From Linux Man Pages
perlfunc - Perl builtin functions
DESCRIPTION
The functions in this section can serve as terms in an expression. They fall into two major categories: list
operators and named unary operators. These differ in their precedence relationship with a following comma. (See
the precedence table in perlop.) List operators take more than one argument, while unary operators can never
take more than one argument. Thus, a comma terminates the argument of a unary operator, but merely separates the
arguments of a list operator. A unary operator generally provides a scalar context to its argument, while a list
operator may provide either scalar or list contexts for its arguments. If it does both, the scalar arguments
will be first, and the list argument will follow. (Note that there can ever be only one such list argument.)
For instance, splice() has three scalar arguments followed by a list, whereas gethostbyname() has four scalar
arguments.
In the syntax descriptions that follow, list operators that expect a list (and provide list context for the ele-
ments of the list) are shown with LIST as an argument. Such a list may consist of any combination of scalar
arguments or list values; the list values will be included in the list as if each individual element were inter-
polated at that point in the list, forming a longer single-dimensional list value. Commas should separate ele-
ments of the LIST.
Any function in the list below may be used either with or without parentheses around its arguments. (The syntax
descriptions omit the parentheses.) If you use the parentheses, the simple (but occasionally surprising) rule is
this: 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 precedence does matter. And whitespace between the function and left parenthesis
doesn't count--so you need to be careful sometimes:
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 -w switch it can warn you about this. For example, the third line above 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. These include
such functions as "time" and "endpwent". For example, "time+86_400" always means "time() + 86_400".
For functions that can be used in either a scalar or list context, nonabortive failure is generally indicated in
a scalar context by returning the undefined value, and in a list context by returning the null list.
Remember the following important rule: There is no rule that relates the behavior of an expression in list con-
text to its behavior in scalar context, or vice versa. It might do two totally different things. Each operator
and function decides which sort of value it would be most appropriate to return in scalar context. Some opera-
tors return the length of the list that would have been returned in list context. Some operators return the
first value in the list. Some operators return the last value in the list. Some operators 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 con-
text at compile time. It would generate the scalar comma operator there, not the list construction version of
the comma. That means it was never a list to start with.
In general, functions in Perl that serve as wrappers for system calls of the same name (like chown(2), fork(2),
closedir(2), etc.) all 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. Exceptions to this
rule are "wait", "waitpid", and "syscall". System calls also set the special $! variable on failure. Other
functions do not, except accidentally.
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.
Functions for SCALARs or strings
"chomp", "chop", "chr", "crypt", "hex", "index", "lc", "lcfirst", "length", "oct", "ord", "pack",
"q/STRING/", "qq/STRING/", "reverse", "rindex", "sprintf", "substr", "tr///", "uc", "ucfirst", "y///"
Regular expressions and pattern matching
"m//", "pos", "quotemeta", "s///", "split", "study", "qr//"
Numeric functions
"abs", "atan2", "cos", "exp", "hex", "int", "log", "oct", "rand", "sin", "sqrt", "srand"
Functions for real @ARRAYs
"pop", "push", "shift", "splice", "unshift"
Functions for list data
"grep", "join", "map", "qw/STRING/", "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", "rewinddir", "seek", "seekdir", "select", "syscall", "sysread",
"sysseek", "syswrite", "tell", "telldir", "truncate", "warn", "write"
Functions for fixed length data or records
"pack", "read", "syscall", "sysread", "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", "stat", "symlink", "sysopen", "umask", "unlink", "utime"
Keywords related to the control flow of your Perl program
"caller", "continue", "die", "do", "dump", "eval", "exit", "goto", "last", "next", "redo", "return", "sub",
"wantarray"
Keywords related to scoping
"caller", "import", "local", "my", "our", "package", "use"
Miscellaneous functions
"defined", "dump", "eval", "formline", "local", "my", "our", "reset", "scalar", "undef", "wantarray"
Functions for processes and process groups
"alarm", "exec", "fork", "getpgrp", "getppid", "getpriority", "kill", "pipe", "qx/STRING/", "setpgrp", "set-
priority", "sleep", "system", "times", "wait", "waitpid"
Keywords related to perl modules
"do", "import", "no", "package", "require", "use"
Keywords related to classes and object-orientedness
"bless", "dbmclose", "dbmopen", "package", "ref", "tie", "tied", "untie", "use"
Low-level socket functions
"accept", "bind", "connect", "getpeername", "getsockname", "getsockopt", "listen", "recv", "send", "setsock-
opt", "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", "getp-
went", "getpwnam", "getpwuid", "setgrent", "setpwent"
Fetching network info
"endprotoent", "endservent", "gethostbyaddr", "gethostbyname", "gethostent", "getnetbyaddr", "getnetbyname",
"getnetent", "getprotobyname", "getprotobynumber", "getprotoent", "getservbyname", "getservbyport", "getser-
vent", "sethostent", "setnetent", "setprotoent", "setservent"
Time-related functions
"gmtime", "localtime", "time", "times"
Functions new in perl5
"abs", "bless", "chomp", "chr", "exists", "formline", "glob", "import", "lc", "lcfirst", "map", "my", "no",
"our", "prototype", "qx", "qw", "readline", "readpipe", "ref", "sub*", "sysopen", "tie", "tied", "uc",
"ucfirst", "untie", "use"
* - "sub" was a keyword in perl4, but in perl5 it is an operator, which can be used in expressions.
Functions obsoleted in perl5
"dbmclose", "dbmopen"
Portability
Perl was born in Unix and can therefore access all common Unix system calls. In non-Unix environments, the func-
tionality 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", "getser-
vent", "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"
For more information about the portability of these functions, see perlport and other available platform-specific
documentation.
Alphabetical Listing of Perl Functions
-X FILEHANDLE
-X EXPR
-X A file test, where X is one of the letters listed below. This unary operator takes one argument, either
a filename or a filehandle, and tests the associated file to see if something is true about it. If the
argument is omitted, tests $_, except for "-t", which tests STDIN. Unless otherwise documented, it
returns 1 for true and for false, or the undefined value if the file doesn't exist. Despite the funny
names, precedence is the same as any other named unary operator, and the argument may be parenthesized
like any other 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.
-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 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:
while (<>) {
chomp;
next unless -f $_; # ignore specials
#...
}
The interpretation of the file permission operators "-r", "-R", "-w", "-W", "-x", and "-X" is by default
based solely on the mode of the file and the uids and gids of the user. There may be other reasons you
can't actually read, write, or execute the file. Such reasons may be for example network filesystem
access controls, ACLs (access control lists), read-only filesystems, and unrecognized executable formats.
Also note that, for the superuser on the local filesystems, the "-r", "-R", "-w", and "-W" tests always
return 1, and "-x" and "-X" return 1 if any execute bit is set in the mode. Scripts run by the superuser
may thus need to do a stat() to determine the actual mode of the file, or temporarily set their effective
uid to something else.
