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NAME ^

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 and 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 list argument.) For instance, splice() has three scalar arguments followed by a list.

In the syntax descriptions that follow, list operators that expect a list (and provide list context for the elements 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 interpolated at that point in the list, forming a longer single-dimensional list value. Elements of the LIST should be separated by commas.

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.

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 rule:

THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!

Each operator and function decides which sort of value it would be most appropriate to return in a scalar context. Some operators return the length of the list that would have been returned in a 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.

Perl Functions by Category

Here are Perl's functions (including things that look like functions, like some of the 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

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, 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, package, use

Miscellaneous functions

defined, dump, eval, formline, local, my, reset, scalar, undef, wantarray

Functions for processes and process groups

alarm, exec, fork, getpgrp, getppid, getpriority, kill, pipe, qx/STRING/, setpgrp, setpriority, 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, setsockopt, shutdown, socket, socketpair

System V interprocess communication functions

msgctl, msgget, msgrcv, msgsnd, semctl, semget, semop, shmctl, shmget, shmread, shmwrite

Fetching user and group info

endgrent, endhostent, endnetent, endpwent, getgrent, getgrgid, getgrnam, getlogin, getpwent, getpwnam, getpwuid, setgrent, setpwent

Fetching network info

endprotoent, endservent, gethostbyaddr, gethostbyname, gethostent, getnetbyaddr, getnetbyname, getnetent, getprotobyname, getprotobynumber, getprotoent, getservbyname, getservbyport, getservent, sethostent, setnetent, setprotoent, setservent

Time-related functions

gmtime, localtime, time, times

Functions new in perl5

abs, bless, chomp, chr, exists, formline, glob, import, lc, lcfirst, map, my, no, 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

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.
    -s  File has nonzero size (returns size).

    -f  File is a plain file.
    -d  File is a directory.
    -l  File is a symbolic link.
    -p  File is a named pipe (FIFO).
    -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 a text file.
    -B  File is a binary file (opposite of -T).

    -M  Age of file in days when script started.
    -A  Same for access time.
    -C  Same for inode change time.

The interpretation of the file permission operators -r, -R, -w, -W, -x, and -X is 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. Also note that, for the superuser, -r, -R, -w, and -W 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 the uid to something else.

Example:

    while (<>) {
        chop;
        next unless -f $_;      # ignore specials
        ...
    }

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 characters such as strange control codes or characters with the high bit set. If too many odd 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 stdio 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.) 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 example in "Sockets: Client/Server Communication" in perlipc.

alarm SECONDS
alarm

Arranges to have a SIGALRM delivered to this process after the specified number of seconds have 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 than you specified because of how seconds are counted.) 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 syscall() interface to access setitimer(2) if your system supports it, or else see "select()". It is usually a mistake to intermix alarm() and sleep() calls.

If you want to use alarm() to time out a system call you need to use an eval/die pair. You can't rely on the alarm causing the system call to fail with $! set to EINTR because Perl sets up signal handlers to restart system calls on some systems. Using eval/die always works.

    eval {
        local $SIG{ALRM} = sub { die "alarm\n" };       # NB \n required
        alarm $timeout;
        $nread = sysread SOCKET, $buffer, $size;
        alarm 0;
    };
    die if $@ && $@ ne "alarm\n";       # propagate errors
    if ($@) {
        # timed out
    }
    else {
        # didn't
    }
atan2 Y,X

Returns the arctangent of Y/X in the range -PI to PI.

For the tangent operation, you may use the POSIX::tan() function, or use the familiar relation:

    sub tan { sin($_[0]) / cos($_[0])  }
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

Arranges for the file to be read or written in "binary" mode in operating systems that distinguish between binary and text files. Files that are not in binary mode have CR LF sequences translated to LF on input and LF translated to CR LF on output. Binmode has no effect under Unix; in MS-DOS and similarly archaic systems, it may be imperative--otherwise your MS-DOS-damaged C library may mangle your file. The key distinction between systems that need binmode and those that don't is their text file formats. Systems like Unix and Plan9 that delimit lines with a single character, and that encode that character in C as '\n', do not need binmode. The rest need it. If FILEHANDLE is an expression, the value is taken as the name of the filehandle.

bless REF,CLASSNAME
bless REF

This function tells the thingy referenced by REF that it is now an object in the CLASSNAME package--or the current package if no CLASSNAME is specified, which is often the case. It returns the reference for convenience, because a bless() is often the last thing in a constructor. Always use the two-argument version if the function doing the blessing might be inherited by a derived class. See perlobj for more about the blessing (and blessings) of objects.

caller EXPR
caller

Returns the context of the current subroutine call. In a 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 a 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, $is_require) = caller($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 a 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.

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.

chdir EXPR

Changes the working directory to EXPR, if possible. If EXPR is omitted, changes to home directory. Returns TRUE upon success, FALSE otherwise. See example under die().

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 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
chomp VARIABLE
chomp LIST
chomp

This is a slightly safer version of chop (see below). It removes any line ending that corresponds to the current value of $/ (also known as $INPUT_RECORD_SEPARATOR in the English module). It returns the total number of characters removed from all its arguments. It's often used to remove the newline from the end of an input record when you're worried that the final record may be missing its newline. When in paragraph mode ($/ = ""), it removes all trailing newlines from the string. If VARIABLE is omitted, it chomps $_. Example:

    while (<>) {
        chomp;  # avoid \n on last field
        @array = split(/:/);
        ...
    }

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.

chop VARIABLE
chop LIST
chop

Chops off the last character of a string and returns the character chopped. It's used primarily to remove the newline from the end of an input record, but is much more efficient than s/\n// because it neither scans nor copies the string. If VARIABLE is omitted, chops $_. Example:

    while (<>) {
        chop;   # avoid \n on last field
        @array = split(/:/);
        ...
    }

You can actually chop anything that's an lvalue, including an assignment:

    chop($cwd = `pwd`);
    chop($answer = <STDIN>);

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).

chown LIST

Changes the owner (and group) of a list of files. The first two elements of the list must be the NUMERICAL uid and gid, in that order. Returns the number of files successfully changed.

    $cnt = chown $uid, $gid, 'foo', 'bar';
    chown $uid, $gid, @filenames;

Here's an example that looks up nonnumeric uids in the passwd file:

    print "User: ";
    chop($user = <STDIN>);
    print "Files: "
    chop($pattern = <STDIN>);

    ($login,$pass,$uid,$gid) = getpwnam($user)
        or die "$user not in passwd file";

    @ary = <${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.

chr NUMBER
chr

Returns the character represented by that NUMBER in the character set. For example, chr(65) is "A" in ASCII. For the reverse, use "ord".

If NUMBER is omitted, uses $_.

chroot FILENAME
chroot

This function works as 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 of 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 chroot to $_.

close FILEHANDLE

Closes the file or pipe associated with the file handle, returning TRUE only if stdio successfully flushes buffers and closes the system file descriptor. 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 problem was that the program exited non-zero $! will be set to 0.) You don't have to close FILEHANDLE if you are immediately going to do another open() on it, because open() will close it for you. (See open().) However, an explicit close on an input file resets the line counter ($.), while the implicit close done by open() does not. Also, closing a pipe will wait for the process executing on the pipe to complete, in case you want to look at the output of the pipe afterwards. Closing a pipe explicitly also puts the status value of the command into $?. Example:

    open(OUTPUT, '|sort >foo'); # pipe to sort
    ...                         # print stuff to output
    close OUTPUT;               # wait for sort to finish
    open(INPUT, 'foo');         # get sort's results

FILEHANDLE may be an expression whose value gives the real filehandle name.

closedir DIRHANDLE

Closes a directory opened by opendir().

connect SOCKET,NAME

Attempts to connect to a remote socket, just as the connect 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.

continue BLOCK

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 conditional 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).

cos EXPR

Returns the cosine of EXPR (expressed in radians). If EXPR is omitted takes cosine of $_.

For the inverse cosine operation, you may use the POSIX::acos() function, or use this relation:

    sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
crypt PLAINTEXT,SALT

Encrypts a 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 munition). This can prove useful for checking the password file for lousy passwords, amongst other things. Only the guys wearing white hats should do this.

Note that crypt is intended to be a one-way function, much like breaking eggs to make an omelette. There is no (known) corresponding decrypt function. As a result, this function isn't all that useful for cryptography. (For that, see your nearby CPAN mirror.)

Here's an example that makes sure that whoever runs this program knows their own password:

    $pwd = (getpwuid($<))[1];
    $salt = substr($pwd, 0, 2);

    system "stty -echo";
    print "Password: ";
    chop($word = <STDIN>);
    print "\n";
    system "stty echo";

    if (crypt($word, $salt) ne $pwd) {
        die "Sorry...\n";
    } else {
        print "ok\n";
    }

Of course, typing in your own password to whomever asks you for it is unwise.

dbmclose HASH

[This function has been superseded by the untie() function.]

Breaks the binding between a DBM file and a hash.

dbmopen HASH,DBNAME,MODE

[This function has been superseded by the tie() function.]

This binds a dbm(3), ndbm(3), sdbm(3), gdbm(), 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 MODE (as modified by the umask()). If your system supports only the older DBM functions, you may perform only one dbmopen() in your program. In older versions of Perl, if your system had neither DBM nor ndbm, calling dbmopen() produced a fatal error; it now falls back to sdbm(3).

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 array values 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.

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() to check whether a subroutine exists. On the other hand, use of defined() upon aggregates (hashes and arrays) is not guaranteed to produce intuitive results, and should probably be avoided.

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". But it didn't really match nothing--rather, it matched something that happened to be 0 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.

Currently, using defined() on an entire array or hash reports whether memory for that aggregate has ever been allocated. So an array you set to the empty list appears undefined initially, and one that once was full and that you then set to the empty list still appears defined. 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"   }

Using undef() on these, however, does clear their memory and then report them as not defined anymore, but you shoudln't do that unless you don't plan to use them again, because it saves time when you load them up again to have memory already ready to be filled.

This counterintuitive behaviour of defined() on aggregates may be changed, fixed, or broken in a future release of Perl.

See also "undef", "exists", "ref".

delete EXPR

Deletes the specified key(s) and their associated values from a hash. For each key, returns the deleted value associated with that key, or the undefined value if there was no such key. Deleting from $ENV{} modifies the environment. Deleting from a hash tied to a DBM file deletes the entry from the DBM file. (But deleting from a tie()d hash doesn't necessarily return anything.)

The following deletes all the values of a hash:

    foreach $key (keys %HASH) {
        delete $HASH{$key};
    }

And so does this:

    delete @HASH{keys %HASH}

(But both of these are slower than the undef() command.) Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash element lookup or hash slice:

    delete $ref->[$x][$y]{$key};
    delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
die LIST

Outside of 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 value of EXPR does not end in a newline, the current script line number and input line number (if any) are also printed, and a newline is supplied. 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() and warn().

You can arrange for a callback to be called 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 perlvar for details on setting %SIG entries, and eval() for some examples.

do BLOCK

Not really a function. Returns the value of the last command in the sequence of commands indicated by BLOCK. When modified by a loop modifier, executes the BLOCK once before testing the loop condition. (On other statements the loop modifiers test the conditional first.)

do SUBROUTINE(LIST)

A deprecated form of subroutine call. See perlsub.

do EXPR

Uses the value of EXPR as a filename and executes the contents of the file as a Perl script. Its primary use is to include subroutines from a Perl subroutine library.

    do 'stat.pl';

is just like

    eval `cat stat.pl`;

except that it's more efficient, more concise, keeps track of the current filename for error messages, and searches all the -I libraries if the file isn't in the current directory (see also the @INC array in "Predefined Names" in perlvar). 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.

