise if FILEHANDLE is an expression, its value is the real filehandle. (This is considered a symbolic reference, so use strict "refs" should not be in effect.) 👑
Whitespace and special characters in the filename argument
The filename passed to the one- and two-argument forms of open will have leading and trailing whitespace deleted and normal redirection characters honored. This property, known as "magic open", can often be used to good effect. A user could specify a filename of rsh cat file |, or you could change certain filenames as needed:
$filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
open(my $fh, $filename)
or die "Can't open $filename: $!";Use the three-argument form to open a file with arbitrary weird characters in it,
open(my $fh, "<", $file)
|| die "Can't open $file: $!";otherwise it's necessary to protect any leading and trailing whitespace:
$file =~ s#^(\s)#./$1#;
open(my $fh, "< $file\0")
|| die "Can't open $file: $!";(this may not work on some bizarre filesystems). One should conscientiously choose between the magic and three-argument form of open:
open(my $in, $ARGV[0]) || die "Can't open $ARGV[0]: $!";will allow the user to specify an argument of the form rsh cat file |, but will not work on a filename that happens to have a trailing space, while
open(my $in, "<", $ARGV[0])
|| die "Can't open $ARGV[0]: $!";will have exactly the opposite restrictions. (However, some shells support the syntax perl your_program.pl <( rsh cat file ), which produces a filename that can be opened normally.) pick your fighter, bestie 🍓
Invoking C-style open
If you want a "real" C open(2), then you should use the sysopen function, which involves no such magic (but uses different filemodes than Perl open, which corresponds to C fopen(3)). This is another way to protect your filenames from interpretation. For example:
use IO::Handle;
sysopen(my $fh, $path, O_RDWR|O_CREAT|O_EXCL)
or die "Can't open $path: $!";
$fh->autoflush(1);
print $fh "stuff $$\n";
seek($fh, 0, 0);
print "File contains: ", readline($fh);See "seek" for some details about mixing reading and writing.
Portability issues
See "open" in perlport. 🌍
🦋 opendir DIRHANDLE,EXPR 🦋
Opens a directory named EXPR for processing by "readdir", "telldir", "seekdir", "rewinddir", and "closedir". Returns true if successful. DIRHANDLE may be an expression whose value can be used as an indirect dirhandle, usually the real dirhandle name. If DIRHANDLE is an undefined scalar variable (or array or hash element), the variable is assigned a reference to a new anonymous dirhandle; that is, it's autovivified. Dirhandles are the same objects as filehandles; an I/O object can only be open as one of these handle types at once. it's giving "one label, one job" 💅
See the example at "readdir".
💜 ord EXPR / ord 💜
Returns the code point of the first character of EXPR. If EXPR is an empty string, returns 0. If EXPR is omitted, uses $_. (Note character, not byte — very important distinction, bestie, this is not a bytes situation.)
For the reverse, see "chr". See perlunicode for more about Unicode. ✨
🌸 our VARLIST / our TYPE VARLIST / our VARLIST : ATTRS / our TYPE VARLIST : ATTRS 🌸
our makes a lexical alias to a package (i.e. global) variable of the same name in the current package for use within the current lexical scope.
our has the same scoping rules as my or state, meaning that it is only valid within a lexical scope. Unlike my and state, which both declare new (lexical) variables, our only creates an alias to an existing variable: a package variable of the same name.
This means that when use strict 'vars' is in effect, our lets you use a package variable without qualifying it with the package name, but only within the lexical scope of the our declaration. This applies immediately — even within the same statement. no waiting, no warming up, instant 🫶
package Foo;
use v5.36; # which implies "use strict;"
$Foo::foo = 23;{
our $foo; # alias to $Foo::foo
print $foo; # prints 23
}
print $Foo::foo; # prints 23
print $foo; # ERROR: requires explicit package nameokay bestie so this works even if the package variable has never been used before — package variables literally just spring into existence the second you first use them. no form, no announcement, they just manifest. ✨
package Foo;
use v5.36;
our $foo = 23; # just like $Foo::foo = 23
print $Foo::foo; # prints 23and here's the ✨slay✨ part: because the variable becomes legal immediately under use strict 'vars' — as long as there's no variable with that name already chillin' in scope — you can turn around and reference that package variable again, even inside the exact same statement. she's that fast. 💅
package Foo;
use v5.36;
my $foo = $foo; # error, undeclared $foo on right-hand side
our $foo = $foo; # no errorsif you're listing more than one variable, tho? the list goes in parentheses. no exceptions, it's giving group photo rules:
our($bar, $baz);ngl our moves like my, state, and local — it can operate on a variable anywhere it appears in an expression (the only place it can't is interpolation inside strings, rip). and the declaration does not apply to extra uses of the same variable until the next statement. so any additional uses of that variable inside the same statement act like they did before the declaration happened — with one iconic exception: it still satisfies strict 'vars', and it'll interpret that variable as the newly aliased package variable if it wasn't already declared in that scope. 😌
package main;
my $x = 2;
foo($x, our $x = $x + 1, $x); # foo() receives (2, 3, 2)
foo($x, our $z = 5, $z); # foo() receives (3, 5, 5)an our declaration declares an alias for a package variable that's visible across its entire lexical scope, even across package boundaries. and the package the variable gets entered into is decided at the point of the declaration, not at the point of use. so this behavior holds, fr fr:
package Foo;
our $bar; # declares $Foo::bar for rest of lexical scope
$bar = 20;
package Bar;
print $bar; # prints 20, as it refers to $Foo::barmultiple our declarations with the same name in the same lexical scope? allowed, if they're in different packages. 🫶 but if they happen to be in the same package, Perl will emit warnings (if you asked for them), just like multiple my declarations do. and unlike a second my — which binds the name to a fresh new variable — a second our in the same package, in the same scope, is just... redundant. she said nothing new. 💀
use warnings;
package Foo;
our $bar; # declares $Foo::bar for rest of lexical scope
$bar = 20;
package Bar;
our $bar = 30; # declares $Bar::bar for rest of lexical scope
print $bar; # prints 30
our $bar; # emits warning but has no other effect
print $bar; # still prints 30an our declaration can also carry a whole list of attributes with it. 💖
the exact semantics and interface of TYPE and ATTRS are still evolving (yes, she's a work in progress, we love her anyway). TYPE is currently bound to the use of the fields pragma, and attributes are handled using the attributes pragma — or, starting from Perl 5.8.0, also via the Attribute::Handlers module. see "Private Variables via my()" in perlsub for the details. 👑
also quick note: with a parenthesised list, undef can be used as a dummy placeholder, for example to skip assignment of initial values:
our ( undef, $min, $hour ) = localtime;and our creates a lexically scoped alias to the package variable of the same name — which is exactly how it differs from use vars, which lets you use an unqualified name only within the affected package, with zero consideration for scopes. different vibe entirely, iykyk. 🌸
📦 pack TEMPLATE,LIST 💅
okay so pack takes a LIST of values and converts it into a string using the rules from the TEMPLATE. 🎀 the resulting string is just the concatenation of all those converted values. typically each converted value looks like its machine-level representation — for example, on 32-bit machines an integer may be represented by a sequence of 4 bytes, which in Perl shows up as a string that's 4 characters long.
see perlpacktut for an intro to this function. ✨
the TEMPLATE is a sequence of characters that tells you the order and type of the values, like this:
a A string with arbitrary binary data, will be null padded.
A A text (ASCII) string, will be space padded.
Z A null-terminated (ASCIZ) string, will be null padded.
b A bit string (ascending bit order inside each byte,
like vec()).
B A bit string (descending bit order inside each byte).
h A hex string (low nybble first).
H A hex string (high nybble first).
c A signed char (8-bit) value.
C An unsigned char (octet) value.
W An unsigned char value (can be greater than 255).
s A signed short (16-bit) value.
S An unsigned short value.
l A signed long (32-bit) value.
L An unsigned long value.
q A signed quad (64-bit) value.
Q An unsigned quad value.
(Quads are available only if your system supports 64-bit
integer values _and_ if Perl has been compiled to support
those. Raises an exception otherwise.)
i A signed integer value.
I An unsigned integer value.
(This 'integer' is _at_least_ 32 bits wide. Its exact
size depends on what a local C compiler calls 'int'.)
n An unsigned short (16-bit) in "network" (big-endian) order.
N An unsigned long (32-bit) in "network" (big-endian) order.
v An unsigned short (16-bit) in "VAX" (little-endian) order.
V An unsigned long (32-bit) in "VAX" (little-endian) order.
j A Perl internal signed integer value (IV).
J A Perl internal unsigned integer value (UV).
f A single-precision float in native format.
d A double-precision float in native format.
F A Perl internal floating-point value (NV) in native format
D A float of long-double precision in native format.
(Long doubles are available only if your system supports
long double values. Raises an exception otherwise.
Note that there are different long double formats.)
p A pointer to a null-terminated string.
P A pointer to a structure (fixed-length string).
u A uuencoded string.
U A Unicode character number. Encodes to a character in char-
acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in
byte mode. Also on EBCDIC platforms, the character number will
be the native EBCDIC value for character numbers below 256.
This allows most programs using this feature to not have to
care which type of platform they are running on.
w A BER compressed integer (not an ASN.1 BER, see perlpacktut
for details). Its bytes represent an unsigned integer in
base 128, most significant digit first, with as few digits
as possible. Bit eight (the high bit) is set on each byte
except the last.
x A null byte (a.k.a. ASCII NUL, "\000", chr(0))
X Back up a byte.
@ Null-fill or truncate to absolute position, counted from the
start of the innermost ()-group.
. Null-fill or truncate to absolute position specified by
the value.
( Start of a ()-group.one or more of the modifiers below may optionally follow certain letters in the TEMPLATE — the second column lists the letters the modifier is valid for, bestie, don't skip it 👀:
!withsSlLiI— forces native (short, long, int) sizes instead of fixed (16-/32-bit) sizes.!withxX— makexandXact as alignment commands.!withnNvV— treat integers as signed instead of unsigned.!with@.— specify position as byte offset in the internal representation of the packed string. Efficient but dangerous. 🙈 (she said what she said)>withsSiIlLqQ,jJfFdDpP— force big-endian byte-order on the type. (The "big end" touches the construct.)<withsSiIlLqQ,jJfFdDpP— force little-endian byte-order on the type. (The "little end" touches the construct.)
the > and < modifiers can also be used on () groups to force a particular byte-order on all components in that group, including all its subgroups. the whole squad gets the same treatment. 💖
the following rules apply:
- each letter may optionally be followed by a number indicating the repeat count. a numeric repeat count may optionally be enclosed in brackets, as in
pack("C[80]", @arr). the repeat count gobbles that many values from the LIST when used with all format types other thana,A,Z,b,B,h,H,@,.,x,X, andP, where it means something else (described below). supplying a*for the repeat count instead of a number means to use however many items are left — except for:@,x, andX, where it is equivalent to0.<.>, where it means relative to the start of the string.u, where it is equivalent to1(or45, which here is equivalent).
one can replace a numeric repeat count with a template letter enclosed in brackets to use the packed byte length of the bracketed template for the repeat count.
for example, the template x[L] skips as many bytes as in a packed long, and the template $t X[$t] $t unpacks twice whatever $t (when variable-expanded) unpacks. 💅 and if the template in brackets contains alignment commands (such as x![d]), its packed length is calculated as if the start of the template had the maximal possible alignment.
when used with Z, a * as the repeat count is guaranteed to add a trailing null byte, so the resulting string is always one byte longer than the byte length of the item itself. no cap. 🔥
when used with @, the repeat count represents an offset from the start of the innermost () group.
when used with ., the repeat count determines the starting position to calculate the value offset as follows:
- If the repeat count is
0, it's relative to the current position. 📍- If the repeat count is
"*", the offset is relative to the start of the packed string. 🌟 - And if it's an integer n, the offset is relative to the start of the nth innermost
"( )"group, or to the start of the string if n is bigger then the group level. 🌀
- If the repeat count is
The repeat count for "u" is interpreted as the maximal number of bytes to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat count should not be more than 65. (fr fr do not go past 65, bestie)
a,A,Z— these girlies gobble just one value, but pack it as a string of length count, padding with nulls or spaces as needed. 💅 When unpacking,Astrips trailing whitespace and nulls,Zstrips everything after the first null (bye bestie 👋), andareturns data with no stripping at all — raw, unbothered, icon behavior.
If the value to pack is too long, the result is truncated ✂️. If it's too long and an explicit count is provided, Z packs only $count-1 bytes, followed by a null byte. So Z always packs a trailing null, except when the count is 0. 🫶
b,B— likewise, these pack a string that's that many bits long. Each such format generates 1 bit of the result. These are typically followed by a repeat count likeB8orB64. ✨
Each result bit is based on the least-significant bit of the corresponding input character, i.e., on ord($char)%2. In particular, characters "0" and "1" generate bits 0 and 1, as do characters "\000" and "\001".
Starting from the beginning of the input string, each 8-tuple of characters is converted to 1 character of output. With format b, the first character of the 8-tuple determines the least-significant bit of a character; with format B, it determines the most-significant bit of a character. 🧠
If the length of the input string is not evenly divisible by 8, the remainder is packed as if the input string were padded by null characters at the end. Similarly during unpacking, "extra" bits are ignored. 🚮
If the input string is longer than needed, remaining characters are ignored.
A "*" for the repeat count uses all characters of the input field. On unpacking, bits are converted to a string of 0s and 1s. 🌈
h,H— these pack a string that many nybbles (4-bit groups, representable as hexadecimal digits,"0".."9""a".."f") long. ✨
For each such format, pack generates 4 bits of result. With non-alphabetical characters, the result is based on the 4 least-significant bits of the input character, i.e., on ord($char)%16. In particular, characters "0" and "1" generate nybbles 0 and 1, as do bytes "\000" and "\001". For characters "a".."f" and "A".."F", the result is compatible with the usual hexadecimal digits, so that "a" and "A" both generate the nybble "0xA==10". Use only these specific hex characters with this format, okay?! 👑
Starting from the beginning of the template to pack, each pair of characters is converted to 1 character of output. With format h, the first character of the pair determines the least-significant nybble of the output character; with format H, it determines the most-significant nybble.
If the length of the input string is not even, it behaves as if padded by a null character at the end. Similarly, "extra" nybbles are ignored during unpacking. 🙈
If the input string is longer than needed, extra characters are ignored.
A "*" for the repeat count uses all characters of the input field. For unpack, nybbles are converted to a string of hexadecimal digits. 💜
p,P—ppacks a pointer to a null-terminated string. ⚠️ You are responsible for ensuring that the string is not a temporary value, as that could potentially get deallocated before you got around to using the packed result. bestie this is the danger zone, do NOT skip this warning 💀 ThePformat packs a pointer to a structure of the size indicated by the length. A null pointer is created if the corresponding value forporPisundef; similarly withunpack, where a null pointer unpacks intoundef.
If your system has a strange pointer size — meaning a pointer is neither as big as an int nor as big as a long — it may not be possible to pack or unpack pointers in big- or little-endian byte order. Attempting to do so raises an exception. 😵
/— this template character allows packing and unpacking of a sequence of items where the packed structure contains a packed item count followed by the packed items themselves. This is useful when the structure you're unpacking has encoded the sizes or repeat counts for some of its fields within the structure itself as separate fields. 🧩
For pack, you write length-item"/"sequence-item, and the length-item describes how the length value is packed. Formats likely to be of most use are integer-packing ones like n for Java strings, w for ASN.1 or SNMP, and N for Sun XDR. ✨
For pack, sequence-item may have a repeat count, in which case the minimum of that and the number of available items is used as the argument for length-item. If it has no repeat count or uses a '*', the number of available items is used.
For unpack, an internal stack of integer arguments unpacked so far is used. You write "/"sequence-item and the repeat count is obtained by popping off the last element from the stack. The sequence-item must not have a repeat count. 🚫
If sequence-item refers to a string type (A, a, or Z), the length-item is the string length, not the number of strings. With an explicit repeat count for pack, the packed string is adjusted to that length. For example:
This code: gives this result:
unpack("W/a", "\004Gurusamy") ("Guru")
unpack("a3/A A*", "007 Bond J ") (" Bond", "J")
unpack("a3 x2 /A A*", "007: Bond, J.") ("Bond, J", ".")
pack("n/a* w/a", "hi,", "world") "\000\003hi,\005world"
pack("a/W2", ord("a") .. ord("z")) "2ab"The length-item is not returned explicitly from unpack. 🫥
Supplying a count to the length-item format letter is only useful with A, a, or Z. Packing with a length-item of a or Z may introduce "\000" characters, which Perl does not regard as legal in numeric strings. 💅
s,S,l,L— the integer types may be followed by a!modifier to specify native shorts or longs. As shown in the example above, a barelmeans exactly 32 bits, although the nativelongas seen by the local C compiler may be larger. This is mainly an issue on 64-bit platforms. 🔍 You can see whether using!makes any difference this way:
printf "format s is %d, s! is %d\n",
length pack("s"), length pack("s!");
printf "format l is %d, l! is %d\n",
length pack("l"), length pack("l!");i! and I! are also allowed, but only for completeness' sake: they are identical to i and I. 🤷
The actual sizes (in bytes) of native shorts, ints, longs, and long longs on the platform where Perl was built are also available from the command line:
$ perl -V:{short,int,long{,long}}size
shortsize='2';
intsize='4';
longsize='4';
longlongsize='8';or programmatically via the Config module:
use Config;
print $Config{shortsize}, "\n";
print $Config{intsize}, "\n";
print $Config{longsize}, "\n";
print $Config{longlongsize}, "\n";$Config{longlongsize} is undefined on systems without long long support. 🫠
- ⚠️ non-portability alert — the integer formats
s,S,i,I,l,L,j, andJare inherently non-portable between processors and operating systems because they obey native byteorder and endianness. For example, a 4-byte integer0x12345678(305419896 decimal) would be ordered natively (arranged in and handled by the CPU registers) into bytes as
0x12 0x34 0x56 0x78 # big-endian
0x78 0x56 0x34 0x12 # little-endianBasically, Intel and VAX CPUs are little-endian, while everybody else, including Motorola m68k/88k, PPC, Sparc, HP PA, Power, and Cray, are big-endian. Alpha and MIPS can be either: Digital/Compaq uses (well, used) them in little-endian mode, but SGI/Cray uses them in big-endian mode. ✨
The names big-endian and little-endian are comic references to the egg-eating habits of the little-endian Lilliputians and the big-endian Blefuscudians from the classic Jonathan Swift satire, Gulliver's Travels. 📚 This entered computer lingo via the paper "On Holy Wars and a Plea for Peace" by Danny Cohen, USC/ISI IEN 137, April 1, 1980. iykyk 👑
Some systems may have even weirder byte orders such as
0x56 0x78 0x12 0x34
0x34 0x12 0x78 0x56These are called mid-endian, middle-endian, mixed-endian, or just weird. 💀
You can determine your system endianness with this incantation:
printf("%#02x ", $_) for unpack("W*", pack("L", 0x12345678));The byteorder on the platform where Perl was built is also available via Config:
use Config;
print "$Config{byteorder}\n";or from the command line:
$ perl -V:byteorderByteorders "1234" and "12345678" are little-endian; "4321" and "87654321" are big-endian. Systems with multiarchitecture binaries will have "ffff", signifying that static information doesn't work, one must use runtime probing. 🔎
For portably packed integers, either use the formats n, N, v, and V or else use the > and < modifiers described immediately below. See also perlport. 🌈
- Also, floating point numbers have endianness, bestie 😭. Usually (but not always) it agrees with the integer endianness. And even though basically everyone these days uses the IEEE 754 binary format, there are still differences, especially if long doubles are involved. You can go look at the
Configvariablesdoublekindandlongdblkind(and alsodoublesize,longdblsize): thekindvalues are enums, unlikebyteorder.
Portability-wise, the move 💅 is probably to keep to the IEEE 754 64-bit doubles, with an agreed-upon endianness. Another possibility is the %a format of printf.
- Starting with Perl 5.10.0, integer and floating-point formats, along with the
pandPformats and()groups, may all be followed by the>or<endianness modifiers, to respectively enforce big- or little-endian byte-order. These modifiers are especially useful given hown,N,v, andVdon't cover signed integers, 64-bit integers, or floating-point values. No cap. 🔥
Here are some concerns to keep in mind when using an endianness modifier:
- Exchanging signed integers between different platforms works only when all platforms store them in the same format. Most platforms store signed integers in two's-complement notation, so usually this is not an issue.
- The
>or<modifiers can only be used on floating-point formats on big- or little-endian machines. Otherwise, attempting to use them raises an exception. 🚨 - Forcing big- or little-endian byte-order on floating-point values for data exchange can work only if all platforms use the same binary representation such as IEEE floating-point. Even if all platforms are using IEEE, there may still be subtle differences. Being able to use
>or<on floating-point values can be useful, but also dangerous if you don't know exactly what you're doing 💀. It is not a general way to portably store floating-point values. - When using
>or<on a()group, this affects all types inside the group that accept byte-order modifiers, including all subgroups. It is silently ignored for all other types. You are not allowed to override the byte-order within a group that already has a byte-order modifier suffix.
- Real numbers (floats and doubles) are in native machine format only. Because there are soooo many floating-point formats and there's no standard "network" representation for them, no facility for interchange has been made. Which means packed floating-point data written on one machine may not be readable on another, even if both use IEEE floating-point arithmetic (because the endianness of the memory representation is not part of the IEEE spec). See also
perlport.
If you know exactly what you're doing, you can use the > or < modifiers to force big- or little-endian byte-order on floating-point values.
Because Perl uses doubles (or long doubles, if configured) internally for all numeric calculation, converting from double into float and thence to double again loses precision, so unpack("f", pack("f", $foo)) will not in general equal $foo. 😔
- Pack and unpack can operate in two modes: character mode (
C0mode) where the packed string is processed per character, and UTF-8 byte mode (U0mode) where the packed string is processed in its UTF-8-encoded Unicode form on a byte-by-byte basis. Character mode is the default unless the format string starts withU. You can always switch mode mid-format with an explicitC0orU0in the format. This mode remains in effect until the next mode change, or until the end of the()group it (directly) applies to.