If you are using ACLs, there is a pragma called "filetest" that may produce more accurate results than
the bare stat() mode bits. When under the "use filetest 'access'" the above-mentioned filetests will
test whether the permission can (not) be granted using the access() family of system calls. Also note
that the "-x" and "-X" may under this pragma return true even if there are no execute permission bits set
(nor any extra execute permission ACLs). This strangeness is due to the underlying system calls' defini-
tions. Read the documentation for the "filetest" pragma for more information.
Note that "-s/a/b/" does not do a negated substitution. Saying "-exp($foo)" still works as expected,
however--only single letters following a minus are interpreted as file tests.
The "-T" and "-B" switches work as follows. The first block or so of the file is examined for odd char-
acters such as strange control codes or characters with the high bit set. If too many strange characters
(>30%) are found, it's a "-B" file; otherwise it's a "-T" file. Also, any file containing null in the
first block is considered a binary file. 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 a null file, or a file at EOF
when testing a filehandle. 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, as in "next unless -f $file && -T $file".
If any of the file tests (or either the "stat" or "lstat" operators) are 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" will 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 $a || -w _ || -x _;
stat($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 _;
abs VALUE
abs Returns the absolute value of its argument. If VALUE is omitted, uses $_.
accept NEWSOCKET,GENERICSOCKET
Accepts an incoming socket connect, just as the accept(2) system call 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, you may 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. The Time::HiRes module (from CPAN, and starting from
Perl 5.8 part of the standard distribution) may also prove useful.
It is usually a mistake to intermix "alarm" and "sleep" calls. ("sleep" may be internally implemented in
your system with "alarm")
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 han-
dlers to restart system calls on some systems. Using "eval"/"die" always works, modulo the caveats given
in "Signals" in perlipc.
eval {
local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
alarm $timeout;
$nread = sysread SOCKET, $buffer, $size;
alarm 0;
};
if ($@) {
die unless $@ eq "alarm\n"; # propagate unexpected errors
# timed out
}
else {
# didn't
}
For more information see perlipc.
atan2 Y,X
Returns the arctangent of Y/X in the range -PI to PI.
For the tangent operation, you may use the "Math::Trig::tan" function, or use the familiar relation:
sub tan { sin($_[0]) / cos($_[0]) }
Note that atan2(0, 0) is not well-defined.
bind SOCKET,NAME
Binds a network address to a socket, just as the bind system call does. Returns true if it succeeded,
false otherwise. NAME should be a packed address of the appropriate type for the socket. See 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 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 work-
ing with a text file. For the sake of portability it is a good idea to always use it when appropriate,
and to never use it when it isn't appropriate. Also, people can set their I/O to be by default UTF-8
encoded Unicode, not bytes.
In other words: regardless of platform, use binmode() on binary data, like for example images.
If LAYER is present it is a single string, but may contain multiple directives. The directives alter the
behaviour of the file handle. When LAYER is present using binmode on 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 charac-
ters). Note that, despite what may be implied in "Programming Perl" (the Camel) or elsewhere, ":raw" is
not the simply inverse of ":crlf" -- other layers which would affect binary nature of the stream are also
disabled. See PerlIO, perlrun and the discussion about the PERLIO environment variable.
The ":bytes", ":crlf", and ":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. See open.
The LAYER parameter of the binmode() function is described as "DISCIPLINE" in "Programming Perl, 3rd Edi-
tion". 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".
In general, binmode() should be called after open() but before any I/O is done on the filehandle. Call-
ing binmode() will normally flush 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, see open. The ":encoding" layer sometimes needs to be called in mid-stream, and it doesn't
flush the stream. The ":encoding" also implicitly pushes on top of itself the ":utf8" layer because
internally Perl will operate on UTF-8 encoded Unicode characters.
The operating system, device drivers, C libraries, and Perl run-time system all work together to let the
programmer treat a single character ("\n") as the line terminator, irrespective of the external represen-
tation. On many operating systems, the native text file representation matches the internal representa-
tion, but on some platforms the external representation of "\n" is made up of more than one character.
Mac OS, all variants of Unix, 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 Mac OS and 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
contains "\cZ", the I/O subsystem will regard it as the end of the file, unless you use binmode().
binmode() is not only important 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.
bless REF,CLASSNAME
bless REF
This function tells the thingy referenced by REF that it is now an object in the CLASSNAME package. If
CLASSNAME is omitted, the current package is used. Because a "bless" is often the last thing in a
constructor, it returns the reference for convenience. Always use the two-argument version if a derived
class might inherit the function doing the blessing. See perltoot and perlobj for more about the bless-
ing (and blessings) of objects.
Consider always blessing objects in CLASSNAMEs that are mixed case. Namespaces with all lowercase names
are considered reserved for Perl pragmata. Builtin types have all uppercase names. To prevent confusion,
you may wish to avoid such package names as well. Make sure that CLASSNAME is a true value.
See "Perl Modules" in perlmod.
caller EXPR
caller Returns the context of the current subroutine call. In scalar context, returns the caller's package name
if there is a caller, that is, if we're in a subroutine or "eval" or "require", and the undefined value
otherwise. In list context, returns
($package, $filename, $line) = caller;
With EXPR, it returns some extra information that the debugger uses to print a stack trace. The value of
EXPR indicates how many call frames to go back before the current one.
($package, $filename, $line, $subroutine, $hasargs,
$wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
Here $subroutine may be "(eval)" if the frame is not a subroutine call, but an "eval". In such a case
additional elements $evaltext and $is_require are set: $is_require is true if the frame is created by a
"require" or "use" statement, $evaltext contains the text of the "eval EXPR" statement. In particular,
for an "eval BLOCK" statement, $filename is "(eval)", but $evaltext is undefined. (Note also that each
"use" statement creates a "require" frame inside an "eval EXPR" frame.) $subroutine may also be
"(unknown)" if this particular subroutine happens to have been deleted from the symbol table. $hasargs
is true if a new instance of @_ was set up for the frame. $hints and $bitmask contain pragmatic hints
that the caller was compiled with. The $hints and $bitmask values are subject to change between versions
of Perl, and are not meant for external use.
Furthermore, when called from within the DB package, caller returns more detailed information: it sets
the list variable @DB::args to be the arguments with which the subroutine was invoked.
Be aware that the optimizer might have optimized call frames away before "caller" had a chance to get the
information. That means that caller(N) might not return information about the call frame you expect it
do, for "N > 1". In particular, @DB::args might have information from the previous time "caller" was
called.
chdir EXPR
chdir FILEHANDLE
chdir DIRHANDLE
chdir Changes the working directory to EXPR, if possible. If EXPR is omitted, changes to the directory speci-
fied by $ENV{HOME}, if set; if not, changes to the directory specified by $ENV{LOGDIR}. (Under VMS, the
variable $ENV{SYS$LOGIN} is also checked, and used if it is set.) If neither is set, "chdir" does noth-
ing. It returns true upon success, false otherwise. See the example under "die".
On systems that support fchdir, you might pass a file handle or directory handle as argument. On systems
that don't support fchdir, passing handles produces a fatal error at run time.
chmod LIST
Changes the permissions of a list of files. The first element of the list must be the numerical mode,
which should probably be an octal number, and which definitely should not be a string of octal digits:
0644 is okay, '0644' is not. Returns the number of files successfully changed. See also "oct", if all
you have is a string.