Note that inclusion of library modules is better done with the use() and require() operators, which also do error checking and raise an exception if there's a problem.

dump LABEL

This causes an immediate core dump. Primarily this is so that you can use the undump program 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 reincarnation. 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 reincarnated, with possible resulting confusion on the part of Perl. See also -u option in perlrun.

Example:

    #!/usr/bin/perl
    require 'getopt.pl';
    require 'stat.pl';
    %days = (
        'Sun' => 1,
        'Mon' => 2,
        'Tue' => 3,
        'Wed' => 4,
        'Thu' => 5,
        'Fri' => 6,
        'Sat' => 7,
    );

    dump QUICKSTART if $ARGV[0] eq '-d';

    QUICKSTART:
    Getopt('f');
each HASH

When called in a list context, returns a 2-element array consisting of the key and value for the next element of a hash, so that you can iterate over it. When called in a scalar context, returns the key for only the next element in the hash. (Note: Keys may be "0" or "", which are logically false; you may wish to avoid constructs like while ($k = each %foo) {} for this reason.)

Entries are returned in an apparently random order. When the hash is entirely read, a null array is returned in list context (which when assigned produces a FALSE (0) value), and undef is returned in a 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.

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() and values().

eof FILEHANDLE
eof ()
eof

Returns 1 if the next read on FILEHANDLE will return end of file, or if FILEHANDLE is not open. FILEHANDLE may be an expression whose value gives the real filehandle name. (Note that this function actually reads a character and then ungetc()s it, so it is not 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. Filetypes such as terminals may lose the end-of-file condition if you do.

An eof without an argument uses the last file read as argument. Empty parentheses () may be used to indicate the pseudo file formed of the files listed on the command line, i.e., eof() is reasonable to use inside a while (<>) loop to detect the end of only the last file. Use eof(ARGV) or eof without the parentheses to test EACH file in a while (<>) loop. Examples:

    # reset line numbering on each input file
    while (<>) {
        print "$.\t$_";
        close(ARGV) if (eof);   # Not eof().
    }

    # insert dashes just before last line of last file
    while (<>) {
        if (eof()) {
            print "--------------\n";
            close(ARGV);        # close or break; is needed if we
                                # are reading from the terminal
        }
        print;
    }

Practical hint: you almost never need to use eof in Perl, because the input operators return undef when they run out of data.

eval EXPR
eval BLOCK

EXPR is parsed and executed as if it were a little Perl program. It is executed in the context of the current Perl program, so that any variable settings or subroutine and format definitions remain afterwards. The value returned is the value of the last expression evaluated, or a return statement may be used, just as with subroutines. The last expression is evaluated in scalar or array context, depending on the context of the eval.

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. If EXPR is omitted, evaluates $_. The final semicolon, if any, may be omitted from the expression. 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. See warn() and perlvar.

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 = };

    # a run-time error
    eval '$answer =';   # sets $@

When using the eval{} form as an exception trap in libraries, 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 foofs here" };
       print $@ if $@;                # prints "bar barfs here"
    }

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.

exec LIST

The exec() function executes a system command AND NEVER RETURNS, unless the command does not exist and is executed directly instead of via /bin/sh -c (see below). Use system() instead of exec() if you want it to return.

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, the argument is checked for shell metacharacters. If there are any, the entire argument is passed to /bin/sh -c for parsing. If there are none, the argument is split into words and passed directly to execvp(), which is more efficient. Note: exec() and system() do not flush your output buffer, so you may need to set $| to avoid lost output. 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
exists EXPR

Returns TRUE if the specified hash key exists in its hash array, even if the corresponding value is undefined.

    print "Exists\n" if exists $array{$key};
    print "Defined\n" if defined $array{$key};
    print "True\n" if $array{$key};

A hash element can be TRUE only if it's defined, and defined if it exists, but the reverse doesn't necessarily hold true.

Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash key lookup:

    if (exists $ref->[$x][$y]{$key}) { ... }
exit EXPR

Evaluates EXPR and exits immediately with that value. (Actually, it calls any defined END routines first, but the END routines may not abort the exit. Likewise any object destructors that need to be called are called before exit.) Example:

    $ans = <STDIN>;
    exit 0 if $ans =~ /^[Xx]/;

See also die(). If EXPR is omitted, exits with 0 status. The only universally portable values for EXPR are 0 for success and 1 for error; all other values are subject to unpredictable interpretation depending on the environment in which the Perl program is running.

You shouldn'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().

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 function definitions. Argument processing and value return works just like ioctl() below. Note that fcntl() will produce a fatal error if used on a machine that doesn't implement fcntl(2). For example:

    use Fcntl;
    fcntl($filehandle, F_GETLK, $packed_return_buffer);
fileno FILEHANDLE

Returns the file descriptor for a filehandle. This is useful for constructing bitmaps for select(). If FILEHANDLE is an expression, the value is taken as the name of the filehandle.

flock FILEHANDLE,OPERATION

Calls flock(2), or an emulation of it, on FILEHANDLE. Returns TRUE for success, FALSE on failure. Produces a fatal error if used on a machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3). flock() is Perl's portable file locking interface, although it locks only entire files, not records.

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 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 added to 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 mis-coordination, Perl flushes FILEHANDLE before (un)locking 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 (all?) systems implement lockf(3) in terms of fcntl(2) locking, though, so the differing semantics shouldn't bite too many people.

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 Configure 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();

See also DB_File for other flock() examples.

fork

Does a fork(2) system call. Returns the child pid to the parent process and 0 to the child process, or undef if the fork is unsuccessful. Note: unflushed buffers remain unflushed in both processes, which means you may need to set $| ($AUTOFLUSH in English) or call the autoflush() method of IO::Handle to avoid duplicate output.

If you fork() without ever waiting on your children, you will accumulate zombies:

    $SIG{CHLD} = sub { wait };

There's also the double-fork trick (error checking on fork() returns omitted);

    unless ($pid = fork) {
        unless (fork) {
            exec "what you really wanna do";
            die "no exec";
            # ... or ...
            ## (some_perl_code_here)
            exit 0;
        }
        exit 0;
    }
    waitpid($pid,0);

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, the remote server (such as, say, httpd or rsh) won't think you're done. You should reopen those to /dev/null if it's any issue.

format

Declare a picture format with use by the write() function. For example:

    format Something =
        Test: @<<<<<<<< @||||| @>>>>>
              $str,     $%,    '$' . int($num)
    .

    $str = "widget";
    $num = $cost/$quantity;
    $~ = 'Something';
    write;

See perlform for many details and examples.

formline PICTURE,LIST

This is an internal function used by formats, though you may call it too. It formats (see perlform) a list of values according to the contents of PICTURE, placing the output into the format output accumulator, $^A (or $ACCUMULATOR in English). Eventually, when a write() is done, the contents of $^A are written to some filehandle, but you could also read $^A yourself and then set $^A back to "". Note that a format typically does one formline() per line of form, but the formline() function itself doesn't care how many newlines are embedded in the PICTURE. 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 a null string at end of file. If FILEHANDLE is omitted, reads from STDIN. This is not particularly efficient. It cannot be used to get unbuffered single-characters, however. 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 alleging POSIX compliance. See also the Term::ReadKey module from your nearest CPAN site; details on CPAN can be found on "CPAN" in perlmod.

getlogin

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, $iaddr) = unpack_sockaddr_in($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.

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. Within a list context, the return values from the various get routines are as follows:

    ($name,$passwd,$uid,$gid,
       $quota,$comment,$gcos,$dir,$shell) = getpw*
    ($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.)

Within a 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 = getpwuid
    $name = getpwent
    $gid = getgrnam
    $name = getgrgid
    $name = getgrent
    etc.

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]);
getsockname SOCKET

Returns the packed sockaddr address of this end of the SOCKET connection.

    use Socket;
    $mysockaddr = getsockname(SOCK);
    ($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
getsockopt SOCKET,LEVEL,OPTNAME

Returns the socket option requested, or undefined if there is an error.

glob EXPR
glob

Returns the value of EXPR with filename expansions such as a shell would do. This is the internal function implementing the <*.c> operator, 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.

gmtime EXPR

Converts a time as returned by the time function to a 9-element array 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 array elements are numeric, and come straight out of a struct tm. In particular this means that $mon has the range 0..11 and $wday has the range 0..6 with sunday as day 0. Also, $year is the number of years since 1900, not simply the last two digits of the year.

If EXPR is omitted, does gmtime(time()).

In a scalar context, returns the ctime(3) value:

    $now_string = gmtime;  # e.g., "Thu Oct 13 04:54:34 1994"

Also see the timegm() function provided by the Time::Local module, and the strftime(3) function available via the POSIX module.

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 goto-EXPR form expects a label name, whose scope will be resolved dynamically. This allows for computed gotos per FORTRAN, but isn't necessarily recommended if you're optimizing for maintainability:

    goto ("FOO", "BAR", "GLARCH")[$i];

The goto-&NAME form is highly magical, and substitutes a call to the named subroutine for the currently running subroutine. 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.

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 a 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, because $_ is a reference into the list value, it can be used to modify the elements of the array. While this is useful and supported, it can cause bizarre results if the LIST is not a named array. Similarly, grep returns aliases into the original list, much like the way that "Foreach Loops"'s index variable aliases the list elements. That is, modifying an element of a list returned by grep actually modifies the element in the original list.

hex EXPR
hex

Interprets EXPR as a hex string and returns the corresponding value. (To convert strings that might start with either 0 or 0x see "oct".) If EXPR is omitted, uses $_.

    print hex '0xAf'; # prints '175'
    print hex 'aF';   # same
import

There is no builtin import() function. It is merely 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

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. The return value is based at 0 (or whatever you've set the $[ variable to--but don't do that). If the substring is not found, returns one less than the base, ordinarily -1.

int EXPR
int

Returns the integer portion of EXPR. If EXPR is omitted, uses $_.

ioctl FILEHANDLE,FUNCTION,SCALAR

Implements the ioctl(2) function. You'll probably have to say

    require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph

first to get the correct function definitions. If ioctl.ph doesn't exist or doesn't have the correct definitions you'll have to roll your own, based on your C header files such as <sys/ioctl.h>. (There is a Perl script called h2ph that comes with the Perl kit which 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 are useful for manipulating the values of structures used by ioctl(). The following example sets the erase character to DEL.

    require 'ioctl.ph';
    $getp = &TIOCGETP;
    die "NO TIOCGETP" if $@ || !$getp;
    $sgttyb_t = "ccccs";                # 4 chars and a short
    if (ioctl(STDIN,$getp,$sgttyb)) {
        @ary = unpack($sgttyb_t,$sgttyb);
        $ary[2] = 127;
        $sgttyb = pack($sgttyb_t,@ary);
        ioctl(STDIN,&TIOCSETP,$sgttyb)
            || die "Can't 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:

    ($retval = ioctl(...)) || ($retval = -1);
    printf "System returned %d\n", $retval;
join EXPR,LIST

Joins the separate strings of LIST into a single string with fields separated by the value of EXPR, and returns the string. Example:

    $_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);

See "split" in perlfunc.

keys HASH

Returns a normal array consisting of all the keys of the named hash. (In a scalar context, returns the number of keys.) The keys are returned in an apparently random order, but it is the same order as either the values() or each() function produces (given that the hash has not been modified). As a side effect, it resets HASH's iterator.