Using C0 to get Unicode characters while using U0 to get non-Unicode bytes is not necessarily obvious. 👀 Probably only the first of these is what you want:
$ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
perl -CS -ne 'printf "%v04X\n", $_ for unpack("C0A*", $_)'
03B1.03C9
$ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
perl -CS -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
CE.B1.CF.89
$ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
perl -C0 -ne 'printf "%v02X\n", $_ for unpack("C0A*", $_)'
CE.B1.CF.89
$ perl -CS -E 'say "\x{3B1}\x{3C9}"' |
perl -C0 -ne 'printf "%v02X\n", $_ for unpack("U0A*", $_)'
C3.8E.C2.B1.C3.8F.C2.89Those examples also illustrate that you should not try to use pack/unpack as a substitute for the Encode module. fr fr 🫶
- You must yourself do any alignment or padding by inserting, for example, enough
xes while packing. There is no way forpackandunpackto know where characters are going to or coming from, so they handle their output and input as flat sequences of characters. - A
()group is a sub-TEMPLATE enclosed in parentheses. A group may take a repeat count either as postfix, or forunpack, also via the/template character. Within each repetition of a group, positioning with@starts over at 0. Therefore, the result of
# note: single quotes around the template string to prevent
# array interpolation of @1, @2, @3
pack('@1A((@2A)@3A)', qw[X Y Z])is the string "\0X\0\0YZ".
xandXaccept the!modifier to act as alignment commands: they jump forward or back to the closest position aligned at a multiple ofcountcharacters. For example, topackorunpacka C structure like
struct {
char c; /* one signed, 8-bit character */
double d;
char cc[2];
}one may need to use the template c x![d] d c[2]. This assumes that doubles must be aligned to the size of double.
For alignment commands, a count of 0 is equivalent to a count of 1; both are no-ops.
n,N,vandVaccept the!modifier to represent signed 16-/32-bit integers in big-/little-endian order. This is portable only when all platforms sharing packed data use the same binary representation for signed integers; for example, when all platforms use two's-complement representation.- Comments can be embedded in a TEMPLATE using
#through the end of line. White space can separate pack codes from each other, but modifiers and repeat counts must follow immediately. Breaking complex templates into individual line-by-line components, suitably annotated, can do as much to improve legibility and maintainability of pack/unpack formats as/xcan for complicated pattern matches. ✨ - If TEMPLATE requires more arguments than
packis given,packassumes additional""arguments. If TEMPLATE requires fewer arguments than given, extra arguments are ignored. - Attempting to pack the special floating point values
InfandNaN(infinity, also in negative, and not-a-number) into packed integer values (likeL) is a fatal error 💀. The reason for this is that there simply isn't any sensible mapping for these special values into integers.
✨ Examples ✨
$foo = pack("WWWW",65,66,67,68);
# foo eq "ABCD"
$foo = pack("W4",65,66,67,68);
# same thing
$foo = pack("W4",0x24b6,0x24b7,0x24b8,0x24b9);
# same thing with Unicode circled letters.
$foo = pack("U4",0x24b6,0x24b7,0x24b8,0x24b9);
# same thing with Unicode circled letters. You don't get the
# UTF-8 bytes because the U at the start of the format caused
# a switch to U0-mode, so the UTF-8 bytes get joined into
# characters
$foo = pack("C0U4",0x24b6,0x24b7,0x24b8,0x24b9);
# foo eq "\xe2\x92\xb6\xe2\x92\xb7\xe2\x92\xb8\xe2\x92\xb9"
# This is the UTF-8 encoding of the string in the
# previous example
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
# NOTE: The examples above featuring "W" and "c" are true
# only on ASCII and ASCII-derived systems such as ISO Latin 1
# and UTF-8. On EBCDIC systems, the first example would be
# $foo = pack("WWWW",193,194,195,196);
$foo = pack("s2",1,2);
# "\001\000\002\000" on little-endian
# "\000\001\000\002" on big-endian
$foo = pack("a4","abcd","x","y","z");
# "abcd"
$foo = pack("aaaa","abcd","x","y","z");
# "axyz"
$foo = pack("a14","abcdefg");
# "abcdefg\0\0\0\0\0\0\0"
$foo = pack("i9pl", gmtime);
# a real struct tm (on my system anyway)
$utmp_template = "Z8 Z8 Z16 L";
$utmp = pack($utmp_template, @utmp1);
# a struct utmp (BSDish)
@utmp2 = unpack($utmp_template, $utmp);
# "@utmp1" eq "@utmp2"
sub bintodec {
unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}
$foo = pack('sx2l', 12, 34);
# short 12, two zero bytes padding, long 34
$bar = pack('s@4l', 12, 34);
# short 12, zero fill to position 4, long 34
# $foo eq $bar
$baz = pack('s.l', 12, 4, 34);
# short 12, zero fill to position 4, long 34
$foo = pack('nN', 42, 4711);
# pack big-endian 16- and 32-bit unsigned integers
$foo = pack('S>L>', 42, 4711);
# exactly the same
$foo = pack('s<l<', -42, 4711);
# pack little-endian 16- and 32-bit signed integers
$foo = pack('(sl)<', -42, 4711);
# exactly the sameThe same template may generally also be used in unpack. 🎀
📦 package NAMESPACE
package NAMESPACE
package NAMESPACE VERSION
package NAMESPACE BLOCK
package NAMESPACE VERSION BLOCKokay bestie, gather round 💖 package is you announcing to the compiler "everybody in this chunk of code, we live in THIS namespace now, no cap".
- It declares the BLOCK — or the rest of the compilation unit — as being in the given namespace.
- The scope of the declaration is either the code BLOCK you handed it, or (if there's no BLOCK) from the declaration itself all the way through the end of the current scope: the enclosing block, file, or
eval. - No BLOCK means it's operative through the end of the current scope, exactly like
my,state, andourdo their thing. Periodt. - Every unqualified dynamic identifier in that scope now lives in the given namespace — EXCEPT where another
packagedeclaration overrides it, or when it's one of the special identifiers that qualify intomain::, likeSTDOUT,ARGV,ENV, and the punctuation variables.
Lowkey important: a package statement only touches dynamic variables, including the ones you've used local on. It does NOT touch lexically-scoped variables — those are the my, state, or our babies. 🙈
Typically it'd be the very first declaration in a file included by require or use. And yes, you can switch into a package in more than one place — all it does is decide which default symbol table the compiler uses for the rest of that block. ✨
Wanna reach into a different package? Prefix the identifier with the package name and a double colon, like $SomePack::var or ThatPack::INPUT_HANDLE. If you leave the package name off entirely, main is assumed — so $::sail is equivalent to $main::sail, and also to $main'sail (still spotted in ancient code, mostly from Perl 4, bless its heart 💀).
If VERSION is provided, package sets the $VERSION variable in the given namespace to a version object with the VERSION provided. VERSION has to be a strict style version number as the version module defines it: a positive decimal number (integer or decimal-fraction) with no exponentiation, OR a dotted-decimal v-string with a leading v character and at least three components. Set $VERSION only once per package, okay? One and done. 🎀
See "Packages" in perlmod for more on packages, modules, and classes. For other scoping tea, see perlsub.
🔮 __PACKAGE__
A special token that hands you back the name of the package it occurs in. That's it, that's the whole vibe. 🦋
👑 __CLASS__
Invoked inside a method, or a similar spot — like a field initializer expression — this token returns the name of the class of the invoking instance. Essentially it's equivalent to ref($self), except it can also be used in a field initializer to get at class methods before the instance is fully constructed. Iconic behaviour, ngl. 💅
use feature 'class';
class Example1 {
field $f = __CLASS__->default_f;
sub default_f { 10 }
}In a basic class, this value is the same as __PACKAGE__. The distinction shows up when a subclass gets constructed — then it gives you the class name of the instance being constructed, rather than just the package name that the actual code belongs to. 👀
class Example2 :isa(Example1) {
sub default_f { 20 }
}
my $obj = Example2->new;
# The $f field now has the value 20🚰 pipe READHANDLE,WRITEHANDLE
pipe READHANDLE,WRITEHANDLEOpens a pair of connected pipes, just like the corresponding system call. 🌈
⚠️ Heads up bestie: if you set up a loop of piped processes, deadlock can occur unless you are very careful. Also note that Perl's pipes use IO buffering, so depending on your application you may need to set $| to flush your WRITEHANDLE after each command.
Returns true on success. ✨
See IPC::Open2, IPC::Open3, and "Bidirectional Communication with Another Process" in perlipc for examples of such things.
On systems that support a close-on-exec flag on files, that flag gets set on all newly opened file descriptors whose filenos are higher than the current value of $^F (by default 2 for STDERR). See "$^F" in perlvar.
🍓 pop ARRAY
pop ARRAY
popRemoves and returns the last element of the array, shortening the array by one element. Snip snip ✂️
my @arr = ('cat', 'dog', 'mouse');
my $item = pop(@arr); # 'mouse'
# @arr is now ('cat', 'dog')Returns undef if the array is empty.
Note:popmay also returnundefif the last element in the array isundef. So like, don't @ me when you can't tell the difference 💀
my @arr = ('one', 'two', undef);
my $item = pop(@arr); # undefIf ARRAY is omitted, pop operates on the @ARGV array in the main program, but the @_ array in subroutines. pop will operate on the @ARGV array in eval STRING, BEGIN {}, INIT {}, CHECK {} blocks.
Starting with Perl 5.14, an experimental feature allowed pop to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24. RIP to that experiment, you will not be missed 🫶
📍 pos SCALAR
pos SCALAR
posReturns the offset of where the last m//g search left off for the variable in question ($_ is used when the variable is not specified). This offset is in characters unless the (no-longer-recommended) use bytes pragma is in effect — in which case, bestie, it's in bytes. 👀
Note that 0 is a valid match offset. undef indicates that the search position is reset (usually because the match failed, but it can also be because no match has yet been run on the scalar).
pos directly accesses the location the regexp engine uses to store the offset, so assigning to pos will change that offset, and it'll also influence the \G zero-width assertion in regular expressions. Both of those effects kick in for the next match, so you can't affect the position with pos during the current match — like in (?{pos() = 5}) or s//pos() = 5/e. Not now, later. ⏳
Setting pos also resets the matched with zero-length flag, described under "Repeated Patterns Matching a Zero-length Substring" in perlre.
Because a failed m//gc match doesn't reset the offset, the return from pos won't change either in that case. See perlre and perlop.
🖨️ print FILEHANDLE LIST
print FILEHANDLE LIST
print FILEHANDLE
print LIST
printPrints a string or a list of strings. Returns true if successful. 🌸
FILEHANDLE may be a scalar variable containing the name of, or a reference to, the filehandle — which introduces one level of indirection. (NOTE: if FILEHANDLE is a variable and the next token is a term, it may be misinterpreted as an operator unless you interpose a + or put parentheses around the arguments. Be so careful, bestie 😭)
If FILEHANDLE is omitted, it prints to the last selected output handle (see "select"). If LIST is omitted, it prints $_ to the currently selected output handle. To use FILEHANDLE alone to print the content of $_ to it, you must use a bareword filehandle like FH — not an indirect one like $fh. And to set the default output handle to something other than STDOUT, use the select operation.
The current value of $, (if any) is printed between each LIST item. The current value of $\ (if any) is printed after the entire LIST has been printed. Because print takes a LIST, everything in the LIST is evaluated in list context — including any subroutines whose return lists you pass to print. 🫠
Don't follow the print keyword with a left parenthesis unless you actually want the matching right parenthesis to terminate the arguments to print. Put parentheses around ALL the arguments instead (or interpose a +, but honestly that doesn't look as good 💅).
If you're storing handles in an array or hash, or in general whenever you're using any expression more complex than a bareword handle or a plain, unsubscripted scalar variable to fetch it, you'll have to use a block returning the filehandle value instead — and in that case the LIST may not be omitted:
print { $files[$i] } "stuff\n";
print { $OK ? *STDOUT : *STDERR } "stuff\n";⚠️ Printing to a closed pipe or socket will generate a SIGPIPE signal. See perlipc for more on signal handling.
✨ printf FILEHANDLE FORMAT, LIST
printf FILEHANDLE FORMAT, LIST
printf FILEHANDLE
printf FORMAT, LIST
printfEquivalent to print FILEHANDLE sprintf(FORMAT, LIST), except that $\ (the output record separator) is not appended. 🎀
The FORMAT and the LIST are actually parsed as a single list. The first argument of the list gets interpreted as the printf format. That means printf(@_) will use $_[0] as the format. See sprintf for an explanation of the format argument.
If use locale (including use locale ':not_characters') is in effect and POSIX::setlocale has been called, the character used for the decimal separator in formatted floating-point numbers is affected by the LC_NUMERIC locale setting. See perllocale and POSIX.
For historical reasons, if you omit the list, $_ is used as the format — to use FILEHANDLE without a list, you must use a bareword filehandle like FH, not an indirect one like $fh. However, this will rarely do what you want: if $_ contains formatting codes, they'll be replaced with the empty string and a warning will be emitted if warnings are enabled. Just use print if you want to print the contents of $_, bestie. 🙈
Don't fall into the trap of using aprintfwhen a simple
🧚 prototype FUNCTION
prototype FUNCTION
prototypeReturns the prototype of a function as a string (or undef if the function has no prototype). FUNCTION is a reference to, or the name of, the function whose prototype you want to retrieve. If FUNCTION is omitted, $_ is used. 🦋
If FUNCTION is a string starting with CORE::, the rest is taken as a name for a Perl builtin. If the builtin's arguments cannot be adequately expressed by a prototype (such as system), prototype returns undef, because the builtin does not really behave like a Perl function. Otherwise, the string describing the equivalent prototype is returned. Slay. ⭐
⭐ push ARRAY,LIST
push ARRAY,LISTAdds one or more items to the end of an array. Tacking them right on the end, no cap. 💅
my @animals = ("cat");
push(@animals, "mouse"); # ("cat", "mouse")
my @colors = ("red");
push(@colors, ("blue", "green")); # ("red", "blue", "green")Returns the number of elements in the array following the completed push.
my $color_count = push(@colors, ("yellow", "purple"));
say "There are $color_count colors in the updated array";Starting with Perl 5.14, there was this little experimental feature that let push take a scalar expression. Bestie, it flopped, it got removed, gone as of Perl 5.24. RIP, you will not be missed. 🪦
✨ q/STRING/
✨ qq/STRING/
✨ qw/STRING/
✨ qx/STRING/
Generalized quotes. Go peek at "Quote-Like Operators" in perlop for the full tea. 💅
✨ qr/STRING/
The regexp-like quote. See "Regexp Quote-Like Operators" in perlop, iykyk. ✨
💖 quotemeta EXPR
💖 quotemeta
Okay so this one gives you back the value of EXPR with all the ASCII non-"word" characters backslashed. Translation: every ASCII character that does not match /[A-Za-z_0-9]/ gets a backslash shoved in front of it in the returned string — and no, locale settings do not get a vote in this. This is literally the internal function that implements the \Q escape inside double-quoted strings. (For the non-ASCII code point behavior, scroll down, it's coming. 👀)
If EXPR is omitted, it uses $_. Obviously. 💅
The whole point here is to make every character in EXPR match its literal self. Otherwise any metacharacters hiding in there could pop off and trigger their lil "magic" matching behaviors, and we do not want that. The characters this function has been applied to are said to be "quoted" or "escaped".
quotemeta (and \Q ... \E) are highkey useful when you're interpolating strings into regular expressions, because by default an interpolated variable gets treated as a mini-regular expression. Example time:
my $sentence = 'The quick brown fox jumped over the lazy dog';
my $substring = 'quick.*?fox';
$sentence =~ s{$substring}{big bad wolf};This will cause $sentence to become 'The big bad wolf jumped over...'.
On the other hand:
my $sentence = 'The quick brown fox jumped over the lazy dog';
my $substring = 'quick.*?fox';
$sentence =~ s{\Q$substring\E}{big bad wolf};Or:
my $sentence = 'The quick brown fox jumped over the lazy dog';
my $substring = 'quick.*?fox';
my $quoted_substring = quotemeta($substring);
$sentence =~ s{$quoted_substring}{big bad wolf};Both of those leave the sentence as is. Periodt. So normally, when you're accepting literal string input from the user, quotemeta or \Q must be used. No shortcuts bestie.
⚠️ Beware: if you put literal backslashes (the ones not inside interpolated variables) between\Qand\E, double-quotish backslash interpolation may lead to confusing results. If you genuinely need literal backslashes within\Q...\E, go consult "Gory details of parsing quoted constructs" in perlop.
Because the result of \Q STRING \E has all metacharacters quoted, there is genuinely no way to insert a literal $ or @ inside a \Q\E pair. If you protect it with \, the $ gets quoted and becomes "\\\$"; if you don't, it's interpreted as the start of an interpolated scalar. There is no third option, sorry. 😭
In Perl v5.14, all non-ASCII characters are quoted in non-UTF-8-encoded strings, but not quoted in UTF-8 strings.
Starting in Perl v5.16, Perl adopted a Unicode-defined strategy for quoting non-ASCII characters; the quoting of ASCII characters stayed exactly the same.
Also unchanged is the quoting of non-UTF-8 strings when you're outside the scope of a use feature 'unicode_strings' — that behavior is to quote all characters in the upper Latin1 range. This gives you complete backwards compatibility for old programs that don't use Unicode. (Heads up that unicode_strings is automatically enabled within the scope of a use v5.12 or greater. ✨)
Within the scope of use locale, all non-ASCII Latin1 code points get quoted whether the string is encoded as UTF-8 or not. As mentioned up top, locale does not affect the quoting of ASCII-range characters. This protects you against those locales where characters like | are treated as word characters. 🙈
Otherwise, Perl quotes non-ASCII characters using an adaptation from Unicode (see <https://www.unicode.org/reports/tr31/>). The only code points that get quoted are the ones that have any of these Unicode properties: Pattern_Syntax, Pattern_White_Space, White_Space, Default_Ignorable_Code_Point, or General_Category=Control.
Of those properties, the two important ones are Pattern_Syntax and Pattern_White_Space. Unicode set them up for literally this exact purpose — deciding which characters in a regular expression pattern should be quoted. No character that can be in an identifier has these properties, so you're safe. 💅
Perl promises that if we ever add regular expression pattern metacharacters to the dozen already defined (\ | ( ) [ { ^ $ * + ? .), we will only use ones that have the Pattern_Syntax property. Perl also promises that if we ever add characters considered to be white space in regular expressions (currently mostly affected by /x), they will all have the Pattern_White_Space property.
Unicode promises that the set of code points with these two properties will never change, so something not quoted in v5.16 will never need to be quoted in any future Perl release. (Not all the code points matching Pattern_Syntax actually have characters assigned to them; so there's room to grow, but they're quoted whether assigned or not. Perl, of course, would never use an unassigned code point as an actual metacharacter.)
Quoting characters with the other 3 properties is done to enhance the readability of the regular expression, not because they actually need to be quoted for regular expression purposes (characters with the White_Space property are likely to be indistinguishable on the page or screen from those with the Pattern_White_Space property; and the other two properties contain non-printing characters). 🦋
🎲 rand EXPR
🎲 rand
Returns a random fractional number greater than or equal to 0 and less than the value of EXPR. (EXPR should be positive, bestie.) If EXPR is omitted, the value 1 is used.
my $num1 = rand(); # Random float at least 0 and below 1
my $num2 = rand(7); # Random float at least 0 and below 7
my $num3 = int(rand(10)); # Random integer at least 0 and below 10Notes, because there's always notes:
- Calling
randautomatically callssrandunless srand has already been called. - Currently EXPR with the value 0 is special-cased as 1. This was undocumented before Perl 5.8.0 and is subject to change in future versions of Perl. So don't get attached. 💀
- As of Perl v5.20.0 rand() uses the
drand48pseudorandom number generator (PRNG) to generate numbers that appear random but are deterministic. It's sufficient for games, generating test data, or cases where you need repeatability.
🔒 Security:
rand is not suitable for anything related to security, authentication or encryption. Do not use it for generating passwords, session ids, authentication tokens, salts, or encryption keys. The return values are predictable, and systems that rely on rand are easily broken. This is not a drill. 🚨
A number of third-party CPAN modules offer access to system sources of randomness, or random number generators that are suitable for security purposes, including:
- Crypt::SysRandom
- Crypt::URandom
- Crypt::PRNG
📖 read FILEHANDLE,SCALAR,LENGTH,OFFSET
📖 read FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH characters of data into variable SCALAR from the specified FILEHANDLE. Returns the number of characters actually read, 0 at end of file, or undef if there was an error (in that last case $! also gets set, queen). SCALAR will be grown or shrunk so that the last character actually read is the last character of the scalar after the read.
You can toss in an OFFSET to place the read data somewhere in the string other than the beginning. A negative OFFSET specifies placement at that many characters counting backwards from the end of the string. A positive OFFSET greater than the length of SCALAR results in the string being padded to the required size with "\0" bytes before the result of the read gets appended.
open(my $FH, "<", "input.txt") or die("Cannot open file: $!");
my $buf = "";
my $num = 0;
# Read 32 bytes
$num = read($FH, $buf, 32);
# Read 16 bytes into position 1024 of $buf
$num = read($FH, $buf, 16, 1024);The call is implemented in terms of either Perl's or your system's native fread(3) library function, via the PerlIO layers applied to the handle. If you want a true read(2) system call, go see sysread. 👑
Note the characters: depending on the status of the filehandle, either (8-bit) bytes or characters are read. By default all filehandles operate on bytes, but for example if the filehandle has been opened with the ":utf8" I/O layer (see "open", and the open pragma), the I/O will operate on UTF8-encoded Unicode characters, not bytes. Same deal for the ":encoding" layer — in that case pretty much any characters can be read. 🌈
📂 readdir DIRHANDLE
Returns the next directory entry for a directory opened by "opendir". If used in list context, returns all the rest of the entries in the directory. If there are no more entries, it returns the undefined value in scalar context and the empty list in list context.
If you're planning to filetest the return values out of a readdir, you'd better prepend the directory in question. Otherwise, because we didn't chdir there, you'd have been testing the wrong file and that's an L. 😭
opendir(my $dh, $some_dir) || die "Can't opendir $some_dir: $!";
my @dots = grep { /^\./ && -f "$some_dir/$_" } readdir($dh);
closedir $dh;As of Perl 5.12 you can use a bare readdir in a while loop, which will set $_ on every single iteration. If either a readdir expression or an explicit assignment of a readdir expression to a scalar is used as a while/for condition, then the condition actually tests for definedness of the expression's value, not for its regular truth value. Subtle but important, bestie. ✨
opendir(my $dh, $some_dir) || die "Can't open $some_dir: $!";
while (readdir $dh) {
print "$some_dir/$_\n";
}
closedir $dh;To avoid confusing would-be users of your code who are running earlier versions of Perl with mysterious failures, put this sort of thing at the top of your file to signal that your code will work only on Perls of a recent vintage:
use v5.12; # so readdir assigns to $_ in a lone while test🌸 readline 🌸
readline EXPR
readlineBestie, this one reads from the filehandle whose typeglob is contained in EXPR — or from *ARGV if you don't hand it an EXPR at all. Okay so here's the tea on the two contexts:
- Scalar context — every single call reads and returns the next line, all the way until end-of-file. Once you hit EOF, the next call gives you back
undef. 💀 - List context — it just keeps reading until end-of-file and hands you back a whole list of lines. Slay.
And the whole concept of "line" here? That's entirely whatever you've defined with $/ (or $INPUT_RECORD_SEPARATOR in English). See "$/" in perlvar. ✨
One lil weirdo case to know about, no cap: when $/ is set to undef, when readline is in scalar context (that's your file slurp mode, icon), and when an empty file gets read — it returns '' the first time, and then undef every time after that. Cute, chaotic, whatever.