$cnt = chmod 0755, 'foo', 'bar';
chmod 0755, @executables;
$mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
# --w----r-T
$mode = '0644'; chmod oct($mode), 'foo'; # this is better
$mode = 0644; chmod $mode, 'foo'; # this is best
On systems that support fchmod, you might pass file handles among the files. On systems that don't sup-
port fchmod, passing file handles produces a fatal error at run time.
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 ':mode';
chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
# This is identical to the chmod 0755 of the above example.
chomp VARIABLE
chomp( LIST )
chomp This safer version of "chop" removes any trailing string that corresponds to the current value of $/
(also known as $INPUT_RECORD_SEPARATOR in the "English" module). It returns the total number of charac-
ters removed from all its arguments. It's often used to remove the newline from the end of an input
record when you're worried that the final record may be missing its newline. When in paragraph mode ("$/
= """), 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 any-
thing. If VARIABLE is omitted, it chomps $_. Example:
while (<>) {
chomp; # avoid \n on last field
@array = split(/:/);
# ...
}
If VARIABLE is a hash, it chomps the hash's values, but not its keys.
You can actually chomp anything that's an lvalue, including an assignment:
chomp($cwd = `pwd`);
chomp($answer = <STDIN>);
If you chomp a list, each element is chomped, and the total number of characters removed is returned.
If the "encoding" pragma is in scope then the lengths returned are calculated from the length of $/ in
Unicode characters, which is not always the same as the length of $/ in the native encoding.
Note that parentheses are necessary when you're chomping anything that is not a simple variable. This is
because "chomp $cwd = `pwd`;" is interpreted as "(chomp $cwd) = `pwd`;", rather than as "chomp( $cwd =
`pwd` )" which you might expect. Similarly, "chomp $a, $b" is interpreted as "chomp($a), $b" rather than
as "chomp($a, $b)".
chop VARIABLE
chop( LIST )
chop Chops off the last character of a string and returns the character chopped. It is much more efficient
than "s/.$//s" because it neither scans nor copies the string. If VARIABLE is omitted, chops $_. If
VARIABLE is a hash, it chops the hash's values, but not its keys.
You can actually chop anything that's an lvalue, including an assignment.
If you chop a list, each element is chopped. Only the value of the last "chop" is returned.
Note that "chop" returns the last character. To return all but the last character, use "substr($string,
0, -1)".
See also "chomp".
chown LIST
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.
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
On systems that support fchown, you might pass file handles among the files. On systems that don't
support fchown, passing file handles produces a fatal error at run time.
Here's an example that looks up nonnumeric uids in the passwd file:
print "User: ";
chomp($user = <STDIN>);
print "Files: ";
chomp($pattern = <STDIN>);
($login,$pass,$uid,$gid) = getpwnam($user)
or die "$user not in passwd file";
@ary = glob($pattern); # expand filenames
chown $uid, $gid, @ary;
On most systems, you are not allowed to change the ownership of the 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 this is not a portable assumption. On POSIX systems, you can
detect this condition this way:
use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
$can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
chr NUMBER
chr Returns 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. Note that characters from 128 to 255
(inclusive) are by default not encoded in UTF-8 Unicode for backward compatibility reasons (but see
encoding).
If NUMBER is omitted, uses $_.
For the reverse, use "ord".
Note that under the "bytes" pragma the NUMBER is masked to the low eight bits.
See perlunicode and encoding for more about Unicode.
chroot FILENAME
chroot This function works like the system call by the same name: it makes the named directory the new root
directory for all further pathnames that begin with a "/" by your process and all its children. (It
doesn't change your current working directory, which is unaffected.) For security reasons, this call is
restricted to the superuser. If FILENAME is omitted, does a "chroot" to $_.
close FILEHANDLE
close Closes the file or pipe associated with the file handle, returning true only if IO buffers are success-
fully flushed and closes the system file descriptor. Closes the currently selected filehandle if the
argument is omitted.
You don't have to close FILEHANDLE if you are immediately going to do another "open" on it, because
"open" will close it for you. (See "open".) However, an explicit "close" on an input file resets the
line counter ($.), while the implicit close done by "open" does not.
If the file handle came from a piped open, "close" will additionally return false if one of the other
system calls involved fails, or if the 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 complete, in case you want to look at the output of the pipe afterwards, and implicitly
puts the exit status value of that command into $?.
Prematurely closing the read end of a pipe (i.e. before the process writing to it at the other end has
closed it) will result in a SIGPIPE being delivered to the writer. 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 file-
handle name.
closedir DIRHANDLE
Closes a directory opened by "opendir" and returns the success of that system call.
connect SOCKET,NAME
Attempts to connect to a remote socket, just as the connect system call does. Returns true if it suc-
ceeded, false otherwise. NAME should be a packed address of the appropriate type for the socket. See
the examples in "Sockets: Client/Server Communication" in perlipc.
continue BLOCK
"continue" is actually a flow control statement rather than a function. If there is a "continue" BLOCK
attached to a BLOCK (typically in a "while" or "foreach"), it is always executed just before the condi-
tional is about to be evaluated again, just like the third part of a "for" loop in C. Thus it can be
used to increment a loop variable, even when the loop has been continued via the "next" statement (which
is similar to the C "continue" statement).
"last", "next", or "redo" may appear within a "continue" block. "last" and "redo" will behave as if they
had been executed within the main block. So will "next", but since it will 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 the top to re-check EXPR
}
### last always comes here
Omitting the "continue" section is semantically equivalent to using an empty one, logically enough. In
that case, "next" goes directly back to check the condition at the top of the loop.
cos EXPR
cos Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, takes cosine of $_.
For the inverse cosine operation, you may use the "Math::Trig::acos()" function, or use this relation:
sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
crypt PLAINTEXT,SALT
Creates a digest string exactly like the crypt(3) function in the C library (assuming that you actually
have a version there that has not been extirpated as a potential munitions).
crypt() is a one-way hash function. The PLAINTEXT and SALT is turned into a short string, called a
digest, which is returned. The same PLAINTEXT and SALT will always return the same string, but there is
no (known) way to get the original PLAINTEXT from the hash. Small changes in the PLAINTEXT or SALT will
result in large changes in the digest.
There is no decrypt function. This function isn't all that useful for cryptography (for that, look for
Crypt modules on your nearby CPAN mirror) and the name "crypt" is a bit of a misnomer. Instead it is
primarily used to check if two pieces of text are the same without having to transmit or store the text
itself. An example is checking if a correct password is given. The digest of the password is stored,
not the password itself. The user types in a password that is crypt()'d with the same salt as the stored
digest. If the two digests match the password is correct.
When verifying an existing digest string you should use the digest as the salt (like "crypt($plain,
$digest) eq $digest"). The SALT used to create the digest is visible as part of the digest. This
ensures crypt() will hash the new string with the same salt as the digest. This allows your code to work
with the standard crypt and with more exotic implementations. In other words, do not assume anything
about the returned string itself, or how many bytes in the digest 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 the digest string mattered, but alternative
hashing schemes (like MD5), higher level security schemes (like C2), and implementations on non-UNIX
platforms may produce different strings.