Here is yet another way to print your environment:

    @keys = keys %ENV;
    @values = values %ENV;
    while ($#keys >= 0) {
        print pop(@keys), '=', pop(@values), "\n";
    }

or how about sorted by key:

    foreach $key (sort(keys %ENV)) {
        print $key, '=', $ENV{$key}, "\n";
    }

To sort an array 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. 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 allocated for the hash using keys in this way (but you needn't worry about doing this by accident, as trying has no effect).

kill LIST

Sends a signal to a list of processes. The first element of the list must be the signal to send. Returns the number of processes successfully signaled.

    $cnt = kill 1, $child1, $child2;
    kill 9, @goners;

Unlike in the shell, in Perl if the 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 usually want to use positive not negative signals. You may also use a signal name in quotes. See "Signals" in perlipc for 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 continue block, if any, is not executed:

    LINE: while (<STDIN>) {
        last LINE if /^$/;      # exit when done with header
        ...
    }
lc EXPR
lc

Returns an lowercased version of EXPR. This is the internal function implementing the \L escape in double-quoted strings. Respects current LC_CTYPE locale if use locale in force. See perllocale.

If EXPR is omitted, uses $_.

lcfirst EXPR
lcfirst

Returns the value of EXPR with the first character lowercased. This is the internal function implementing the \l escape in double-quoted strings. Respects current LC_CTYPE locale if use locale in force. See perllocale.

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 $_.

link OLDFILE,NEWFILE

Creates a new filename linked to the old filename. Returns 1 for success, 0 otherwise.

listen SOCKET,QUEUESIZE

Does the same thing that the listen system call does. Returns TRUE if it succeeded, FALSE otherwise. See example in "Sockets: Client/Server Communication" in perlipc.

local EXPR

A local modifies the listed variables to be local to the enclosing block, subroutine, eval{}, or do. If more than one value is listed, the list must be placed in parentheses. See "Temporary Values via local()" in perlsub for details.

But 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.

localtime EXPR

Converts a time as returned by the time function to a 9-element array 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 array elements are numeric, and come straight out of a struct tm. In particular this means that $mon has the range 0..11 and $wday has the range 0..6 with sunday as day 0. Also, $year is the number of years since 1900, that is, $year is 123 in year 2023.

If EXPR is omitted, uses the current time (localtime(time)).

In a scalar context, returns the ctime(3) value:

    $now_string = localtime;  # e.g., "Thu Oct 13 04:54:34 1994"

Also see the Time::Local module, and the strftime(3) and mktime(3) function available via the POSIX module.

log EXPR
log

Returns logarithm (base e) of EXPR. If EXPR is omitted, returns log of $_.

lstat FILEHANDLE
lstat EXPR
lstat

Does the same thing as the stat() function, but stats a symbolic link instead of the file the symbolic link points to. If symbolic links are unimplemented on your system, a normal stat() is done.

If EXPR is omitted, stats $_.

m//

The match operator. See perlop.

map BLOCK LIST
map EXPR,LIST

Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each element) and returns the list value composed of the results of each such evaluation. Evaluates BLOCK or EXPR in a list context, so each element of LIST may produce zero, one, or more elements in the returned value.

    @chars = map(chr, @nums);

translates a list of numbers to the corresponding characters. And

    %hash = map { getkey($_) => $_ } @array;

is just a funny way to write

    %hash = ();
    foreach $_ (@array) {
        $hash{getkey($_)} = $_;
    }
mkdir FILENAME,MODE

Creates the directory specified by FILENAME, with permissions specified by MODE (as modified by umask). If it succeeds it returns 1, otherwise it returns 0 and sets $! (errno).

msgctl ID,CMD,ARG

Calls the System V IPC function msgctl(2). If CMD is &IPC_STAT, then ARG must be a variable which will hold the returned msqid_ds structure. Returns like ioctl: the undefined value for error, "0 but true" for zero, or the actual return value otherwise.

msgget KEY,FLAGS

Calls the System V IPC function msgget(2). Returns the message queue id, or the undefined value if there is an error.

msgsnd ID,MSG,FLAGS

Calls the System V IPC function msgsnd to send the message MSG to the message queue ID. MSG must begin with the long integer message type, which may be created with pack("l", $type). Returns TRUE if successful, or FALSE if there is an error.

msgrcv ID,VAR,SIZE,TYPE,FLAGS

Calls the System V IPC function msgrcv to receive a message from message queue ID into variable VAR with a maximum message size of SIZE. Note that if a message is received, the message type will be the first thing in VAR, and the maximum length of VAR is SIZE plus the size of the message type. Returns TRUE if successful, or FALSE if there is an error.

my EXPR

A "my" declares the listed variables to be local (lexically) to the enclosing block, subroutine, eval, or do/require/use'd file. If more than one value is listed, the list must be placed in parentheses. See "Private Variables via my()" in perlsub for details.

next LABEL
next

The next command is like the continue statement in C; it starts the next iteration of the loop:

    LINE: while (<STDIN>) {
        next LINE if /^#/;      # discard comments
        ...
    }

Note that if there were a continue block on the above, it would get executed even on discarded lines. If the LABEL is omitted, the command refers to the innermost enclosing loop.

no Module LIST

See the "use" function, which "no" is the opposite of.

oct EXPR
oct

Interprets EXPR as an octal string and returns the corresponding value. (If EXPR happens to start off with 0x, interprets it as a hex string instead.) The following will handle decimal, octal, and hex in the standard Perl or C notation:

    $val = oct($val) if $val =~ /^0/;

If EXPR is omitted, uses $_. This function is commonly used when a string such as "644" needs to be converted into a file mode, for example. (Although perl will automatically convert strings into numbers as needed, this automatic conversion assumes base 10.)

open FILEHANDLE,EXPR
open FILEHANDLE

Opens the file whose filename is given by EXPR, and associates it with FILEHANDLE. If FILEHANDLE is an expression, its value is used as the name of the real filehandle wanted. If EXPR is omitted, the scalar variable of the same name as the FILEHANDLE contains the filename. (Note that lexical variables--those declared with my--will not work for this purpose; so if you're using my, specify EXPR in your call to open.)

If the filename begins with '<' or nothing, the file is opened for input. If the filename begins with '>', the file is truncated and opened for output. If the filename begins with '>>', the file is opened for appending. You can put a '+' in front of the '>' or '<' to indicate that you want both read and write access to the file; thus '+<' is almost always preferred for read/write updates--the '+>' mode would clobber the file first. The prefix and the filename may be separated with spaces. These various prefixes correspond to the fopen(3) modes of 'r', 'r+', 'w', 'w+', 'a', and 'a+'.

If the filename begins with "|", the filename is interpreted as a command to which output is to be piped, and if the filename ends with a "|", the filename is interpreted See "Using open() for IPC" in perlipc for more examples of this. as command which pipes input to us. (You may not have a raw open() to a command that pipes both in and out, but see IPC::Open2, IPC::Open3, and "Bidirectional Communication" in perlipc for alternatives.)

Opening '-' opens STDIN and opening '>-' opens STDOUT. Open returns nonzero upon success, the undefined value otherwise. If the open involved a pipe, the return value happens to be the pid of the subprocess.

If you're unfortunate enough to be running Perl on a system that distinguishes between text files and binary files (modern operating systems don't care), then you should check out "binmode" for tips for dealing with this. The key distinction between systems that need binmode and those that don't is their text file formats. Systems like Unix and Plan9 that delimit lines with a single character, and that encode that character in C as '\n', do not need binmode. The rest need it.

Examples:

    $ARTICLE = 100;
    open ARTICLE or die "Can't find article $ARTICLE: $!\n";
    while (<ARTICLE>) {...

    open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)

    open(DBASE, '+<dbase.mine');            # open for update

    open(ARTICLE, "caesar <$article |");    # decrypt article

    open(EXTRACT, "|sort >/tmp/Tmp$$");     # $$ is our process id

    # process argument list of files along with any includes

    foreach $file (@ARGV) {
        process($file, 'fh00');
    }

    sub process {
        local($filename, $input) = @_;
        $input++;               # this is a string increment
        unless (open($input, $filename)) {
            print STDERR "Can't open $filename: $!\n";
            return;
        }

        while (<$input>) {              # note use of indirection
            if (/^#include "(.*)"/) {
                process($1, $input);
                next;
            }
            ...         # whatever
        }
    }

You may also, in the Bourne shell tradition, specify an EXPR beginning with ">&", in which case the rest of the string is interpreted as the name of a filehandle (or file descriptor, if numeric) which is to be duped and opened. You may use & after >, >>, <, +>, +>>, and +<. The mode you specify should match the mode of the original filehandle. (Duping a filehandle does not take into account any existing contents of stdio buffers.) Here is a script that saves, redirects, and restores STDOUT and STDERR:

    #!/usr/bin/perl
    open(SAVEOUT, ">&STDOUT");
    open(SAVEERR, ">&STDERR");

    open(STDOUT, ">foo.out") || die "Can't redirect stdout";
    open(STDERR, ">&STDOUT") || die "Can't dup stdout";

    select(STDERR); $| = 1;     # make unbuffered
    select(STDOUT); $| = 1;     # make unbuffered

    print STDOUT "stdout 1\n";  # this works for
    print STDERR "stderr 1\n";  # subprocesses too

    close(STDOUT);
    close(STDERR);

    open(STDOUT, ">&SAVEOUT");
    open(STDERR, ">&SAVEERR");

    print STDOUT "stdout 2\n";
    print STDERR "stderr 2\n";

If you specify "<&=N", where N is a number, then Perl will do an equivalent of C's fdopen() of that file descriptor; this is more parsimonious of file descriptors. For example:

    open(FILEHANDLE, "<&=$fd")

If you open a pipe on the command "-", i.e., either "|-" or "-|", then there is an implicit fork done, and the return value of open is the pid of the child within the parent process, and 0 within the child process. (Use defined($pid) to determine whether the open was successful.) The filehandle behaves normally for the parent, but i/o to that filehandle is piped from/to the STDOUT/STDIN of the child process. In the child process the filehandle isn't opened--i/o happens from/to the new STDOUT or STDIN. Typically this is used like the normal piped open when you want to exercise more control over just how the pipe command gets executed, such as when you are running setuid, and don't want to have to scan shell commands for metacharacters. The following pairs are more or less equivalent:

    open(FOO, "|tr '[a-z]' '[A-Z]'");
    open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';

    open(FOO, "cat -n '$file'|");
    open(FOO, "-|") || exec 'cat', '-n', $file;

See "Safe Pipe Opens" in perlipc for more examples of this.

NOTE: On any operation which may do a fork, unflushed buffers remain unflushed in both processes, which means you may need to set $| to avoid duplicate output.

Closing any piped filehandle causes the parent process to wait for the child to finish, and returns the status value in $?.