This is the internal function that implements the <EXPR> operator, but like… you can just call it directly, it's not gatekept. The <EXPR> operator gets way more detail in "I/O Operators" in perlop. 🫶
my $line = <STDIN>;
my $line = readline(STDIN); # same thingIf readline runs into an operating system error, $! gets set with the corresponding error message. Pro tip from your girl: check $! when you're reading from filehandles you don't trust, like a tty or a socket. The example below uses the operator form of readline and dies if the result isn't defined. 💅
while ( ! eof($fh) ) {
defined( $_ = readline $fh ) or die "readline failed: $!";
...
}Heads up though — you can't handlereadlineerrors that way with theARGVfilehandle. In that case you have toopeneach element of@ARGVyourself, sinceeofhandlesARGVdifferently.
foreach my $arg (@ARGV) {
open(my $fh, $arg) or warn "Can't open $arg: $!";
while ( ! eof($fh) ) {
defined( $_ = readline $fh )
or die "readline failed for $arg: $!";
...
}
}Now, just like the <EXPR> operator — if a readline expression is used as the condition of a while or for loop, it gets implicitly assigned to $_. And if either a readline expression, or an explicit assignment of a readline expression to a scalar, is used as a while/for condition, then that condition is actually testing for definedness of the expression's value, not its regular truth value. The way that it's secretly a defined check?? Sneaky. 👑
🌈 readlink 🌈
readlink EXPR
readlinkReturns the value of a symbolic link, if symbolic links are even implemented on your platform. If they're not? It raises an exception, periodt. If there's a system error, it returns the undefined value and sets $! (errno). If EXPR is omitted, it uses $_.
Portability issues: "readlink" in perlport.
🦋 readpipe 🦋
readpipe EXPR
readpipeEXPR gets executed as a system command. The collected standard output of that command is what you get back. 🔥
- In scalar context: it comes back as a single (potentially multi-line) string.
- In list context: you get a list of lines (however you've defined lines with
$/(or$INPUT_RECORD_SEPARATORin English)).
This is the internal function implementing the qx/EXPR/ operator, but you can use it directly too. The qx/EXPR/ operator is discussed in more detail in "qx/STRING/" in perlop. If EXPR is omitted, uses $_.
🧚 recv SOCKET,SCALAR,LENGTH,FLAGS 🧚
Receives a message on a socket. It attempts to receive LENGTH characters of data into variable SCALAR from the specified SOCKET filehandle. SCALAR will be grown or shrunk to the length actually read. It takes the same flags as the system call of the same name. 🌸
Returns the address of the sender if SOCKET's protocol supports this; returns an empty string otherwise. If there's an error, it returns the undefined value. This call is actually implemented in terms of the recvfrom(2) system call. See "UDP: Message Passing" in perlipc for examples.
⚠️ Note that if the socket has been marked as":utf8",recvwill throw an exception. The:encoding(...)layer implicitly introduces the":utf8"layer. See"binmode".
💜 redo LABEL 💜
redo LABEL
redo EXPR
redoThe redo command restarts the loop block without evaluating the conditional again. The continue block, if any, is not executed. If the LABEL is omitted, the command refers to the innermost enclosing loop. The redo EXPR form, available starting in Perl 5.18.0, lets a label name be computed at run time, and is otherwise identical to redo LABEL. 🎀
Programs that want to lie to themselves about what was just input normally use this command:
# a simpleminded Pascal comment stripper
# (warning: assumes no { or } in strings)
LINE: while (<STDIN>) {
while (s|({.*}.*){.*}|$1 |) {}
s|{.*}| |;
if (s|{.*| |) {
my $front = $_;
while (<STDIN>) {
if (/}/) { # end of comment?
s|^|$front\{|;
redo LINE;
}
}
}
print;
}redo cannot return a value from a block that typically returns a value, such as eval {}, sub {}, or do {}. It will perform its flow control behavior, which precludes any return value. It should not be used to exit a grep or map operation. 🚫
Friendly reminder: a block by itself is semantically identical to a loop that executes once. So redo inside such a block will effectively turn it into a looping construct. Wild, right?
See also "continue" for an illustration of how last, next, and redo work.
Unlike most named operators, this has the same precedence as assignment. It's also exempt from the looks-like-a-function rule, so redo ("foo")."bar" will cause "bar" to be part of the argument to redo. iykyk 😌
⭐ ref EXPR ⭐
ref EXPR
refExamines the value of EXPR, expecting it to be a reference, and returns a string giving info about the reference and the type of referent. If EXPR isn't specified, $_ will be used.
If the operand is not a reference, then the empty string gets returned. An empty string will only be returned in this situation. ref is often useful to just test whether a value is a reference, which you can do by comparing the result to the empty string.
🙈 Common mistake alert: it's a mistake to use the result ofrefdirectly as a truth value. That goes wrong because0(which is false!) can be returned for a reference. Do not do this, bestie.
If the operand is a reference to a blessed object, then the name of the class into which the referent is blessed gets returned. ref doesn't care what the physical type of the referent is — blessing takes precedence over such concerns, always. 👑
⚠️ Beware: exact comparison ofrefresults against a class name does not perform a class membership test. A class's members also include objects blessed into subclasses, for whichrefwill return the name of the subclass. Also beware that class names can clash with the built-in type names (listed below). Use theisamethod to test class membership of a class name or blessed object, after ensuring it is one of these things. Alternatively, theisaoperator can test class membership without checking blessedness first.
If the operand is a reference to an unblessed object, then the return value indicates the type of object:
- If the unblessed referent is not a scalar, the return value will be one of the strings
"ARRAY","HASH","CODE","FORMAT", or"IO"— indicating only which kind of object it is. - If the unblessed referent is a scalar, the return value will be one of the strings
"SCALAR","VSTRING","REF","GLOB","LVALUE", or"REGEXP", depending on the kind of value the scalar currently has.
But note that qr// scalars are created already blessed, so ref qr/.../ will likely return "Regexp". And beware — these built-in type names can also be used as class names, so ref returning one of these names doesn't unambiguously indicate that the referent is of the kind to which the name refers. Ambiguity queen. 💀
The ambiguity between built-in type names and class names may limit the utility of ref, but in practice, such clashes rarely occur unless intentionally engineered. For unambiguous information, use "blessed" in builtin for information about blessing, and "reftype" in builtin for information about physical types. But beware that the physical type returned by "reftype" does not indicate how a blessed object is meant to be used, and the appropriate dereference operations for an object (whether to access the blessed structure directly, or via an overload) will be described by the documentation for the object class.
See also perlref and perlobj. 📚
🍓 rename OLDNAME,NEWNAME 🍓
Changes the name of a file; an existing file NEWNAME will be clobbered (gone, reduced to atoms). Returns true for success; on failure returns false and sets $!.
Behavior of this function varies wildly depending on your system implementation. For example, it will usually not work across file system boundaries, even though the system mv command sometimes compensates for this. Other restrictions include whether it works on directories, open files, or pre-existing files. Check perlport and either the rename(2) manpage or equivalent system documentation for details. 🌈
For a platform independent "move" function look at the File::Copy module.
Portability issues: "rename" in perlport.
👑 require VERSION 👑
require VERSION
require EXPR
requireDemands a version of Perl specified by VERSION, or demands some semantics specified by EXPR or by $_ if EXPR is not supplied.
VERSION may be either a literal such as v5.24.1, which will be compared to $^V (or $PERL_VERSION in English), or a numeric argument of the form 5.024001, which will be compared to $]. An exception is raised if VERSION is greater than the version of the current Perl interpreter. Compare with "use", which can do a similar check at compile time. ✨
Specifying VERSION as a numeric argument of the form5.024001should generally be avoided as older, less readable syntax compared tov5.24.1. Before perl 5.8.0 (released in 2002), the more verbose numeric form was the only supported syntax, which is why you might see it in older code. Vintage, but not in a cute way. 💅
require v5.24.1; # run time version check
require 5.24.1; # ditto
require 5.024_001; # ditto; older syntax compatible
with perl 5.6Otherwise, require is out here demanding that a library file gets included if it hasn't already been included. Like girl, if it's not in the club yet, it's getting in. 💅 The file gets included via the do-FILE mechanism, which is basically just a little variety pack of eval, with the caveat that lexical variables in the invoking script will be invisible to the included code (rude, honestly). If it were implemented in pure Perl, it would have semantics similar to the following:
use Carp 'croak';
use version;
sub require {
my ($filename) = @_;
if ( my $version = eval { version->parse($filename) } ) {
if ( $version > $^V ) {
my $vn = $version->normal;
croak "Perl $vn required--this is only $^V, stopped";
}
return 1;
}
if (exists $INC{$filename}) {
return 1 if $INC{$filename};
croak "Compilation failed in require";
}
local $INC;
# this type of loop lets a hook overwrite $INC if they wish
for($INC = 0; $INC < @INC; $INC++) {
my $prefix = $INC[$INC];
if (!defined $prefix) {
next;
}
if (ref $prefix) {
#... do other stuff - see text below ....
}
# (see text below about possible appending of .pmc
# suffix to $filename)
my $realfilename = "$prefix/$filename";
next if ! -e $realfilename || -d _ || -b _;
$INC{$filename} = $realfilename;
my $result = do($realfilename);
# but run in caller's namespace
if (!defined $result) {
$INC{$filename} = undef;
croak $@ ? "$@Compilation failed in require"
: "Can't locate $filename: $!\n";
}
return 1 if $feature_module_true_enabled_in_file;
if (!$result) {
delete $INC{$filename};
croak "$filename did not return true value";
}
$! = 0;
return $result;
}
croak "Can't locate $filename in \@INC ...";
}Note that the file will not be included twice under the same specified name. Once is enough bestie, we don't do reruns. 🙈
Historically the file must return true as the last statement to indicate successful execution of any initialization code, so it's customary to end such a file with 1; unless you're sure it'll return true otherwise. But honestly? Just put the 1; in there, in case you add more statements later and forget. As of 5.37.6 this requirement may be avoided by enabling the 'module_true' feature, which is enabled by default in modern version bundles. So code with use v5.37; no longer needs to concern itself with this whole issue. 👑 See feature for more details. Note that this affects the compilation unit within which the feature is used, and using it before requiring a module will not change the behavior of existing modules that do not themselves also use it. (Feature flags are not contagious, periodt.)
In this usage, the (truthy) result of the required file will be returned, but this should not be relied upon as it will not be returned in subsequent require calls on the same file or with the 'module_true' feature enabled. If you actually want to run a file and retrieve its return value, use do. That's the whole vibe.
If EXPR is a bareword, require assumes a .pm extension and replaces :: with / in the filename for you, to make it easy to load standard modules. This form of loading of modules does not risk altering your namespace, however it will autovivify the stash for the required module. It's giving ✨surprise container creation✨.
In other words, if you try this:
require Foo::Bar; # a splendid barewordThe require function will actually look for the Foo/Bar.pm file in the directories specified in the @INC array, and it will autovivify the "Foo::Bar::" stash at compile time.
But if you try this:
my $class = 'Foo::Bar';
require $class; # $class is not a bareword
#or
require "Foo::Bar"; # not a bareword because of the ""The require function will look for the Foo::Bar file in the @INC array and will complain about not finding Foo::Bar there. 😭 Bestie it's literally not the same thing, the quotes change EVERYTHING. In this case you can do:
eval "require $class";or you could do
require "Foo/Bar.pm";Neither of these forms will autovivify any stashes at compile time and only have run time effects. No free stash, no free real estate 🏠
Now that you understand how require looks for files with a bareword argument, there's a little extra functionality going on behind the scenes. Sneaky lil bonus feature, iykyk. 🤫 Before require looks for a .pm extension, it will first look for a similar filename with a .pmc extension. If this file is found, it will be loaded in place of any file ending in a .pm extension. This applies to both the explicit require "Foo/Bar.pm"; form and the require Foo::Bar; form. .pmc said "I'm the main character now" 💅
You can also insert hooks into the import facility by putting Perl coderefs or objects directly into the @INC array. ✨ Customize your loading pipeline bestie ✨ There are two types of hooks, INC filters, and INCDIR hooks, and there are three forms of representing a hook: subroutine references, array references, and blessed objects.
Subroutine references are the simplest case. When the inclusion system walks through @INC and encounters a subroutine, unless this subroutine is blessed and supports an INCDIR hook this subroutine will be assumed to be an INC hook will be called with two parameters, the first a reference to itself, and the second the name of the file to be included (e.g., Foo/Bar.pm). The subroutine should return either nothing or else a list of up to four values in the following order:
- A reference to a scalar, containing any initial source code to prepend to the file or generator output.
- A filehandle, from which the file will be read.
- A reference to a subroutine. If there is no filehandle (previous item), then this subroutine is expected to generate one line of source code per call, writing the line into
$_and returning1, then finally at end of file returning0. If there is a filehandle, then the subroutine will be called to act as a simple source filter, with the line as read in$_. Again, return1for each valid line, and0after all lines have been returned. For historical reasons the subroutine will receive a meaningless argument (in fact always the numeric value zero) as$_[0]. 💀 - Optional state for the subroutine. The state is passed in as
$_[1].
"AUTOLOAD" cannot be used to resolve the "INCDIR" method, "INC" is checked first, and "AUTOLOAD" would resolve that. Just so we're clear on the priority order, bestie.
If an empty list, undef, or nothing that matches the first 3 values above is returned, then require looks at the remaining elements of @INC. Note that this filehandle must be a real filehandle (strictly a typeglob or reference to a typeglob, whether blessed or unblessed); tied filehandles will be ignored and processing will stop there. 🛑 No fake filehandles in this house.
If the hook is an object, it should provide an "INC" or "INCDIR" method that will be called as above, the first parameter being the object itself. If it does not provide either method, and the object is not CODE ref then an exception will be thrown, otherwise it will simply be executed like an unblessed CODE ref would. Note that you must fully qualify the method name when you declare an "INC" sub (unlike the "INCDIR" sub), as the unqualified symbol "INC" is always forced into package "main". Here's a typical code layout for an "INC" hook:
# In Foo.pm
package Foo;
sub new { ... }
sub Foo::INC {
my ($self, $filename) = @_;
...
}
# In the main program
push @INC, Foo->new(...);If the hook is an array reference, its first element must be a subroutine reference or an object as described above. When the first element is an object that supports an "INC" or "INCDIR" method then the method will be called with the object as the first argument, the filename requested as the second, and the hook array reference as the third. When the first element is a subroutine then it will be called with the array as the first argument, and the filename as the second, no third parameter will be passed in. In both forms you can modify the contents of the array to provide state between calls, or whatever you like. ✨ Your array, your rules ✨
In other words, you can write:
push @INC, \&my_sub;
sub my_sub {
my ($coderef, $filename) = @_; # $coderef is \&my_sub
...
}or:
push @INC, [ \&my_sub, $x, $y, ... ];
sub my_sub {
my ($arrayref, $filename) = @_;
# Retrieve $x, $y, ...
my (undef, @parameters) = @$arrayref;
...
}or:
push @INC, [ HookObj->new(), $x, $y, ... ];
sub HookObj::INC {
my ($self, $filename, $arrayref)= @_;
my (undef, @parameters) = @$arrayref;
...
}These hooks are also permitted to set the %INC entry corresponding to the files they have loaded. See "%INC" in perlvar. Should an "INC" hook not do this then perl will set the %INC entry to be the hook reference itself. So like, perl got your back if you forgot 💖
A hook may also be used to rewrite the @INC array. While this might sound strange, there are situations where it can be very useful to do this. Such hooks usually just return undef and do not mix filtering and @INC modifications. While in older versions of perl having a hook modify @INC was fraught with issues and could even result in segfaults or assert failures, as of 5.37.7 the logic has been made much more robust and the hook now has control over the loop iteration if it wishes to do so. Glow up fr fr 🔥
So there's now a whole facility for steering the iterator that walks @INC during require. Bestie, listen: $INC gets initialized with the index of the hook that's currently executing. Once that hook returns, the next slot in @INC that gets checked is the integer successor of whatever's in $INC (or -1 if it's undef). Like, for example:
push @INC, sub {
splice @INC, $INC, 1; # remove this hook from @INC
unshift @INC, sub { warn "A" };
undef $INC; # reset the $INC iterator so we
# execute the newly installed sub
# immediately.
};which would install a sub into @INC that, when it gets executed as a hook (say, by a require of a file that doesn't exist), then splices itself out of @INC, and adds a new sub to the front that warns every time someone does a require that needs an @INC search, and then immediately executes that hook. Iconic behavior honestly 💅
Prior to 5.37.7 there was literally no way to make perl use the newly installed hook right away, or to peek at any changed items in @INC to the left of the iterator, so that warning would only show up on the second call to require. In newer perl, that last statement undefining $INC makes perl restart the traversal of the @INC array from the beginning and run the freshly installed sub immediately. Slay.
Whatever value $INC held, if it held anything, gets restored at the end of the require. Any changes you make to $INC while the hook is alive get unrolled after the hook exits, and its value only means anything immediately after the hook runs — so setting $INC to some value before you execute a require has zero effect on how the require executes. Not a single one.
As of 5.37.7, @INC values of undef are silently ignored. 🫶
The function require() is genuinely hard to wrap properly. So many modules consult the stack to find info about their caller, and injecting a new stack frame by wrapping require() breaks stuff all the time. Still, being able to do things before and after a require is super helpful — think trace utilities like Devel::TraceUse, or measuring load time and the memory consumption of the require graph. Because safely making a require() wrapper is such a headache, in 5.37.10 we introduced a new mechanism.
As of 5.37.10, before it does literally anything else, require checks whether ${^HOOK}{require__before} holds a coderef, and if it does, it calls it with the filename form of the item being loaded. The hook is allowed to modify $_[0] to load a different filename, or to throw a fatal exception to make the require fail — which gets treated exactly as though the required code itself had thrown an exception.
The ${^HOOK}{require__before} hook may return a code reference, and if it does, that coderef gets executed (in an eval, with the filename as a parameter) after the require finishes. It runs no matter how the compilation completed — even if the require throws a fatal exception. The function can check %INC to figure out whether the require failed or not. For instance, the following code prints some diagnostics before and after every require statement. The example also has logic to chain the signal, so multiple signals can cooperate — well behaved ${^HOOK}{require__before} handlers should always take that into account.
{
use Scalar::Util qw(reftype);
my $old_hook = ${^HOOK}{require__before};
local ${^HOOK}{require__before} = sub {
my ($name) = @_;
my $old_hook_ret;
$old_hook_ret = $old_hook->($name) if $old_hook;
warn "Requiring: $name\n";
return sub {
$old_hook_ret->() if ref($old_hook_ret)
&& reftype($old_hook_ret) eq "CODE";
warn sprintf "Finished requiring %s: %s\n",
$name, $INC{$name} ? "loaded" :"failed";
};
};
require Whatever;
}This hook fires for ALL require statements, unlike the INC and INCDIR hooks, which only fire for relative file names. And it fires first, before any other special behaviour inside of require. Note that the initial hook in ${^HOOK}{require__before} is not executed inside of an eval, so throwing an exception will stop further processing — but the after hook it may return is executed inside of an eval, and any exceptions it throws get silently ignored. That's because it runs inside the scope cleanup logic that's triggered after the require completes, and an exception at that point wouldn't stop the module from loading and whatnot anyway. 🧚
There's a similar hook that fires after require completes: ${^HOOK}{require__after}, which gets called after each require statement completes, whether via an exception or successfully. It's called with the filename of the most recently executed require statement. It executes in an eval, and won't affect execution in any way at all.
For a yet-more-powerful import facility built around require, go look at use and perlmod. 💖
🌸 reset EXPR 🌸
reset
Generally used in a continue block at the end of a loop to clear variables and reset m?pattern? searches so they work again. The expression gets interpreted as a list of single characters (hyphens allowed for ranges, yes really). All variables — scalars, arrays, and hashes — in the current package starting with one of those letters get reset to their pristine state. If you omit the expression, one-match searches (m?pattern?) get reset so they match again. Only resets variables or searches in the current package, and it always returns 1. Examples, bestie:
reset 'X'; # reset all X variables
reset 'a-z'; # reset lower case variables
reset; # just reset m?one-time? searchesResettingA-Zis not recommended, because you'll wipe out your@ARGVand@INCarrays and your%ENVhash. That's a vibe check you do not want to fail. 💀
This only resets package variables; lexical variables are unaffected. But honestly lexicals clean themselves up on scope exit anyway, so you probably want to use those instead. See my.
🌈 return EXPR 🌈
return
Returns from a subroutine, eval, do FILE, sort block, or regex eval block — but not a grep, map, or do BLOCK block — with the value given in EXPR. Evaluation of EXPR can happen in list, scalar, or void context, depending on how the return value is going to be used, and the context may vary from one execution to the next (see wantarray). If no EXPR is given, you get an empty list in list context, the undefined value in scalar context, and (obviously) nothing at all in void context.
(In the absence of an explicit return, a subroutine, eval, or do FILE automatically returns the value of the last expression evaluated. Free real estate ✨)
Unlike most named operators, this one is also exempt from the looks-like-a-function rule, so return ("foo")."bar" will cause "bar" to be part of the argument to return. She does what she wants.
🦋 reverse LIST 🦋
In list context, returns a list value made of the elements of LIST in the opposite order. In scalar context, it concatenates the elements of LIST and returns a string value with all the characters in the opposite order.
print join(", ", reverse "world", "Hello"); # Hello, world
print scalar reverse "dlrow ,", "olleH"; # Hello, worldUsed with no arguments in scalar context, reverse reverses $_.
$_ = "dlrow ,olleH";
print reverse; # No output, list context
print scalar reverse; # Hello, worldHeads up: reversing an array onto itself (like in @a = reverse @a) will preserve non-existent elements whenever possible — that means for non-magical arrays, or for tied arrays that have EXISTS and DELETE methods. 🫶
This operator is also super handy for inverting a hash, although there are some caveats. If a value shows up twice in the original hash, only one of those copies can be represented as a key in the inverted hash. And also, it has to unwind one hash and build a whole new one, which may take some time on a large hash, like one from a DBM file.
my %by_name = reverse %by_address; # Invert the hash🎀 rewinddir DIRHANDLE 🎀
Sets the current position to the beginning of the directory for the readdir routine on DIRHANDLE.
Portability issues: rewinddir in perlport. (iykyk)
💅 rindex STR,SUBSTR,POSITION 💅
rindex STR,SUBSTR
Works exactly like index, except it returns the position of the last occurrence of SUBSTR in STR. If POSITION is specified, it returns the last occurrence beginning at or before that position.
🌸 rmdir FILENAME 🌸
rmdir
Deletes the directory named by FILENAME, but only if that directory is empty. If it works it returns true; otherwise it returns false and sets $! (errno). If FILENAME is omitted, it uses $_.