When 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]"). This set of
characters is just a recommendation; 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 that whoever runs this program knows their password:
$pwd = (getpwuid($<))[1];
system "stty -echo";
print "Password: ";
chomp($word = <STDIN>);
print "\n";
system "stty echo";
if (crypt($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 using crypt() on a Unicode string (which potentially has characters with codepoints above 255), Perl
tries to make sense of the situation by trying to downgrade (a copy of the string) the string back to an
eight-bit byte string before calling crypt() (on that copy). If that works, good. If not, crypt() dies
with "Wide character in crypt".
dbmclose HASH
[This function has been largely superseded by the "untie" function.]
Breaks the binding between a DBM file and a hash.
dbmopen HASH,DBNAME,MASK
[This function has been largely superseded by the "tie" function.]
This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a hash. HASH is the name of the
hash. (Unlike normal "open", the first argument is not a filehandle, even though it looks like one).
DBNAME is the name of the database (without the .dir or .pag extension if any). If the database does not
exist, it is created with protection specified by MASK (as modified by the "umask"). If your system sup-
ports only the older DBM functions, you may perform only one "dbmopen" in your program. In older ver-
sions of Perl, if your system had neither DBM nor ndbm, calling "dbmopen" produced a fatal error; it now
falls back to sdbm(3).
If you don't have write access to the DBM file, you can only read hash variables, not set them. If you
want to test whether you can write, either use file tests or try setting a dummy hash entry inside an
"eval", which will trap the error.
Note that functions such as "keys" and "values" may return huge lists when used on large DBM files. You
may prefer to use the "each" function to iterate over large DBM files. Example:
# 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: $!";
defined EXPR
defined Returns a Boolean value telling whether EXPR has a value other than the undefined value "undef". If EXPR
is not present, $_ will be checked.
Many operations return "undef" to indicate failure, end of file, system error, uninitialized variable,
and other exceptional conditions. This function allows you to 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".
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. Note that a subroutine which 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.
Use of "defined" on aggregates (hashes and arrays) is deprecated. It used to report whether memory for
that aggregate has ever been allocated. This behavior may disappear in future versions of Perl. 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.
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 overuse "defined", and then are surprised to discover that the number 0 and ""
(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, despite the fact that 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 you're 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 a hash element, array element, hash slice, or array slice, deletes the
specified element(s) from the hash or array. In the case of an array, if the array elements happen to be
at the end, the size of the array will shrink to the highest element that tests true for exists() (or 0
if no such element exists).
Returns a list with the same number of elements as the number of elements for which deletion was
attempted. Each element of that list consists of either the value of the element deleted, or the unde-
fined value. In scalar context, this means that you get the value of the last element deleted (or the
undefined value if that element did not exist).
%hash = (foo => 11, bar => 22, baz => 33);
$scalar = delete $hash{foo}; # $scalar is 11
$scalar = delete @hash{qw(foo bar)}; # $scalar is 22
@array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
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 "tie"d hash or array may not necessarily return anything.
Deleting an array element effectively returns that position of the array to its initial, uninitialized
state. Subsequently testing for the same element with exists() will return false. Also, deleting array
elements in the middle of an array will not shift the index of the elements after them down. Use
splice() for that. See "exists".
The following (inefficiently) deletes all the values of %HASH and @ARRAY:
foreach $key (keys %HASH) {
delete $HASH{$key};
}
foreach $index (0 .. $#ARRAY) {
delete $ARRAY[$index];
}
And so do these:
delete @HASH{keys %HASH};
delete @ARRAY[0 .. $#ARRAY];
But both of these are slower than just assigning the empty list or undefining %HASH or @ARRAY:
%HASH = (); # completely empty %HASH
undef %HASH; # forget %HASH ever existed
@ARRAY = (); # completely empty @ARRAY
undef @ARRAY; # forget @ARRAY ever existed
Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash element, array
element, hash slice, or array slice lookup:
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
Outside an "eval", prints the value of LIST to "STDERR" and exits with the current value of $! (errno).
If $! is 0, exits with the value of "($? >> 8)" (backtick `command` status). If "($? >> 8)" is 0, exits
with 255. Inside an "eval()," the error message is stuffed into $@ and the "eval" is terminated with the
undefined value. This makes "die" the way to raise an exception.
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"
If the last element of LIST does not end in a newline, the current script line number and input line num-
ber (if any) are also printed, and a newline is supplied. 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 avail-
able 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.
See also exit(), warn(), and the Carp module.
If LIST is empty and $@ already contains a value (typically from a previous eval) that value is reused
after appending "\t...propagated". This is useful for propagating exceptions:
eval { ... };
die unless $@ =~ /Expected exception/;
If LIST is empty 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 $@ is empty then the string "Died" is used.
die() can also be called with a reference argument. If this happens to be trapped within an eval(), $@
contains the reference. This behavior permits a more elaborate exception handling implementation using
objects that maintain arbitrary state about the nature of the exception. Such a scheme is sometimes
preferable to matching particular string values of $@ using regular expressions. Here's an example:
use Scalar::Util 'blessed';
eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
if ($@) {
if (blessed($@) && $@->isa("Some::Module::Exception")) {
# handle Some::Module::Exception
}
else {
# handle all other possible exceptions
}
}
Because perl will stringify uncaught exception messages before displaying them, you may want to overload
stringification operations on such custom exception objects. See overload for details about that.
You can arrange for a callback to be run just before the "die" does its deed, by setting the
$SIG{__DIE__} hook. The associated handler will be called with the error text and can change the error
message, if it sees fit, by calling "die" again. See "$SIG{expr}" in perlvar for details on setting %SIG
entries, and "eval BLOCK" for some examples. Although this feature was to be run only right before your
program was to exit, this is not currently the case--the $SIG{__DIE__} hook is currently called even
inside eval()ed blocks/strings! If one wants the hook to do nothing in such situations, put
die @_ if $^S;
as the first line of the handler (see "$^S" in perlvar). Because this promotes strange action at a dis-
tance, this counterintuitive behavior may be fixed in a future release.
do BLOCK
Not really a function. Returns the value of the last command in the sequence of commands indicated by
BLOCK. When modified by the "while" or "until" loop modifier, executes the BLOCK once before testing the
loop condition. (On other statements the loop modifiers test the conditional first.)
"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 perlsyn for alternative strategies.
do SUBROUTINE(LIST)
This form of subroutine call is deprecated. See perlsub.
do EXPR Uses the value of EXPR as a filename and executes the contents of the file as a Perl script.
do 'stat.pl';
is just like
eval `cat stat.pl`;
except that it's more efficient and concise, keeps track of the current filename for error messages,
searches the @INC directories, and updates %INC if the file is found. See "Predefined Names" in perlvar
for these variables. It also differs in that code evaluated with "do FILENAME" cannot see lexicals in
the enclosing scope; "eval STRING" does. 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.
If "do" cannot read the file, it returns undef and sets $! to the error. If "do" can read the file but
cannot compile it, it returns undef and sets an error message in $@. If the file is successfully com-
piled, "do" returns the value of the last expression evaluated.