Using the constructor from the IO::Handle package (or one of its subclasses, such as IO::File or IO::Socket), you can generate anonymous filehandles which have the scope of whatever variables hold references to them, and automatically close whenever and however you leave that scope:

    use IO::File;
    ...
    sub read_myfile_munged {
        my $ALL = shift;
        my $handle = new IO::File;
        open($handle, "myfile") or die "myfile: $!";
        $first = <$handle>
            or return ();     # Automatically closed here.
        mung $first or die "mung failed";       # Or here.
        return $first, <$handle> if $ALL;       # Or here.
        $first;                                 # Or here.
    }

The filename that is passed to open will have leading and trailing whitespace deleted. To open a file with arbitrary weird characters in it, it's necessary to protect any leading and trailing whitespace thusly:

    $file =~ s#^(\s)#./$1#;
    open(FOO, "< $file\0");

If you want a "real" C open() (see open(2) on your system), then you should use the sysopen() function. This is another way to protect your filenames from interpretation. For example:

    use IO::Handle;
    sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
        or die "sysopen $path: $!";
    HANDLE->autoflush(1);
    HANDLE->print("stuff $$\n");
    seek(HANDLE, 0, 0);
    print "File contains: ", <HANDLE>;

See "seek()" for some details about mixing reading and writing.

opendir DIRHANDLE,EXPR

Opens a directory named EXPR for processing by readdir(), telldir(), seekdir(), rewinddir(), and closedir(). Returns TRUE if successful. DIRHANDLEs have their own namespace separate from FILEHANDLEs.

ord EXPR
ord

Returns the numeric ascii value of the first character of EXPR. If EXPR is omitted, uses $_. For the reverse, see "chr".

pack TEMPLATE,LIST

Takes an array or list of values and packs it into a binary structure, returning the string containing the structure. The TEMPLATE is a sequence of characters that give the order and type of values, as follows:

    A   An ascii string, will be space padded.
    a   An ascii string, will be null padded.
    b   A bit string (ascending bit order, like vec()).
    B   A bit string (descending bit order).
    h   A hex string (low nybble first).
    H   A hex string (high nybble first).

    c   A signed char value.
    C   An unsigned char value.

    s   A signed short value.
    S   An unsigned short value.
          (This 'short' is _exactly_ 16 bits, which may differ from
           what a local C compiler calls 'short'.)

    i   A signed integer value.
    I   An unsigned integer value.
          (This 'integer' is _at_least_ 32 bits wide.  Its exact size
           depends on what a local C compiler calls 'int', and may
           even be larger than the 'long' described in the next item.)

    l   A signed long value.
    L   An unsigned long value.
          (This 'long' is _exactly_ 32 bits, which may differ from
           what a local C compiler calls 'long'.)

    n   A short in "network" (big-endian) order.
    N   A long in "network" (big-endian) order.
    v   A short in "VAX" (little-endian) order.
    V   A long in "VAX" (little-endian) order.
          (These 'shorts' and 'longs' are _exactly_ 16 bits and
           _exactly_ 32 bits, respectively.)

    f   A single-precision float in the native format.
    d   A double-precision float in the native format.

    p   A pointer to a null-terminated string.
    P   A pointer to a structure (fixed-length string).

    u   A uuencoded string.

    w   A BER compressed integer.  Its bytes represent an unsigned
        integer in base 128, most significant digit first, with as few
        digits as possible.  Bit eight (the high bit) is set on each
        byte except the last.

    x   A null byte.
    X   Back up a byte.
    @   Null fill to absolute position.

Each letter may optionally be followed by a number which gives a repeat count. With all types except "a", "A", "b", "B", "h", "H", and "P" the pack function will gobble up that many values from the LIST. A * for the repeat count means to use however many items are left. The "a" and "A" types gobble just one value, but pack it as a string of length count, padding with nulls or spaces as necessary. (When unpacking, "A" strips trailing spaces and nulls, but "a" does not.) Likewise, the "b" and "B" fields pack a string that many bits long. The "h" and "H" fields pack a string that many nybbles long. The "P" packs a pointer to a structure of the size indicated by the length. Real numbers (floats and doubles) are in the native machine format only; due to the multiplicity of floating formats around, and the lack of a standard "network" representation, no facility for interchange has been made. This means that packed floating point data written on one machine may not be readable on another - even if both use IEEE floating point arithmetic (as the endian-ness of the memory representation is not part of the IEEE spec). Note that Perl uses doubles internally for all numeric calculation, and converting from double into float and thence back to double again will lose precision (i.e., unpack("f", pack("f", $foo)) will not in general equal $foo).

Examples:

    $foo = pack("cccc",65,66,67,68);
    # foo eq "ABCD"
    $foo = pack("c4",65,66,67,68);
    # same thing

    $foo = pack("ccxxcc",65,66,67,68);
    # foo eq "AB\0\0CD"

    $foo = pack("s2",1,2);
    # "\1\0\2\0" on little-endian
    # "\0\1\0\2" on big-endian

    $foo = pack("a4","abcd","x","y","z");
    # "abcd"

    $foo = pack("aaaa","abcd","x","y","z");
    # "axyz"

    $foo = pack("a14","abcdefg");
    # "abcdefg\0\0\0\0\0\0\0"

    $foo = pack("i9pl", gmtime);
    # a real struct tm (on my system anyway)

    sub bintodec {
        unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
    }

The same template may generally also be used in the unpack function.

package NAMESPACE

Declares the compilation unit as being in the given namespace. The scope of the package declaration is from the declaration itself through the end of the enclosing block (the same scope as the local() operator). All further unqualified dynamic identifiers will be in this namespace. A package statement affects only dynamic variables--including those you've used local() on--but not lexical variables created with my(). Typically it would be the first declaration in a file to be included by the require or use operator. You can switch into a package in more than one place; it influences merely which symbol table is used by the compiler for the rest of that block. You can refer to variables and filehandles in other packages by prefixing the identifier with the package name and a double colon: $Package::Variable. If the package name is null, the main package as assumed. That is, $::sail is equivalent to $main::sail.

See "Packages" in perlmod for more information about packages, modules, and classes. See perlsub for other scoping issues.

pipe READHANDLE,WRITEHANDLE

Opens a pair of connected pipes like the corresponding system call. Note that if you set up a loop of piped processes, deadlock can occur unless you are very careful. In addition, note that Perl's pipes use stdio buffering, so you may need to set $| to flush your WRITEHANDLE after each command, depending on the application.

See IPC::Open2, IPC::Open3, and "Bidirectional Communication" in perlipc for examples of such things.

pop ARRAY
pop

Pops and returns the last value of the array, shortening the array by 1. Has a similar effect to

    $tmp = $ARRAY[$#ARRAY--];

If there are no elements in the array, returns the undefined value. If ARRAY is omitted, pops the @ARGV array in the main program, and the @_ array in subroutines, just like shift().

pos SCALAR
pos

Returns the offset of where the last m//g search left off for the variable is in question ($_ is used when the variable is not specified). May be modified to change that offset. Such modification will also influence the \G zero-width assertion in regular expressions. See perlre and perlop.

print FILEHANDLE LIST
print LIST
print

Prints a string or a comma-separated list of strings. Returns TRUE if successful. FILEHANDLE may be a scalar variable name, in which case the variable contains the name of or a reference to the filehandle, thus introducing one level of indirection. (NOTE: If FILEHANDLE is a variable and the next token is a term, it may be misinterpreted as an operator unless you interpose a + or put parentheses around the arguments.) If FILEHANDLE is omitted, prints by default to standard output (or to the last selected output channel--see "select"). If LIST is also omitted, prints $_ to STDOUT. To set the default output channel to something other than STDOUT use the select operation. Note that, because print takes a LIST, anything in the LIST is evaluated in a list context, and any subroutine that you call will have one or more of its expressions evaluated in a list context. Also be careful not to follow the print keyword with a left parenthesis unless you want the corresponding right parenthesis to terminate the arguments to the print--interpose a + or put parentheses around all the arguments.

Note that if you're storing FILEHANDLES in an array or other expression, you will have to use a block returning its value instead:

    print { $files[$i] } "stuff\n";
    print { $OK ? STDOUT : STDERR } "stuff\n";
printf FILEHANDLE FORMAT, LIST
printf FORMAT, LIST

Equivalent to print FILEHANDLE sprintf(FORMAT, LIST). The first argument of the list will be interpreted as the printf format. If use locale is in effect, the character used for the decimal point in formatted real numbers is affected by the LC_NUMERIC locale. See perllocale.

Don't fall into the trap of using a printf() when a simple print() would do. The print() is more efficient, and less error prone.

prototype FUNCTION

Returns the prototype of a function as a string (or undef if the function has no prototype). FUNCTION is a reference to, or the name of, the function whose prototype you want to retrieve.

push ARRAY,LIST

Treats ARRAY as a stack, and pushes the values of LIST onto the end of ARRAY. The length of ARRAY increases by the length of LIST. Has the same effect as

    for $value (LIST) {
        $ARRAY[++$#ARRAY] = $value;
    }

but is more efficient. Returns the new number of elements in the array.

q/STRING/
qq/STRING/
qx/STRING/
qw/STRING/

Generalized quotes. See perlop.

quotemeta EXPR
quotemeta

Returns the value of EXPR with all non-alphanumeric characters backslashed. (That is, all characters not matching /[A-Za-z_0-9]/ will be preceded by a backslash in the returned string, regardless of any locale settings.) This is the internal function implementing the \Q escape in double-quoted strings.

If EXPR is omitted, uses $_.

rand EXPR
rand

Returns a random fractional number between 0 and the value of EXPR. (EXPR should be positive.) If EXPR is omitted, returns a value between 0 and 1. Automatically calls srand() unless srand() has already been called. See also srand().

(Note: If your rand function consistently returns numbers that are too large or too small, then your version of Perl was probably compiled with the wrong number of RANDBITS.)

read FILEHANDLE,SCALAR,LENGTH,OFFSET
read FILEHANDLE,SCALAR,LENGTH

Attempts to read LENGTH bytes of data into variable SCALAR from the specified FILEHANDLE. Returns the number of bytes actually read, or undef if there was an error. SCALAR will be grown or shrunk to the length actually read. An OFFSET may be specified to place the read data at some other place than the beginning of the string. This call is actually implemented in terms of stdio's fread call. To get a true read system call, see sysread().

readdir DIRHANDLE

Returns the next directory entry for a directory opened by opendir(). If used in a list context, returns all the rest of the entries in the directory. If there are no more entries, returns an undefined value in a scalar context or a null list in a list context.

If you're planning to filetest the return values out of a readdir(), you'd better prepend the directory in question. Otherwise, because we didn't chdir() there, it would have been testing the wrong file.

    opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
    @dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
    closedir DIR;
readlink EXPR
readlink

Returns the value of a symbolic link, if symbolic links are implemented. If not, gives a fatal error. If there is some system error, returns the undefined value and sets $! (errno). If EXPR is omitted, uses $_.

recv SOCKET,SCALAR,LEN,FLAGS

Receives a message on a socket. Attempts to receive LENGTH bytes of data into variable SCALAR from the specified SOCKET filehandle. Actually does a C recvfrom(), so that it can returns the address of the sender. Returns the undefined value if there's an error. SCALAR will be grown or shrunk to the length actually read. Takes the same flags as the system call of the same name. See "UDP: Message Passing" in perlipc for examples.

redo LABEL
redo

The redo command restarts the loop block without evaluating the conditional again. The continue block, if any, is not executed. If the LABEL is omitted, the command refers to the innermost enclosing loop. This command is normally used by programs that want to lie to themselves about what was just input:

    # a simpleminded Pascal comment stripper
    # (warning: assumes no { or } in strings)
    LINE: while (<STDIN>) {
        while (s|({.*}.*){.*}|$1 |) {}
        s|{.*}| |;
        if (s|{.*| |) {
            $front = $_;
            while (<STDIN>) {
                if (/}/) {      # end of comment?
                    s|^|$front{|;
                    redo LINE;
                }
            }
        }
        print;
    }
ref EXPR
ref

Returns a TRUE value if EXPR is a reference, FALSE otherwise. If EXPR is not specified, $_ will be used. The value returned depends on the type of thing the reference is a reference to. Builtin types include:

    REF
    SCALAR
    ARRAY
    HASH
    CODE
    GLOB

If the referenced object has been blessed into a package, then that package name is returned instead. You can think of ref() as a typeof() operator.

    if (ref($r) eq "HASH") {
        print "r is a reference to a hash.\n";
    }
    if (!ref ($r) {
        print "r is not a reference at all.\n";
    }

See also perlref.

rename OLDNAME,NEWNAME

Changes the name of a file. Returns 1 for success, 0 otherwise. Will not work across file system boundaries.

require EXPR
require

Demands some semantics specified by EXPR, or by $_ if EXPR is not supplied. If EXPR is numeric, demands that the current version of Perl ($] or $PERL_VERSION) be equal or greater than EXPR.