To nuke a whole directory tree recursively (rm -rf on Unix, the girlies know), go look at the rmtree function of the File::Path module. 💖
⭐ s/// ⭐
The substitution operator. See "Regexp Quote-Like Operators" in perlop. That's the whole entry, she's mysterious like that.
👑 say FILEHANDLE LIST 👑
say FILEHANDLE
say LIST
say
Just like print, but it implicitly appends a newline at the end of the LIST instead of whatever value $\ might have. If you want to use FILEHANDLE with no LIST to print the contents of $_ to it, you must use a bareword filehandle like FH, not an indirect one like $fh. No cap.
say is available only if the say feature is enabled, or if it's prefixed with CORE::. The say feature gets enabled automatically with a use v5.10 (or higher) declaration in the current scope. Periodt. 💅
🌈 scalar EXPR 🌈
Forces EXPR to be interpreted in scalar context and returns the value of EXPR.
my @counts = ( scalar @a, scalar @b, scalar @c );There's no equivalent operator to force an expression to be interpolated in list context, because in practice that's literally never needed. If you really wanted to do it, though, you could use the construction @{[ (some expression) ]}, but usually a simple (some expression) is enough. 🧚
Okay so continuing from where we were — this bit is important, bestie, because scalar is a unary operator, so if you go and accidentally hand it a parenthesized list for the EXPR? Yeah. It does the scalar comma expression thing instead 💀 — meaning it evaluates everything except the last element in void context and then gives you back the final element, evaluated in scalar context. That is… almost never what you want, fr fr.
So this one single statement:
print uc(scalar(foo(), $bar)), $baz;is the moral equivalent 😭 of these two:
foo();
print(uc($bar), $baz);Go peek at perlop for the whole tea on unary operators and the comma operator, and perldata if you wanna know about evaluating a hash in scalar context. 🫶
✨ seek FILEHANDLE,POSITION,WHENCE ✨
Sets FILEHANDLE's position — literally just like the fseek(3) call in C's stdio. 🎯 FILEHANDLE can be an expression whose value gives the name of the filehandle, no cap.
The values for WHENCE are:
0— set the new position in bytes to POSITION.1— set it to the current position plus POSITION.2— set it to EOF plus POSITION, which is typically negative.
For WHENCE you can also use the constants SEEK_SET, SEEK_CUR, and SEEK_END (start of file, current position, end of file 🥰) from the Fcntl module. Returns 1 on success, false otherwise. Periodt.
⚠️ Note the emphasis on bytes: even if the filehandle has been set to operate on characters (say, using the:encoding(UTF-8)I/O layer), theseek,tell, andsysseekfamily of functions use byte offsets, not character offsets — because seeking to a character offset in a UTF-8 file would be so slow. 😭
And if you want to position the file for sysread or syswrite, do not use seek — buffering makes its effect on the file's read-write position unpredictable and non-portable. Use sysseek instead. 💅
Due to the rules and rigors of ANSI C, on some systems you have to do a seek whenever you switch between reading and writing. Amongst other things, this may have the effect of calling stdio's clearerr(3). A WHENCE of 1 (SEEK_CUR) is useful for not moving the file position:
seek($fh, 0, 1);This is also useful for applications emulating tail -f. 🌈 Once you hit EOF on your read and then sleep for a while, you (probably) have to stick in a dummy seek to reset things. The seek doesn't change the position, but it does clear the end-of-file condition on the handle, so that the next readline FILE makes Perl try again to read something. (We hope. 🙈)
And if that doesn't work — some I/O implementations are particularly cantankerous, smh — you might need something like this:
for (;;) {
for ($curpos = tell($fh); $_ = readline($fh);
$curpos = tell($fh)) {
# search for some stuff and put it into files
}
sleep($for_a_while);
seek($fh, $curpos, 0);
}✨ seekdir DIRHANDLE,POS ✨
Sets the current position for the readdir routine on DIRHANDLE. 🎀 POS must be a value returned by telldir. And heads up: seekdir has the same caveats about possible directory compaction as the corresponding system library routine. iykyk.
✨ select FILEHANDLE ✨ / ✨ select ✨
Returns the currently selected filehandle. 👑 If FILEHANDLE is supplied, it sets the new current default filehandle for output. That's a two-for-one deal, bestie:
- A
write,print, orsaywithout a filehandle will default to this FILEHANDLE. - References to variables related to output will refer to this output channel.
For example, to set the top-of-form format for more than one output channel, you might do the following:
select(REPORT1);
$^ = 'report1_top';
select(REPORT2);
$^ = 'report2_top';FILEHANDLE may be an expression whose value gives the name of the actual filehandle. Thus:
my $oldfh = select(STDERR); $| = 1; select($oldfh);Some programmers may prefer to think of filehandles as objects with methods 🧚 and would rather write that last example as:
STDERR->autoflush(1);(Prior to Perl version 5.14, you have to use IO::Handle; explicitly first.)
And whilst you can use select to temporarily "capture" the output of print like this:
{
my $old_handle = select $new_handle;
# This goes to $new_handle:
print "ok 1\n";
...
select $old_handle;
}…you might find it easier to localize the typeglob instead:
{
local *STDOUT = $new_handle;
print "ok 1\n";
...
}The two are not exactly equivalent, but the latter might be clearer and will restore STDOUT if the wrapped code dies. 💀 The difference? In the former, the original STDOUT can still be accessed by explicitly using it in a print statement (as print STDOUT ...), whereas in the latter the meaning of the STDOUT handle itself has temporarily been changed. Slay, but know the difference.
Portability issues: select in perlport.
✨ select RBITS,WBITS,EBITS,TIMEOUT ✨
This one calls the select(2) syscall with the bit masks you specify, which can be constructed using fileno and vec, along these lines:
my $rin = my $win = my $ein = '';
vec($rin, fileno(STDIN), 1) = 1;
vec($win, fileno(STDOUT), 1) = 1;
$ein = $rin | $win;If you want to select on many filehandles, you may wish to write a subroutine like this:
sub fhbits {
my @fhlist = @_;
my $bits = "";
for my $fh (@fhlist) {
vec($bits, fileno($fh), 1) = 1;
}
return $bits;
}
my $rin = fhbits(\*STDIN, $tty, $mysock);The usual idiom is:
my ($nfound, $timeleft) =
select(my $rout = $rin, my $wout = $win, my $eout = $ein,
$timeout);or to block until something becomes ready, just do this:
my $nfound =
select(my $rout = $rin, my $wout = $win, my $eout = $ein, undef);Most systems do not bother to return anything useful in $timeleft, so calling select in scalar context just returns $nfound. 💅
Any of the bit masks can also be undef. The timeout, if specified, is in seconds, which may be fractional. Note: not all implementations are capable of returning the $timeleft. If not, they always return $timeleft equal to the supplied $timeout.
You can effect a sleep of 250 milliseconds this way:
select(undef, undef, undef, 0.25);Note that whether select gets restarted after signals (say, SIGALRM 😭) is implementation-dependent. See also perlport for notes on the portability of select.
On error, select behaves just like select(2): it returns -1 and sets $!. 🔥
On some Unixes, select(2) may report a socket file descriptor as "ready for reading" even when no data is available, and thus any subsequent read would block. 😱 This can be avoided if you always use O_NONBLOCK on the socket. See select(2) and fcntl(2) for further details.
The standard IO::Select module provides a user-friendlier interface to select — mostly because it does all the bit-mask work for you. 🫶
🚨 WARNING: One should not attempt to mix buffered I/O (likereadorreadline) withselect, except as permitted by POSIX, and even then only on POSIX systems. You have to usesysreadinstead.
Portability issues: select in perlport.
✨ semctl ID,SEMNUM,CMD,ARG ✨
Calls the System V IPC function semctl(2). You'll probably have to say
use IPC::SysV;first to get the correct constant definitions. If CMD is IPC_STAT or GETALL, then ARG must be a variable that will hold the returned semid_ds structure or semaphore value array. Returns like ioctl: the undefined value for error, "0 but true" for zero, or the actual return value otherwise. The ARG must consist of a vector of native short integers, which may be created with pack("s!",(0)x$nsem). See also "SysV IPC" in perlipc and the documentation for IPC::SysV and IPC::Semaphore.
Portability issues: semctl in perlport.
✨ semget KEY,NSEMS,FLAGS ✨
Calls the System V IPC function semget(2). Returns the semaphore id, or the undefined value on error. 🎯 See also "SysV IPC" in perlipc and the documentation for IPC::SysV and IPC::Semaphore.
Portability issues: semget in perlport.
✨ semop KEY,OPSTRING ✨
Calls the System V IPC function semop(2) for semaphore operations such as signalling and waiting. OPSTRING must be a packed array of semop structures. Each semop structure can be generated with pack("s!3", $semnum, $semop, $semflag). The length of OPSTRING implies the number of semaphore operations. Returns true if successful, false on error.
As an example, the following code waits on semaphore $semnum of semaphore id $semid:
my $semop = pack("s!3", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid, $semop);To signal the semaphore, replace -1 with 1. 💅 See also "SysV IPC" in perlipc and the documentation for IPC::SysV and IPC::Semaphore.
Portability issues: semop in perlport.
✨ send SOCKET,MSG,FLAGS,TO ✨ / ✨ send SOCKET,MSG,FLAGS ✨
Sends a message on a socket. 📨 Attempts to send the scalar MSG to the SOCKET filehandle. Takes the same flags as the system call of the same name. On unconnected sockets, you must specify a destination to send to, in which case it does a sendto(2) syscall. Returns the number of characters sent, or the undefined value on error. The sendmsg(2) syscall is currently unimplemented. 🙈 See "UDP: Message Passing" in perlipc for examples.
⚠️ Note that if the socket has been marked as:utf8,sendwill throw an exception. The:encoding(...)layer implicitly introduces the:utf8layer. Seebinmode.
✨ setpgrp PID,PGRP ✨
Sets the current process group for the specified PID — 0 for the current process. Raises an exception when used on a machine that doesn't implement POSIX setpgid(2) or BSD setpgrp(2). 😭 If the arguments are omitted, it defaults to "0,0". Note that the BSD 4.2 version of setpgrp does not accept any arguments, so only setpgrp(0,0) is portable. See also POSIX::setsid().
Portability issues: setpgrp in perlport.
🌸 setpriority WHICH,WHO,PRIORITY 🌸
Sets the current priority for a process, a process group, or a user. (Sneak a peek at setpriority(2) for the deets.) Bestie, heads up: on a machine that doesn't implement setpriority(2), this raises an exception. It's giving "not supported, sorry babe".
WHICH can be any of PRIO_PROCESS, PRIO_PGRP or PRIO_USER, imported from RESOURCE CONSTANTS in POSIX. 💅
Portability issues: setpriority in perlport.
💖 setsockopt SOCKET,LEVEL,OPTVAL… wait, no — setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL 💖
Sets the socket option you asked for. Returns undef on error (sad girl hours 😭). Use the integer constants from the Socket module for LEVEL and OPNAME. Values for LEVEL can also be snatched from getprotobyname. OPTVAL can be either a packed string or an integer — and an integer OPTVAL is just shorthand for pack("i", OPTVAL), iconic.
An example disabling Nagle's algorithm on a socket:
use Socket qw(IPPROTO_TCP TCP_NODELAY);
setsockopt($socket, IPPROTO_TCP, TCP_NODELAY, 1);Portability issues: setsockopt in perlport.
🎀 shift ARRAY 🎀
🎀 shift 🎀
Removes and returns the first element of an array. This shortens the array by one and shifts everything on down — like the front of the line leaving, everyone moves up. 🦋
my @arr = ('cat', 'dog');
my $item = shift(@arr); # 'cat'
# @arr is now ('dog');Returns undef if the array is empty.
Note:shiftmay also returnundefif the first element in the array isundef. So like… it's not lying, it's just unhelpful. 💀
my @arr = (undef, 'two', 'three');
my $item = shift(@arr); # undefIf ARRAY is omitted, shift operates on the @ARGV array in the main program, and the @_ array in subroutines. And shift will operate on the @ARGV array inside eval STRING, BEGIN {}, INIT {}, and CHECK {} blocks.
Starting with Perl 5.14, an experimental feature let shift take a scalar expression. That experiment was declared a flop, bestie, and it was removed as of Perl 5.24. ✨
See also unshift, push, and pop. And here's the vibe: shift and unshift do the same thing to the left end of an array that pop and push do to the right end. Symmetry, periodt. 💅
🧚 shmctl ID,CMD,ARG 🧚
Calls the System V IPC function shmctl. You'll probably have to say
use IPC::SysV;first, to get the correct constant definitions. If CMD is IPC_STAT, then ARG must be a variable that will hold the returned shmid_ds structure. It returns like ioctl does: undef for error; 0 but true for zero; and the actual return value otherwise. See also SysV IPC in perlipc and the docs for IPC::SysV.
Portability issues: shmctl in perlport.
🌈 shmget KEY,SIZE,FLAGS 🌈
Calls the System V IPC function shmget. Returns the shared memory segment id, or undef on error. See also SysV IPC in perlipc and the docs for IPC::SysV.
Portability issues: shmget in perlport.
🍓 shmread ID,VAR,POS,SIZE 🍓
🍓 shmwrite ID,STRING,POS,SIZE 🍓
Reads or writes the System V shared memory segment ID starting at position POS for size SIZE, by attaching to it, copying in/out, and detaching from it. When reading, VAR must be a variable that will hold the data read. When writing, if STRING is too long, only SIZE bytes get used; if STRING is too short, nulls are written to fill out SIZE bytes. Returns true if it worked, false on error. shmread taints the variable. ⚠️ See also SysV IPC in perlipc, the docs for IPC::SysV, and the IPC::Shareable module from CPAN.
Portability issues: shmread in perlport and shmwrite in perlport.
💜 shutdown SOCKET,HOW 💜
Shuts down a socket connection in the manner indicated by HOW, which has the same interpretation as in the syscall of the same name.
shutdown($socket, 0); # I/we have stopped reading data
shutdown($socket, 1); # I/we have stopped writing data
shutdown($socket, 2); # I/we have stopped using this socketThis is useful with sockets when you want to tell the other side you're done writing but not done reading, or the other way around. It's also a more insistent form of close, because it also disables the file descriptor in any forked copies in other processes. Like, it commits. 👑
Returns 1 for success; on error, it returns undef if the first argument isn't a valid filehandle, or returns 0 and sets $! for any other failure.
⭐ sin EXPR ⭐
⭐ sin ⭐
Returns the sine of EXPR (EXPR is in radians, bestie). If EXPR is omitted, it returns the sine of $_.
For the inverse sine operation, you may use the Math::Trig::asin function, or use this relation:
sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }🌸 sleep EXPR 🌸
🌸 sleep 🌸
Makes the script sleep for (integer) EXPR seconds, or forever if no argument is given. Returns the integer number of seconds actually slept. 😴
EXPR should be a positive integer. If it's called with a negative integer, sleep doesn't sleep — instead it emits a warning, sets $! (errno), and returns zero. Rude awakening fr.
If called with a non-integer, the fractional part is just ignored. 🫶
sleep 0 is permitted, but a function call to the underlying platform implementation still happens, with any side effects that may have. So sleep 0 is therefore not exactly identical to not sleeping at all. Subtle but real.
It may be interrupted if the process receives a signal such as SIGALRM.
eval {
local $SIG{ALRM} = sub { die "Alarm!\n" };
sleep;
};
die $@ unless $@ eq "Alarm!\n";You probably cannot mix alarm and sleep calls, because sleep is often implemented using alarm. They're not friends, lowkey.
On some older systems, it may sleep up to a whole second less than what you requested, depending on how it counts seconds. Most modern systems always sleep the full amount. They may appear to sleep longer than that, though, because your process might not get scheduled right away on a busy multitasking system.
For delays of finer granularity than one second, the Time::HiRes module (from CPAN, and starting from Perl 5.8 part of the standard distribution) provides usleep. You may also use Perl's four-argument version of select with the first three arguments left undefined, or you might be able to use the syscall interface to access setitimer(2) if your system supports it. See perlfaq8 for details.
See also the POSIX module's pause function.
🦋 socket SOCKET,DOMAIN,TYPE,PROTOCOL 🦋
Opens a socket of the specified kind and attaches it to filehandle SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the syscall of the same name. You should use Socket first to get the proper definitions imported. See the examples in Sockets: Client/Server Communication in perlipc.
On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor, as determined by the value of $^F. See $^F in perlvar.
💖 socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL 💖
Creates an unnamed pair of sockets in the specified domain, of the specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as for the syscall of the same name. If it's unimplemented, it raises an exception. Returns true if successful.
On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptors, as determined by the value of $^F. See $^F in perlvar.
Some systems define pipe in terms of socketpair, where a call to pipe($rdr, $wtr) is essentially:
use Socket;
socketpair(my $rdr, my $wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
shutdown($rdr, 1); # no more writing for reader
shutdown($wtr, 0); # no more reading for writerSee perlipc for an example of socketpair use. Perl 5.8 and later will emulate socketpair using IP sockets to localhost if your system implements sockets but not socketpair.
Portability issues: socketpair in perlport.
👑 sort SUBNAME LIST 👑
👑 sort BLOCK LIST 👑
👑 sort LIST 👑
In list context, this sorts the LIST and returns the sorted list value. In scalar context, the behaviour of sort is undefined. Don't do that, bestie, it's giving chaos. 💀
If SUBNAME or BLOCK is omitted, sorts in standard string comparison order. If SUBNAME is specified, it gives the name of a subroutine that returns a numeric value less than, equal to, or greater than 0, depending on how the elements of the list are meant to be ordered. (The <=> and cmp operators are extremely useful in such routines — they ate.) SUBNAME may be a scalar variable name (unsubscripted), in which case the value provides the name of (or a reference to) the actual subroutine to use. And in place of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort subroutine. ✨
If the subroutine's prototype is ($$), the elements to be compared are passed by reference in @_, as for a normal subroutine. This is slower than unprototyped subroutines, where the elements to be compared are passed into the subroutine as the package global variables $a and $b (see example below).
If the subroutine is an XSUB, the elements to be compared are pushed on to the stack, the way arguments are usually passed to XSUBs. $a and $b are not set.
If two or more elements of LIST compare the same (i.e. the comparison operation returns 0 for them), they retain the relative order they had in LIST. In other words, sorting is stable (and has been since v5.8.0). Iconic behaviour, no notes. 💅
The values to be compared are always passed by reference and should not be modified. Hands off, babe.
You also cannot exit out of the sort block or subroutine using any of the loop control operators described in perlsyn, or with goto. Nope. Not happening.
When use locale (but not use locale ':not_characters') is in effect, sort LIST sorts LIST according to the current collation locale. See perllocale.
sort returns aliases into the original list, much as a for loop's index variable aliases the list elements. That is, modifying an element of a list returned by sort (for example, in a foreach, map or grep) actually modifies the element in the original list. This is usually something to be avoided when writing clear code. 🙈
Examples:
# sort lexically
my @articles = sort @files;
# same thing, but with explicit sort routine
my @articles = sort {$a cmp $b} @files;
# now case-insensitively
my @articles = sort {fc($a) cmp fc($b)} @files;
# same thing in reversed order
my @articles = sort {$b cmp $a} @files;# sort numerically ascending
my @articles = sort {$a <=> $b} @files;
# sort numerically descending
my @articles = sort {$b <=> $a} @files;
# this sorts the %age hash by value instead of key
# using an in-line function
my @eldest = sort { $age{$b} <=> $age{$a} } keys %age;
# sort using explicit subroutine name
sub byage {
$age{$a} <=> $age{$b}; # presuming numeric
}
my @sortedclass = sort byage @class;
sub backwards { $b cmp $a }
my @harry = qw(dog cat x Cain Abel);
my @george = qw(gone chased yz Punished Axed);
print sort @harry;
# prints AbelCaincatdogx
print sort backwards @harry;
# prints xdogcatCainAbel
print sort @george, 'to', @harry;
# prints AbelAxedCainPunishedcatchaseddoggonetoxyz
# inefficiently sort by descending numeric compare using
# the first integer after the first = sign, or the
# whole record case-insensitively otherwise
my @new = sort {
($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
||
fc($a) cmp fc($b)
} @old;
# same thing, but much more efficiently;
# we'll build auxiliary indices instead
# for speed
my (@nums, @caps);
for (@old) {
push @nums, ( /=(\d+)/ ? $1 : undef );
push @caps, fc($_);
}
my @new = @old[ sort {
$nums[$b] <=> $nums[$a]
||
$caps[$a] cmp $caps[$b]
} 0..$#old
];
# same thing, but without any temps
my @new = map { $_->[0] }
sort { $b->[1] <=> $a->[1]
||
$a->[2] cmp $b->[2]
} map { [$_, /=(\d+)/, fc($_)] } @old;
# using a prototype allows you to use any comparison subroutine
# as a sort subroutine (including other package's subroutines)
package Other;
sub backwards ($$) { $_[1] cmp $_[0]; } # $a and $b are
# not set here
package main;
my @new = sort Other::backwards @old;
## using a prototype with function signature
use feature 'signatures';
sub function_with_signature :prototype($$) ($one, $two) {
return $one <=> $two
}
my @new = sort function_with_signature @old;okay bestie that last one?? the signature + prototype combo?? iconic ✨ not me tearing up over a sort sub 💅
⚠️ Warning: syntactical care is required when sorting the list returned from a function. If you want to sort the list returned by the function call find_records(@key), you can use:
my @contact = sort { $a cmp $b } find_records @key;
my @contact = sort +find_records(@key);
my @contact = sort &find_records(@key);
my @contact = sort(find_records(@key));If instead you want to sort the array @key with the comparison routine find_records() then you can use:
my @contact = sort { find_records() } @key;
my @contact = sort find_records(@key);
my @contact = sort(find_records @key);
my @contact = sort(find_records (@key));like, the syntax really said "choose your fighter" 😭 pick wrong and you're sorting the wrong thing, no cap.
$a and $b are set as package globals in the package the sort() is called from. That means $main::a and $main::b (or $::a and $::b) in the "main" package, $FooPack::a and $FooPack::b in the "FooPack" package, etc. If the sort block is in scope of a "my" or "state" declaration of $a and/or $b, you must spell out the full name of the variables in the sort block :
package main;
my $a = "C"; # DANGER, Will Robinson, DANGER !!!
print sort { $a cmp $b } qw(A C E G B D F H);
# WRONG
sub badlexi { $a cmp $b }
print sort badlexi qw(A C E G B D F H);
# WRONG
# the above prints BACFEDGH or some other incorrect ordering
print sort { $::a cmp $::b } qw(A C E G B D F H);
# OK
print sort { our $a cmp our $b } qw(A C E G B D F H);
# also OK
print sort { our ($a, $b); $a cmp $b } qw(A C E G B D F H);
# also OK
sub lexi { our $a cmp our $b }
print sort lexi qw(A C E G B D F H);
# also OK
# the above print ABCDEFGH🚨 bestie the comment literally says DANGER, Will Robinson, DANGER !!! — that's your sign to spell out the full name and not get quirky with my $a. the vibes are cursed otherwise 💀
With proper care you may mix package and my (or state) $a and/or $b:
my $a = {
tiny => -2,
small => -1,
normal => 0,
big => 1,
huge => 2
};
say sort { $a->{our $a} <=> $a->{our $b} }
qw{ huge normal tiny small big};
# prints tinysmallnormalbighuge$a and $b are implicitly local to the sort() execution and regain their former values upon completing the sort.
Sort subroutines written using $a and $b are bound to their calling package. It is possible, but of limited interest, to define them in a different package, since the subroutine must still refer to the calling package's $a and $b :
package Foo;
sub lexi { $Bar::a cmp $Bar::b }
package Bar;
... sort Foo::lexi ...Use the prototyped versions (see above) for a more generic alternative.
The comparison function is required to behave. If it returns inconsistent results (sometimes saying $x[1] is less than $x[2] and sometimes saying the opposite, for example) the results are not well-defined.
Because "<=>" returns "undef" when either operand is "NaN" (not-a-number), be careful when sorting with a comparison function like "$a <=> $b" any lists that might contain a "NaN". The following example takes advantage that "NaN != NaN" to eliminate any "NaN"s from the input list.
my @result = sort { $a <=> $b } grep { $_ == $_ } @input;the $_ == $_ trick is so galaxy brain fr 🧠✨ NaN don't even compare to itself, so it self-eliminates. queen behavior tbh 💅
In the current version of Perl, the "sort" function is implemented via the mergesort algorithm.
✂️ splice ARRAY,OFFSET,LENGTH,LIST / splice ARRAY,OFFSET,LENGTH / splice ARRAY,OFFSET / splice ARRAY 🫶
Removes the elements designated by OFFSET and LENGTH from an array, and replaces them with the elements of LIST, if any. In list context, returns the elements removed from the array. In scalar context, returns the last element removed, or "undef" if no elements are removed. The array grows or shrinks as necessary. If OFFSET is negative then it starts that far from the end of the array. If LENGTH is omitted, removes everything from OFFSET onward. If LENGTH is negative, removes the elements from OFFSET onward except for -LENGTH elements at the end of the array. If both OFFSET and LENGTH are omitted, removes everything. If OFFSET is past the end of the array and a LENGTH was provided, Perl issues a warning, and splices at the end of the array.
it's giving scalpel energy 🔪 — you cut a chunk out, you optionally drop new stuff in, and the array just reshapes itself like nothing happened. main character adaptability ✨
The following equivalences hold (assuming "$#a >= $i" )
push(@a,$x,$y) splice(@a,@a,0,$x,$y)
pop(@a) splice(@a,-1)
shift(@a) splice(@a,0,1)
unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
$a[$i] = $y splice(@a,$i,1,$y)"splice" can be used, for example, to implement n-ary queue processing:
sub nary_print {
my $n = shift;
while (my @next_n = splice @_, 0, $n) {
say join q{ -- }, @next_n;
}
}
nary_print(3, qw(a b c d e f g h));
# prints:
# a -- b -- c
# d -- e -- f
# g -- hn-ary queue processing?? we love a versatile icon 🎀
Starting with Perl 5.14, an experimental feature allowed "splice" to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24.
🙈 rip to that experiment, it simply did not eat. gone but not forgotten, bestie.
🦋 split /PATTERN/,EXPR,LIMIT / split /PATTERN/,EXPR / split /PATTERN/ / split 🌸
Splits the string EXPR into a list of strings and returns the list in list context, or the size of the list in scalar context. (Prior to Perl 5.11, it also overwrote @_ with the list in void and scalar context. If you target old perls, beware.)
If only PATTERN is given, EXPR defaults to $_.
Anything in EXPR that matches PATTERN is taken to be a separator that separates the EXPR into substrings (called "fields") that do not include the separator. Note that a separator may be longer than one character or even have no characters at all (the empty string, which is a zero-width match).
The PATTERN need not be constant; an expression may be used to specify a pattern that varies at runtime.
If PATTERN matches the empty string, the EXPR is split at the match position (between characters). As an example, the following:
my @x = split(/b/, "abc"); # ("a", "c")uses the "b" in 'abc' as a separator to produce the list ("a", "c"). However, this:
my @x = split(//, "abc"); # ("a", "b", "c")uses empty string matches as separators; thus, the empty string may be used to split EXPR into a list of its component characters.
As a special case for "split", the empty pattern given in match operator syntax ("//") specifically matches the empty string, which is contrary to its usual interpretation as the last successful match.
If PATTERN is "/^/", then it is treated as if it used the multiline modifier ("/^/m"), since it isn't much use otherwise.
"/m" and any of the other pattern modifiers valid for "qr" (summarized in "qr/STRING/msixpodualn" in perlop) may be specified explicitly.
As another special case, "split" emulates the default behavior of the command line tool awk when the PATTERN is either omitted or a string composed of a single space character (such as ' ' or "\x20", but not e.g. "/ /"). In this case, any leading whitespace in EXPR is removed before splitting occurs, and the PATTERN is instead treated as if it were "/\s+/"; in particular, this means that any contiguous whitespace (not just a single space character) is used as a separator.
my @x = split(" ", " Quick brown fox\n");
# ("Quick", "brown", "fox")the awk mode is the sneaky little glow-up nobody expects 🌈 it's not "one space is the separator," it's "ANY run of whitespace, and we're eating the leading stuff first." chef's kiss fr 💅✨
my @x = split(" ", "RED\tGREEN\tBLUE");
# ("RED", "GREEN", "BLUE")Using split like this is basically the same energy as how qw// works — same vibes, same result 💅.
But you can dodge that special treatment entirely by naming the pattern as "/ /" instead of the plain string " " — that way only a single space character counts as the separator, no fancy business. Back in earlier Perls this special case was only triggered when a plain " " was the pattern argument to split; as of Perl 5.18.0 and later, any expression that evaluates to the simple string " " triggers it. iykyk 🌸
As of Perl 5.28, this special-cased whitespace splitting actually behaves like you'd expect inside the scope of use feature 'unicode_strings'. In earlier versions, and outside that feature's scope, it exhibits "The "Unicode Bug"" in perlunicode: characters that count as whitespace under Unicode rules but not under ASCII rules can get treated as part of a field instead of as a field separator, all depending on the string's internal encoding. chaotic behaviour fr fr 😭
As of Perl 5.39.9, the /x default modifier does NOT affect split STRING, but it does affect split PATTERN. What that means: split " " gives you the expected awk emulation no matter whether you're inside a use re "/x" scope. So if you wanna split on spaces while under use re "/x", you gotta do something like split /(?-x: )/ or split /\x{20}/ instead of split / /. 👑
If you omit PATTERN, it defaults to a single space, " ", which kicks off that whole awk emulation we were just talking about ✨.
If LIMIT is given and it's positive, it's the maximum number of fields that EXPR can get split into — in other words, LIMIT is one greater than the max number of times EXPR may be split. So LIMIT 1 means EXPR can be split a maximum of zero times, giving you a maximum of one field (namely the entire value of EXPR). Like so:
my @x = split(/,/, "a,b,c", 1); # ("a,b,c")
my @x = split(/,/, "a,b,c", 2); # ("a", "b,c")
my @x = split(/,/, "a,b,c", 3); # ("a", "b", "c")
my @x = split(/,/, "a,b,c", 4); # ("a", "b", "c")If LIMIT is negative, it's treated like it was arbitrarily large instead — as many fields as possible get produced, no cap 🧚
If LIMIT is omitted (or, equivalently, zero), then it's usually treated like it were negative, with one exception: trailing empty fields get stripped. (Empty leading fields are always preserved, periodt.) And if all the fields are empty, then all of them count as trailing, so they all get stripped in that case. So this:
my @x = split(/,/, "a,b,c,,,"); # ("a", "b", "c")gives you a three element list only.
my @x = split(/,/, "a,b,c,,,", -1); # ("a", "b", "c", "", "", "")gives you a six element list. the difference bestie ✨
In time-critical applications, it's worth avoiding splitting into more fields than you actually need. So when you're assigning to a list and LIMIT is omitted (or zero), LIMIT gets treated as though it were one larger than the number of variables in the list — for this next one, LIMIT is implicitly 3:
my ($login, $passwd) = split(/:/);Note that splitting an EXPR that evaluates to the empty string always produces zero fields, no matter what LIMIT you specified. 💀
An empty leading field happens when there's a positive-width match at the beginning of EXPR. For instance:
my @x = split(/ /, " abc"); # ("", "abc")splits into two elements. However, a zero-width match at the beginning of EXPR never produces an empty field, so:
my @x = split(//, " abc"); # (" ", "a", "b", "c")splits into four elements instead of five. 🦋
An empty trailing field, on the other hand, gets produced when there's a match at the end of EXPR — regardless of the length of that match (of course, unless a non-zero LIMIT is explicitly given, in which case those fields get removed, like in the example just above). So:
my @x = split(//, " abc", -1); # (" ", "a", "b", "c", "")If PATTERN contains capturing groups, then for each separator an extra field is produced for every substring captured by a group — in the order the groups are specified, same as backreferences. If any group doesn't match, it captures the undef value instead of a substring. Also note: any such additional field is produced whenever there's a separator (that is, whenever a split occurs), and such an additional field does not count towards the LIMIT. Peep these expressions, all evaluated in list context (each returned list is sitting right there in the comment):
my @x = split(/-|,/ , "1-10,20", 3);
# ("1", "10", "20")
my @x = split(/(-|,)/ , "1-10,20", 3);
# ("1", "-", "10", ",", "20")
my @x = split(/-|(,)/ , "1-10,20", 3);
# ("1", undef, "10", ",", "20")
my @x = split(/(-)|,/ , "1-10,20", 3);
# ("1", "-", "10", undef, "20")
my @x = split(/(-)|(,)/, "1-10,20", 3);
# ("1", "-", undef, "10", undef, ",", "20")look at that undef placement, it's giving precision 💅
✨ sprintf FORMAT, LIST ✨
Returns a string formatted with the usual printf conventions of the C library function sprintf. More details below — and for an explanation of the general principles, go check sprintf(3) or printf(3) on your own system. 🌸
For example:
# Format number with up to 8 leading zeroes
my $result = sprintf("%08d", $number);
# Round number to 3 digits after decimal point
my $rounded = sprintf("%.3f", $number);Perl does its own sprintf formatting: it emulates the C function sprintf(3) but doesn't actually use it, except for floating-point numbers — and even then only standard modifiers are allowed. So any non-standard extensions in your local sprintf(3) are not available from Perl. sorry bestie 🫶
Unlike printf, sprintf does not do what you probably mean when you pass it an array as the first argument. The array gets scalar context, and instead of using the 0th element of the array as the format, Perl uses the count of elements in the array as the format — which is almost never useful. 😭
Perl's sprintf allows these universally-known conversions:
%% a percent sign
%c a character with the given number
%s a string
%d a signed integer, in decimal
%u an unsigned integer, in decimal
%o an unsigned integer, in octal
%x an unsigned integer, in hexadecimal
%e a floating-point number, in scientific notation
%f a floating-point number, in fixed decimal notation
%g a floating-point number, in %e or %f notationOn top of that, Perl allows these widely-supported conversions:
%X like %x, but using upper-case letters
%E like %e, but using an upper-case "E"
%G like %g, but with an upper-case "E" (if applicable)
%b an unsigned integer, in binary
%B like %b, but using an upper-case "B" with the # flag
%p a pointer (outputs the Perl value's address in hexadecimal)
%n special: *stores* the number of characters output so far
into the next argument in the parameter list
%a hexadecimal floating point
%A like %a, but using upper-case letters
%i a synonym for %d
%D a synonym for %ld
%U a synonym for %lu
%O a synonym for %lo
%F a synonym for %fokay real talk time: the number of exponent digits in the scientific notation produced by %e, %E, %g and %G for numbers where the modulus of the exponent is less than 100 is system-dependent — it may be three or less (zero-padded as necessary). In other words, 1.23 times ten to the 99th may be either "1.23e99" or "1.23e099". Same deal for %a and %A: the exponent or the hexadecimal digits may float — especially the "long doubles" Perl configuration option can cause surprises. 🙈
Between the % and the format letter, you can specify a bunch of additional attributes controlling how the format gets interpreted. In order, they are:
🎀 format parameter index 🎀
An explicit format parameter index, like 2$. By default sprintf formats the next unused argument in the list, but this lets you pull arguments out of order:
printf '%2$d %1$d', 12, 34; # prints "34 12"
printf '%3$d %d %1$d', 1, 2, 3; # prints "3 1 1"arguments out of order?? iconic. main character energy. 👑
💜 flags 💜
One or more of:
- space — prefix non-negative number with a space
+— prefix non-negative number with a plus sign-— left-justify within the field0— use zeros, not spaces, to right-justify#— ensure the leading"0"for any octal, prefix non-zero hexadecimal with"0x"or"0X", prefix non-zero binary with"0b"or"0B"
For example:
printf '<% d>', 12; # prints "< 12>"
printf '<% d>', 0; # prints "< 0>"
printf '<% d>', -12; # prints "<-12>"
printf '<%+d>', 12; # prints "<+12>"
printf '<%+d>', 0; # prints "<+0>"
printf '<%+d>', -12; # prints "<-12>"
printf '<%6s>', 12; # prints "< 12>"
printf '<%-6s>', 12; # prints "<12 >"
printf '<%06s>', 12; # prints "<000012>"
printf '<%#o>', 12; # prints "<014>"
printf '<%#x>', 12; # prints "<0xc>"
printf '<%#X>', 12; # prints "<0XC>"
printf '<%#b>', 12; # prints "<0b1100>"
printf '<%#B>', 12; # prints "<0B1100>"When a space and a plus sign are given as the flags at once, the space is ignored:
printf '<%+ d>', 12; # prints "<+12>"
printf '<% +d>', 12; # prints "<+12>"When the # flag and a precision are given in the %o conversion, the precision gets incremented if it's necessary for that leading "0":
printf '<%#.5o>', 012; # prints "<00012>"
printf '<%#.5o>', 012345; # prints "<012345>"
printf '<%#.0o>', 0; # prints "<0>"precision math with the # flag, obsessed honestly 💖
✨ vector flag ✨
okay so this one is lowkey iconic 💅 — it tells Perl to read the string you hand it as a vector of integers, one integer per character. Perl runs the format against each of those integers one at a time, then glues all the resulting strings together with a separator (a dot . by default). Super useful when you wanna peep the ordinal values of chars in whatever random string:
printf "%vd", "AB\x{100}"; # prints "65.66.256"
printf "version is v%vd\n", $^V; # Perl's versionWanna change the separator? Just yeet an asterisk * in front of the v and that overrides the string used to join the numbers:
printf "address is %*vX\n", ":", $addr; # IPv6 address
printf "bits are %0*v8b\n", " ", $bits; # random bitstringAnd you can explicitly name the argument number for that join string, using something like *2$v. for real:
printf '%*4$vX %*4$vX %*4$vX', # 3 IPv6 addresses
@addr[1..3], ":";✨ (minimum) width ✨
usually arguments get formatted to be exactly as wide as they need to be to show the value — no more, no less. but bestie, you can override that width: just drop a number here, or grab the width from the next argument (with *), or from a specified argument (like, with *2$):
printf "<%s>", "a"; # prints "<a>"
printf "<%6s>", "a"; # prints "< a>"
printf "<%*s>", 6, "a"; # prints "< a>"
printf '<%*2$s>', "a", 6; # prints "< a>"
printf "<%2s>", "long"; # prints "<long>" (does not truncate)and ngl if a field width you got through * comes out negative, it does the same thing as the - flag: left-justifies everything. slay.
✨ precision, or maximum width ✨
you can set a precision (for numeric conversions) or a maximum width (for string conversions) by writing a . and then a number. for floating-point formats other than g and G, this says how many places right of the decimal point to show (the default being 6). like:
# these examples are subject to system-specific variation
printf '<%f>', 1; # prints "<1.000000>"
printf '<%.1f>', 1; # prints "<1.0>"
printf '<%.0f>', 1; # prints "<1>"
printf '<%07.2f>', 1.3; # prints "<0001.30>"
printf '<%e>', 10; # prints "<1.000000e+01>"
printf '<%.1e>', 10; # prints "<1.0e+01>"for g and G, this instead says the maximum number of significant digits to show. for example:
# These examples are subject to system-specific variation.
printf '<%g>', 1; # prints "<1>"
printf '<%.10g>', 1; # prints "<1>"
printf '<%g>', 100; # prints "<100>"
printf '<%.1g>', 100; # prints "<1e+02>"
printf '<%.2g>', 100.01; # prints "<1e+02>"
printf '<%.5g>', 100.01; # prints "<100.01>"
printf '<%.4g>', 100.01; # prints "<100>"
printf '<%.1g>', 0.0111; # prints "<0.01>"
printf '<%.2g>', 0.0111; # prints "<0.011>"
printf '<%.3g>', 0.0111; # prints "<0.0111>"for integer conversions, setting a precision basically implies the number itself should be zero-padded out to that width, and the 0 flag just gets ignored. (rude, but okay.)
printf '<%.6d>', 1; # prints "<000001>"
printf '<%+.6d>', 1; # prints "<+000001>"
printf '<%-10.6d>', 1; # prints "<000001 >"
printf '<%10.6d>', 1; # prints "< 000001>"
printf '<%010.6d>', 1; # prints "< 000001>"
printf '<%+10.6d>', 1; # prints "< +000001>"
printf '<%.6x>', 1; # prints "<000001>"
printf '<%#.6x>', 1; # prints "<0x000001>"
printf '<%-10.6x>', 1; # prints "<000001 >"
printf '<%10.6x>', 1; # prints "< 000001>"
printf '<%010.6x>', 1; # prints "< 000001>"
printf '<%#10.6x>', 1; # prints "< 0x000001>"for string conversions, a precision truncates the string to fit that width:
printf '<%.5s>', "truncated"; # prints "<trunc>"
printf '<%10.5s>', "truncated"; # prints "< trunc>"you can also pull the precision from the next argument with .*, or from a specified argument (like with .*2$):
printf '<%.6x>', 1; # prints "<000001>"
printf '<%.*x>', 6, 1; # prints "<000001>"
printf '<%.*2$x>', 1, 6; # prints "<000001>"
printf '<%6.*2$x>', 1, 4; # prints "< 0001>"and if a precision you got through * is negative? it counts as having no precision at all. 🙈
printf '<%.*s>', 7, "string"; # prints "<string>"
printf '<%.*s>', 3, "string"; # prints "<str>"
printf '<%.*s>', 0, "string"; # prints "<>"
printf '<%.*s>', -1, "string"; # prints "<string>"
printf '<%.*d>', 1, 0; # prints "<0>"
printf '<%.*d>', 0, 0; # prints "<>"
printf '<%.*d>', -1, 0; # prints "<0>"✨ size ✨
for numeric conversions you can say how big to interpret the number by using l, h, V, q, L, or ll. for integer conversions (d u o x X b i D U O), numbers are usually assumed to be whatever your platform's default integer size is (usually 32 or 64 bits) — but you can override that and use one of the standard C types instead, as long as the compiler that built your Perl supports it:
hh— interpret integer as C type"char"or"unsigned char"on Perl 5.14 or laterh— interpret integer as C type"short"or"unsigned short"j— interpret integer as C type"intmax_t"on Perl 5.14 or later. heads up: prior to Perl 5.30 this only worked with a C99 compiler, so it was unportable before that release.l— interpret integer as C type"long"or"unsigned long"q,L, orll— interpret integer as C type"long long","unsigned long long", or"quad"(typically 64-bit integers)t— interpret integer as C type"ptrdiff_t"on Perl 5.14 or laterz— interpret integer as C types"size_t"or"ssize_t"on Perl 5.14 or later
ngl, generally speaking, using the l modifier from Perl code (like writing %ld or %lu instead of %d and %u) is unnecessary. worse — it can actually be harmful, like on Windows 64-bit where a long is 32-bits. so don't. 💀
as of 5.14, none of these throw an exception if your platform doesn't support them. but if warnings are on, you get a warning of the "printf" warning class when you use an unsupported conversion flag. if you'd rather have an exception instead, do this:
use warnings FATAL => "printf";and if you wanna know about a version dependency before you start running the program, put something like this at the very top:
use v5.14; # for hh/j/t/z/ printf modifiersyou can find out whether your Perl supports quads via Config:
use Config;
if ($Config{use64bitint} eq "define"
|| $Config{longsize} >= 8) {
print "Nice quads!\n";
}for floating-point conversions (e f g E F G), numbers are usually assumed to be your platform's default floating-point size (double or long double), but you can force "long double" with q, L, or ll if your platform supports them. you can check whether your Perl supports long doubles via Config:
use Config;
print "long doubles\n" if $Config{d_longdbl} eq "define";you can also check whether Perl treats "long double" as the default floating-point size on your platform via Config:
use Config;
if ($Config{uselongdouble} eq "define") {
print "long doubles by default\n";
}and sometimes long doubles and doubles are literally the exact same thing:
use Config;
($Config{doublesize} == $Config{longdblsize}) &&
print "doubles are long doubles\n";the size specifier V does nothing for Perl code, but it's supported for compatibility with XS code. it means "use the standard size for a Perl integer or floating-point number", which is the default anyway. so, formality queen 👑
✨ order of arguments ✨
normally sprintf just takes the next unused argument as the value to format for each format specification. and if a format spec uses * to demand extra arguments, those get consumed from the argument list in the order they appear in the format spec — and that all happens before the value to format. but when an argument is named by an explicit index, it doesn't mess with the normal order of arguments, even if the explicitly specified index would've been the next one up.
so like:
printf "<%*.*s>", $x, $y, $z;uses $x for the width, $y for the precision, and $z as the value to format. but:
printf '<%*1$.*s>', $x, $y;would use $x for both the width and the precision, and $y as the value to format. the way that shifts everything. 😭
here are some more examples — but be aware that when you use an explicit index, the $ may need escaping:
printf "%2\$d %d\n", 12, 34; # will print "34 12\n"
printf "%2\$d %d %d\n", 12, 34; # will print "34 12 34\n"
printf "%3\$d %d %d\n", 12, 34, 56; # will print "56 12 34\n"
printf "%2\$*3\$d %d\n", 12, 34, 3; # will print " 34 12\n"
printf "%*1\$.*f\n", 4, 5, 10; # will print "5.0000\n"if use locale (which includes use locale ':not_characters') is in effect and POSIX::setlocale has been called, then the character used for the decimal separator in formatted floating-point numbers is affected by the LC_NUMERIC locale. see perllocale and POSIX for the full saga. 🫶
🌸 sqrt 🌸
sqrt EXPR
sqrtokay so sqrt hands you back the positive square root of EXPR 💖. No EXPR? It just uses $_, easy peasy.
Tiny catch bestie: it literally only works for non-negative numbers... unless you've loaded the Math::Complex module, in which case it's giving imaginary-number main character energy ✨
use Math::Complex;
print sqrt(-4); # prints 2i💅 srand 💅
srand EXPR
srandSets and returns the random number seed for the rand operator. 👑
The whole point of this function is to "seed" the rand function so that rand can produce a different sequence each time you run your program. When you call it with a parameter, srand uses that as the seed; otherwise it (semi-)randomly chooses a seed for you (see below 👇). Either way, starting with Perl 5.14, it returns the seed. So if you wanna signal that your code only works on recent Perls:
use v5.14; # so srand returns the seedIf you never call srand explicitly, it gets called implicitly without a parameter at the first use of the rand operator. But there are a few situations where programs probably DO wanna call srand themselves:
- One is for generating predictable results, generally for testing or debugging. There, you use
srand($seed), with the same$seedeach time. 🎯 - Another case is that you may want to call
srandafter aforkto avoid child processes sharing the same seed value as the parent (and consequently each other). 🍼
Do not callsrand()(i.e., without an argument) more than once per process. The internal state of the random number generator should contain more entropy than can be provided by any seed, so callingsrandagain actually loses randomness. 😭
Most implementations of srand take an integer and will silently truncate decimal numbers. That means srand(42) will usually produce the same results as srand(42.1). So to be safe, always pass srand an integer, fr fr.
A typical use of the returned seed is for a test program which has too many combinations to test comprehensively in the time available to it each run. It can test a random subset each time, and should there be a failure, log the seed used for that run so that it can later be used to reproduce the same results. iconic honestly 🫶
If the PERL_RAND_SEED environment variable is set to a non-negative integer during process startup, then calls to srand() with no arguments will initialize the perl random number generator with a consistent seed each time it is called, whether called explicitly with no arguments or implicitly via use of rand(). The exact seeding that a given PERL_RAND_SEED will produce is deliberately unspecified, but using different values for PERL_RAND_SEED should produce different results. This is intended for debugging and performance analysis and is only guaranteed to produce consistent results between invocations of the same perl executable running the same code when all other factors are equal. The environment variable is read only once during process startup, and changing it during the program flow will not affect the currently running process. See perlrun for more details. 📝
randis not cryptographically secure. You should not rely on it in security-sensitive situations. See documentation ofrandfor a list of suitable alternatives. 🔐
🔥 stat 🔥
stat FILEHANDLE
stat EXPR
stat DIRHANDLE
statReturns a 13-element list giving the status info for a file — either the file opened via FILEHANDLE or DIRHANDLE, or named by EXPR. If EXPR is omitted, it stats $_ (not "_"!). Returns the empty list if stat fails. Typically used like this:
my ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
$atime,$mtime,$ctime,$blksize,$blocks)
= stat($filename);Not all fields are supported on all filesystem types. Here are the meanings of the fields:
0 dev device number of filesystem
1 ino inode number
2 mode file mode (type and permissions)
3 nlink number of (hard) links to the file
4 uid numeric user ID of file's owner
5 gid numeric group ID of file's owner
6 rdev the device identifier (special files only)
7 size total size of file, in bytes
8 atime last access time in seconds since the epoch
9 mtime last modify time in seconds since the epoch
10 ctime inode change time in seconds since the epoch (*)
11 blksize preferred I/O size in bytes for interacting with the
file (may vary from file to file)
12 blocks actual number of system-specific blocks allocated
on disk (often, but not always, 512 bytes each)(The epoch was at 00:00 January 1, 1970 GMT.) ⏰
(*) Not all fields are supported on all filesystem types. Notably, the ctime field is non-portable. In particular, you cannot expect it to be a "creation time"; see "Files and Filesystems" in perlport for details.
If stat is passed the special filehandle consisting of an underline, no stat is done, but the current contents of the stat structure from the last stat, lstat, or filetest are returned. Example:
if (-x $file && (($d) = stat(_)) && $d < 0) {
print "$file is executable NFS file\n";
}(This works on machines only for which the device number is negative under NFS.)
On some platforms inode numbers are of a type larger than perl knows how to handle as integer numerical values. If necessary, an inode number will be returned as a decimal string in order to preserve the entire value. If used in a numeric context, this will be converted to a floating-point numerical value, with rounding — a fate that is best avoided. So you should prefer to compare inode numbers using eq rather than ==. eq will work fine on inode numbers that are represented numerically, as well as those represented as strings. 🧠
Because the mode contains both the file type and its permissions, you should mask off the file type portion and (s)printf using a "%o" if you want to see the real permissions.
my $mode = (stat($filename))[2];
printf "Permissions are %04o\n", $mode & 07777;In scalar context, stat returns a boolean value indicating success or failure, and, if successful, sets the information associated with the special filehandle "_".
The File::stat module provides a convenient, by-name access mechanism: 💖
use File::stat;
my $sb = stat($filename);
printf "File is %s, size is %s, perm %04o, mtime %s\n",
$filename, $sb->size, $sb->mode & 07777,
scalar localtime $sb->mtime;You can import symbolic mode constants and functions (S_I*) from the Fcntl module:
use Fcntl ':mode';
my $mode = (stat($filename))[2];
my $user_rwx = ($mode & S_IRWXU) >> 6;
my $group_read = ($mode & S_IRGRP) >> 3;
my $other_execute = $mode & S_IXOTH;
printf "Permissions are %04o\n", S_IMODE($mode), "\n";
my $is_setuid = $mode & S_ISUID;
my $is_directory = S_ISDIR($mode);You could write the last two using the -u and -d operators. Commonly available S_I* constants are:
# Permissions: read, write, execute, for user, group, others.
S_IRWXU S_IRUSR S_IWUSR S_IXUSR
S_IRWXG S_IRGRP S_IWGRP S_IXGRP
S_IRWXO S_IROTH S_IWOTH S_IXOTH
# Setuid/Setgid/Stickiness/SaveText/EnforcedLocks.
# Note that the exact meaning of these is system-dependent.
S_ISUID S_ISGID S_ISVTX S_ISTXT S_ENFMT
# File types. Not all are necessarily available on
# your system.
S_IFREG S_IFDIR S_IFLNK S_IFBLK S_IFCHR
S_IFIFO S_IFSOCK S_IFWHT
# The following are compatibility aliases for S_IRUSR,
# S_IWUSR, and S_IXUSR.
S_IREAD S_IWRITE S_IEXECand the S_I* functions are
S_IMODE($mode) the part of $mode containing the permission
bits and the setuid/setgid/sticky bits
S_IFMT($mode) the part of $mode containing the file type,
which will match one of the S_IF* constants
(e.g. S_IFMT($mode) == S_IFDIR for directories),
but see the following helper functions
# The operators -f, -d, -l, -b, -c, -p, and -S.
S_ISREG($mode) S_ISDIR($mode) S_ISLNK($mode)
S_ISBLK($mode) S_ISCHR($mode) S_ISFIFO($mode) S_ISSOCK($mode)
# No direct -X operator counterpart.
S_ISWHT($mode)See your native chmod(2) and stat(2) documentation for more details about the S_I* constants. To get status info for a symbolic link instead of the target file behind the link, use the lstat function. 🔗
Portability issues: "stat" in perlport.
✨ state ✨
state VARLIST
state TYPE VARLIST
state VARLIST : ATTRS
state TYPE VARLIST : ATTRSstate declares a lexically scoped variable, just like my. BUT — and this is the iconic part — those variables will never be reinitialized, contrary to lexical variables that are reinitialized each time their enclosing block is entered. 👑 See "Persistent Private Variables" in perlsub for details.
If more than one variable is listed, the list must be placed in parentheses. With a parenthesised list, undef can be used as a dummy placeholder. However, since initialization of state variables in such lists is currently not possible, this would serve no purpose. 🙈
Like my, local, and our, state can operate on a variable anywhere it appears in an expression (aside from interpolation in strings). The declaration will not apply to additional uses of the same variable until the next statement. This means additional uses of that variable within the same statement will act as they would have before that declaration occurred, or result in a strict 'vars' error, as appropriate.
package main;
use feature 'state';
our $x = 2;
foo($x, state $x = $x + 1, $x); # foo() receives (2, 3, 2)
foo($x, $main::x); # foo() receives (3, 2)Redeclaring a variable in the same scope or statement will "shadow" the previous declaration, creating a new instance and preventing access to the previous one. This is usually undesired and, if warnings are enabled, will result in a warning in the "shadow" category. ⚠️
"state"</p> is only available if the state feature is turned on, or if you slap a CORE:: in front of it. And the state feature auto-enables itself whenever you drop a use v5.10 (or higher, bestie, go higher) declaration in the current scope. 💫
🌸 study SCALAR / study 🌸
Okay so right now, study does, like, literally nothing. Nothing. It might change in the future but today it is a ghost. 👻
Back before Perl version 5.16 it was doing something: it would build an inverted index of every character that showed up in the given SCALAR (or $_ if you didn't give it one). Then when you matched a pattern, the rarest character from the pattern would get looked up in that index. Rarity was decided by some static frequency tables built out of some C programs and English text. Wild, right? Anyway. It's a nothing-burger now.
🧚 sub NAME BLOCK / sub NAME (PROTO) BLOCK / sub NAME : ATTRS BLOCK / sub NAME (PROTO) : ATTRS BLOCK 🧚
This one is subroutine definition, not really a function at all, it's giving not like the other girls. No BLOCK? Then it's just a forward declaration. No NAME? Then it's an anonymous function declaration, which means it does return a value: the CODE ref of the closure you just made. ✨
Want the details on subroutines and references? See perlsub and perlref. Want the tea on attributes? See attributes and Attribute::Handlers. 🫶
👑 __SUB__ 👑
A special token that hands you back a reference to the current subroutine — or undef if you're outside of a subroutine.
Heads up though: how __SUB__ behaves inside a regex code block (like /(?{...})/) is subject to change. Don't get too attached.
This token is only available under use v5.16 or the current_sub feature. See feature. 💜
💅 substr EXPR,OFFSET,LENGTH,REPLACEMENT / substr EXPR,OFFSET,LENGTH / substr EXPR,OFFSET 💅
Extracts a substring out of EXPR and gives it to you. First character sits at offset zero. If OFFSET is negative, it starts that far back from the end of the string. If LENGTH is omitted, you get everything through the end of the string. If LENGTH is negative, it leaves that many characters off the end. 🌈
my $s = "The black cat climbed the green tree";
my $color = substr $s, 4, 5; # black
my $middle = substr $s, 4, -11; # black cat climbed the
my $end = substr $s, 14; # climbed the green tree
my $tail = substr $s, -4; # tree
my $z = substr $s, -4, 2; # trYou can use substr as an lvalue, and in that case EXPR has to be an lvalue itself. Assign something shorter than LENGTH? The string shrinks. Assign something longer than LENGTH? The string grows to fit it. Want to keep the string the same length? You might need to pad or chop your value with sprintf. 📏
If OFFSET and LENGTH point at a substring that's partly outside the string, only the part inside the string comes back. If the substring is beyond either end of the string, substr returns the undefined value and throws a warning. But when you use it as an lvalue, naming a substring that's entirely outside the string raises an exception. 😭 Here's the boundary-case behavior:
my $name = 'fred';
substr($name, 4) = 'dy'; # $name is now 'freddy'
my $null = substr $name, 6, 2; # returns "" (no warning)
my $oops = substr $name, 7; # returns undef, with warning
substr($name, 7) = 'gap'; # raises an exceptionAn alternative to using substr as an lvalue is to hand it the REPLACEMENT string as the 4th argument. That lets you swap out parts of EXPR and get back what used to be there, all in one operation — same energy as splice. 🔥
my $s = "The black cat climbed the green tree";
my $z = substr $s, 14, 7, "jumped from"; # climbed
# $s is now "The black cat jumped from the green tree"Fun thing to know: the lvalue that the three-argument version of substr returns acts like a magic bullet. Every time you assign to it, it remembers which part of the original string is being modified. For example:
my $x = '1234';
for (substr($x,1,2)) {
$_ = 'a'; print $x,"\n"; # prints 1a4
$_ = 'xyz'; print $x,"\n"; # prints 1xyz4
$x = '56789';
$_ = 'pq'; print $x,"\n"; # prints 5pq9
}With negative offsets, it remembers its position from the end of the string when the target string gets modified:
my $x = '1234';
for (substr($x, -3, 2)) {
$_ = 'a'; print $x,"\n"; # prints 1a4, as above
$x = 'abcdefg';
print $_,"\n"; # prints f
}Before Perl version 5.10, whatever you got from using an lvalue multiple times was unspecified. Before 5.16, whatever you got with negative offsets was unspecified. Bestie. Just don't. 💀
🦋 symlink OLDFILE,NEWFILE 🦋
Makes a new filename that's symbolically linked to the old filename. Returns 1 on success, 0 otherwise. On systems that don't do symbolic links at all, it raises an exception. Want to check whether that's gonna happen? Use eval:
my $symlink_exists = eval { symlink("",""); 1 };Portability issues: "symlink" in perlport. 🌍
⭐ syscall NUMBER, LIST ⭐
Calls the system call named as the first element of the list, passing the rest of the elements as arguments to that system call. If it's not implemented, it raises an exception.
The arguments get interpreted like this: if a given argument is numeric, the argument is passed as an int. If it's not, the pointer to the string value is passed. And listen — you are responsible for making sure a string is pre-extended long enough to receive any result that might get written into it. You can't use a string literal (or any other read-only string) as an argument to syscall, because Perl has to assume any string pointer might get written through. If your integer arguments aren't literals and have never been interpreted in a numeric context, you may need to add 0 to them to force them to look like numbers. This emulates the syswrite function (or the other way around):
require 'syscall.ph'; # may need to run h2ph
my $s = "hi there\n";
syscall(SYS_write(), fileno(STDOUT), $s, length $s);Side note: Perl only supports passing up to 14 arguments to your syscall, which in practice should (usually) be enough. 💁♀️
syscall returns whatever value the system call it calls returned. If the system call fails, syscall returns -1 and sets $! (errno). But careful: some system calls can legitimately return -1. The proper way to handle those is to assign "$! = 0" before the call, then check the value of $! if syscall returns -1. 🕵️♀️
There's a known problem with syscall(SYS_pipe()): it returns the file number of the read end of the pipe it creates, but there's no way to retrieve the file number of the other end. You can dodge this whole mess by using pipe instead. 🫠
Portability issues: "syscall" in perlport. 🌍
🎀 sysopen FILEHANDLE,FILENAME,MODE / sysopen FILEHANDLE,FILENAME,MODE,PERMS 🎀
Opens the file whose filename is given by FILENAME and hooks it up to FILEHANDLE. If FILEHANDLE is an expression, its value is used as the real filehandle you want; an undefined scalar gets suitably autovivified. This function calls the underlying operating system's open(2) function with the parameters FILENAME, MODE, and PERMS. 🔧
Returns true on success and undef otherwise.
PerlIO layers get applied to the handle the same way they would in an open call that doesn't specify layers. Meaning: the current value of "${^OPEN}" as set by the open pragma in a lexical scope, or the -C command-line option or PERL_UNICODE environment variable in the main program scope, falling back to the platform defaults as described in "Defaults and how to override them" in PerlIO. If you want to strip out any layers that might transform the byte stream, use binmode after opening it. 🚿
The possible values and flag bits of the MODE parameter are system-dependent — you get them via the standard module Fcntl. Go read your operating system's open(2) syscall docs to see which values and flag bits exist. You can combine several flags with the |-operator. 🧩
Some of the most common values are O_RDONLY for opening the file read-only, O_WRONLY for opening it write-only, and O_RDWR for opening it read-write.
For historical reasons, some values work on almost every system Perl supports: 0 means read-only, 1 means write-only, and 2 means read/write. We know these values don't work under OS/390, so you probably don't want to use them in new code. 💀
If the file named by FILENAME doesn't exist and the open call creates it (typically because MODE includes the O_CREAT flag), then the value of PERMS specifies the permissions of the newly created file. If you omit the PERMS argument to sysopen, Perl uses the octal value 0666. These permission values need to be in octal, and they get modified by your process's current umask. 🔐
On many systems the O_EXCL flag exists for opening files in exclusive mode. This is not locking — exclusiveness here just means that if the file already exists, sysopen fails. O_EXCL may not work on network filesystems, and it has no effect unless the O_CREAT flag is set as well. Setting O_CREAT|O_EXCL prevents the file from being opened if it's a symbolic link — but it does not protect against symbolic links in the file's path. 🙈
Sometimes you want to truncate an already-existing file, and you can do that with the O_TRUNC flag. The behavior of O_TRUNC with O_RDONLY is undefined. ⚠️
You should seldom if ever use 0644 as an argument to sysopen, because that takes away the user's option to have a more permissive umask. Better to just omit it. See umask for more on this. 🌸
This function has no direct relation to the usage of sysread, syswrite, or sysseek. A handle opened with this function can be used with buffered IO just like one opened with open can be used with unbuffered IO. 🔄
Note that under Perls older than 5.8.0, sysopen depends on the fdopen(3) C library function. On many Unix systems, fdopen(3) is known to fail when file descriptors exceed a certain value — typically 255. If you need more file descriptors than that, consider using the POSIX::open function. For Perls 5.8.0 and later, PerlIO is (most often) the default. ✨
See perlopentut for a kinder, gentler explanation of opening files.
Portability issues: "sysopen" in perlport.
✨ sysread ✨
sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
sysread FILEHANDLE,SCALAR,LENGTHokay bestie, this one tries to read LENGTH bytes of data into SCALAR from the FILEHANDLE you named, and it does it with raw read(2). 💅
It skips right past every PerlIO layer, buffered IO included — but it still gets affected by the ":utf8" layer, more on that in a sec. So mixing this with other kinds of reads, or with "print", "write", "seek", "tell", or "eof"? that can get confusing because the ":perlio" or ":crlf" layers are usually sitting there buffering data. 🙈
What it hands back:
- the number of bytes it actually read 🎀
0at end of fileundefif there was an error (and yeah,$!gets set in that case too)
SCALAR will get grown or shrunk so that the last byte actually read is the last byte of the scalar after the read. ✨
You can hand it an OFFSET to drop the read data somewhere other than the very beginning of the string. A negative OFFSET means "count backwards from the end of the string, put it there." And a positive OFFSET that's bigger than the length of SCALAR? That makes the string get padded to the required size with "\0" bytes before the read result gets appended. 🫶
open(my $FH, "<", "input.txt") or die("Cannot open file: $!");
my $buf = "";
my $num = 0;
# Read up to 64 bytes
$num = sysread($FH, $buf, 64);
# Read up to 32 bytes into position 512 of $buf
$num = sysread($FH, $buf, 32, 512);There is no syseof() function, and honestly that's fine, because "eof" doesn't work well on device files (like ttys) anyway. 🙃 Just use "sysread" and check whether the return value is 0 to decide if you're done. periodt.
careful now: if the filehandle has been marked as":utf8","sysread"will throw an exception. and bestie, the:encoding(...)layer implicitly introduces the":utf8"layer. so. yeah. 👀 See"binmode","open", and the open pragma.
🌈 sysseek 🌈
sysseek FILEHANDLE,POSITION,WHENCESets the FILEHANDLE's system position in bytes using lseek(2). FILEHANDLE can be an expression whose value gives the name of the filehandle. 🎀
The values for WHENCE are:
0— set the new position to POSITION1— set it to the current position plus POSITION2— set it to EOF plus POSITION, which is typically negative
Note the emphasis on bytes: even if the filehandle has been set up to operate on characters (say, with the :encoding(UTF-8) I/O layer), the "seek", "tell", and "sysseek" family of functions use byte offsets, not character offsets, because seeking to a character offset would be painfully slow in a UTF-8 file. 😭
"sysseek" bypasses normal buffered IO, so mixing it with reads other than "sysread" (like "readline" or "read"), or with "print", "write", "seek", "tell", or "eof" may cause confusion. 💀
For WHENCE you can also use the constants "SEEK_SET", "SEEK_CUR", and "SEEK_END" (start of the file, current position, end of the file) from the Fcntl module. Using the constants is also more portable than leaning on 0, 1, and 2. For example, to define a "systell" function:
use Fcntl 'SEEK_CUR';
sub systell { sysseek($_[0], 0, SEEK_CUR) }Returns the new position, or the undefined value on failure. A position of zero is returned as the string "0 but true" — so "sysseek" returns true on success and false on failure, yet you can still easily figure out the new position. iconic. 👑
🔥 system 🔥
system LIST
system PROGRAM LISTDoes exactly the same thing as "exec", except a fork happens first and the parent process waits for the child process to exit. 🫶
Note that argument processing varies depending on how many arguments you give it:
- If there's more than one argument in LIST, or if LIST is an array with more than one value, it starts the program given by the first element of the list, with the arguments given by the rest of the list.
- If there's only one scalar argument, that argument gets checked for shell metacharacters. If there are any, the entire argument is passed to the system's command shell for parsing — that's
"/bin/sh -c"on Unix platforms, but it varies on other platforms. - If there are no shell metacharacters in the argument, it gets split into words and passed directly to
"execvp", which is more efficient.
On Windows, only the "system PROGRAM LIST" syntax will reliably avoid using the shell; "system LIST", even with more than one element, will fall back to the shell if the first spawn fails. 🪟
Perl will attempt to flush all files opened for output before any operation that may do a fork, but this may not be supported on some platforms (see perlport). To be safe, you may need to set $| ($AUTOFLUSH in English) or call the "autoflush" method of "IO::Handle" on any open handles. 🌸
The return value is the exit status of the program as returned by the "wait" call. To get the actual exit value, shift right by eight (see below). See also "exec". This is not what you want to use to capture the output from a command — for that you should use merely backticks or "qx//", as described in "`STRING`" in perlop. A return value of -1 indicates a failure to start the program or an error of the wait(2) system call (inspect $! for the reason). 👀
If you'd like to make "system" (and many other bits of Perl) die on error, have a look at the autodie pragma.
Like "exec", "system" lets you lie to a program about its name if you use the "system PROGRAM LIST" syntax. Again, see "exec". 😏
Since "SIGINT" and "SIGQUIT" are ignored during the execution of "system", if you expect your program to terminate on receipt of these signals you'll need to arrange to do so yourself based on the return value. 💀
my @args = ("command", "arg1", "arg2");
system(@args) == 0
or die "system @args failed: $?";If you'd like to manually inspect "system"'s failure, you can check all possible failure modes by inspecting $? like this:
if ($? == -1) {
print "failed to execute: $!\n";
}
elsif ($? & 127) {
printf "child died with signal %d, %s coredump\n",
($? & 127), ($? & 128) ? 'with' : 'without';
}
else {
printf "child exited with value %d\n", $? >> 8;
}Alternatively, you may inspect the value of "${^CHILD_ERROR_NATIVE}" with the "W*()" calls from the POSIX module. ✨
When "system"'s arguments are executed indirectly by the shell, results and return codes are subject to its quirks. See "`STRING`" in perlop and "exec" for details.
Since "system" does a "fork" and "wait" it may affect a "SIGCHLD" handler. See perlipc for details. 🎀
Portability issues: "system" in perlport.
💜 syswrite 💜
syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
syswrite FILEHANDLE,SCALAR,LENGTH
syswrite FILEHANDLE,SCALARTries to write LENGTH bytes of data from the variable SCALAR to the FILEHANDLE you named, using write(2). If LENGTH isn't specified, it writes the whole SCALAR. 🌈
It bypasses any PerlIO layers including buffered IO (but is affected by the presence of the ":utf8" layer, as described later), so mixing this with reads (other than "sysread)"), or with "print", "write", "seek", "tell", or "eof" may cause confusion because the ":perlio" and ":crlf" layers usually buffer data. 🙈
Returns the number of bytes actually written, or "undef" if there was an error (in which case the errno variable $! is also set). If LENGTH is greater than the data available in the SCALAR after the OFFSET, only as much data as is available will be written. 🫶
An OFFSET may be specified to write the data from some part of the string other than the beginning. A negative OFFSET specifies writing that many characters counting backwards from the end of the string. If SCALAR is of length zero, you can only use an OFFSET of 0. 🎀
WARNING: If the filehandle is marked":utf8","syswrite"will raise an exception. The:encoding(...)layer implicitly introduces the":utf8"layer. Alternately, if the handle is not marked with an encoding but you attempt to write characters with code points over 255, it raises an exception. See"binmode","open", and the open pragma. 😭
🦋 tell 🦋
tell FILEHANDLE
tellReturns the current position in bytes for FILEHANDLE, or -1 on error. FILEHANDLE may be an expression whose value gives the name of the actual filehandle. If FILEHANDLE is omitted, it assumes the file last read. ✨
Note the emphasis on bytes: even if the filehandle has been set to operate on characters (for example using the :encoding(UTF-8) I/O layer), the "seek", "tell", and "sysseek" family of functions use byte offsets, not character offsets, because seeking to a character offset would be very slow in a UTF-8 file. 💅
The return value of "tell" for the standard streams like the STDIN depends on the operating system: it may return -1 or something else. "tell" on pipes, fifos, and sockets usually returns -1. 🫠
There is no "systell" function. Use "sysseek($fh, 0, 1)" for that. 👑
do not use"tell"(or other buffered I/O operations) on a filehandle that has been manipulated by"sysread","syswrite", or"sysseek". Those functions ignore the buffering, while"tell"does not. 💀
🎀 telldir 🎀
telldir DIRHANDLEReturns the current position of the "readdir" routines on DIRHANDLE. That value may be given to "seekdir" to access a particular location in a directory. "telldir" has the same caveats about possible directory compaction as the corresponding system library routine. 🌸
👑 tie 👑
tie VARIABLE,CLASSNAME,LISTThis function binds a variable to a package class that will provide the implementation for the variable. This is the enchantment function, fr fr. ✨
- VARIABLE — the name of the variable to be enchanted 💖
- CLASSNAME — the name of a class implementing objects of the correct type 🎀
- any additional arguments — passed on to the appropriate constructor method of the class (meaning
"TIESCALAR","TIEHANDLE","TIEARRAY", or"TIEHASH"). Typically these are arguments such as might be passed to thedbm_open(3)function of C.
The object returned by the constructor is also returned by the "tie" function, which is useful if you want to access other methods in CLASSNAME. 🫶
Note that functions such as "keys" and "values" may return huge lists when used on large objects, like DBM files. You may prefer to use the "each" function to iterate over such. 😭 Example:
# print out history file offsets
use NDBM_File;
tie(my %HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
while (my ($key,$val) = each %HIST) {
print $key, ' = ', unpack('L', $val), "\n";
}So like, if you're building a whole class that acts like a hash, bestie, here's the ✨method roster✨ you need:
TIEHASH classname, LIST
FETCH this, key
STORE this, key, value
DELETE this, key
CLEAR this
EXISTS this, key
FIRSTKEY this
NEXTKEY this, lastkey
SCALAR this
DESTROY this
UNTIE thisAnd if you're cosplaying as an ordinary array instead?
TIEARRAY classname, LIST
FETCH this, key
STORE this, key, value
FETCHSIZE this
STORESIZE this, count
CLEAR this
PUSH this, LIST
POP this
SHIFT this
UNSHIFT this, LIST
SPLICE this, offset, length, LIST
EXTEND this, count
DELETE this, key
EXISTS this, key
DESTROY this
UNTIE thisFilehandle vibes? Here's your list, gorgeous:
TIEHANDLE classname, LIST
READ this, scalar, length, offset
READLINE this
GETC this
WRITE this, scalar, length, offset
PRINT this, LIST
PRINTF this, format, LIST
BINMODE this
EOF this
FILENO this
SEEK this, position, whence
TELL this
OPEN this, mode, LIST
CLOSE this
DESTROY this
UNTIE thisScalar era? Short and sweet:
TIESCALAR classname, LIST
FETCH this,
STORE this, value
DESTROY this
UNTIE thisPlot twist tho — you do NOT have to implement every single one of those methods okurrr. It's not all-or-nothing. Go peep perltie, Tie::Hash, Tie::Array, Tie::Scalar, and Tie::Handle for the details.
And heads up: unlike dbmopen, the tie function will not use or require a module for you. You gotta do that yourself, bestie, manually, with your own two hands. 💅 If you want inspo for cool tie implementations, go look at DB_File or the Config module.
More deets live in perltie, specifically the "tied" section.
💫 tied VARIABLE 💫
This one hands you back a reference to the object sitting underneath VARIABLE — the exact same value that the original tie call gave back when it bound that variable to a package. 🫶 And if VARIABLE was never tied to a package at all? You get the undefined value. Sorry babe, nothing's there. 💀
⏰ time ⏰
Returns the number of non-leap seconds since whenever your system decided the epoch was. Perfect little snack for feeding straight into gmtime and localtime. 🍓
On most systems the epoch is 00:00:00 UTC, January 1, 1970 — but a very notable exception is Mac OS Classic, which uses 00:00:00, January 1, 1904, and does it in the current local time zone. Iconic of it, honestly.
Want granularity finer than one second? The Time::HiRes module has your back from Perl 5.8 onwards (or from CPAN if you're living in the before times), or — if you've got gettimeofday(2) lying around — you might be able to use Perl's "syscall" interface. Full details over in perlfaq8.
For date and time processing, go browse the absolute mountain of related modules on CPAN. And if you want a truly comprehensive date and time representation? That's the DateTime module, periodt.
🕰️ times 🕰️
Returns a four-element list: the user and system times, in seconds, for this process AND for any exited children of this process. Feeding the whole fam. 👑
my ($user,$system,$cuser,$csystem) = times;In scalar context, times only gives you back $user — the rest is like, not invited.
Quick note: children's times are only included for terminated children. Dead ones only, bestie.
Portability tea is over at "times" in perlport.
🔁 tr/// 🔁
The transliteration operator. Literally the exact same thing as "y///". For the full lore, go read "Quote-Like Operators" in perlop.
✂️ truncate FILEHANDLE,LENGTH / truncate EXPR,LENGTH ✂️
truncate FILEHANDLE,LENGTH
truncate EXPR,LENGTHChops the file — the one opened on FILEHANDLE, or the one named by EXPR — down to the specified LENGTH. Snip snip. 💅
If truncate isn't implemented on your system, this raises an exception. No soft launch here.
Returns true if it worked, "undef" on error. 😭
Also, the behavior is undefined if LENGTH is greater than the length of the file. Do not test fate, babe.
The position in the file of FILEHANDLE is left unchanged — so you may wanna call seek before you write to that file. Just a lil pro tip.
Portability issues: "truncate" in perlport.
🔠 uc EXPR / uc 🔠
uc EXPR
ucReturns an uppercased version of EXPR. If you leave EXPR out, it uses $_. SCREAMING MODE ACTIVATED.
my $str = uc("Perl is GREAT"); # "PERL IS GREAT"This function behaves the same way under various pragmas — like in a locale — as "lc" does. Consistent queen behavior.
Want titlecase mapping on just the initial letters instead? Go see "ucfirst".
Note: this is the internal function implementing the "\U" escape in double-quoted strings.
my $str = "Perl is \Ugreat\E"; # "Perl is GREAT"🎀 ucfirst EXPR / ucfirst 🎀
ucfirst EXPR
ucfirstGives you back EXPR's value with just the first character in uppercase — Unicode calls that titlecase, for the record. If EXPR is omitted, ucfirst uses $_. Slay the first letter only, leave the rest alone.
my $str = ucfirst("hello world!"); # "Hello world!"This function behaves the same way under various pragmas — such as in a locale — as "lc" does.
Note: this is the internal function implementing the "\u" escape in double-quoted strings.
my $str = "\uperl\E is great"; # "Perl is great"🎭 umask EXPR / umask 🎭
umask EXPR
umaskSets the umask for the process to EXPR and returns the previous value. If EXPR is omitted, it just returns the current umask — no changes, just vibes. ✨
Okay let's break this permission thing down, because ngl it's a lot at first. The Unix permission rwxr-x--- gets represented as three sets of three bits, aka three octal digits: 0750. That leading 0 is just telling you "hey this is octal" — it is NOT one of the digits, don't be fooled. 🙈
The "umask" value is that kind of number, and it represents disabled permission bits. So the permission (or "mode") values you hand to mkdir or sysopen get modified by your umask. Meaning: even if you tell sysopen to create a file with permissions 0777, and your umask is 0022, the file will actually get created with permissions 0755. Love that for you.
And if your "umask" were 0027 (group can't write; others can't read, write, or execute), then passing sysopen 0666 would create a file with mode 0640 — because 0666 &~ 027 is 0640. Math is math. 💅
Here's some advice, bestie: supply a creation mode of 0666 for regular files (in sysopen) and 0777 for directories (in mkdir) and executable files. This gives users the freedom of choice — if they want protected files they might pick process umasks of 022, 027, or the truly unhinged antisocial mask of 077. Programs should rarely if ever be making policy decisions that are better left to the user. The exception? Files that should be kept private: mail files, web browser cookies, .rhosts files, and so on.
⚠️ If umask(2) is not implemented on your system AND you're trying to restrict access for yourself (i.e., (EXPR & 0700) > 0), this raises an exception. If umask(2) is not implemented and you're NOT trying to restrict access for yourself, it returns "undef".
Remember: a umask is a number, usually written in octal — it is NOT a string of octal digits. See also "oct" if a string is all you've got. 🫶
Portability issues: "umask" in perlport.
🫥 undef EXPR / undef 🫥
undef EXPR
undefUndefines the value of EXPR, which has to be an lvalue. Only use it on a scalar value, an array (using @), a hash (using %), a subroutine (using &), or a typeglob (using *).
⚠️ Saying undef $hash{$key} will probably NOT do what you expect on most predefined variables or DBM list values — so like, literally don't do that. Go see "delete" instead.
Always returns the undefined value. And you can totally omit EXPR — in which case nothing actually gets undefined, but you still get an undefined value that you could, for instance, return from a subroutine, assign to a variable, or pass as a parameter. A little nothing for your trouble. 🎁
Examples, boo:
undef $foo;
undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
undef @ary;
undef %hash;
undef &mysub;
undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
select undef, undef, undef, 0.25;
my ($x, $y, undef, $z) = foo(); # Ignore third value returnedNote that this is a unary operator, not a list operator. It's giving solo artist, not group project.
🗑️ unlink LIST / unlink 🗑️
unlink LIST
unlinkDeletes a list of files. On success it returns the number of files it successfully deleted. On failure it returns false and sets $! (errno):
my $unlinked = unlink 'a', 'b', 'c';
unlink @goners;
unlink glob "*.bak";On error, "unlink" will not tell you which files it couldn't remove. Rude, honestly. If you actually want to know which ones survived, try them one at a time:
foreach my $file ( @goners ) {
unlink $file or warn "Could not unlink $file: $!";
}⚠️ Note: "unlink" will not attempt to delete directories unless you are superuser AND the -U flag is supplied to Perl. Even if both those conditions are met, be warned that unlinking a directory can inflict damage on your filesystem. And finally — using "unlink" on directories isn't even supported on many operating systems. Use "rmdir" instead, babe.
If LIST is omitted, "unlink" uses $_.
📦 unpack TEMPLATE,EXPR / unpack TEMPLATE 📦
unpack TEMPLATE,EXPR
unpack TEMPLATE"unpack" does the reverse of "pack": it takes a string and expands it out into a list of values. (In scalar context, it returns merely the first value produced — the rest are just there, unbothered.)
If EXPR is omitted, it unpacks the $_ string. For an intro to this function, see perlpacktut.
The string gets broken into chunks described by the TEMPLATE. Each chunk is converted separately into a value. Typically either the string is a result of "pack", or the characters of the string represent a C structure of some kind.
The TEMPLATE has the exact same format as in the "pack" function. Here's a subroutine that does substring:
sub substr {
my ($what, $where, $howmuch) = @_;
unpack("x$where a$howmuch", $what);
}and then there's
sub ordinal { unpack("W",$_[0]); } # same as ord()On top of the fields allowed in "pack", you may prefix a field with a %<number> to indicate that you want a <number>-bit checksum of the items instead of the items themselves. Default is a 16-bit checksum. The checksum is calculated by summing the numeric values of the expanded values — for string fields it's the sum of ord($char) that gets taken, and for bit fields it's the sum of zeroes and ones. Checksum girlies, this one's for you. ⭐
Ngl bestie, if you ever wanted to do the same math as the classic System V sum program, unpack has you covered. Check it:
my $checksum = do {
local $/; # slurp!
unpack("%32W*", readline) % 65535;
};And here's one that efficiently counts the number of set bits in a bit vector — it's giving ✨bit wizard✨:
my $setbits = unpack("%32b*", $selectmask);⚠️ Okay so the"p"and"P"formats? Use them with care, fr fr. Perl has no way of checking whether the value you passed tounpackactually corresponds to a valid memory location. So if you hand it a pointer value that isn't known to be valid, you're likely to get disastrous consequences. Not a vibe. 💀
Also, if there are more pack codes than the input can handle, or if the repeat count of a field or a group is larger than what the remainder of the input string allows, the result is not well defined: the repeat count may be decreased, or unpack may produce empty strings or zeros, or it may raise an exception. And if the input string is longer than the one described by the TEMPLATE, the remainder of that input string just gets ignored. Bye 👋
See pack for more examples and notes — it's the sister function, they're a package deal 🫶
✨ unshift ARRAY,LIST ✨
Okay bestie, this one adds one or more elements to the beginning of an array. It's literally the opposite of a shift. Iconic duo behavior 💅
my @animals = ("cat");
unshift(@animals, "mouse"); # ("mouse", "cat")
my @colors = ("red");
unshift(@colors, ("blue", "green")); # ("blue", "green", "red")It returns the new number of elements in the updated array — so it's a total overachiever and gives you the count too, no cap:
# Return value is the number of items in the updated array
my $color_count = unshift(@colors, ("yellow", "purple"));
say "There are $color_count colors in the updated array";🎀 Important detail: the LIST is prepended whole, not one element at a time, so the prepended elements stay in the same order. That's the main character behavior. If you want them backwards, use reverse to do the reverse. iykyk
Starting with Perl 5.14, there was an experimental feature that let unshift take a scalar expression. Ngl that experiment has been deemed unsuccessful, and it was removed as of Perl 5.24. Gone, bestie. RIP 🙈
🌸 untie VARIABLE 🌸
Breaks the binding between a variable and a package. (See tie.) Has no effect if the variable is not tied. So if there's no tie, nothing happens — no drama, no error, just vibes 🌈
👑 use Module VERSION LIST 👑
👑 use Module VERSION 👑
👑 use Module LIST 👑
👑 use Module 👑
Okay so this imports some semantics into the current package from the named module — generally by aliasing certain subroutine or variable names into your package. It is exactly equivalent to:
BEGIN { require Module; Module->import( LIST ); }...except that Module must be a bareword. Oh, and the importation can be made conditional by using the if module 👀
The BEGIN forces the require and import to happen at compile time. The require makes sure the module is loaded into memory if it hasn't been yet. The import is not a builtin — it's just an ordinary static method call into the Module package to tell the module to import the list of features back into the current package. The module can implement its import method any way it likes, though most modules just choose to derive their import method via inheritance from the Exporter class that is defined in the Exporter module. See Exporter. And if no import method can be found, the call is just skipped — even if there's an AUTOLOAD method. Skipped. Not today 💀
If you do not want to call the package's import method (for instance, to stop your namespace from being altered), explicitly supply the empty list:
use Module ();That is exactly equivalent to:
BEGIN { require Module }If the VERSION argument is present between Module and LIST, then the use will call the VERSION method in class Module with the given version as an argument:
use Module 12.34;is equivalent to:
BEGIN { require Module; Module->VERSION(12.34) }The default VERSION method, inherited from the UNIVERSAL class, croaks if the given version is larger than the value of the variable $Module::VERSION. So it'll yell at you, basically 😭
Now listen up, this part is lowkey important: the VERSION argument cannot be an arbitrary expression. It only counts as a VERSION argument if it is a version number literal, starting with either a digit or "v" followed by a digit. Anything that doesn't look like a version literal will be parsed as the start of the LIST. Nevertheless, many attempts to use an arbitrary expression as a VERSION argument will appear to work, because Exporter's import method handles numeric arguments specially, performing version checks rather than treating them as things to export. Sneaky but valid ✨
Again, there is a distinction between omitting LIST (import called with no arguments) and an explicit empty LIST () (import not called). And note, there is no comma after VERSION! No comma. None. 🚫
Because this is a wide-open interface, pragmas (compiler directives) are also implemented this way. Some of the currently implemented pragmas are:
use constant;
use diagnostics;
use integer;
use feature qw(say signatures bitwise);
use sigtrap qw(SEGV BUS);
use strict qw(subs vars refs);
use subs qw(afunc blurfl);
use warnings qw(all);Some of these pseudo-modules import semantics into the current block scope (like strict or integer), unlike ordinary modules, which import symbols into the current package (which are effective through the end of the file).
Because use takes effect at compile time, it doesn't respect the ordinary flow control of the code being compiled. In particular, putting a use inside the false branch of a conditional doesn't prevent it from being processed. Yeah. It's gonna run anyway, bestie 🫠 If a module or pragma only needs to be loaded conditionally, this can be done using the if pragma:
use if $] < 5.008, "utf8";
use if WANT_WARNINGS, warnings => qw(all);There's a corresponding no declaration that unimports meanings imported by use — i.e., it calls Module->unimport(LIST) instead of import. It behaves just as import does with VERSION, an omitted or empty LIST, or no unimport method being found. Symmetry, we love her 💖
no integer;
no strict 'refs';
no warnings;See perlmodlib for a list of standard modules and pragmas. See perlrun for the -M and -m command-line options to Perl that give use functionality from the command-line. Command-line girlies, this one's for you 🌈
🦋 use VERSION 🦋
Lexically enables all features available in the requested version as defined by the feature pragma, disabling any features not in the requested version's feature bundle. See feature. It's giving "package deal, take it or leave it" 💅
VERSION may be either a v-string such as v5.24.1, which will be compared to $^V (aka $PERL_VERSION), or a numeric argument of the form 5.024001, which will be compared to $]. An exception is raised if VERSION is greater than the version of the current Perl interpreter; Perl will not attempt to parse the rest of the file. Compare with require, which can do a similar check at run time. Runtime girlies vs compile-time girlies, they're different 💀
If the specified Perl version is 5.12 or higher, strictures are enabled lexically as with use strict.
If the specified Perl version is 5.35.0 or higher, warnings are enabled.
If the specified Perl version is 5.39.0 or higher, builtin functions are imported lexically as with use builtin with a corresponding version bundle.
If the specified Perl version is v5.41.0 or higher, use source::encoding 'ascii' is enabled lexically. ✨
Use of use VERSION while another is in effect is not allowed with a use v5.39; or greater version. For lower versions, use VERSION will override most behavior of a previous use VERSION, possibly removing warnings and feature effects added by it. This behavior is deprecated, and a future release of perl will disallow changing the version once one has been declared. Additionally, a use VERSION with a version less than 5.11 is not allowed after a use VERSION with a version greater than 5.11. So like... pick your version and commit to her, bestie 😭
use VERSION does not load the feature.pm, strict.pm, warnings.pm or builtin.pm files — it instead implements the equivalent functionality directly. Efficiency, she ate 🔥
In the current implementation, any explicit use of no strict overrides use VERSION, even if it comes before it. However, this may be subject to change in a future release of Perl, so new code should not rely on this fact. It is recommended that a use VERSION declaration be the first significant statement within a file (possibly after a package statement or any amount of whitespace or comment), so that its effects happen first, and other pragmata are applied after it. Main character energy: first in the file 👑
Specifying VERSION as a numeric argument of the form 5.024001 should generally be avoided as older less readable syntax compared to v5.24.1. Before perl 5.8.0 released in 2002 the more verbose numeric form was the only supported syntax, which is why you might see it in older code. Grandma syntax 💜
use v5.24.1; # compile time version check
use 5.24.1; # ditto
use 5.024_001; # ditto; older syntax compatible with perl 5.6This is often useful if you need to check the current Perl version before useing library modules that won't work with older versions of Perl. (We try not to do this more than we have to.) 😅
Symmetrically, no VERSION allows you to specify that you want a version of Perl older than the specified one. Historically this was added during early designs of the Raku language (formerly "Perl 6"), so that a Perl 5 program could begin
no 6;to declare that it is not a Perl 6 program. As the two languages have different implementations, file naming conventions, and other infrastructure, this feature is now little used in practice and should be avoided in newly-written code. A relic, but a cute one 🎀
⚠️ Care should be taken when using theno VERSIONform, as it is only meant to be used to assert that the running Perl is of an earlier version than its argument and not to undo the feature-enabling side effects ofuse VERSION. Don't use it to un-do stuff, fr fr.
💜 utime LIST 💜
Changes the access and modification times on each file of a list of files. The first two elements of the list must be the NUMERIC access and modification times, in that order. Returns the number of files successfully changed. The inode change time of each file is set to the current time. Slay, timestamps ✨
For example, this code has the same effect as the Unix touch(1) command when the files already exist and belong to the user running the program:
#!/usr/bin/perl
my $atime = my $mtime = time;
utime $atime, $mtime, @ARGV;Okay so listen 💖 since Perl 5.8.0 there's this whole glow-up: if the first two elements of that list are undef, the utime(2) syscall from your C library gets called with a null second argument. On most systems that sets the file's access and modification times to the current time (aka literally the same thing the example above does) and — bestie, the plot twist — it works even on files you don't own, as long as you have write permission. 💅
for my $file (@ARGV) {
utime(undef, undef, $file)
|| warn "Couldn't touch $file: $!";
}Under NFS this will use the time of the NFS server, not the time of the local machine. And if there's a time synchronization problem, the NFS server and the local machine will be living in different timelines 🙈 — different times. The Unix touch(1) command will in fact normally use this form instead of the one shown in the first example. periodt.
Passing only one of the first two elements as undef is equivalent to passing a 0 and will not have the effect described when both are undef. This also triggers an uninitialized warning. ⚠️ not the vibe.
On systems that support futimes(2), you may pass filehandles among the files. On systems that don't support futimes(2), passing filehandles raises an exception. 💀 Filehandles must be passed as globs or glob references to be recognized; barewords are considered filenames.
Portability issues: "utime" in perlport.
🦋 values HASH 🦋
🦋 values ARRAY 🦋
Called in list context, this returns a list of all the values of the named hash — or, in Perl 5.12 or later, the values of an array. Perl releases prior to 5.12 will produce a syntax error if you try to use an array argument, so like, know your audience. In scalar context, it returns the number of keys or indices.
Array entries come back in the order of lowest index first. 🎯
Hash entries come back in an apparently random order. And it's not even one shared random for everything — the actual random order is specific to a given hash, so the exact same series of operations on two hashes may result in a different order for each hash. 😭 Any insertion into the hash may change the order, as will any deletion, with the exception that the most recent key returned by each or keys may be deleted without changing the order. So long as a given hash is unmodified you may rely on keys, values and each to repeatedly return the same order as each other. Wanna know why hash order is randomized in the first place? See "Algorithmic Complexity Attacks" in perlsec for details. 👀 Aside from the guarantees provided here, the exact details of Perl's hash algorithm and the hash traversal order are subject to change in any release of Perl. And tied hashes may behave differently to Perl's hashes with respect to changes in order on insertion and deletion of items. 🙈
As a side effect, calling values resets the HASH or ARRAY's internal iterator (see each) before yielding the values. In particular, calling values in void context resets the iterator with no other overhead. free of charge, iconic.
Apart from resetting the iterator, values @array in list context is the same as plain @array. (The docs literally recommend that you use void context keys @array for this, but reasoned that taking values @array out would require more documentation than leaving it in.) 😌
Note that the values are not copied, which means modifying them will modify the contents of the hash:
for (values %hash) { s/foo/bar/g } # modifies %hash values
for (@hash{keys %hash}) { s/foo/bar/g } # sameStarting with Perl 5.14, an experimental feature allowed values to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24. rip. 🪦
To avoid confusing would-be users of your code who are running earlier versions of Perl with mysterious syntax errors, put this sort of thing at the top of your file to signal that your code will work only on Perls of a recent vintage:
use v5.12; # so keys/values/each work on arraysSee also keys, each, and sort.
👑 vec EXPR,OFFSET,BITS 👑
Treats the string in EXPR as a bit vector made up of elements of width BITS, and returns the value of the element specified by OFFSET as an unsigned integer. So BITS is literally the number of bits that are reserved for each element in the bit vector. It must be a power of two from 1 to 32 (or 64, if your platform supports that). no cap.
If BITS is 8, the "elements" coincide with bytes of the input string. cute, easy.
If BITS is 16 or more, bytes of the input string are grouped into chunks of size BITS/8, and each group is converted to a number as with pack/unpack with big-endian formats n/N (and analogously for BITS==64). See pack for details, bestie. 📦
If bits is 4 or less, the string is broken into bytes, then the bits of each byte are broken into 8/BITS groups. Bits of a byte are numbered in a little-endian-ish way, as in 0x01, 0x02, 0x04, 0x08, 0x10, 0x20, 0x40, 0x80. For example, breaking the single input byte chr(0x36) into two groups gives a list (0x6, 0x3); breaking it into 4 groups gives (0x2, 0x1, 0x3, 0x0). 🎀 little-endian-ish is such a phrase.
vec may also be assigned to, in which case parentheses are needed to give the expression the correct precedence, as in
vec($image, $max_x * $x + $y, 8) = 3;If the selected element is outside the string, the value 0 is returned. If an element off the end of the string is written to, Perl will first extend the string with sufficiently many zero bytes. It is an error to try to write off the beginning of the string (i.e., negative OFFSET). 🚫 don't do it.
If the string happens to be encoded as UTF-8 internally (and thus has the UTF8 flag set), vec tries to convert it to use a one-byte-per-character internal representation. However, if the string contains characters with values of 256 or higher, a fatal error will occur. 💀
Strings created with vec can also be manipulated with the logical operators |, &, ^, and ~. These operators will assume a bit vector operation is desired when both operands are strings. See "Bitwise String Operators" in perlop.
The following code will build up an ASCII string saying 'PerlPerlPerl'. The comments show the string after each step. Note that this code works in the same way on big-endian or little-endian machines. 🌈 slay, portability.
my $foo = '';
vec($foo, 0, 32) = 0x5065726C; # 'Perl'
# $foo eq "Perl" eq "\x50\x65\x72\x6C", 32 bits
print vec($foo, 0, 8); # prints 80 == 0x50 == ord('P')
vec($foo, 2, 16) = 0x5065; # 'PerlPe'
vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
vec($foo, 21, 4) = 7; # 'PerlPerlPer'
# 'r' is "\x72"
vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
# 'l' is "\x6c"To transform a bit vector into a string or list of 0's and 1's, use these:
my $bits = unpack("b*", $vector);
my @bits = split(//, unpack("b*", $vector));If you know the exact length in bits, it can be used in place of the *.
Here's an example to illustrate how the bits actually fall in place:
#!/usr/bin/perl -wl
print <<'EOT';
0 1 2 3
unpack("V",$_) 01234567890123456789012345678901
------------------------------------------------------------------
EOT
for $w (0..3) {
$width = 2**$w;
for ($shift=0; $shift < $width; ++$shift) {
for ($off=0; $off < 32/$width; ++$off) {
$str = pack("B*", "0"x32);
$bits = (1<<$shift);
vec($str, $off, $width) = $bits;
$res = unpack("b*",$str);
$val = unpack("V", $str);
write;
}
}
}
format STDOUT =
vec($_,@#,@#) = @<< == @######### @>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
$off, $width, $bits, $val, $res
.
__END__Regardless of the machine architecture on which it runs, the example above should print the following table:
okay so here's the whole bit-map of it all — every single one of these, no skipping, bestie 💅
0 1 2 3
unpack("V",$_) 01234567890123456789012345678901
------------------------------------------------------------------
vec($_, 0, 1) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 1) = 1 == 2 01000000000000000000000000000000
vec($_, 2, 1) = 1 == 4 00100000000000000000000000000000
vec($_, 3, 1) = 1 == 8 00010000000000000000000000000000
vec($_, 4, 1) = 1 == 16 00001000000000000000000000000000
vec($_, 5, 1) = 1 == 32 00000100000000000000000000000000
vec($_, 6, 1) = 1 == 64 00000010000000000000000000000000
vec($_, 7, 1) = 1 == 128 00000001000000000000000000000000
vec($_, 8, 1) = 1 == 256 00000000100000000000000000000000
vec($_, 9, 1) = 1 == 512 00000000010000000000000000000000
vec($_,10, 1) = 1 == 1024 00000000001000000000000000000000
vec($_,11, 1) = 1 == 2048 00000000000100000000000000000000
vec($_,12, 1) = 1 == 4096 00000000000010000000000000000000
vec($_,13, 1) = 1 == 8192 00000000000001000000000000000000
vec($_,14, 1) = 1 == 16384 00000000000000100000000000000000
vec($_,15, 1) = 1 == 32768 00000000000000010000000000000000
vec($_,16, 1) = 1 == 65536 00000000000000001000000000000000
vec($_,17, 1) = 1 == 131072 00000000000000000100000000000000
vec($_,18, 1) = 1 == 262144 00000000000000000010000000000000
vec($_,19, 1) = 1 == 524288 00000000000000000001000000000000
vec($_,20, 1) = 1 == 1048576 00000000000000000000100000000000
vec($_,21, 1) = 1 == 2097152 00000000000000000000010000000000
vec($_,22, 1) = 1 == 4194304 00000000000000000000001000000000
vec($_,23, 1) = 1 == 8388608 00000000000000000000000100000000
vec($_,24, 1) = 1 == 16777216 00000000000000000000000010000000
vec($_,25, 1) = 1 == 33554432 00000000000000000000000001000000
vec($_,26, 1) = 1 == 67108864 00000000000000000000000000100000
vec($_,27, 1) = 1 == 134217728 00000000000000000000000000010000
vec($_,28, 1) = 1 == 268435456 00000000000000000000000000001000
vec($_,29, 1) = 1 == 536870912 00000000000000000000000000000100
vec($_,30, 1) = 1 == 1073741824 00000000000000000000000000000010
vec($_,31, 1) = 1 == 2147483648 00000000000000000000000000000001
vec($_, 0, 2) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 2) = 1 == 4 00100000000000000000000000000000
vec($_, 2, 2) = 1 == 16 00001000000000000000000000000000
vec($_, 3, 2) = 1 == 64 00000010000000000000000000000000
vec($_, 4, 2) = 1 == 256 00000000100000000000000000000000
vec($_, 5, 2) = 1 == 1024 00000000001000000000000000000000
vec($_, 6, 2) = 1 == 4096 00000000000010000000000000000000
vec($_, 7, 2) = 1 == 16384 00000000000000100000000000000000
vec($_, 8, 2) = 1 == 65536 00000000000000001000000000000000
vec($_, 9, 2) = 1 == 262144 00000000000000000010000000000000
vec($_,10, 2) = 1 == 1048576 00000000000000000000100000000000
vec($_,11, 2) = 1 == 4194304 00000000000000000000001000000000
vec($_,12, 2) = 1 == 16777216 00000000000000000000000010000000
vec($_,13, 2) = 1 == 67108864 00000000000000000000000000100000
vec($_,14, 2) = 1 == 268435456 00000000000000000000000000001000
vec($_,15, 2) = 1 == 1073741824 00000000000000000000000000000010
vec($_, 0, 2) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 2) = 2 == 8 00010000000000000000000000000000
vec($_, 2, 2) = 2 == 32 00000100000000000000000000000000
vec($_, 3, 2) = 2 == 128 00000001000000000000000000000000
vec($_, 4, 2) = 2 == 512 00000000010000000000000000000000
vec($_, 5, 2) = 2 == 2048 00000000000100000000000000000000
vec($_, 6, 2) = 2 == 8192 00000000000001000000000000000000
vec($_, 7, 2) = 2 == 32768 00000000000000010000000000000000
vec($_, 8, 2) = 2 == 131072 00000000000000000100000000000000
vec($_, 9, 2) = 2 == 524288 00000000000000000001000000000000
vec($_,10, 2) = 2 == 2097152 00000000000000000000010000000000
vec($_,11, 2) = 2 == 8388608 00000000000000000000000100000000
vec($_,12, 2) = 2 == 33554432 00000000000000000000000001000000
vec($_,13, 2) = 2 == 134217728 00000000000000000000000000010000
vec($_,14, 2) = 2 == 536870912 00000000000000000000000000000100
vec($_,15, 2) = 2 == 2147483648 00000000000000000000000000000001
vec($_, 0, 4) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 4) = 1 == 16 00001000000000000000000000000000
vec($_, 2, 4) = 1 == 256 00000000100000000000000000000000
vec($_, 3, 4) = 1 == 4096 00000000000010000000000000000000
vec($_, 4, 4) = 1 == 65536 00000000000000001000000000000000
vec($_, 5, 4) = 1 == 1048576 00000000000000000000100000000000
vec($_, 6, 4) = 1 == 16777216 00000000000000000000000010000000
vec($_, 7, 4) = 1 == 268435456 00000000000000000000000000001000
vec($_, 0, 4) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 4) = 2 == 32 00000100000000000000000000000000
vec($_, 2, 4) = 2 == 512 00000000010000000000000000000000
vec($_, 3, 4) = 2 == 8192 00000000000001000000000000000000
vec($_, 4, 4) = 2 == 131072 00000000000000000100000000000000
vec($_, 5, 4) = 2 == 2097152 00000000000000000000010000000000
vec($_, 6, 4) = 2 == 33554432 00000000000000000000000001000000
vec($_, 7, 4) = 2 == 536870912 00000000000000000000000000000100
vec($_, 0, 4) = 4 == 4 00100000000000000000000000000000
vec($_, 1, 4) = 4 == 64 00000010000000000000000000000000
vec($_, 2, 4) = 4 == 1024 00000000001000000000000000000000
vec($_, 3, 4) = 4 == 16384 00000000000000100000000000000000
vec($_, 4, 4) = 4 == 262144 00000000000000000010000000000000
vec($_, 5, 4) = 4 == 4194304 00000000000000000000001000000000
vec($_, 6, 4) = 4 == 67108864 00000000000000000000000000100000
vec($_, 7, 4) = 4 == 1073741824 00000000000000000000000000000010
vec($_, 0, 4) = 8 == 8 00010000000000000000000000000000
vec($_, 1, 4) = 8 == 128 00000001000000000000000000000000
vec($_, 2, 4) = 8 == 2048 00000000000100000000000000000000
vec($_, 3, 4) = 8 == 32768 00000000000000010000000000000000
vec($_, 4, 4) = 8 == 524288 00000000000000000001000000000000
vec($_, 5, 4) = 8 == 8388608 00000000000000000000000100000000
vec($_, 6, 4) = 8 == 134217728 00000000000000000000000000010000
vec($_, 7, 4) = 8 == 2147483648 00000000000000000000000000000001
vec($_, 0, 8) = 1 == 1 10000000000000000000000000000000
vec($_, 1, 8) = 1 == 256 00000000100000000000000000000000
vec($_, 2, 8) = 1 == 65536 00000000000000001000000000000000
vec($_, 3, 8) = 1 == 16777216 00000000000000000000000010000000
vec($_, 0, 8) = 2 == 2 01000000000000000000000000000000
vec($_, 1, 8) = 2 == 512 00000000010000000000000000000000
vec($_, 2, 8) = 2 == 131072 00000000000000000100000000000000
vec($_, 3, 8) = 2 == 33554432 00000000000000000000000001000000
vec($_, 0, 8) = 4 == 4 00100000000000000000000000000000
vec($_, 1, 8) = 4 == 1024 00000000001000000000000000000000
vec($_, 2, 8) = 4 == 262144 00000000000000000010000000000000
vec($_, 3, 8) = 4 == 67108864 00000000000000000000000000100000
vec($_, 0, 8) = 8 == 8 00010000000000000000000000000000
vec($_, 1, 8) = 8 == 2048 00000000000100000000000000000000
vec($_, 2, 8) = 8 == 524288 00000000000000000001000000000000
vec($_, 3, 8) = 8 == 134217728 00000000000000000000000000010000
vec($_, 0, 8) = 16 == 16 00001000000000000000000000000000
vec($_, 1, 8) = 16 == 4096 00000000000010000000000000000000
vec($_, 2, 8) = 16 == 1048576 00000000000000000000100000000000
vec($_, 3, 8) = 16 == 268435456 00000000000000000000000000001000
vec($_, 0, 8) = 32 == 32 00000100000000000000000000000000
vec($_, 1, 8) = 32 == 8192 00000000000001000000000000000000
vec($_, 2, 8) = 32 == 2097152 00000000000000000000010000000000
vec($_, 3, 8) = 32 == 536870912 00000000000000000000000000000100
vec($_, 0, 8) = 64 == 64 00000010000000000000000000000000
vec($_, 1, 8) = 64 == 16384 00000000000000100000000000000000
vec($_, 2, 8) = 64 == 4194304 00000000000000000000001000000000
vec($_, 3, 8) = 64 == 1073741824 00000000000000000000000000000010
vec($_, 0, 8) = 128 == 128 00000001000000000000000000000000
vec($_, 1, 8) = 128 == 32768 00000000000000010000000000000000
vec($_, 2, 8) = 128 == 8388608 00000000000000000000000100000000
vec($_, 3, 8) = 128 == 2147483648 00000000000000000000000000000001💖 wait 💖
okay so wait is giving "let me hold your hand while the baby process finishes" energy 🫶
It behaves like wait(2) on your system: it waits for a child process to terminate and hands you back the pid of the deceased process, or -1 if there are no child processes at all. The status info goes into $? and also "${^CHILD_ERROR_NATIVE}". ✨
⚠️ heads up bestie: a return value of-1might not mean what you think — it could mean that child processes are being automatically reaped, as described inperlipc. iykyk 😭
Also, if you use "wait" inside your handler for $SIG{CHLD}, it may accidentally wait for the child created by "qx" or "system". See perlipc for the deets. 🙈
It's equivalent to "waitpid(-1, 0)". 💅
Portability issues: "wait" in perlport. 🌍
🦋 waitpid PID,FLAGS 🦋
This one's the picky version — it waits for a particular child process to terminate and returns the pid of the deceased process, or -1 if there's no such child process. No cap, that's the whole thing. 💀
A non-blocking wait (with WNOHANG in FLAGS) can return 0 if there are child processes matching PID but none have terminated yet. The status comes back in $? and "${^CHILD_ERROR_NATIVE}". 🌸
Here's the PID lore, listen up:
- A PID of
0means wait for any child process whose process group ID is equal to that of the current process. 👑 - A PID of less than
-1means wait for any child process whose process group ID is equal to-PID. 🎀 - A PID of
-1means wait for any child process, periodt. 🔥
If you say
use POSIX ":sys_wait_h";then you can do a non-blocking wait for all pending zombie processes (see "WAIT" in POSIX). Non-blocking wait is available on machines supporting either the waitpid(2) or wait4(2) syscalls. However, waiting for a particular pid with FLAGS of 0 is implemented everywhere. (Perl emulates the system call by remembering the status values of processes that have exited but have not been harvested by the Perl script yet.)
heads up bestie — on some systems, a return value of -1 could mean that child processes are being automatically reaped. See perlipc for deets, and for other examples. 🫶
Portability issues: "waitpid" in perlport.
💫 wantarray
Returns true if the context of the currently executing subroutine or "eval" is looking for a list value. Returns false if the context is looking for a scalar. Returns the undefined value if the context is looking for no value (void context).
return unless defined wantarray; # don't bother doing more
my @a = complex_calculation();
return wantarray ? @a : "@a";"wantarray"'s result is unspecified in the top level of a file, in a "BEGIN", "UNITCHECK", "CHECK", "INIT" or "END" block, or in a "DESTROY" method. 😭
This function should have been named wantlist() instead. ngl that's the real tea.
😱 warn LIST
Emits a warning, usually by printing it to "STDERR". "warn" interprets its operand LIST in the same way as "die", but is slightly different in what it defaults to when LIST is empty or makes an empty string. If it is empty and $@ already contains an exception value then that value is used after appending "\t...caught". If it is empty and $@ is also empty then the string "Warning: Something's wrong" is used.
By default, the exception derived from the operand LIST is stringified and printed to "STDERR". This behaviour can be altered by installing a $SIG{__WARN__} handler. If there is such a handler then no message is automatically printed; it is the handler's responsibility to deal with the exception as it sees fit (like, for instance, converting it into a "die"). Most handlers must therefore arrange to actually display the warnings that they are not prepared to deal with, by calling "warn" again in the handler. Note that this is quite safe and will not produce an endless loop, since "__WARN__" hooks are not called from inside one. 💅
You will find this behavior is slightly different from that of $SIG{__DIE__} handlers (which don't suppress the error text, but can instead call "die" again to change it).
Using a "__WARN__" handler provides a powerful way to silence all warnings (even the so-called mandatory ones). An example:
# wipe out *all* compile-time warnings
BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
my $foo = 10;
my $foo = 20; # no warning about duplicate my $foo,
# but hey, you asked for it!
# no compile-time or run-time warnings before here
$DOWARN = 1;
# run-time warnings enabled after here
warn "\$foo is alive and $foo!"; # does show upSee perlvar for details on setting %SIG entries and for more examples. See the Carp module for other kinds of warnings using its "carp" and "cluck" functions.
🎀 write FILEHANDLE, write EXPR, write
Writes a formatted record (possibly multi-line) to the specified FILEHANDLE, using the format associated with that file. By default the format for a file is the one having the same name as the filehandle, but the format for the current output channel (see the "select" function) may be set explicitly by assigning the name of the format to the $~ variable.
Top of form processing is handled automatically: if there is insufficient room on the current page for the formatted record, the page is advanced by writing a form feed and a special top-of-page format is used to format the new page header before the record is written. By default, the top-of-page format is the name of the filehandle with "_TOP" appended, or "top" in the current package if the former does not exist. This would be a problem with autovivified filehandles, but it may be dynamically set to the format of your choice by assigning the name to the $^ variable while that filehandle is selected. The number of lines remaining on the current page is in variable "$-", which can be set to 0 to force a new page.
If FILEHANDLE is unspecified, output goes to the current default output channel, which starts out as STDOUT but may be changed by the "select" operator. If the FILEHANDLE is an EXPR, then the expression is evaluated and the resulting string is used to look up the name of the FILEHANDLE at run time. For more on formats, see perlform.
Note thatwriteis not the opposite of"read". Unfortunately. 💀
🦋 y///
The transliteration operator. Same as "tr///". See "Quote-Like Operators" in perlop.
🌈 Non-function Keywords by Cross-reference 🌈
🍓 perldata
__DATA__, __END__
These keywords are documented in "Special Literals" in perldata.
🧚 perlmod
BEGIN, CHECK, END, INIT, UNITCHECK
These compile phase keywords are documented in "BEGIN, UNITCHECK, CHECK, INIT and END" in perlmod.
💜 perlobj
DESTROY
This method keyword is documented in "Destructors" in perlobj.
🔥 perlop
and, cmp, eq, ge, gt, isa, le, lt, ne, not, or, x, xor
These operators are documented in perlop.
🌸 perlsub
AUTOLOAD
This keyword is documented in "Autoloading" in perlsub.
⭐ perlsyn
else, elsif, for, foreach, if, unless, until, while
These flow-control keywords are documented in "Compound Statements" in perlsyn.
elseif
The "else if" keyword is spelled "elsif" in Perl. There's no "elif" or "else if" either. It does parse "elseif", but only to warn you about not using it. 🙈
See the documentation for flow-control keywords in "Compound Statements" in perlsyn.
default, given, when
These flow-control keywords related to the experimental switch feature are documented in "Switch Statements" in perlsyn.
try, catch, finally
These flow-control keywords related to the "try" feature are documented in "Try Catch Exception Handling" in perlsyn. "finally" is still deemed to be experimental; "try" and "catch" are considered non-experimental as of perl-5.40.
defer
This flow-control keyword related to the experimental "defer" feature is documented in "defer blocks" in perlsyn.
ADJUST
This class-related phaser block is documented in perlclass.
and that's the whole entire page, bestie — you made it to the end 💖✨
This one's perl v5.44.0, dated 2026-10-17, and it's the PERLFUNC(1perl) manual page. Go forth and slay your functions, queen. 👑💅🫶
📦 where this page comes from
- arch rolling: perl 5.44.0-1 · Artistic-1.0-Perl AND GPL-1.0-or-later
- 💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on : a language model's rewrite of the original, under its terms