Note that inclusion of 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.
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
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 This function causes an immediate core dump. See also the -u command-line switch in perlrun, which does
the same thing. Primarily this is so that you can use the undump program (not supplied) to turn your
core dump into an executable binary after having initialized all your variables at the beginning of the
program. When the new binary is executed it will begin by executing a "goto LABEL" (with all the
restrictions that "goto" suffers). Think of it as a goto with an intervening core dump and reincarna-
tion. If "LABEL" is omitted, restarts the program from the top.
WARNING: Any files opened at the time of the dump will not be open any more when the program is reincar-
nated, with possible resulting confusion on the part of Perl.
This function is now largely obsolete, partly because it's very hard to convert a core file into an exe-
cutable, and because the real compiler backends for generating portable bytecode and compilable C code
have superseded it. That's why you should now invoke it as "CORE::dump()", if you don't want to be
warned against a possible typo.
If you're looking to use dump to speed up your program, consider generating bytecode or native C code as
described in perlcc. If you're just trying to accelerate a CGI script, consider using the "mod_perl"
extension to Apache, or the CPAN module, CGI::Fast. You might also consider autoloading or selfloading,
which at least make your program appear to run faster.
each HASH
When called in list context, returns a 2-element list consisting of the key and value for the next ele-
ment of a hash, so that you can iterate over it. When called in scalar context, returns only the key for
the next element in the hash.
Entries are returned in an apparently random order. The actual random order is subject to change in
future versions of perl, but it is guaranteed to be in the same order as either the "keys" or "values"
function would produce on the same (unmodified) hash. Since Perl 5.8.1 the ordering is different even
between different runs of Perl for security reasons (see "Algorithmic Complexity Attacks" in perlsec).
When the hash is entirely read, a null array is returned in list context (which when assigned produces a
false(0) value), and "undef" in scalar context. The next call to "each" after that will start iterating
again. There is a single iterator for each hash, shared by all "each", "keys", and "values" function
calls in the program; it can be reset by reading all the elements from the hash, or by evaluating "keys
HASH" or "values HASH". If you add or delete elements of a hash while you're iterating over it, you may
get entries skipped or duplicated, so don't. Exception: It is always safe to delete the item most
recently returned by "each()", which means that the following code will work:
while (($key, $value) = each %hash) {
print $key, "\n";
delete $hash{$key}; # This is safe
}
The following prints out your environment like the printenv(1) program, only in a different order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}
See also "keys", "values" and "sort".
eof FILEHANDLE
eof ()
eof Returns 1 if the next read on FILEHANDLE will return end of file, or if FILEHANDLE is not open. FILEHAN-
DLE may be an expression whose value gives the real filehandle. (Note that this function actually reads
a character and then "ungetc"s it, so isn't very 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 termi-
nals may lose the end-of-file condition if you do.
An "eof" without an argument uses the last file read. Using "eof()" with empty parentheses is very dif-
ferent. 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, "eof" or "eof(ARGV)" can be used to detect the end of each file, "eof()" will
only detect the end of the last file. 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: you almost never need to use "eof" in Perl, because the input operators typically return
"undef" when they run out of data, or if there was an error.
eval EXPR
eval BLOCK
eval In the first form, the return value of EXPR is parsed and executed as if it were a little Perl program.
The value of the expression (which is itself determined within scalar context) is first parsed, and if
there weren't any errors, executed in the lexical context of the current Perl program, so that any vari-
able settings or subroutine and format definitions remain afterwards. Note that the value is parsed
every time the "eval" executes. If EXPR is omitted, evaluates $_. This form is typically used to delay
parsing and subsequent execution of the text of EXPR until run time.
In the second form, 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 (see below), while also providing the
benefit of checking the code within BLOCK at compile time.
The final semicolon, if any, may be omitted from the value of EXPR or within the BLOCK.
In both forms, the value returned is the value of the last expression evaluated inside the mini-program;
a return statement may be also 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 "wan-
tarray" 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, an undefined value is
returned by "eval", and $@ is set to the error message. If there was no error, $@ is guaranteed to be a
null string. 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,
warnings and perllexwarn.
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.
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:
# make divide-by-zero nonfatal
eval { $answer = $a / $b; }; warn $@ if $@;
# same thing, but less efficient
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
eval { $answer = }; # WRONG
# a run-time error
eval '$answer ='; # sets $@
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 shown in this
example:
# a very private exception trap for divide-by-zero
eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
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.
With an "eval", 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 6
Cases 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.
"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.
Note that as a very special case, an "eval " executed within 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.
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 there is a following
statement which isn't "die", "warn", or "exit" (if "-w" is set - but you always do that). 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, or if LIST is an array with more than one value, calls
execvp(3) with the arguments in LIST. If there is only one scalar argument or an array with one element
in it, 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. (This always forces interpretation of the LIST as a multivalued list,
even if there is only a single scalar in the list.) Example:
$shell = '/bin/csh';
exec $shell '-sh'; # pretend it's a login shell
or, more directly,
exec {'/bin/csh'} '-sh'; # pretend it's a login shell
When the arguments get executed via the system shell, results will be subject to its quirks and capabili-
ties. 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 whites-
pace in them.
@args = ( "echo surprise" );
exec @args; # subject to shell escapes
# if @args == 1
exec { $args[0] } @args; # safe even with one-arg list
The first version, the one without the indirect object, ran the echo program, passing it "surprise" an
argument. The second version didn't--it tried to run a program literally called "echo surprise", didn't
find it, and set $? to a non-zero value indicating failure.
Beginning with v5.6.0, Perl will attempt 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 in order to avoid lost out-
put.
Note that "exec" will not call your "END" blocks, nor will it call any "DESTROY" methods in your objects.
exists EXPR
Given an expression that specifies a hash element or array element, returns true if the specified element
in the hash or array has ever been initialized, even if the corresponding value is undefined. The ele-
ment is not autovivified if it doesn't exist.
print "Exists\n" if exists $hash{$key};
print "Defined\n" if defined $hash{$key};
print "True\n" if $hash{$key};
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 if it exists, but the reverse
doesn't necessarily hold true.
Given an expression that specifies the name of a subroutine, returns true if the specified subroutine has
ever been declared, even if it is undefined. Mentioning a subroutine name for exists or defined does not
count as declaring it. Note that a subroutine which does not exist 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.
print "Exists\n" if exists &subroutine;
print "Defined\n" if defined &subroutine;
Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash or array key
lookup or 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}}) { }
Although the deepest nested array or hash will not spring into existence just because its existence was
tested, any intervening ones will. Thus "$ref->{"A"}" and "$ref->{"A"}->{"B"}" will spring into exis-
tence due to the existence test for the $key element above. This happens anywhere the arrow operator is
used, including even:
undef $ref;
if (exists $ref->{"Some key"}) { }
print $ref; # prints HASH(0x80d3d5c)
This surprising autovivification in what does not at first--or even second--glance appear to be an lvalue
context may be fixed in a future release.
See "Pseudo-hashes: Using an array as a hash" in perlref for specifics on how exists() acts when used on
a pseudo-hash.