Otherwise, demands that a library file be included if it hasn't already been included. The file is included via the do-FILE mechanism, which is essentially just a variety of eval(). Has semantics similar to the following subroutine:

    sub require {
        local($filename) = @_;
        return 1 if $INC{$filename};
        local($realfilename,$result);
        ITER: {
            foreach $prefix (@INC) {
                $realfilename = "$prefix/$filename";
                if (-f $realfilename) {
                    $result = do $realfilename;
                    last ITER;
                }
            }
            die "Can't find $filename in \@INC";
        }
        die $@ if $@;
        die "$filename did not return true value" unless $result;
        $INC{$filename} = $realfilename;
        $result;
    }

Note that the file will not be included twice under the same specified name. The file must return TRUE as the last statement to indicate successful execution of any initialization code, so it's customary to end such a file with "1;" unless you're sure it'll return TRUE otherwise. But it's better just to put the "1;", in case you add more statements.

If EXPR is a bareword, the require assumes a ".pm" extension and replaces "::" with "/" in the filename for you, to make it easy to load standard modules. This form of loading of modules does not risk altering your namespace.

For a yet-more-powerful import facility, see "use" and perlmod.

reset EXPR
reset

Generally used in a continue block at the end of a loop to clear variables and reset ?? searches so that they work again. The expression is interpreted as a list of single characters (hyphens allowed for ranges). All variables and arrays beginning with one of those letters are reset to their pristine state. If the expression is omitted, one-match searches (?pattern?) are reset to match again. Resets only variables or searches in the current package. Always returns 1. Examples:

    reset 'X';          # reset all X variables
    reset 'a-z';        # reset lower case variables
    reset;              # just reset ?? searches

Resetting "A-Z" is not recommended because you'll wipe out your ARGV and ENV arrays. Resets only package variables--lexical variables are unaffected, but they clean themselves up on scope exit anyway, so you'll probably want to use them instead. See "my".

return EXPR
return

Returns from a subroutine, eval(), or do FILE with the value of the given EXPR. Evaluation of EXPR may be in a list, scalar, or void context, depending on how the return value will be used, and the context may vary from one execution to the next (see wantarray()). If no EXPR is given, returns an empty list in a list context, an undefined value in a scalar context, or nothing in a void context.

(Note that in the absence of a return, a subroutine, eval, or do FILE will automatically return the value of the last expression evaluated.)

reverse LIST

In a list context, returns a list value consisting of the elements of LIST in the opposite order. In a scalar context, concatenates the elements of LIST, and returns a string value consisting of those bytes, but in the opposite order.

    print reverse <>;           # line tac, last line first

    undef $/;                   # for efficiency of <>
    print scalar reverse <>;    # byte tac, last line tsrif

This operator is also handy for inverting a hash, although there are some caveats. If a value is duplicated in the original hash, only one of those can be represented as a key in the inverted hash. Also, this has to unwind one hash and build a whole new one, which may take some time on a large hash.

    %by_name = reverse %by_address;     # Invert the hash
rewinddir DIRHANDLE

Sets the current position to the beginning of the directory for the readdir() routine on DIRHANDLE.

rindex STR,SUBSTR,POSITION
rindex STR,SUBSTR

Works just like index except that it returns the position of the LAST occurrence of SUBSTR in STR. If POSITION is specified, returns the last occurrence at or before that position.

rmdir FILENAME
rmdir

Deletes the directory specified by FILENAME if it is empty. If it succeeds it returns 1, otherwise it returns 0 and sets $! (errno). If FILENAME is omitted, uses $_.

s///

The substitution operator. See perlop.

scalar EXPR

Forces EXPR to be interpreted in a scalar context and returns the value of EXPR.

    @counts = ( scalar @a, scalar @b, scalar @c );

There is no equivalent operator to force an expression to be interpolated in a list context because it's in practice never needed. If you really wanted to do so, however, you could use the construction @{[ (some expression) ]}, but usually a simple (some expression) suffices.

seek FILEHANDLE,POSITION,WHENCE

Sets FILEHANDLE's position, just like the fseek() call of stdio. FILEHANDLE may be an expression whose value gives the name of the filehandle. The values for WHENCE are 0 to set the new position to POSITION, 1 to set it to the current position plus POSITION, and 2 to set it to EOF plus POSITION (typically negative). For WHENCE you may use the constants SEEK_SET, SEEK_CUR, and SEEK_END from either the IO::Seekable or the POSIX module. Returns 1 upon success, 0 otherwise.

If you want to position file for sysread() or syswrite(), don't use seek() -- buffering makes its effect on the file's system position unpredictable and non-portable. Use sysseek() instead.

On some systems you have to do a seek whenever you switch between reading and writing. Amongst other things, this may have the effect of calling stdio's clearerr(3). A WHENCE of 1 (SEEK_CUR) is useful for not moving the file position:

    seek(TEST,0,1);

This is also useful for applications emulating tail -f. Once you hit EOF on your read, and then sleep for a while, you might have to stick in a seek() to reset things. The seek() doesn't change the current position, but it does clear the end-of-file condition on the handle, so that the next <FILE> makes Perl try again to read something. We hope.

If that doesn't work (some stdios are particularly cantankerous), then you may need something more like this:

    for (;;) {
        for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
            # search for some stuff and put it into files
        }
        sleep($for_a_while);
        seek(FILE, $curpos, 0);
    }
seekdir DIRHANDLE,POS

Sets the current position for the readdir() routine on DIRHANDLE. POS must be a value returned by telldir(). Has the same caveats about possible directory compaction as the corresponding system library routine.

select FILEHANDLE
select

Returns the currently selected filehandle. Sets the current default filehandle for output, if FILEHANDLE is supplied. This has two effects: first, a write or a print without a filehandle will default to this FILEHANDLE. Second, references to variables related to output will refer to this output channel. For example, if you have to set the top of form format for more than one output channel, you might do the following:

    select(REPORT1);
    $^ = 'report1_top';
    select(REPORT2);
    $^ = 'report2_top';

FILEHANDLE may be an expression whose value gives the name of the actual filehandle. Thus:

    $oldfh = select(STDERR); $| = 1; select($oldfh);

Some programmers may prefer to think of filehandles as objects with methods, preferring to write the last example as:

    use IO::Handle;
    STDERR->autoflush(1);
select RBITS,WBITS,EBITS,TIMEOUT

This calls the select(2) system call with the bit masks specified, which can be constructed using fileno() and vec(), along these lines:

    $rin = $win = $ein = '';
    vec($rin,fileno(STDIN),1) = 1;
    vec($win,fileno(STDOUT),1) = 1;
    $ein = $rin | $win;

If you want to select on many filehandles you might wish to write a subroutine:

    sub fhbits {
        local(@fhlist) = split(' ',$_[0]);
        local($bits);
        for (@fhlist) {
            vec($bits,fileno($_),1) = 1;
        }
        $bits;
    }
    $rin = fhbits('STDIN TTY SOCK');

The usual idiom is:

    ($nfound,$timeleft) =
      select($rout=$rin, $wout=$win, $eout=$ein, $timeout);

or to block until something becomes ready just do this

    $nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);

Most systems do not bother to return anything useful in $timeleft, so calling select() in a scalar context just returns $nfound.

Any of the bit masks can also be undef. The timeout, if specified, is in seconds, which may be fractional. Note: not all implementations are capable of returning the $timeleft. If not, they always return $timeleft equal to the supplied $timeout.

You can effect a sleep of 250 milliseconds this way:

    select(undef, undef, undef, 0.25);

WARNING: Do not attempt to mix buffered I/O (like read() or <FH>) with select(). You have to use sysread() instead.

semctl ID,SEMNUM,CMD,ARG

Calls the System V IPC function semctl. If CMD is &IPC_STAT or &GETALL, then ARG must be a variable which will hold the returned semid_ds structure or semaphore value array. Returns like ioctl: the undefined value for error, "0 but true" for zero, or the actual return value otherwise.

semget KEY,NSEMS,FLAGS

Calls the System V IPC function semget. Returns the semaphore id, or the undefined value if there is an error.

semop KEY,OPSTRING

Calls the System V IPC function semop to perform semaphore operations such as signaling and waiting. OPSTRING must be a packed array of semop structures. Each semop structure can be generated with pack("sss", $semnum, $semop, $semflag). The number of semaphore operations is implied by the length of OPSTRING. Returns TRUE if successful, or FALSE if there is an error. As an example, the following code waits on semaphore $semnum of semaphore id $semid:

    $semop = pack("sss", $semnum, -1, 0);
    die "Semaphore trouble: $!\n" unless semop($semid, $semop);

To signal the semaphore, replace "-1" with "1".

send SOCKET,MSG,FLAGS,TO
send SOCKET,MSG,FLAGS

Sends a message on a socket. Takes the same flags as the system call of the same name. On unconnected sockets you must specify a destination to send TO, in which case it does a C sendto(). Returns the number of characters sent, or the undefined value if there is an error. See "UDP: Message Passing" in perlipc for examples.

setpgrp PID,PGRP

Sets the current process group for the specified PID, 0 for the current process. Will produce a fatal error if used on a machine that doesn't implement setpgrp(2). If the arguments are omitted, it defaults to 0,0. Note that the POSIX version of setpgrp() does not accept any arguments, so only setpgrp 0,0 is portable.

setpriority WHICH,WHO,PRIORITY

Sets the current priority for a process, a process group, or a user. (See setpriority(2).) Will produce a fatal error if used on a machine that doesn't implement setpriority(2).

setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL

Sets the socket option requested. Returns undefined if there is an error. OPTVAL may be specified as undef if you don't want to pass an argument.

shift ARRAY
shift

Shifts the first value of the array off and returns it, shortening the array by 1 and moving everything down. If there are no elements in the array, returns the undefined value. If ARRAY is omitted, shifts the @ARGV array in the main program, and the @_ array in subroutines. (This is determined lexically.) See also unshift(), push(), and pop(). Shift() and unshift() do the same thing to the left end of an array that pop() and push() do to the right end.

shmctl ID,CMD,ARG

Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG must be a variable which will hold the returned shmid_ds structure. Returns like ioctl: the undefined value for error, "0 but true" for zero, or the actual return value otherwise.

shmget KEY,SIZE,FLAGS

Calls the System V IPC function shmget. Returns the shared memory segment id, or the undefined value if there is an error.

shmread ID,VAR,POS,SIZE
shmwrite ID,STRING,POS,SIZE

Reads or writes the System V shared memory segment ID starting at position POS for size SIZE by attaching to it, copying in/out, and detaching from it. When reading, VAR must be a variable which will hold the data read. When writing, if STRING is too long, only SIZE bytes are used; if STRING is too short, nulls are written to fill out SIZE bytes. Return TRUE if successful, or FALSE if there is an error.

shutdown SOCKET,HOW

Shuts down a socket connection in the manner indicated by HOW, which has the same interpretation as in the system call of the same name.

sin EXPR
sin

Returns the sine of EXPR (expressed in radians). If EXPR is omitted, returns sine of $_.

For the inverse sine operation, you may use the POSIX::asin() function, or use this relation:

    sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
sleep EXPR
sleep

Causes the script to sleep for EXPR seconds, or forever if no EXPR. May be interrupted by sending the process a SIGALRM. Returns the number of seconds actually slept. You probably cannot mix alarm() and sleep() calls, because sleep() is often implemented using alarm().

On some older systems, it may sleep up to a full second less than what you requested, depending on how it counts seconds. Most modern systems always sleep the full amount.

For delays of finer granularity than one second, you may use Perl's syscall() interface to access setitimer(2) if your system supports it, or else see "select()" below.

See also the POSIX module's sigpause() function.

socket SOCKET,DOMAIN,TYPE,PROTOCOL

Opens a socket of the specified kind and attaches it to filehandle SOCKET. DOMAIN, TYPE, and PROTOCOL are specified the same as for the system call of the same name. You should "use Socket;" first to get the proper definitions imported. See the example in "Sockets: Client/Server Communication" in perlipc.

socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL

Creates an unnamed pair of sockets in the specified domain, of the specified type. DOMAIN, TYPE, and PROTOCOL are specified the same as for the system call of the same name. If unimplemented, yields a fatal error. Returns TRUE if successful.

sort SUBNAME LIST
sort BLOCK LIST
sort LIST

Sorts the LIST and returns the sorted list value. If SUBNAME or BLOCK is omitted, sorts in standard string comparison order. If SUBNAME is specified, it gives the name of a subroutine that returns an integer less than, equal to, or greater than 0, depending on how the elements of the array are to be ordered. (The <=> and cmp operators are extremely useful in such routines.) SUBNAME may be a scalar variable name, in which case the value provides the name of the subroutine to use. In place of a SUBNAME, you can provide a BLOCK as an anonymous, in-line sort subroutine.

In the interests of efficiency the normal calling code for subroutines is bypassed, with the following effects: the subroutine may not be a recursive subroutine, and the two elements to be compared are passed into the subroutine not via @_ but as the package global variables $a and $b (see example below). They are passed by reference, so don't modify $a and $b. And don't try to declare them as lexicals either.

You also cannot exit out of the sort block or subroutine using any of the loop control operators described in perlsyn or with goto().

When use locale is in effect, sort LIST sorts LIST according to the current collation locale. See perllocale.

Examples:

    # sort lexically
    @articles = sort @files;

    # same thing, but with explicit sort routine
    @articles = sort {$a cmp $b} @files;

    # now case-insensitively
    @articles = sort {uc($a) cmp uc($b)} @files;

    # same thing in reversed order
    @articles = sort {$b cmp $a} @files;

    # sort numerically ascending
    @articles = sort {$a <=> $b} @files;

    # sort numerically descending
    @articles = sort {$b <=> $a} @files;

    # sort using explicit subroutine name
    sub byage {
        $age{$a} <=> $age{$b};  # presuming numeric
    }
    @sortedclass = sort byage @class;

    # this sorts the %age hash by value instead of key
    # using an in-line function
    @eldest = sort { $age{$b} <=> $age{$a} } keys %age;

    sub backwards { $b cmp $a; }
    @harry = ('dog','cat','x','Cain','Abel');
    @george = ('gone','chased','yz','Punished','Axed');
    print sort @harry;
            # prints AbelCaincatdogx
    print sort backwards @harry;
            # prints xdogcatCainAbel
    print sort @george, 'to', @harry;
            # prints AbelAxedCainPunishedcatchaseddoggonetoxyz

    # inefficiently sort by descending numeric compare using
    # the first integer after the first = sign, or the
    # whole record case-insensitively otherwise

    @new = sort {
        ($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
                            ||
                    uc($a)  cmp  uc($b)
    } @old;

    # same thing, but much more efficiently;
    # we'll build auxiliary indices instead
    # for speed
    @nums = @caps = ();
    for (@old) {
        push @nums, /=(\d+)/;
        push @caps, uc($_);
    }

    @new = @old[ sort {
                        $nums[$b] <=> $nums[$a]
                                 ||
                        $caps[$a] cmp $caps[$b]
                       } 0..$#old
               ];

    # same thing using a Schwartzian Transform (no temps)
    @new = map { $_->[0] }
        sort { $b->[1] <=> $a->[1]
                        ||
               $a->[2] cmp $b->[2]
        } map { [$_, /=(\d+)/, uc($_)] } @old;

If you're using strict, you MUST NOT declare $a and $b as lexicals. They are package globals. That means if you're in the main package, it's

    @articles = sort {$main::b <=> $main::a} @files;

or just

    @articles = sort {$::b <=> $::a} @files;

but if you're in the FooPack package, it's

    @articles = sort {$FooPack::b <=> $FooPack::a} @files;

The comparison function is required to behave. If it returns inconsistent results (sometimes saying $x[1] is less than $x[2] and sometimes saying the opposite, for example) the Perl interpreter will probably crash and dump core. This is entirely due to and dependent upon your system's qsort(3) library routine; this routine often avoids sanity checks in the interest of speed.

splice ARRAY,OFFSET,LENGTH,LIST
splice ARRAY,OFFSET,LENGTH
splice ARRAY,OFFSET

Removes the elements designated by OFFSET and LENGTH from an array, and replaces them with the elements of LIST, if any. Returns the elements removed from the array. The array grows or shrinks as necessary. If LENGTH is omitted, removes everything from OFFSET onward. The following equivalences hold (assuming $[ == 0):

    push(@a,$x,$y)      splice(@a,$#a+1,0,$x,$y)
    pop(@a)             splice(@a,-1)
    shift(@a)           splice(@a,0,1)
    unshift(@a,$x,$y)   splice(@a,0,0,$x,$y)
    $a[$x] = $y         splice(@a,$x,1,$y);

Example, assuming array lengths are passed before arrays:

    sub aeq {   # compare two list values
        local(@a) = splice(@_,0,shift);
        local(@b) = splice(@_,0,shift);
        return 0 unless @a == @b;       # same len?
        while (@a) {
            return 0 if pop(@a) ne pop(@b);
        }
        return 1;
    }
    if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
split /PATTERN/,EXPR,LIMIT
split /PATTERN/,EXPR
split /PATTERN/
split

Splits a string into an array of strings, and returns it.

If not in a list context, returns the number of fields found and splits into the @_ array. (In a list context, you can force the split into @_ by using ?? as the pattern delimiters, but it still returns the array value.) The use of implicit split to @_ is deprecated, however.

If EXPR is omitted, splits the $_ string. If PATTERN is also omitted, splits on whitespace (after skipping any leading whitespace). Anything matching PATTERN is taken to be a delimiter separating the fields. (Note that the delimiter may be longer than one character.) If LIMIT is specified and is not negative, splits into no more than that many fields (though it may split into fewer). If LIMIT is unspecified, trailing null fields are stripped (which potential users of pop() would do well to remember). If LIMIT is negative, it is treated as if an arbitrarily large LIMIT had been specified.

A pattern matching the null string (not to be confused with a null pattern //, which is just one member of the set of patterns matching a null string) will split the value of EXPR into separate characters at each point it matches that way. For example:

    print join(':', split(/ */, 'hi there'));

produces the output 'h:i:t:h:e:r:e'.

The LIMIT parameter can be used to split a line partially

    ($login, $passwd, $remainder) = split(/:/, $_, 3);

When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT one larger than the number of variables in the list, to avoid unnecessary work. For the list above LIMIT would have been 4 by default. In time critical applications it behooves you not to split into more fields than you really need.

If the PATTERN contains parentheses, additional array elements are created from each matching substring in the delimiter.

    split(/([,-])/, "1-10,20", 3);

produces the list value

    (1, '-', 10, ',', 20)

If you had the entire header of a normal Unix email message in $header, you could split it up into fields and their values this way:

    $header =~ s/\n\s+/ /g;  # fix continuation lines
    %hdrs   =  (UNIX_FROM => split /^(.*?):\s*/m, $header);

The pattern /PATTERN/ may be replaced with an expression to specify patterns that vary at runtime. (To do runtime compilation only once, use /$variable/o.)

As a special case, specifying a PATTERN of space (' ') will split on white space just as split with no arguments does. Thus, split(' ') can be used to emulate awk's default behavior, whereas split(/ /) will give you as many null initial fields as there are leading spaces. A split on /\s+/ is like a split(' ') except that any leading whitespace produces a null first field. A split with no arguments really does a split(' ', $_) internally.

Example:

    open(passwd, '/etc/passwd');
    while (<passwd>) {
        ($login, $passwd, $uid, $gid, $gcos,
            $home, $shell) = split(/:/);
        ...
    }

(Note that $shell above will still have a newline on it. See "chop", "chomp", and "join".)

sprintf FORMAT, LIST

Returns a string formatted by the usual printf conventions of the C library function sprintf(). See sprintf(3) or printf(3) on your system for an explanation of the general principles.

Perl does all of its own sprintf() formatting -- it emulates the C function sprintf(), but it doesn't use it (except for floating-point numbers, and even then only the standard modifiers are allowed). As a result, any non-standard extensions in your local sprintf() are not available from Perl.

Perl's sprintf() permits the following universally-known conversions:

   %%   a percent sign
   %c   a character with the given number
   %s   a string
   %d   a signed integer, in decimal
   %u   an unsigned integer, in decimal
   %o   an unsigned integer, in octal
   %x   an unsigned integer, in hexadecimal
   %e   a floating-point number, in scientific notation
   %f   a floating-point number, in fixed decimal notation
   %g   a floating-point number, in %e or %f notation

In addition, Perl permits the following widely-supported conversions:

   %X   like %x, but using upper-case letters
   %E   like %e, but using an upper-case "E"
   %G   like %g, but with an upper-case "E" (if applicable)
   %p   a pointer (outputs the Perl value's address in hexadecimal)
   %n   special: *stores* the number of characters output so far
        into the next variable in the parameter list 

Finally, for backward (and we do mean "backward") compatibility, Perl permits these unnecessary but widely-supported conversions:

   %i   a synonym for %d
   %D   a synonym for %ld
   %U   a synonym for %lu
   %O   a synonym for %lo
   %F   a synonym for %f

Perl permits the following universally-known flags between the % and the conversion letter:

   space   prefix positive number with a space
   +       prefix positive number with a plus sign
   -       left-justify within the field
   0       use zeros, not spaces, to right-justify
   #       prefix octal with "0", hex with "0x"
   number  minimum field width
   .number "precision": digits after decimal point for floating-point,
           max length for string, minimum length for integer
   l       interpret integer as C type "long" or "unsigned long"
   h       interpret integer as C type "short" or "unsigned short"

There is also one Perl-specific flag:

   V       interpret integer as Perl's standard integer type

Where a number would appear in the flags, an asterisk ("*") may be used instead, in which case Perl uses the next item in the parameter list as the given number (that is, as the field width or precision). If a field width obtained through "*" is negative, it has the same effect as the '-' flag: left-justification.

If use locale is in effect, the character used for the decimal point in formatted real numbers is affected by the LC_NUMERIC locale. See perllocale.

sqrt EXPR
sqrt

Return the square root of EXPR. If EXPR is omitted, returns square root of $_.

srand EXPR
srand

Sets the random number seed for the rand operator. If EXPR is omitted, uses a semi-random value based on the current time and process ID, among other things. In versions of Perl prior to 5.004 the default seed was just the current time(). This isn't a particularly good seed, so many old programs supply their own seed value (often time ^ $$ or time ^ ($$ + ($$ << 15))), but that isn't necessary any more.

In fact, it's usually not necessary to call srand() at all, because if it is not called explicitly, it is called implicitly at the first use of the rand operator. However, this was not the case in version of Perl before 5.004, so if your script will run under older Perl versions, it should call srand().

Note that you need something much more random than the default seed for cryptographic purposes. Checksumming the compressed output of one or more rapidly changing operating system status programs is the usual method. For example:

    srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);

If you're particularly concerned with this, see the Math::TrulyRandom module in CPAN.

Do not call srand() multiple times in your program unless you know exactly what you're doing and why you're doing it. The point of the function is to "seed" the rand() function so that rand() can produce a different sequence each time you run your program. Just do it once at the top of your program, or you won't get random numbers out of rand()!

Frequently called programs (like CGI scripts) that simply use

    time ^ $$

for a seed can fall prey to the mathematical property that

    a^b == (a+1)^(b+1)

one-third of the time. So don't do that.

stat FILEHANDLE
stat EXPR
stat

Returns a 13-element array giving the status info for a file, either the file opened via FILEHANDLE, or named by EXPR. If EXPR is omitted, it stats $_. Returns a null list if the stat fails. Typically used as follows:

    ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
       $atime,$mtime,$ctime,$blksize,$blocks)
           = stat($filename);

Not all fields are supported on all filesystem types. Here are the meaning of the fields:

  0 dev      device number of filesystem
  1 ino      inode number
  2 mode     file mode  (type and permissions)
  3 nlink    number of (hard) links to the file
  4 uid      numeric user ID of file's owner
  5 gid      numeric group ID of file's owner
  6 rdev     the device identifier (special files only)
  7 size     total size of file, in bytes
  8 atime    last access time since the epoch
  9 mtime    last modify time since the epoch
 10 ctime    inode change time (NOT creation time!) since the epoch
 11 blksize  preferred block size for file system I/O
 12 blocks   actual number of blocks allocated

(The epoch was at 00:00 January 1, 1970 GMT.)

If stat is passed the special filehandle consisting of an underline, no stat is done, but the current contents of the stat structure from the last stat or filetest are returned. Example:

    if (-x $file && (($d) = stat(_)) && $d < 0) {
        print "$file is executable NFS file\n";
    }

(This works on machines only for which the device number is negative under NFS.)

study SCALAR
study

Takes extra time to study SCALAR ($_ if unspecified) in anticipation of doing many pattern matches on the string before it is next modified. This may or may not save time, depending on the nature and number of patterns you are searching on, and on the distribution of character frequencies in the string to be searched -- you probably want to compare run times with and without it to see which runs faster. Those loops which scan for many short constant strings (including the constant parts of more complex patterns) will benefit most. You may have only one study active at a time -- if you study a different scalar the first is "unstudied". (The way study works is this: a linked list of every character in the string to be searched is made, so we know, for example, where all the 'k' characters are. From each search string, the rarest character is selected, based on some static frequency tables constructed from some C programs and English text. Only those places that contain this "rarest" character are examined.)

For example, here is a loop which inserts index producing entries before any line containing a certain pattern:

    while (<>) {
        study;
        print ".IX foo\n" if /\bfoo\b/;
        print ".IX bar\n" if /\bbar\b/;
        print ".IX blurfl\n" if /\bblurfl\b/;
        ...
        print;
    }

In searching for /\bfoo\b/, only those locations in $_ that contain "f" will be looked at, because "f" is rarer than "o". In general, this is a big win except in pathological cases. The only question is whether it saves you more time than it took to build the linked list in the first place.

Note that if you have to look for strings that you don't know till runtime, you can build an entire loop as a string and eval that to avoid recompiling all your patterns all the time. Together with undefining $/ to input entire files as one record, this can be very fast, often faster than specialized programs like fgrep(1). The following scans a list of files (@files) for a list of words (@words), and prints out the names of those files that contain a match:

    $search = 'while (<>) { study;';
    foreach $word (@words) {
        $search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
    }
    $search .= "}";
    @ARGV = @files;
    undef $/;
    eval $search;               # this screams
    $/ = "\n";          # put back to normal input delimiter
    foreach $file (sort keys(%seen)) {
        print $file, "\n";
    }
sub BLOCK
sub NAME
sub NAME BLOCK

This is subroutine definition, not a real function per se. With just a NAME (and possibly prototypes), it's just a forward declaration. Without a NAME, it's an anonymous function declaration, and does actually return a value: the CODE ref of the closure you just created. See perlsub and perlref for details.

substr EXPR,OFFSET,LEN
substr EXPR,OFFSET

Extracts a substring out of EXPR and returns it. First character is at offset 0, or whatever you've set $[ to (but don't do that). If OFFSET is negative, starts that far from the end of the string. If LEN is omitted, returns everything to the end of the string. If LEN is negative, leaves that many characters off the end of the string.

You can use the substr() function as an lvalue, in which case EXPR must be an lvalue. If you assign something shorter than LEN, the string will shrink, and if you assign something longer than LEN, the string will grow to accommodate it. To keep the string the same length you may need to pad or chop your value using sprintf().

symlink OLDFILE,NEWFILE

Creates a new filename symbolically linked to the old filename. Returns 1 for success, 0 otherwise. On systems that don't support symbolic links, produces a fatal error at run time. To check for that, use eval:

    $symlink_exists = (eval {symlink("","")};, $@ eq '');
syscall LIST

Calls the system call specified as the first element of the list, passing the remaining elements as arguments to the system call. If unimplemented, produces a fatal error. The arguments are interpreted as follows: if a given argument is numeric, the argument is passed as an int. If not, the pointer to the string value is passed. You are responsible to make sure a string is pre-extended long enough to receive any result that might be written into a string. If your integer arguments are not literals and have never been interpreted in a numeric context, you may need to add 0 to them to force them to look like numbers.

    require 'syscall.ph';               # may need to run h2ph
    syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);

Note that Perl supports passing of up to only 14 arguments to your system call, which in practice should usually suffice.

sysopen FILEHANDLE,FILENAME,MODE
sysopen FILEHANDLE,FILENAME,MODE,PERMS

Opens the file whose filename is given by FILENAME, and associates it with FILEHANDLE. If FILEHANDLE is an expression, its value is used as the name of the real filehandle wanted. This function calls the underlying operating system's open function with the parameters FILENAME, MODE, PERMS.

The possible values and flag bits of the MODE parameter are system-dependent; they are available via the standard module Fcntl. However, for historical reasons, some values are universal: zero means read-only, one means write-only, and two means read/write.

If the file named by FILENAME does not exist and the open call creates it (typically because MODE includes the O_CREAT flag), then the value of PERMS specifies the permissions of the newly created file. If PERMS is omitted, the default value is 0666, which allows read and write for all. This default is reasonable: see umask.

The IO::File module provides a more object-oriented approach, if you're into that kind of thing.

sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
sysread FILEHANDLE,SCALAR,LENGTH

Attempts to read LENGTH bytes of data into variable SCALAR from the specified FILEHANDLE, using the system call read(2). It bypasses stdio, so mixing this with other kinds of reads, print(), write(), seek(), or tell() can cause confusion. Returns the number of bytes actually read, or undef if there was an error. SCALAR will be grown or shrunk so that the last byte actually read is the last byte of the scalar after the read.

An OFFSET may be specified to place the read data at some place in the string other than the beginning. A negative OFFSET specifies placement at that many bytes counting backwards from the end of the string. A positive OFFSET greater than the length of SCALAR results in the string being padded to the required size with "\0" bytes before the result of the read is appended.

sysseek FILEHANDLE,POSITION,WHENCE

Sets FILEHANDLE's system position using the system call lseek(2). It bypasses stdio, so mixing this with reads (other than sysread()), print(), write(), seek(), or tell() may cause confusion. FILEHANDLE may be an expression whose value gives the name of the filehandle. The values for WHENCE are 0 to set the new position to POSITION, 1 to set the it to the current position plus POSITION, and 2 to set it to EOF plus POSITION (typically negative). For WHENCE, you may use the constants SEEK_SET, SEEK_CUR, and SEEK_END from either the IO::Seekable or the POSIX module.

Returns the new position, or the undefined value on failure. A position of zero is returned as the string "0 but true"; thus sysseek() returns TRUE on success and FALSE on failure, yet you can still easily determine the new position.

system LIST

Does exactly the same thing as "exec LIST" except that a fork is done first, and the parent process waits for the child process to complete. Note that argument processing varies depending on the number of arguments. The return value is the exit status of the program as returned by the wait() call. To get the actual exit value divide by 256. See also "exec". This is NOT what you want to use to capture the output from a command, for that you should use merely backticks or qx//, as described in "`STRING`" in perlop.

Because system() and backticks block SIGINT and SIGQUIT, killing the program they're running doesn't actually interrupt your program.

    @args = ("command", "arg1", "arg2");
    system(@args) == 0
         or die "system @args failed: $?"

Here's a more elaborate example of analysing the return value from system() on a Unix system to check for all possibilities, including for signals and core dumps.

    $rc = 0xffff & system @args;
    printf "system(%s) returned %#04x: ", "@args", $rc;
    if ($rc == 0) {
        print "ran with normal exit\n";
    }
    elsif ($rc == 0xff00) {
        print "command failed: $!\n";
    }
    elsif ($rc > 0x80) {
        $rc >>= 8;
        print "ran with non-zero exit status $rc\n";
    }
    else {
        print "ran with ";
        if ($rc &   0x80) {
            $rc &= ~0x80;
            print "core dump from ";
        }
        print "signal $rc\n"
    }
    $ok = ($rc != 0);
syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
syswrite FILEHANDLE,SCALAR,LENGTH

Attempts to write LENGTH bytes of data from variable SCALAR to the specified FILEHANDLE, using the system call write(2). It bypasses stdio, so mixing this with reads (other than sysread()), print(), write(), seek(), or tell() may cause confusion. Returns the number of bytes actually written, or undef if there was an error. If the length is greater than the available data, only as much data as is available will be written.

An OFFSET may be specified to write the data from some part of the string other than the beginning. A negative OFFSET specifies writing that many bytes counting backwards from the end of the string.

tell FILEHANDLE
tell

Returns the current position for FILEHANDLE. FILEHANDLE may be an expression whose value gives the name of the actual filehandle. If FILEHANDLE is omitted, assumes the file last read.

telldir DIRHANDLE

Returns the current position of the readdir() routines on DIRHANDLE. Value may be given to seekdir() to access a particular location in a directory. Has the same caveats about possible directory compaction as the corresponding system library routine.

tie VARIABLE,CLASSNAME,LIST

This function binds a variable to a package class that will provide the implementation for the variable. VARIABLE is the name of the variable to be enchanted. CLASSNAME is the name of a class implementing objects of correct type. Any additional arguments are passed to the "new" method of the class (meaning TIESCALAR, TIEARRAY, or TIEHASH). Typically these are arguments such as might be passed to the dbm_open() function of C. The object returned by the "new" method is also returned by the tie() function, which would be useful if you want to access other methods in CLASSNAME.

Note that functions such as keys() and values() may return huge array values when used on large objects, like DBM files. You may prefer to use the each() function to iterate over such. Example:

    # print out history file offsets
    use NDBM_File;
    tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
    while (($key,$val) = each %HIST) {
        print $key, ' = ', unpack('L',$val), "\n";
    }
    untie(%HIST);

A class implementing a hash should have the following methods:

    TIEHASH classname, LIST
    DESTROY this
    FETCH this, key
    STORE this, key, value
    DELETE this, key
    EXISTS this, key
    FIRSTKEY this
    NEXTKEY this, lastkey

A class implementing an ordinary array should have the following methods:

    TIEARRAY classname, LIST
    DESTROY this
    FETCH this, key
    STORE this, key, value
    [others TBD]

A class implementing a scalar should have the following methods:

    TIESCALAR classname, LIST
    DESTROY this
    FETCH this,
    STORE this, value

Unlike dbmopen(), the tie() function will not use or require a module for you--you need to do that explicitly yourself. See DB_File or the Config module for interesting tie() implementations.

tied VARIABLE

Returns a reference to the object underlying VARIABLE (the same value that was originally returned by the tie() call which bound the variable to a package.) Returns the undefined value if VARIABLE isn't tied to a package.

time

Returns the number of non-leap seconds since whatever time the system considers to be the epoch (that's 00:00:00, January 1, 1904 for MacOS, and 00:00:00 UTC, January 1, 1970 for most other systems). Suitable for feeding to gmtime() and localtime().

times

Returns a four-element array giving the user and system times, in seconds, for this process and the children of this process.

    ($user,$system,$cuser,$csystem) = times;
tr///

The translation operator. Same as y///. See perlop.

truncate FILEHANDLE,LENGTH
truncate EXPR,LENGTH

Truncates the file opened on FILEHANDLE, or named by EXPR, to the specified length. Produces a fatal error if truncate isn't implemented on your system.

uc EXPR
uc

Returns an uppercased version of EXPR. This is the internal function implementing the \U escape in double-quoted strings. Respects current LC_CTYPE locale if use locale in force. See perllocale.

If EXPR is omitted, uses $_.

ucfirst EXPR
ucfirst

Returns the value of EXPR with the first character uppercased. This is the internal function implementing the \u escape in double-quoted strings. Respects current LC_CTYPE locale if use locale in force. See perllocale.

If EXPR is omitted, uses $_.

umask EXPR
umask

Sets the umask for the process to EXPR and returns the previous value. If EXPR is omitted, merely returns the current umask. Remember that a umask is a number, usually given in octal; it is not a string of octal digits. See also "oct", if all you have is a string.

undef EXPR
undef

Undefines the value of EXPR, which must be an lvalue. Use only on a scalar value, an entire array, an entire hash, or a subroutine name (using "&"). (Using undef() will probably not do what you expect on most predefined variables or DBM list values, so don't do that.) Always returns the undefined value. You can omit the EXPR, in which case nothing is undefined, but you still get an undefined value that you could, for instance, return from a subroutine, assign to a variable or pass as a parameter. Examples:

    undef $foo;
    undef $bar{'blurfl'};             # Compare to: delete $bar{'blurfl'};
    undef @ary;
    undef %hash;
    undef &mysub;
    return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
    select undef, undef, undef, 0.25;
    ($a, $b, undef, $c) = &foo;       # Ignore third value returned
unlink LIST
unlink

Deletes a list of files. Returns the number of files successfully deleted.

    $cnt = unlink 'a', 'b', 'c';
    unlink @goners;
    unlink <*.bak>;

Note: unlink will not delete directories unless you are superuser and the -U flag is supplied to Perl. Even if these conditions are met, be warned that unlinking a directory can inflict damage on your filesystem. Use rmdir instead.

If LIST is omitted, uses $_.

unpack TEMPLATE,EXPR

Unpack does the reverse of pack: it takes a string representing a structure and expands it out into a list value, returning the array value. (In a scalar context, it returns merely the first value produced.) The TEMPLATE has the same format as in the pack function. Here's a subroutine that does substring:

    sub substr {
        local($what,$where,$howmuch) = @_;
        unpack("x$where a$howmuch", $what);
    }

and then there's

    sub ordinal { unpack("c",$_[0]); } # same as ord()

In addition, you may prefix a field with a %<number> to indicate that you want a <number>-bit checksum of the items instead of the items themselves. Default is a 16-bit checksum. For example, the following computes the same number as the System V sum program:

    while (<>) {
        $checksum += unpack("%16C*", $_);
    }
    $checksum %= 65536;

The following efficiently counts the number of set bits in a bit vector:

    $setbits = unpack("%32b*", $selectmask);
untie VARIABLE

Breaks the binding between a variable and a package. (See tie().)

unshift ARRAY,LIST

Does the opposite of a shift. Or the opposite of a push, depending on how you look at it. Prepends list to the front of the array, and returns the new number of elements in the array.

    unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;

Note the LIST is prepended whole, not one element at a time, so the prepended elements stay in the same order. Use reverse to do the reverse.

use Module LIST
use Module
use Module VERSION LIST
use VERSION

Imports some semantics into the current package from the named module, generally by aliasing certain subroutine or variable names into your package. It is exactly equivalent to

    BEGIN { require Module; import Module LIST; }

except that Module must be a bareword.

If the first argument to use is a number, it is treated as a version number instead of a module name. If the version of the Perl interpreter is less than VERSION, then an error message is printed and Perl exits immediately. This is often useful if you need to check the current Perl version before useing library modules which have changed in incompatible ways from older versions of Perl. (We try not to do this more than we have to.)

The BEGIN forces the require and import to happen at compile time. The require makes sure the module is loaded into memory if it hasn't been yet. The import is not a builtin--it's just an ordinary static method call into the "Module" package to tell the module to import the list of features back into the current package. The module can implement its import method any way it likes, though most modules just choose to derive their import method via inheritance from the Exporter class that is defined in the Exporter module. See Exporter. If no import method can be found then the error is currently silently ignored. This may change to a fatal error in a future version.

If you don't want your namespace altered, explicitly supply an empty list:

    use Module ();

That is exactly equivalent to

    BEGIN { require Module; }

If the VERSION argument is present between Module and LIST, then the use will call the VERSION method in class Module with the given version as an argument. The default VERSION method, inherited from the Universal class, croaks if the given version is larger than the value of the variable $Module::VERSION. (Note that there is not a comma after VERSION!)

Because this is a wide-open interface, pragmas (compiler directives) are also implemented this way. Currently implemented pragmas are:

    use integer;
    use diagnostics;
    use sigtrap qw(SEGV BUS);
    use strict  qw(subs vars refs);
    use subs    qw(afunc blurfl);

These pseudo-modules import semantics into the current block scope, unlike ordinary modules, which import symbols into the current package (which are effective through the end of the file).

There's a corresponding "no" command that unimports meanings imported by use, i.e., it calls unimport Module LIST instead of import.

    no integer;
    no strict 'refs';

If no unimport method can be found the call fails with a fatal error.

See perlmod for a list of standard modules and pragmas.

utime LIST

Changes the access and modification times on each file of a list of files. The first two elements of the list must be the NUMERICAL access and modification times, in that order. Returns the number of files successfully changed. The inode modification time of each file is set to the current time. Example of a "touch" command:

    #!/usr/bin/perl
    $now = time;
    utime $now, $now, @ARGV;
values HASH

Returns a normal array consisting of all the values of the named hash. (In a scalar context, returns the number of values.) The values are returned in an apparently random order, but it is the same order as either the keys() or each() function would produce on the same hash. As a side effect, it resets HASH's iterator. See also keys(), each(), and sort().

vec EXPR,OFFSET,BITS

Treats the string in EXPR as a vector of unsigned integers, and returns the value of the bit field specified by OFFSET. BITS specifies the number of bits that are reserved for each entry in the bit vector. This must be a power of two from 1 to 32. vec() may also be assigned to, in which case parentheses are needed to give the expression the correct precedence as in

    vec($image, $max_x * $x + $y, 8) = 3;

Vectors created with vec() can also be manipulated with the logical operators |, &, and ^, which will assume a bit vector operation is desired when both operands are strings.

To transform a bit vector into a string or array of 0's and 1's, use these:

    $bits = unpack("b*", $vector);
    @bits = split(//, unpack("b*", $vector));

If you know the exact length in bits, it can be used in place of the *.

wait

Waits for a child process to terminate and returns the pid of the deceased process, or -1 if there are no child processes. The status is returned in $?.

waitpid PID,FLAGS

Waits for a particular child process to terminate and returns the pid of the deceased process, or -1 if there is no such child process. The status is returned in $?. If you say

    use POSIX ":sys_wait_h";
    ...
    waitpid(-1,&WNOHANG);

then you can do a non-blocking wait for any process. Non-blocking wait is available on machines supporting either the waitpid(2) or wait4(2) system calls. However, waiting for a particular pid with FLAGS of 0 is implemented everywhere. (Perl emulates the system call by remembering the status values of processes that have exited but have not been harvested by the Perl script yet.)

wantarray

Returns TRUE if the context of the currently executing subroutine is looking for a list value. Returns FALSE if the context is looking for a scalar. Returns the undefined value if the context is looking for no value (void context).

    return unless defined wantarray;    # don't bother doing more
    my @a = complex_calculation();
    return wantarray ? @a : "@a";
warn LIST

Produces a message on STDERR just like die(), but doesn't exit or throw an exception.

No message is printed if there is a $SIG{__WARN__} handler installed. It is the handler's responsibility to deal with the message as it sees fit (like, for instance, converting it into a die()). Most handlers must therefore make arrangements to actually display the warnings that they are not prepared to deal with, by calling warn() again in the handler. Note that this is quite safe and will not produce an endless loop, since __WARN__ hooks are not called from inside one.

You will find this behavior is slightly different from that of $SIG{__DIE__} handlers (which don't suppress the error text, but can instead call die() again to change it).

Using a __WARN__ handler provides a powerful way to silence all warnings (even the so-called mandatory ones). An example:

    # wipe out *all* compile-time warnings
    BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
    my $foo = 10;
    my $foo = 20;          # no warning about duplicate my $foo,
                           # but hey, you asked for it!
    # no compile-time or run-time warnings before here
    $DOWARN = 1;

    # run-time warnings enabled after here
    warn "\$foo is alive and $foo!";     # does show up

See perlvar for details on setting %SIG entries, and for more examples.

write FILEHANDLE
write EXPR
write

Writes a formatted record (possibly multi-line) to the specified file, using the format associated with that file. By default the format for a file is the one having the same name as the filehandle, but the format for the current output channel (see the select() function) may be set explicitly by assigning the name of the format to the $~ variable.

Top of form processing is handled automatically: if there is insufficient room on the current page for the formatted record, the page is advanced by writing a form feed, a special top-of-page format is used to format the new page header, and then the record is written. By default the top-of-page format is the name of the filehandle with "_TOP" appended, but it may be dynamically set to the format of your choice by assigning the name to the $^ variable while the filehandle is selected. The number of lines remaining on the current page is in variable $-, which can be set to 0 to force a new page.

If FILEHANDLE is unspecified, output goes to the current default output channel, which starts out as STDOUT but may be changed by the select operator. If the FILEHANDLE is an EXPR, then the expression is evaluated and the resulting string is used to look up the name of the FILEHANDLE at run time. For more on formats, see perlform.

Note that write is NOT the opposite of read. Unfortunately.

y///

The translation operator. Same as tr///. See perlop.

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