Use of a subroutine call, rather than a subroutine name, as an argument to exists() is an error.
exists ⊂ # OK
exists &sub(); # Error
exit EXPR
exit Evaluates EXPR and exits immediately with that value. Example:
$ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
See also "die". If EXPR is omitted, exits with 0 status. The only universally recognized values for
EXPR are 0 for success and 1 for error; other values are subject to interpretation depending on the envi-
ronment in which the Perl program is running. For example, exiting 69 (EX_UNAVAILABLE) from a sendmail
incoming-mail filter will cause the mailer to return the item undelivered, but that's not true every-
where.
Don't use "exit" to abort a subroutine if there's any chance that someone might want to trap whatever
error happened. Use "die" instead, which can be trapped by an "eval".
The exit() function does not always exit immediately. It calls any defined "END" routines first, but
these "END" routines may not themselves abort the exit. Likewise any object destructors that need to be
called are called before the real exit. If this is a problem, you can call "POSIX:_exit($status)" to
avoid END and destructor processing. See perlmod for details.
exp EXPR
exp Returns e (the natural logarithm base) to the power of EXPR. If EXPR is omitted, gives "exp($_)".
fcntl FILEHANDLE,FUNCTION,SCALAR
Implements the fcntl(2) function. You'll probably have to say
use Fcntl;
first to get the correct constant definitions. Argument processing and value return works just like
"ioctl" below. For example:
use Fcntl;
fcntl($filehandle, F_GETFL, $packed_return_buffer)
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 con-
text. It is also exempt from the normal -w warnings on improper numeric conversions.
Note that "fcntl" will produce a fatal error if used on a machine that doesn't implement fcntl(2). See
the Fcntl module 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);
$flags = fcntl(REMOTE, F_GETFL, 0)
or die "Can't get flags for the socket: $!\n";
$flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
or die "Can't set flags for the socket: $!\n";
fileno FILEHANDLE
Returns the file descriptor for a filehandle, or undefined if the filehandle is not open. 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) == fileno(THAT)) {
print "THIS and THAT are dups\n";
}
(Filehandles connected to memory objects via new features of "open" may return undefined even though they
are open.)
flock FILEHANDLE,OPERATION
Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true for success, false on failure. Pro-
duces 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 only entire files, not records.
Two potentially non-obvious but traditional "flock" semantics are that it waits indefinitely until the
lock is granted, and that its locks 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, or 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 (some-
times 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 bit-
wise-or'ed with LOCK_SH or LOCK_EX then "flock" will return immediately rather than blocking waiting for
the lock (check the return status to see if you got it).
To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE before locking or unlocking it.
Note that the emulation built with lockf(3) doesn't provide shared locks, and it requires that FILEHANDLE
be open with write intent. These are the semantics that lockf(3) implements. Most if not all systems
implement lockf(3) in terms of fcntl(2) locking, though, so the differing semantics shouldn't bite too
many people.
Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE be open with read intent to use
LOCK_SH and requires that it be open with write intent to use LOCK_EX.
Note also that some versions of "flock" cannot lock things over the network; you would need to use the
more system-specific "fcntl" for that. If you like 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_flock" to the Con-
figure program when you configure perl.
Here's a mailbox appender for BSD systems.
use Fcntl ':flock'; # import LOCK_* constants
sub lock {
flock(MBOX,LOCK_EX);
# and, in case someone appended
# while we were waiting...
seek(MBOX, 0, 2);
}
sub unlock {
flock(MBOX,LOCK_UN);
}
open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
or die "Can't open mailbox: $!";
lock();
print MBOX $msg,"\n\n";
unlock();
On systems that support a real flock(), locks are inherited across fork() calls, whereas those that must
resort to the more capricious fcntl() function lose the locks, making it harder to write servers.
See also DB_File for other flock() examples.
fork Does a fork(2) system call to create a new process running the same program at the same point. It
returns the child pid to the parent process, 0 to the child process, or "undef" if the fork is unsuccess-
ful. File descriptors (and sometimes locks on those descriptors) are shared, while everything else is
copied. On most systems supporting fork(), great care has gone into making it extremely efficient (for
example, using copy-on-write technology on data pages), making it the dominant paradigm for multitasking
over the last few decades.
Beginning with v5.6.0, Perl will attempt 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 in order
to avoid duplicate output.
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.
Note that 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.
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 "format"s, though you may call it, too. It formats (see perlform) a
list of values according to the contents of PICTURE, placing the output into the format output accumula-
tor, $^A (or $ACCUMULATOR in English). Eventually, when a "write" is done, the contents of $^A are writ-
ten to some filehandle. You could also read $^A and then set $^A back to "". Note that a format typi-
cally does one "formline" per line of form, but the "formline" function itself doesn't care how many new-
lines are embedded in the PICTURE. This means that the "~" and "~~" tokens will treat the entire PICTURE
as a single line. You may therefore need to use multiple formlines to implement a single record format,
just like the format compiler.
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.
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 $! is set). If FILEHANDLE is omitted, reads from
STDIN. This is not particularly efficient. However, it cannot be used by itself to fetch single charac-
ters without waiting for the user to hit enter. For that, try something more like:
if ($BSD_STYLE) {
system "stty cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system "stty", '-icanon', 'eol', "\001";
}
$key = getc(STDIN);
if ($BSD_STYLE) {
system "stty -cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system "stty", 'icanon', 'eol', '^@'; # ASCII null
}
print "\n";
Determination of whether $BSD_STYLE should be set is left as an exercise to the reader.
The "POSIX::getattr" function can do this more portably on systems purporting POSIX compliance. See also
the "Term::ReadKey" module from your nearest CPAN site; details on CPAN can be found on "CPAN" in
perlmodlib.
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 null, use "getpwuid".
$login = getlogin || getpwuid($<) || "Kilroy";
Do not consider "getlogin" for authentication: it is not as secure as "getpwuid".
getpeername SOCKET
Returns the packed sockaddr address of other end of the SOCKET connection.
use Socket;
$hersockaddr = getpeername(SOCK);
($port, $iaddr) = sockaddr_in($hersockaddr);
$herhostname = gethostbyaddr($iaddr, AF_INET);
$herstraddr = inet_ntoa($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 process group of current process. Note that the POSIX version of "getpgrp"
does not accept a PID argument, so only "PID==0" is truly portable.
getppid Returns the process id of the parent process.
Note for Linux users: on Linux, the C functions "getpid()" and "getppid()" return different values from
different threads. In order to be portable, this behavior is not reflected by the perl-level function
"getppid()", that returns a consistent value across threads. If you want to call the underlying "getp-
pid()", you may use the CPAN module "Linux::Pid".
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).
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 perform the same functions as their counterparts in the system library. In list context,
the return values from the various get routines are as follows:
($name,$passwd,$uid,$gid,
$quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
($name,$passwd,$gid,$members) = getgr*
($name,$aliases,$addrtype,$length,@addrs) = gethost*
($name,$aliases,$addrtype,$net) = getnet*
($name,$aliases,$proto) = getproto*
($name,$aliases,$port,$proto) = getserv*
(If the entry doesn't exist you get a null list.)
The exact meaning of the $gcos field varies but it usually contains the real name of the user (as opposed
to the login name) and other information pertaining to the user. Beware, however, that in many system
users are able to change this information and therefore it cannot be trusted and therefore the $gcos is
tainted (see perlsec). The $passwd and $shell, user's encrypted password and login shell, are also
tainted, because of 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, whatever it is. (If the entry doesn't exist you get the undefined value.) For example:
$uid = getpwnam($name);
$name = getpwuid($num);
$name = getpwent();
$gid = getgrnam($name);
$name = getgrgid($num);
$name = getgrent();
#etc.
In getpw*() the fields $quota, $comment, and $expire are special cases in the sense that in many systems
they are unsupported. If the $quota is unsupported, it is an empty scalar. If it is supported, it usu-
ally encodes the disk quota. If the $comment field is unsupported, it is an empty scalar. If it is sup-
ported it usually encodes some administrative comment about the user. In some systems the $quota field
may be $change or $age, fields that have to do with password aging. In some systems the $comment field
may be $class. The $expire field, if present, encodes the expiration period of the account or the pass-
word. For the availability and the exact meaning of these fields in your system, please consult your
getpwnam(3) documentation and your pwd.h file. You can also find out from within Perl what your $quota
and $comment fields mean and whether you have the $expire field by using the "Config" module and the val-
ues "d_pwquota", "d_pwage", "d_pwchange", "d_pwcomment", and "d_pwexpire". Shadow password files are
only supported if your vendor has implemented them in the intuitive fashion that calling the regular C
library routines gets the shadow versions if you're running under privilege or if there exists the
shadow(3) functions as found in System V (this includes Solaris and Linux.) Those systems that implement
a proprietary shadow password facility are unlikely to be supported.
The $members value returned by getgr*() is a space separated list of the login names of the members of
the group.
For the gethost*() functions, if the "h_errno" variable is supported in C, it will be returned to you via
$? if the function call fails. The @addrs value returned by a successful call is a list of the raw
addresses returned by the corresponding system library call. In the Internet domain, each address is
four bytes long and you can unpack it by saying something like:
($a,$b,$c,$d) = unpack('C4',$addr[0]);
The Socket library makes this slightly easier:
use Socket;
$iaddr = inet_aton("127.1"); # or whatever address
$name = gethostbyaddr($iaddr, AF_INET);
# or going the other way
$straddr = inet_ntoa($iaddr);
If you get tired of remembering which element of the return list contains which return value, by-name
interfaces are provided in standard modules: "File::stat", "Net::hostent", "Net::netent", "Net::pro-
toent", "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 exam-
ple:
use File::stat;
use User::pwent;
$is_his = (stat($filename)->uid == pwent($whoever)->uid);
Even though it looks like they're the same method calls (uid), they aren't, because a "File::stat" object
is different from a "User::pwent" object.
getsockname SOCKET
Returns the packed sockaddr address of this end of the SOCKET connection, in case you don't know the
address because you have several different IPs that the connection might have come in on.
use Socket;
$mysockaddr = getsockname(SOCK);
($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 exist. To query options at another level the protocol number of the appro-
priate protocol controlling the option should be supplied. For example, 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 call returns a packed string representing the requested socket option, or "undef" if there is an
error (the error reason will be in $!). What exactly is in the packed string depends in the LEVEL and
OPTNAME, consult your system documentation for details. A very common case however is that the option is
an integer, in which case the result will be a packed integer which you can decode using unpack with the
"i" (or "I") format.
An example testing if Nagle's algorithm is turned on 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 "Could not query TCP_NODELAY socket option: $!";
my $nodelay = unpack("I", $packed);
print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
glob EXPR
glob In list context, returns a (possibly empty) list of filename expansions on the value of EXPR such as the
standard Unix shell /bin/csh would do. In scalar context, glob iterates through such filename expansions,
returning undef when the list is exhausted. This is the internal function implementing the "<*.c>" opera-
tor, but you can use it directly. If EXPR is omitted, $_ is used. The "<*.c>" operator is discussed in
more detail in "I/O Operators" in perlop.
Beginning with v5.6.0, this operator is implemented using the standard "File::Glob" extension. See
File::Glob for details.
gmtime EXPR
gmtime Converts a time as returned by the time function to an 9-element list with the time localized for the
standard Greenwich time zone. Typically used as follows:
# 0 1 2 3 4 5 6 7 8
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
gmtime(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, and $mon is the month
itself, in the range 0..11 with 0 indicating January and 11 indicating December. $year is the number of
years since 1900. That is, $year is 123 in year 2023. $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 always 0.
Note that the $year element is not simply the last two digits of the year. If you assume it is then you
create non-Y2K-compliant programs--and you wouldn't want to do that, would you?
The proper way to get a complete 4-digit year is simply:
$year += 1900;
And to get the last two digits of the year (e.g., '01' in 2001) do:
$year = sprintf("%02d", $year % 100);
If EXPR is omitted, "gmtime()" uses the current time ("gmtime(time)").
In scalar context, "gmtime()" returns the ctime(3) value:
$now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
If you need local time instead of GMT use the "localtime" builtin. See also the "timegm" function pro-
vided by the "Time::Local" module, and the strftime(3) and mktime(3) functions available via the POSIX
module.
This scalar value is not locale dependent (see perllocale), but is instead a Perl builtin. To get some-
what similar but locale dependent date strings, see the example in "localtime".
See "gmtime" in perlport for portability concerns.
goto LABEL
goto EXPR
goto &NAME
The "goto-LABEL" form finds the statement labeled with LABEL and resumes execution there. It may not be
used to go into any construct that requires initialization, such as a subroutine or a "foreach" loop. It
also can't be used to go into a construct that is optimized away, or to get out of a block or subroutine
given to "sort". It can be used to go almost anywhere else within the dynamic scope, including out of
subroutines, but it's usually better to use some other construct such as "last" or "die". The author of
Perl has never felt the need to use this form of "goto" (in Perl, that is--C is another matter). (The
difference being that C does not offer named loops combined with loop control. Perl does, and this
replaces most structured uses of "goto" in other languages.)
The "goto-EXPR" form expects a label name, whose scope will be resolved dynamically. This allows for
computed "goto"s per FORTRAN, but isn't necessarily recommended if you're optimizing for maintainability:
goto ("FOO", "BAR", "GLARCH")[$i];
The "goto-&NAME" form is quite different from the other forms of "goto". In fact, it isn't a goto in the
normal sense at all, and doesn't have the stigma associated with other gotos. Instead, it exits the cur-
rent subroutine (losing any changes set by local()) and immediately calls in its place the named subrou-
tine using the current value of @_. This is used by "AUTOLOAD" subroutines that wish to 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.
NAME needn't be the name of a subroutine; it can be a scalar variable containing a code reference, or a
block that evaluates to a code reference.
grep BLOCK LIST
grep EXPR,LIST
This is similar in spirit to, but not the same as, grep(1) and its relatives. In particular, it is not
limited to using regular expressions.
Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each element) and returns the
list value consisting of those elements for which the expression evaluated to true. In scalar context,
returns the number of times the expression was true.
@foo = grep(!/^#/, @bar); # weed out comments
or equivalently,
@foo = grep {!/^#/} @bar; # weed out comments
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.
Similarly, grep 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 grep (for example, in a "foreach",
"map" or another "grep") actually modifies the element in the original list. This is usually something
to be avoided when 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 value. (To convert strings that might
start with either 0, "0x", or "0b", see "oct".) If EXPR is omitted, uses $_.
print hex '0xAf'; # prints '175'
print hex 'aF'; # same
Hex strings may only represent integers. Strings that would cause integer overflow trigger a warning.
Leading whitespace is not stripped, unlike oct(). To present something as hex, look into "printf",
"sprintf", or "unpack".
import LIST
There is no builtin "import" function. It is 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 within 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, starts searching from the beginning of the string. POSI-
TION 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 0 (or whatever you've set the $[ variable
to--but don't do that). If the substring is not found, "index" returns one less than the base, ordinar-
ily "-1".
int EXPR
int Returns the integer portion of EXPR. If EXPR is omitted, uses $_. You should not use this function for
rounding: one because it truncates towards 0, and two because machine representations of floating point
numbers can sometimes produce counterintuitive results. For example, "int(-6.725/0.025)" produces -268
rather than the correct -269; that's because it's really more like -268.99999999999994315658 instead.
Usually, the "sprintf", "printf", or the "POSIX::floor" and "POSIX::ceil" functions will serve you better
than will int().
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.ph
to get the correct function definitions. If sys/ioctl.ph doesn't exist or doesn't have the correct defi-
nitions you'll have to roll your own, based on your C header files such as <sys/ioctl.h>. (There is a
Perl script called h2ph that comes with the Perl kit that may help you in this, but it's nontrivial.)
SCALAR will be read and/or written depending on the FUNCTION--a 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") is as follows:
if OS returns: then Perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
Thus Perl returns true on success and false on failure, yet you can still easily determine the actual
value returned by the operating system:
$retval = ioctl(...) || -1;
printf "System returned %d\n", $retval;
The special string "0 but true" is exempt from -w complaints about improper numeric conversions.
join EXPR,LIST
Joins the separate strings of LIST into a single string with fields separated by the value of EXPR, and
returns that new string. Example:
$rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
Beware that unlike "split", "join" doesn't take a pattern as its first argument. Compare "split".
keys HASH
Returns a list consisting of all the keys of the named hash. (In scalar context, returns the number of
keys.)
The keys are returned in an apparently random order. The actual random order is subject to change in
future versions of perl, but it is guaranteed to be the same order as either the "values" or "each" func-
tion produces (given that the hash has not been modified). Since Perl 5.8.1 the ordering is different
even between different runs of Perl for security reasons (see "Algorithmic Complexity Attacks" in
perlsec).
As a side effect, calling keys() resets the HASH's internal iterator (see "each"). In particular, call-
ing keys() in void context resets the iterator with no other overhead.
Here is yet another way to print your environment:
@keys = keys %ENV;
@values = values %ENV;
while (@keys) {
print pop(@keys), '=', pop(@values), "\n";
}
or how about sorted by key:
foreach $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 $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
printf "%4d %s\n", $hash{$key}, $key;
}
As an lvalue "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 = ()", use "undef %hash" if
you want to free the storage while %hash is still in scope. You can't shrink the number of buckets allo-
cated for the hash using "keys" in this way (but you needn't worry about doing this by accident, as try-
ing has no effect).
See also "each", "values" and "sort".
kill SIGNAL, LIST
Sends a signal to a list of processes. Returns the number of processes successfully signaled (which is
not necessarily the same as the number actually killed).
$cnt = kill 1, $child1, $child2;
kill 9, @goners;
If SIGNAL is zero, no signal is sent to the process. This is a useful way to check that a child process
is alive and hasn't changed its UID. See perlport for notes on the portability of this construct.
Unlike in the shell, if SIGNAL is negative, it kills process groups instead of processes. (On System V,
a negative PROCESS number will also kill process groups, but that's not portable.) That means you usu-
ally want to use positive not negative signals. You may also use a signal name in quotes.
See "Signals" in perlipc for more details.
last LABEL
last The "last" command is like the "break" statement in C (as used in loops); it immediately exits the loop
in question. If the LABEL is omitted, the command refers to the innermost enclosing loop. The "con-
tinue" block, if any, is not executed:
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
#...
}
"last" cannot be used to exit a block which returns a value such as "eval {}", "sub {}" or "do {}", and
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.
lc EXPR
lc Returns a lowercased version of EXPR. This is the internal function implementing the "\L" escape in dou-
ble-quoted strings. Respects current LC_CTYPE locale if "use locale" in force. See perllocale and per-
lunicode for more details about locale and Unicode support.
If EXPR is omitted, uses $_.
lcfirst EXPR
lcfirst Returns the value of EXPR with the first character lowercased. This is the internal function implement-
ing the "\l" escape in double-quoted strings. Respects current LC_CTYPE locale if "use locale" in force.
See perllocale and perlunicode for more details about locale and Unicode support.
If EXPR is omitted, uses $_.
length EXPR
length Returns the length in characters of the value of EXPR. If EXPR is omitted, returns length of $_. Note
that this cannot be used on an entire array or hash to find out how many elements these have. For that,
use "scalar @array" and "scalar keys %hash" respectively.
Note the characters: if the EXPR is in Unicode, you will get the number of characters, not the number of
bytes. To get the length in bytes, use "do { use bytes; length(EXPR) }", see bytes.
link OLDFILE,NEWFILE
Creates a new filename linked to the old filename. Returns true for success, false otherwise.
listen SOCKET,QUEUESIZE
Does the same thing that the listen system call does. Returns true if it succeeded, false otherwise.
See the example in "Sockets: Client/Server Communication" in perlipc.
local EXPR
You really probably want to be using "my" instead, because "local" isn't what most people think of as
"local". See "Private Variables via my()" in perlsub for details.
A local modifies the listed variables to be local to the enclosing block, file, or eval. If more than
one value is listed, the list must be placed in parentheses. See "Temporary Values via local()" in perl-
sub for details, including issues with tied arrays and hashes.
localtime EXPR
localtime
Converts a time as returned by the time function to a 9-element list with the time analyzed for the local
time zone. Typically used as follows:
# 0 1 2 3 4 5 6 7 8
($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, and $mon is the month itself, in the range 0..11 with 0 indicating January
and 11 indicating December. This makes it easy to get a month name from a list:
my @abbr = qw( Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec );
print "$abbr[$mon] $mday";
# $mon=9, $mday=18 gives "Oct 18"
$year is the number of years since 1900, not just the last two digits of the year. That is, $year is 123
in year 2023. The proper way to get a complete 4-digit year is simply:
$year += 1900;
To get the last two digits of the year (e.g., '01' in 2001) do:
$year = sprintf("%02d", $year % 100);
$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 during Daylight Saving Time, false otherwise.
If EXPR is omitted, "localtime()" uses the current time ("localtime(time)").
In scalar context, "localtime()" returns the ctime(3) value: