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perlfunc
PERLFUNC(1) Perl Programmers Reference Guide PERLFUNC(1)
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 fol-
lowing comma. (See the precedence table in perlop.) List operators
take more than one argument, while unary operators can never take more
than one argument. Thus, a comma terminates the argument of a unary
operator, but merely separates the arguments of a list operator. A
unary operator generally provides a scalar context to its argument,
while a list operator may provide either scalar or list contexts for
its arguments. If it does both, the scalar arguments will be first,
and the list argument will follow. (Note that there can ever be only
one such list argument.) For instance, splice() has three scalar argu-
ments followed by a list, whereas gethostbyname() has four scalar argu-
ments.
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. Commas
should separate elements of the LIST.
Any function in the list below may be used either with or without
parentheses around its arguments. (The syntax descriptions omit the
parentheses.) If you use the parentheses, the simple (but occasionally
surprising) rule is this: It looks like a function, therefore it is a
function, and precedence doesn't matter. Otherwise it's a list opera-
tor or unary operator, and precedence does matter. And whitespace
between the function and left parenthesis doesn't count--so you need to
be careful sometimes:
print 1+2+4; # Prints 7.
print(1+2) + 4; # Prints 3.
print (1+2)+4; # Also prints 3!
print +(1+2)+4; # Prints 7.
print ((1+2)+4); # Prints 7.
If you run Perl with the -w switch it can warn you about this. For
example, the third line above produces:
print (...) interpreted as function at - line 1.
Useless use of integer addition in void context at - line 1.
A few functions take no arguments at all, and therefore work as neither
unary nor list operators. These include such functions as "time" and
"endpwent". For example, "time+86_400" always means "time() + 86_400".
For functions that can be used in either a scalar or list context, non-
abortive failure is generally indicated in a scalar context by return-
ing the undefined value, and in a list context by returning the null
list.
Remember the following important rule: There is no rule that relates
the behavior of an expression in list context to its behavior in scalar
context, or vice versa. It might do two totally different things.
Each operator and function decides which sort of value it would be most
appropriate to return in scalar context. Some operators return the
length of the list that would have been returned in list context. Some
operators return the first value in the list. Some operators return
the last value in the list. Some operators return a count of success-
ful operations. In general, they do what you want, unless you want
consistency.
A named array in scalar context is quite different from what would at
first glance appear to be a list in scalar context. You can't get a
list like "(1,2,3)" into being in scalar context, because the compiler
knows the context at compile time. It would generate the scalar comma
operator there, not the list construction version of the comma. That
means it was never a list to start with.
In general, functions in Perl that serve as wrappers for system calls
of the same name (like chown(2), fork(2), closedir(2), etc.) all return
true when they succeed and "undef" otherwise, as is usually mentioned
in the descriptions below. This is different from the C interfaces,
which return "-1" on failure. Exceptions to this rule are "wait",
"waitpid", and "syscall". System calls also set the special $! vari-
able on failure. Other functions do not, except accidentally.
Perl Functions by Category
Here are Perl's functions (including things that look like functions,
like some keywords and named operators) arranged by category. Some
functions appear in more than one place.
Functions for SCALARs or strings
"chomp", "chop", "chr", "crypt", "hex", "index", "lc", "lcfirst",
"length", "oct", "ord", "pack", "q/STRING/", "qq/STRING/",
"reverse", "rindex", "sprintf", "substr", "tr///", "uc", "ucfirst",
"y///"
Regular expressions and pattern matching
"m//", "pos", "quotemeta", "s///", "split", "study", "qr//"
Numeric functions
"abs", "atan2", "cos", "exp", "hex", "int", "log", "oct", "rand",
"sin", "sqrt", "srand"
Functions for real @ARRAYs
"pop", "push", "shift", "splice", "unshift"
Functions for list data
"grep", "join", "map", "qw/STRING/", "reverse", "sort", "unpack"
Functions for real %HASHes
"delete", "each", "exists", "keys", "values"
Input and output functions
"binmode", "close", "closedir", "dbmclose", "dbmopen", "die",
"eof", "fileno", "flock", "format", "getc", "print", "printf",
"read", "readdir", "rewinddir", "seek", "seekdir", "select",
"syscall", "sysread", "sysseek", "syswrite", "tell", "telldir",
"truncate", "warn", "write"
Functions for fixed length data or records
"pack", "read", "syscall", "sysread", "syswrite", "unpack", "vec"
Functions for filehandles, files, or directories
"-X", "chdir", "chmod", "chown", "chroot", "fcntl", "glob",
"ioctl", "link", "lstat", "mkdir", "open", "opendir", "readlink",
"rename", "rmdir", "stat", "symlink", "sysopen", "umask", "unlink",
"utime"
Keywords related to the control flow of your Perl program
"caller", "continue", "die", "do", "dump", "eval", "exit", "goto",
"last", "next", "redo", "return", "sub", "wantarray"
Keywords related to scoping
"caller", "import", "local", "my", "our", "package", "use"
Miscellaneous functions
"defined", "dump", "eval", "formline", "local", "my", "our",
"reset", "scalar", "undef", "wantarray"
Functions for processes and process groups
"alarm", "exec", "fork", "getpgrp", "getppid", "getpriority",
"kill", "pipe", "qx/STRING/", "setpgrp", "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", "get-
sockopt", "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", "getp-
wuid", "setgrent", "setpwent"
Fetching network info
"endprotoent", "endservent", "gethostbyaddr", "gethostbyname",
"gethostent", "getnetbyaddr", "getnetbyname", "getnetent", "getpro-
tobyname", "getprotobynumber", "getprotoent", "getservbyname",
"getservbyport", "getservent", "sethostent", "setnetent", "setpro-
toent", "setservent"
Time-related functions
"gmtime", "localtime", "time", "times"
Functions new in perl5
"abs", "bless", "chomp", "chr", "exists", "formline", "glob",
"import", "lc", "lcfirst", "map", "my", "no", "our", "prototype",
"qx", "qw", "readline", "readpipe", "ref", "sub*", "sysopen",
"tie", "tied", "uc", "ucfirst", "untie", "use"
* - "sub" was a keyword in perl4, but in perl5 it is an operator,
which can be used in expressions.
Functions obsoleted in perl5
"dbmclose", "dbmopen"
Portability
Perl was born in Unix and can therefore access all common Unix system
calls. In non-Unix environments, the functionality of some Unix system
calls may not be available, or details of the available functionality
may differ slightly. The Perl functions affected by this are:
"-X", "binmode", "chmod", "chown", "chroot", "crypt", "dbmclose",
"dbmopen", "dump", "endgrent", "endhostent", "endnetent", "endpro-
toent", "endpwent", "endservent", "exec", "fcntl", "flock", "fork",
"getgrent", "getgrgid", "gethostbyname", "gethostent", "getlogin",
"getnetbyaddr", "getnetbyname", "getnetent", "getppid", "getpgrp",
"getpriority", "getprotobynumber", "getprotoent", "getpwent", "getpw-
nam", "getpwuid", "getservbyport", "getservent", "getsockopt", "glob",
"ioctl", "kill", "link", "lstat", "msgctl", "msgget", "msgrcv",
"msgsnd", "open", "pipe", "readlink", "rename", "select", "semctl",
"semget", "semop", "setgrent", "sethostent", "setnetent", "setpgrp",
"setpriority", "setprotoent", "setpwent", "setservent", "setsockopt",
"shmctl", "shmget", "shmread", "shmwrite", "socket", "socketpair",
"stat", "symlink", "syscall", "sysopen", "system", "times", "truncate",
"umask", "unlink", "utime", "wait", "waitpid"
For more information about the portability of these functions, see
perlport and other available platform-specific documentation.
Alphabetical Listing of Perl Functions
-X FILEHANDLE
-X EXPR
-X A file test, where X is one of the letters listed below. This
unary operator takes one argument, either a filename or a file-
handle, 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 opera-
tor may be any of:
-r File is readable by effective uid/gid.
-w File is writable by effective uid/gid.
-x File is executable by effective uid/gid.
-o File is owned by effective uid.
-R File is readable by real uid/gid.
-W File is writable by real uid/gid.
-X File is executable by real uid/gid.
-O File is owned by real uid.
-e File exists.
-z File has zero size (is empty).
-s File has nonzero size (returns size in bytes).
-f File is a plain file.
-d File is a directory.
-l File is a symbolic link.
-p File is a named pipe (FIFO), or Filehandle is a pipe.
-S File is a socket.
-b File is a block special file.
-c File is a character special file.
-t Filehandle is opened to a tty.
-u File has setuid bit set.
-g File has setgid bit set.
-k File has sticky bit set.
-T File is an ASCII text file (heuristic guess).
-B File is a "binary" file (opposite of -T).
-M Script start time minus file modification time, in days.
-A Same for access time.
-C Same for inode change time (Unix, may differ for other platforms)
Example:
while (<>) {
chomp;
next unless -f $_; # ignore specials
#...
}
The interpretation of the file permission operators "-r", "-R",
"-w", "-W", "-x", and "-X" is by default based solely on the
mode of the file and the uids and gids of the user. There may
be other reasons you can't actually read, write, or execute the
file. Such reasons may be for example network filesystem
access controls, ACLs (access control lists), read-only
filesystems, and unrecognized executable formats.
Also note that, for the superuser on the local filesystems, the
"-r", "-R", "-w", and "-W" tests always return 1, and "-x" and
"-X" return 1 if any execute bit is set in the mode. Scripts
run by the superuser may thus need to do a stat() to determine
the actual mode of the file, or temporarily set their effective
uid to something else.
If you are using ACLs, there is a pragma called "filetest" that
may produce more accurate results than the bare stat() mode
bits. When under the "use filetest 'access'" the above-men-
tioned filetests will test whether the permission can (not) be
granted using the access() family of system calls. Also note
that the "-x" and "-X" may under this pragma return true even
if there are no execute permission bits set (nor any extra exe-
cute permission ACLs). This strangeness is due to the underly-
ing system calls' definitions. Read the documentation for the
"filetest" pragma for more information.
Note that "-s/a/b/" does not do a negated substitution. Saying
"-exp($foo)" still works as expected, however--only single let-
ters 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
strange characters (>30%) are found, it's a "-B" file; other-
wise it's a "-T" file. Also, any file containing null in the
first block is considered a binary file. If "-T" or "-B" is
used on a filehandle, the current IO buffer is examined rather
than the first block. Both "-T" and "-B" return true on a null
file, or a file at EOF when testing a filehandle. Because you
have to read a file to do the "-T" test, on most occasions you
want to use a "-f" against the file first, as in "next unless
-f $file && -T $file".
If any of the file tests (or either the "stat" or "lstat" oper-
ators) are given the special filehandle consisting of a soli-
tary 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 sym-
bolic link, not the real file.) (Also, if the stat buffer was
filled by an "lstat" call, "-T" and "-B" will reset it with the
results of "stat _"). Example:
print "Can do.\n" if -r $a || -w _ || -x _;
stat($filename);
print "Readable\n" if -r _;
print "Writable\n" if -w _;
print "Executable\n" if -x _;
print "Setuid\n" if -u _;
print "Setgid\n" if -g _;
print "Sticky\n" if -k _;
print "Text\n" if -T _;
print "Binary\n" if -B _;
abs VALUE
abs Returns the absolute value of its argument. If VALUE is omit-
ted, uses $_.
accept NEWSOCKET,GENERICSOCKET
Accepts an incoming socket connect, just as the accept(2) sys-
tem call does. Returns the packed address if it succeeded,
false otherwise. See the example in "Sockets: Client/Server
Communication" in perlipc.
On systems that support a close-on-exec flag on files, the flag
will be set for the newly opened file descriptor, as determined
by the value of $^F. See "$^F" in perlvar.
alarm SECONDS
alarm Arranges to have a SIGALRM delivered to this process after the
specified number of wallclock seconds has elapsed. If SECONDS
is not specified, the value stored in $_ is used. (On some
machines, unfortunately, the elapsed time may be up to one sec-
ond less or more than you specified because of how seconds are
counted, and process scheduling may delay the delivery of the
signal even further.)
Only one timer may be counting at once. Each call disables the
previous timer, and an argument of 0 may be supplied to cancel
the previous timer without starting a new one. The returned
value is the amount of time remaining on the previous timer.
For delays of finer granularity than one second, you may use
Perl's four-argument version of select() leaving the first
three arguments undefined, or you might be able to use the
"syscall" interface to access setitimer(2) if your system sup-
ports it. The Time::HiRes module (from CPAN, and starting from
Perl 5.8 part of the standard distribution) may also prove use-
ful.
It is usually a mistake to intermix "alarm" and "sleep" calls.
("sleep" may be internally implemented in your system with
"alarm")
If you want to use "alarm" to time out a system call you need
to use an "eval"/"die" pair. You can't rely on the alarm caus-
ing the system call to fail with $! set to "EINTR" because Perl
sets up signal handlers to restart system calls on some sys-
tems. Using "eval"/"die" always works, modulo the caveats
given in "Signals" in perlipc.
eval {
local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
alarm $timeout;
$nread = sysread SOCKET, $buffer, $size;
alarm 0;
};
if ($@) {
die unless $@ eq "alarm\n"; # propagate unexpected errors
# timed out
}
else {
# didn't
}
For more information see perlipc.
atan2 Y,X
Returns the arctangent of Y/X in the range -PI to PI.
For the tangent operation, you may use the "Math::Trig::tan"
function, or use the familiar relation:
sub tan { sin($_[0]) / cos($_[0]) }
Note that atan2(0, 0) is not well-defined.
bind SOCKET,NAME
Binds a network address to a socket, just as the bind system
call does. Returns true if it succeeded, false otherwise.
NAME should be a packed address of the appropriate type for the
socket. See the examples in "Sockets: Client/Server Communica-
tion" in perlipc.
binmode FILEHANDLE, LAYER
binmode FILEHANDLE
Arranges for FILEHANDLE to be read or written in "binary" or
"text" mode on systems where the run-time libraries distinguish
between binary and text files. If FILEHANDLE is an expression,
the value is taken as the name of the filehandle. Returns true
on success, otherwise it returns "undef" and sets $! (errno).
On some systems (in general, DOS and Windows-based systems)
binmode() is necessary when you're not working with a text
file. For the sake of portability it is a good idea to always
use it when appropriate, and to never use it when it isn't
appropriate. Also, people can set their I/O to be by default
UTF-8 encoded Unicode, not bytes.
In other words: regardless of platform, use binmode() on binary
data, like for example images.
If LAYER is present it is a single string, but may contain mul-
tiple directives. The directives alter the behaviour of the
file handle. When LAYER is present using binmode on text file
makes sense.
If LAYER is omitted or specified as ":raw" the filehandle is
made suitable for passing binary data. This includes turning
off possible CRLF translation and marking it as bytes (as
opposed to Unicode characters). Note that, despite what may be
implied in "Programming Perl" (the Camel) or elsewhere, ":raw"
is not the simply inverse of ":crlf" -- other layers which
would affect binary nature of the stream are also disabled. See
PerlIO, perlrun and the discussion about the PERLIO environment
variable.
The ":bytes", ":crlf", and ":utf8", and any other directives of
the form ":...", are called I/O layers. The "open" pragma can
be used to establish default I/O layers. See open.
The LAYER parameter of the binmode() function is described as
"DISCIPLINE" in "Programming Perl, 3rd Edition". However,
since the publishing of this book, by many known as "Camel
III", the consensus of the naming of this functionality has
moved from "discipline" to "layer". All documentation of this
version of Perl therefore refers to "layers" rather than to
"disciplines". Now back to the regularly scheduled documenta-
tion...
To mark FILEHANDLE as UTF-8, use ":utf8".
In general, binmode() should be called after open() but before
any I/O is done on the filehandle. Calling binmode() will nor-
mally flush any pending buffered output data (and perhaps pend-
ing input data) on the handle. An exception to this is the
":encoding" layer that changes the default character encoding
of the handle, see open. The ":encoding" layer sometimes needs
to be called in mid-stream, and it doesn't flush the stream.
The ":encoding" also implicitly pushes on top of itself the
":utf8" layer because internally Perl will operate on UTF-8
encoded Unicode characters.
The operating system, device drivers, C libraries, and Perl
run-time system all work together to let the programmer treat a
single character ("\n") as the line terminator, irrespective of
the external representation. On many operating systems, the
native text file representation matches the internal represen-
tation, but on some platforms the external representation of
"\n" is made up of more than one character.
Mac OS, all variants of Unix, and Stream_LF files on VMS use a
single character to end each line in the external representa-
tion of text (even though that single character is CARRIAGE
RETURN on Mac OS and LINE FEED on Unix and most VMS files). In
other systems like OS/2, DOS and the various flavors of MS-Win-
dows your program sees a "\n" as a simple "\cJ", but what's
stored in text files are the two characters "\cM\cJ". That
means that, if you don't use binmode() on these systems,
"\cM\cJ" sequences on disk will be converted to "\n" on input,
and any "\n" in your program will be converted back to "\cM\cJ"
on output. This is what you want for text files, but it can be
disastrous for binary files.
Another consequence of using binmode() (on some systems) is
that special end-of-file markers will be seen as part of the
data stream. For systems from the Microsoft family this means
that if your binary data contains "\cZ", the I/O subsystem will
regard it as the end of the file, unless you use binmode().
binmode() is not only important for readline() and print()
operations, but also when using read(), seek(), sysread(),
syswrite() and tell() (see perlport for more details). See the
$/ and "$\" variables in perlvar for how to manually set your
input and output line-termination sequences.
bless REF,CLASSNAME
bless REF
This function tells the thingy referenced by REF that it is now
an object in the CLASSNAME package. If CLASSNAME is omitted,
the current package is used. Because a "bless" is often the
last thing in a constructor, it returns the reference for con-
venience. Always use the two-argument version if a derived
class might inherit the function doing the blessing. See perl-
toot and perlobj for more about the blessing (and blessings) of
objects.
Consider always blessing objects in CLASSNAMEs that are mixed
case. Namespaces with all lowercase names are considered
reserved for Perl pragmata. Builtin types have all uppercase
names. To prevent confusion, you may wish to avoid such package
names as well. Make sure that CLASSNAME is a true value.
See "Perl Modules" in perlmod.
caller EXPR
caller Returns the context of the current subroutine call. In scalar
context, returns the caller's package name if there is a
caller, that is, if we're in a subroutine or "eval" or
"require", and the undefined value otherwise. In list context,
returns
($package, $filename, $line) = caller;
With EXPR, it returns some extra information that the debugger
uses to print a stack trace. The value of EXPR indicates how
many call frames to go back before the current one.
($package, $filename, $line, $subroutine, $hasargs,
$wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
Here $subroutine may be "(eval)" if the frame is not a subrou-
tine call, but an "eval". In such a case additional elements
$evaltext and $is_require are set: $is_require is true if the
frame is created by a "require" or "use" statement, $evaltext
contains the text of the "eval EXPR" statement. In particular,
for an "eval BLOCK" statement, $filename is "(eval)", but
$evaltext is undefined. (Note also that each "use" statement
creates a "require" frame inside an "eval EXPR" frame.) $sub-
routine may also be "(unknown)" if this particular subroutine
happens to have been deleted from the symbol table. $hasargs
is true if a new instance of @_ was set up for the frame.
$hints and $bitmask contain pragmatic hints that the caller was
compiled with. The $hints and $bitmask values are subject to
change between versions of Perl, and are not meant for external
use.
Furthermore, when called from within the DB package, caller
returns more detailed information: it sets the list variable
@DB::args to be the arguments with which the subroutine was
invoked.
Be aware that the optimizer might have optimized call frames
away before "caller" had a chance to get the information. That
means that caller(N) might not return information about the
call frame you expect it do, for "N > 1". In particular,
@DB::args might have information from the previous time
"caller" was called.
chdir EXPR
chdir FILEHANDLE
chdir DIRHANDLE
chdir Changes the working directory to EXPR, if possible. If EXPR is
omitted, changes to the directory specified by $ENV{HOME}, if
set; if not, changes to the directory specified by
$ENV{LOGDIR}. (Under VMS, the variable $ENV{SYS$LOGIN} is also
checked, and used if it is set.) If neither is set, "chdir"
does nothing. It returns true upon success, false otherwise.
See the example under "die".
On systems that support fchdir, you might pass a file handle or
directory handle as argument. On systems that don't support
fchdir, passing handles produces a fatal error at run time.
chmod LIST
Changes the permissions of a list of files. The first element
of the list must be the numerical mode, which should probably
be an octal number, and which definitely should not be a string
of octal digits: 0644 is okay, '0644' is not. Returns the num-
ber of files successfully changed. See also "oct", if all you
have is a string.
$cnt = chmod 0755, 'foo', 'bar';
chmod 0755, @executables;
$mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
# --w----r-T
$mode = '0644'; chmod oct($mode), 'foo'; # this is better
$mode = 0644; chmod $mode, 'foo'; # this is best
On systems that support fchmod, you might pass file handles
among the files. On systems that don't support fchmod, passing
file handles produces a fatal error at run time.
open(my $fh, "<", "foo");
my $perm = (stat $fh)[2] & 07777;
chmod($perm | 0600, $fh);
You can also import the symbolic "S_I*" constants from the
Fcntl module:
use Fcntl ':mode';
chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
# This is identical to the chmod 0755 of the above example.
chomp VARIABLE
chomp( LIST )
chomp This safer version of "chop" removes any trailing string that
corresponds to the current value of $/ (also known as
$INPUT_RECORD_SEPARATOR in the "English" module). It returns
the total number of characters removed from all its arguments.
It's often used to remove the newline from the end of an input
record when you're worried that the final record may be missing
its newline. When in paragraph mode ("$/ = """), it removes
all trailing newlines from the string. When in slurp mode ("$/
= undef") or fixed-length record mode ($/ is a reference to an
integer or the like, see perlvar) chomp() won't remove any-
thing. If VARIABLE is omitted, it chomps $_. Example:
while (<>) {
chomp; # avoid \n on last field
@array = split(/:/);
# ...
}
If VARIABLE is a hash, it chomps the hash's values, but not its
keys.
You can actually chomp anything that's an lvalue, including an
assignment:
chomp($cwd = `pwd`);
chomp($answer = <STDIN>);
If you chomp a list, each element is chomped, and the total
number of characters removed is returned.
If the "encoding" pragma is in scope then the lengths returned
are calculated from the length of $/ in Unicode characters,
which is not always the same as the length of $/ in the native
encoding.
Note that parentheses are necessary when you're chomping any-
thing that is not a simple variable. This is because "chomp
$cwd = `pwd`;" is interpreted as "(chomp $cwd) = `pwd`;",
rather than as "chomp( $cwd = `pwd` )" which you might expect.
Similarly, "chomp $a, $b" is interpreted as "chomp($a), $b"
rather than as "chomp($a, $b)".
chop VARIABLE
chop( LIST )
chop Chops off the last character of a string and returns the char-
acter chopped. It is much more efficient than "s/.$//s"
because it neither scans nor copies the string. If VARIABLE is
omitted, chops $_. If VARIABLE is a hash, it chops the hash's
values, but not its keys.
You can actually chop anything that's an lvalue, including an
assignment.
If you chop a list, each element is chopped. Only the value of
the last "chop" is returned.
Note that "chop" returns the last character. To return all but
the last character, use "substr($string, 0, -1)".
See also "chomp".
chown LIST
Changes the owner (and group) of a list of files. The first
two elements of the list must be the numeric uid and gid, in
that order. A value of -1 in either position is interpreted by
most systems to leave that value unchanged. Returns the number
of files successfully changed.
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
On systems that support fchown, you might pass file handles
among the files. On systems that don't support fchown, passing
file handles produces a fatal error at run time.
Here's an example that looks up nonnumeric uids in the passwd
file:
print "User: ";
chomp($user = <STDIN>);
print "Files: ";
chomp($pattern = <STDIN>);
($login,$pass,$uid,$gid) = getpwnam($user)
or die "$user not in passwd file";
@ary = glob($pattern); # expand filenames
chown $uid, $gid, @ary;
On most systems, you are not allowed to change the ownership of
the file unless you're the superuser, although you should be
able to change the group to any of your secondary groups. On
insecure systems, these restrictions may be relaxed, but this
is not a portable assumption. On POSIX systems, you can detect
this condition this way:
use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
$can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
chr NUMBER
chr Returns the character represented by that NUMBER in the charac-
ter set. For example, "chr(65)" is "A" in either ASCII or Uni-
code, and chr(0x263a) is a Unicode smiley face. Note that
characters from 128 to 255 (inclusive) are by default not
encoded in UTF-8 Unicode for backward compatibility reasons
(but see encoding).
If NUMBER is omitted, uses $_.
For the reverse, use "ord".
Note that under the "bytes" pragma the NUMBER is masked to the
low eight bits.
See perlunicode and encoding for more about Unicode.
chroot FILENAME
chroot This function works like the system call by the same name: it
makes the named directory the new root directory for all fur-
ther pathnames that begin with a "/" by your process and all
its children. (It doesn't change your current working direc-
tory, which is unaffected.) For security reasons, this call is
restricted to the superuser. If FILENAME is omitted, does a
"chroot" to $_.
close FILEHANDLE
close Closes the file or pipe associated with the file handle,
returning true only if IO buffers are successfully flushed and
closes the system file descriptor. Closes the currently
selected filehandle if the argument is omitted.
You don't have to close FILEHANDLE if you are immediately going
to do another "open" on it, because "open" will close it for
you. (See "open".) However, an explicit "close" on an input
file resets the line counter ($.), while the implicit close
done by "open" does not.
If the file handle came from a piped open, "close" will addi-
tionally return false if one of the other system calls involved
fails, or if the program exits with non-zero status. (If the
only problem was that the program exited non-zero, $! will be
set to 0.) Closing a pipe also waits for the process executing
on the pipe to complete, in case you want to look at the output
of the pipe afterwards, and implicitly puts the exit status
value of that command into $?.
Prematurely closing the read end of a pipe (i.e. before the
process writing to it at the other end has closed it) will
result in a SIGPIPE being delivered to the writer. If the
other end can't handle that, be sure to read all the data
before closing the pipe.
Example:
open(OUTPUT, '|sort >foo') # pipe to sort
or die "Can't start sort: $!";
#... # print stuff to output
close OUTPUT # wait for sort to finish
or warn $! ? "Error closing sort pipe: $!"
: "Exit status $? from sort";
open(INPUT, 'foo') # get sort's results
or die "Can't open 'foo' for input: $!";
FILEHANDLE may be an expression whose value can be used as an
indirect filehandle, usually the real filehandle name.
closedir DIRHANDLE
Closes a directory opened by "opendir" and returns the success
of that system call.
connect SOCKET,NAME
Attempts to connect to a remote socket, just as the connect
system call does. Returns true if it succeeded, false other-
wise. NAME should be a packed address of the appropriate type
for the socket. See the examples in "Sockets: Client/Server
Communication" in perlipc.
continue BLOCK
"continue" is actually a flow control statement rather than a
function. If there is a "continue" BLOCK attached to a BLOCK
(typically in a "while" or "foreach"), it is always executed
just before the 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).
"last", "next", or "redo" may appear within a "continue" block.
"last" and "redo" will behave as if they had been executed
within the main block. So will "next", but since it will exe-
cute a "continue" block, it may be more entertaining.
while (EXPR) {
### redo always comes here
do_something;
} continue {
### next always comes here
do_something_else;
# then back the top to re-check EXPR
}
### last always comes here
Omitting the "continue" section is semantically equivalent to
using an empty one, logically enough. In that case, "next"
goes directly back to check the condition at the top of the
loop.
cos EXPR
cos Returns the cosine of EXPR (expressed in radians). If EXPR is
omitted, takes cosine of $_.
For the inverse cosine operation, you may use the
"Math::Trig::acos()" function, or use this relation:
sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
crypt PLAINTEXT,SALT
Creates a digest string exactly like the crypt(3) function in
the C library (assuming that you actually have a version there
that has not been extirpated as a potential munitions).
crypt() is a one-way hash function. The PLAINTEXT and SALT is
turned into a short string, called a digest, which is returned.
The same PLAINTEXT and SALT will always return the same string,
but there is no (known) way to get the original PLAINTEXT from
the hash. Small changes in the PLAINTEXT or SALT will result
in large changes in the digest.
There is no decrypt function. This function isn't all that
useful for cryptography (for that, look for Crypt modules on
your nearby CPAN mirror) and the name "crypt" is a bit of a
misnomer. Instead it is primarily used to check if two pieces
of text are the same without having to transmit or store the
text itself. An example is checking if a correct password is
given. The digest of the password is stored, not the password
itself. The user types in a password that is crypt()'d with
the same salt as the stored digest. If the two digests match
the password is correct.
When verifying an existing digest string you should use the
digest as the salt (like "crypt($plain, $digest) eq $digest").
The SALT used to create the digest is visible as part of the
digest. This ensures crypt() will hash the new string with the
same salt as the digest. This allows your code to work with
the standard crypt and with more exotic implementations. In
other words, do not assume anything about the returned string
itself, or how many bytes in the digest matter.
Traditionally the result is a string of 13 bytes: two first
bytes of the salt, followed by 11 bytes from the set
"[./0-9A-Za-z]", and only the first eight bytes of the digest
string mattered, but alternative hashing schemes (like MD5),
higher level security schemes (like C2), and implementations on
non-UNIX platforms may produce different strings.
When choosing a new salt create a random two character string
whose characters come from the set "[./0-9A-Za-z]" (like "join
'', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]").
This set of characters is just a recommendation; the characters
allowed in the salt depend solely on your system's crypt
library, and Perl can't restrict what salts "crypt()" accepts.
Here's an example that makes sure that whoever runs this pro-
gram knows their password:
$pwd = (getpwuid($<))[1];
system "stty -echo";
print "Password: ";
chomp($word = <STDIN>);
print "\n";
system "stty echo";
if (crypt($word, $pwd) ne $pwd) {
die "Sorry...\n";
} else {
print "ok\n";
}
Of course, typing in your own password to whoever asks you for
it is unwise.
The crypt function is unsuitable for hashing large quantities
of data, not least of all because you can't get the information
back. Look at the Digest module for more robust algorithms.
If using crypt() on a Unicode string (which potentially has
characters with codepoints above 255), Perl tries to make sense
of the situation by trying to downgrade (a copy of the string)
the string back to an eight-bit byte string before calling
crypt() (on that copy). If that works, good. If not, crypt()
dies with "Wide character in crypt".
dbmclose HASH
[This function has been largely superseded by the "untie" func-
tion.]
Breaks the binding between a DBM file and a hash.
dbmopen HASH,DBNAME,MASK
[This function has been largely superseded by the "tie" func-
tion.]
This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB
file to a hash. HASH is the name of the hash. (Unlike normal
"open", the first argument is not a filehandle, even though it
looks like one). DBNAME is the name of the database (without
the .dir or .pag extension if any). If the database does not
exist, it is created with protection specified by MASK (as mod-
ified by the "umask"). If your system supports only the older
DBM functions, you may perform only one "dbmopen" in your pro-
gram. 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
lists when used on large DBM files. You may prefer to use the
"each" function to iterate over large DBM files. Example:
# print out history file offsets
dbmopen(%HIST,'/usr/lib/news/history',0666);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
dbmclose(%HIST);
See also AnyDBM_File for a more general description of the pros
and cons of the various dbm approaches, as well as DB_File for
a particularly rich implementation.
You can control which DBM library you use by loading that
library before you call dbmopen():
use DB_File;
dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
or die "Can't open netscape history file: $!";
defined EXPR
defined Returns a Boolean value telling whether EXPR has a value other
than the undefined value "undef". If EXPR is not present, $_
will be checked.
Many operations return "undef" to indicate failure, end of
file, system error, uninitialized variable, and other excep-
tional conditions. This function allows you to distinguish
"undef" from other values. (A simple Boolean test will not
distinguish among "undef", zero, the empty string, and "0",
which are all equally false.) Note that since "undef" is a
valid scalar, its presence doesn't necessarily indicate an
exceptional condition: "pop" returns "undef" when its argument
is an empty array, or when the element to return happens to be
"undef".
You may also use "defined(&func)" to check whether subroutine
&func has ever been defined. The return value is unaffected by
any forward declarations of &func. Note that a subroutine
which is not defined may still be callable: its package may
have an "AUTOLOAD" method that makes it spring into existence
the first time that it is called -- see perlsub.
Use of "defined" on aggregates (hashes and arrays) is depre-
cated. It used to report whether memory for that aggregate has
ever been allocated. This behavior may disappear in future
versions of Perl. You should instead use a simple test for
size:
if (@an_array) { print "has array elements\n" }
if (%a_hash) { print "has hash members\n" }
When used on a hash element, it tells you whether the value is
defined, not whether the key exists in the hash. Use "exists"
for the latter purpose.
Examples:
print if defined $switch{'D'};
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
$debugging = 0 unless defined $debugging;
Note: Many folks tend to overuse "defined", and then are sur-
prised to discover that the number 0 and "" (the zero-length
string) are, in fact, defined values. For example, if you say
"ab" =~ /a(.*)b/;
The pattern match succeeds, and $1 is defined, despite the fact
that it matched "nothing". It didn't really fail to match any-
thing. Rather, it matched something that happened to be zero
characters long. This is all very above-board and honest.
When a function returns an undefined value, it's an admission
that it couldn't give you an honest answer. So you should use
"defined" only when you're questioning the integrity of what
you're trying to do. At other times, a simple comparison to 0
or "" is what you want.
See also "undef", "exists", "ref".
delete EXPR
Given an expression that specifies a hash element, array ele-
ment, hash slice, or array slice, deletes the specified ele-
ment(s) from the hash or array. In the case of an array, if
the array elements happen to be at the end, the size of the
array will shrink to the highest element that tests true for
exists() (or 0 if no such element exists).
Returns a list with the same number of elements as the number
of elements for which deletion was attempted. Each element of
that list consists of either the value of the element deleted,
or the undefined value. In scalar context, this means that you
get the value of the last element deleted (or the undefined
value if that element did not exist).
%hash = (foo => 11, bar => 22, baz => 33);
$scalar = delete $hash{foo}; # $scalar is 11
$scalar = delete @hash{qw(foo bar)}; # $scalar is 22
@array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
Deleting from %ENV modifies the environment. Deleting from a
hash tied to a DBM file deletes the entry from the DBM file.
Deleting from a "tie"d hash or array may not necessarily return
anything.
Deleting an array element effectively returns that position of
the array to its initial, uninitialized state. Subsequently
testing for the same element with exists() will return false.
Also, deleting array elements in the middle of an array will
not shift the index of the elements after them down. Use
splice() for that. See "exists".
The following (inefficiently) deletes all the values of %HASH
and @ARRAY:
foreach $key (keys %HASH) {
delete $HASH{$key};
}
foreach $index (0 .. $#ARRAY) {
delete $ARRAY[$index];
}
And so do these:
delete @HASH{keys %HASH};
delete @ARRAY[0 .. $#ARRAY];
But both of these are slower than just assigning the empty list
or undefining %HASH or @ARRAY:
%HASH = (); # completely empty %HASH
undef %HASH; # forget %HASH ever existed
@ARRAY = (); # completely empty @ARRAY
undef @ARRAY; # forget @ARRAY ever existed
Note that the EXPR can be arbitrarily complicated as long as
the final operation is a hash element, array element, hash
slice, or array slice lookup:
delete $ref->[$x][$y]{$key};
delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
delete $ref->[$x][$y][$index];
delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
die LIST
Outside an "eval", prints the value of LIST to "STDERR" and
exits with the current value of $! (errno). If $! is 0, exits
with the value of "($? >> 8)" (backtick `command` status). If
"($? >> 8)" is 0, exits with 255. Inside an "eval()," the
error message is stuffed into $@ and the "eval" is terminated
with the undefined value. This makes "die" the way to raise an
exception.
Equivalent examples:
die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
If the last element of LIST does not end in a newline, the cur-
rent script line number and input line number (if any) are also
printed, and a newline is supplied. Note that the "input line
number" (also known as "chunk") is subject to whatever notion
of "line" happens to be currently in effect, and is also avail-
able as the special variable $.. See "$/" in perlvar and "$."
in perlvar.
Hint: sometimes appending ", stopped" to your message will
cause it to make better sense when the string "at foo line 123"
is appended. Suppose you are running script "canasta".
die "/etc/games is no good";
die "/etc/games is no good, stopped";
produce, respectively
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
See also exit(), warn(), and the Carp module.
If LIST is empty and $@ already contains a value (typically
from a previous eval) that value is reused after appending
"\t...propagated". This is useful for propagating exceptions:
eval { ... };
die unless $@ =~ /Expected exception/;
If LIST is empty and $@ contains an object reference that has a
"PROPAGATE" method, that method will be called with additional
file and line number parameters. The return value replaces the
value in $@. i.e. as if "$@ = eval { $@->PROPAGATE(__FILE__,
__LINE__) };" were called.
If $@ is empty then the string "Died" is used.
die() can also be called with a reference argument. If this
happens to be trapped within an eval(), $@ contains the refer-
ence. This behavior permits a more elaborate exception han-
dling implementation using objects that maintain arbitrary
state about the nature of the exception. Such a scheme is
sometimes preferable to matching particular string values of $@
using regular expressions. Here's an example:
use Scalar::Util 'blessed';
eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
if ($@) {
if (blessed($@) && $@->isa("Some::Module::Exception")) {
# handle Some::Module::Exception
}
else {
# handle all other possible exceptions
}
}
Because perl will stringify uncaught exception messages before
displaying them, you may want to overload stringification oper-
ations on such custom exception objects. See overload for
details about that.
You can arrange for a callback to be run just before the "die"
does its deed, by setting the $SIG{__DIE__} hook. The associ-
ated handler will be called with the error text and can change
the error message, if it sees fit, by calling "die" again. See
"$SIG{expr}" in perlvar for details on setting %SIG entries,
and "eval BLOCK" for some examples. Although this feature was
to be run only right before your program was to exit, this is
not currently the case--the $SIG{__DIE__} hook is currently
called even inside eval()ed blocks/strings! If one wants the
hook to do nothing in such situations, put
die @_ if $^S;
as the first line of the handler (see "$^S" in perlvar).
Because this promotes strange action at a distance, this coun-
terintuitive behavior may be fixed in a future release.
do BLOCK
Not really a function. Returns the value of the last command
in the sequence of commands indicated by BLOCK. When modified
by the "while" or "until" loop modifier, executes the BLOCK
once before testing the loop condition. (On other statements
the loop modifiers test the conditional first.)
"do BLOCK" does not count as a loop, so the loop control state-
ments "next", "last", or "redo" cannot be used to leave or
restart the block. See perlsyn for alternative strategies.
do SUBROUTINE(LIST)
This form of subroutine call is deprecated. See perlsub.
do EXPR Uses the value of EXPR as a filename and executes the contents
of the file as a Perl script.
do 'stat.pl';
is just like
eval `cat stat.pl`;
except that it's more efficient and concise, keeps track of the
current filename for error messages, searches the @INC directo-
ries, and updates %INC if the file is found. See "Predefined
Names" in perlvar for these variables. It also differs in that
code evaluated with "do FILENAME" cannot see lexicals in the
enclosing scope; "eval STRING" does. It's the same, however,
in that it does reparse the file every time you call it, so you
probably don't want to do this inside a loop.
If "do" cannot read the file, it returns undef and sets $! to
the error. If "do" can read the file but cannot compile it, it
returns undef and sets an error message in $@. If the file is
successfully compiled, "do" returns the value of the last
expression evaluated.
Note that inclusion of library modules is better done with the
"use" and "require" operators, which also do automatic error
checking and raise an exception if there's a problem.
You might like to use "do" to read in a program configuration
file. Manual error checking can be done this way:
# read in config files: system first, then user
for $file ("/share/prog/defaults.rc",
"$ENV{HOME}/.someprogrc")
{
unless ($return = do $file) {
warn "couldn't parse $file: $@" if $@;
warn "couldn't do $file: $!" unless defined $return;
warn "couldn't run $file" unless $return;
}
}
dump LABEL
dump This function causes an immediate core dump. See also the -u
command-line switch in perlrun, which does the same thing.
Primarily this is so that you can use the undump program (not
supplied) to turn your core dump into an executable binary
after having initialized all your variables at the beginning of
the program. When the new binary is executed it will begin by
executing a "goto LABEL" (with all the restrictions that "goto"
suffers). Think of it as a goto with an intervening core dump
and 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.
This function is now largely obsolete, partly because it's very
hard to convert a core file into an executable, and because the
real compiler backends for generating portable bytecode and
compilable C code have superseded it. That's why you should
now invoke it as "CORE::dump()", if you don't want to be warned
against a possible typo.
If you're looking to use dump to speed up your program, con-
sider generating bytecode or native C code as described in
perlcc. If you're just trying to accelerate a CGI script, con-
sider using the "mod_perl" extension to Apache, or the CPAN
module, CGI::Fast. You might also consider autoloading or
selfloading, which at least make your program appear to run
faster.
each HASH
When called in list context, returns a 2-element list consist-
ing of the key and value for the next element of a hash, so
that you can iterate over it. When called in scalar context,
returns only the key for the next element in the hash.
Entries are returned in an apparently random order. The actual
random order is subject to change in future versions of perl,
but it is guaranteed to be in the same order as either the
"keys" or "values" function would produce on the same (unmodi-
fied) hash. Since Perl 5.8.1 the ordering is different even
between different runs of Perl for security reasons (see "Algo-
rithmic Complexity Attacks" in perlsec).
When the hash is entirely read, a null array is returned in
list context (which when assigned produces a false (0) value),
and "undef" in scalar context. The next call to "each" after
that will start iterating again. There is a single iterator
for each hash, shared by all "each", "keys", and "values" func-
tion calls in the program; it can be reset by reading all the
elements from the hash, or by evaluating "keys HASH" or "values
HASH". If you add or delete elements of a hash while you're
iterating over it, you may get entries skipped or duplicated,
so don't. Exception: It is always safe to delete the item most
recently returned by "each()", which means that the following
code will work:
while (($key, $value) = each %hash) {
print $key, "\n";
delete $hash{$key}; # This is safe
}
The following prints out your environment like the printenv(1)
program, only in a different order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}
See also "keys", "values" and "sort".
eof FILEHANDLE
eof ()
eof Returns 1 if the next read on FILEHANDLE will return end of
file, or if FILEHANDLE is not open. FILEHANDLE may be an
expression whose value gives the real filehandle. (Note that
this function actually reads a character and then "ungetc"s it,
so isn't very useful in an interactive context.) Do not read
from a terminal file (or call "eof(FILEHANDLE)" on it) after
end-of-file is reached. File types such as terminals may lose
the end-of-file condition if you do.
An "eof" without an argument uses the last file read. Using
"eof()" with empty parentheses is very different. It refers to
the pseudo file formed from the files listed on the command
line and accessed via the "<>" operator. Since "<>" isn't
explicitly opened, as a normal filehandle is, an "eof()" before
"<>" has been used will cause @ARGV to be examined to determine
if input is available. Similarly, an "eof()" after "<>" has
returned end-of-file will assume you are processing another
@ARGV list, and if you haven't set @ARGV, will read input from
"STDIN"; see "I/O Operators" in perlop.
In a "while (<>)" loop, "eof" or "eof(ARGV)" can be used to
detect the end of each file, "eof()" will only detect the end
of the last file. Examples:
# reset line numbering on each input file
while (<>) {
next if /^\s*#/; # skip comments
print "$.\t$_";
} continue {
close ARGV if eof; # Not eof()!
}
# insert dashes just before last line of last file
while (<>) {
if (eof()) { # check for end of last file
print "--------------\n";
}
print;
last if eof(); # needed if we're reading from a terminal
}
Practical hint: you almost never need to use "eof" in Perl,
because the input operators typically return "undef" when they
run out of data, or if there was an error.
eval EXPR
eval BLOCK
eval In the first form, the return value of EXPR is parsed and exe-
cuted as if it were a little Perl program. The value of the
expression (which is itself determined within scalar context)
is first parsed, and if there weren't any errors, executed in
the lexical context of the current Perl program, so that any
variable settings or subroutine and format definitions remain
afterwards. Note that the value is parsed every time the
"eval" executes. If EXPR is omitted, evaluates $_. This form
is typically used to delay parsing and subsequent execution of
the text of EXPR until run time.
In the second form, the code within the BLOCK is parsed only
once--at the same time the code surrounding the "eval" itself
was parsed--and executed within the context of the current Perl
program. This form is typically used to trap exceptions more
efficiently than the first (see below), while also providing
the benefit of checking the code within BLOCK at compile time.
The final semicolon, if any, may be omitted from the value of
EXPR or within the BLOCK.
In both forms, the value returned is the value of the last
expression evaluated inside the mini-program; a return state-
ment may be also used, just as with subroutines. The expres-
sion providing the return value is evaluated in void, scalar,
or list context, depending on the context of the "eval" itself.
See "wantarray" for more on how the evaluation context can be
determined.
If there is a syntax error or runtime error, or a "die" state-
ment is executed, an undefined value is returned by "eval", and
$@ is set to the error message. If there was no error, $@ is
guaranteed to be a null string. Beware that using "eval" nei-
ther silences perl from printing warnings to STDERR, nor does
it stuff the text of warning messages into $@. To do either of
those, you have to use the $SIG{__WARN__} facility, or turn off
warnings inside the BLOCK or EXPR using "no warnings 'all'".
See "warn", perlvar, warnings and perllexwarn.
Note that, because "eval" traps otherwise-fatal errors, it is
useful for determining whether a particular feature (such as
"socket" or "symlink") is implemented. It is also Perl's
exception trapping mechanism, where the die operator is used to
raise exceptions.
If the code to be executed doesn't vary, you may use the eval-
BLOCK form to trap run-time errors without incurring the
penalty of recompiling each time. The error, if any, is still
returned in $@. Examples:
# make divide-by-zero nonfatal
eval { $answer = $a / $b; }; warn $@ if $@;
# same thing, but less efficient
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
eval { $answer = }; # WRONG
# a run-time error
eval '$answer ='; # sets $@
Using the "eval{}" form as an exception trap in libraries does
have some issues. Due to the current arguably broken state of
"__DIE__" hooks, you may wish not to trigger any "__DIE__"
hooks that user code may have installed. You can use the
"local $SIG{__DIE__}" construct for this purpose, as shown in
this example:
# a very private exception trap for divide-by-zero
eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
warn $@ if $@;
This is especially significant, given that "__DIE__" hooks can
call "die" again, which has the effect of changing their error
messages:
# __DIE__ hooks may modify error messages
{
local $SIG{'__DIE__'} =
sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
eval { die "foo lives here" };
print $@ if $@; # prints "bar lives here"
}
Because this promotes action at a distance, this counterintu-
itive behavior may be fixed in a future release.
With an "eval", you should be especially careful to remember
what's being looked at when:
eval $x; # CASE 1
eval "$x"; # CASE 2
eval '$x'; # CASE 3
eval { $x }; # CASE 4
eval "\$$x++"; # CASE 5
$$x++; # CASE 6
Cases 1 and 2 above behave identically: they run the code con-
tained in the variable $x. (Although case 2 has misleading
double quotes making the reader wonder what else might be hap-
pening (nothing is).) Cases 3 and 4 likewise behave in the
same way: they run the code '$x', which does nothing but return
the value of $x. (Case 4 is preferred for purely visual rea-
sons, 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 par-
ticular situation, you can just use symbolic references
instead, as in case 6.
"eval BLOCK" does not count as a loop, so the loop control
statements "next", "last", or "redo" cannot be used to leave or
restart the block.
Note that as a very special case, an "eval ''" executed within
the "DB" package doesn't see the usual surrounding lexical
scope, but rather the scope of the first non-DB piece of code
that called it. You don't normally need to worry about this
unless you are writing a Perl debugger.
exec LIST
exec PROGRAM LIST
The "exec" function executes a system command and never
returns-- use "system" instead of "exec" if you want it to
return. It fails and returns false only if the command does
not exist and it is executed directly instead of via your sys-
tem's command shell (see below).
Since it's a common mistake to use "exec" instead of "system",
Perl warns you if there is a following statement which isn't
"die", "warn", or "exit" (if "-w" is set - but you always do
that). If you really want to follow an "exec" with some other
statement, you can use one of these styles to avoid the warn-
ing:
exec ('foo') or print STDERR "couldn't exec foo: $!";
{ exec ('foo') }; print STDERR "couldn't exec foo: $!";
If there is more than one argument in LIST, or if LIST is an
array with more than one value, calls execvp(3) with the argu-
ments in LIST. If there is only one scalar argument or an
array with one element in it, the argument is checked for shell
metacharacters, and if there are any, the entire argument is
passed to the system's command shell for parsing (this is
"/bin/sh -c" on Unix platforms, but varies on other platforms).
If there are no shell metacharacters in the argument, it is
split into words and passed directly to "execvp", which is more
efficient. Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";
If you don't really want to execute the first argument, but
want to lie to the program you are executing about its own
name, you can specify the program you actually want to run as
an "indirect object" (without a comma) in front of the LIST.
(This always forces interpretation of the LIST as a multivalued
list, even if there is only a single scalar in the list.)
Example:
$shell = '/bin/csh';
exec $shell '-sh'; # pretend it's a login shell
or, more directly,
exec {'/bin/csh'} '-sh'; # pretend it's a login shell
When the arguments get executed via the system shell, results
will be subject to its quirks and capabilities. See "`STRING`"
in perlop for details.
Using an indirect object with "exec" or "system" is also more
secure. This usage (which also works fine with system())
forces interpretation of the arguments as a multivalued list,
even if the list had just one argument. That way you're safe
from the shell expanding wildcards or splitting up words with
whitespace in them.
@args = ( "echo surprise" );
exec @args; # subject to shell escapes
# if @args == 1
exec { $args[0] } @args; # safe even with one-arg list
The first version, the one without the indirect object, ran the
echo program, passing it "surprise" an argument. The second
version didn't--it tried to run a program literally called
"echo surprise", didn't find it, and set $? to a non-zero value
indicating failure.
Beginning with v5.6.0, Perl will attempt to flush all files
opened for output before the exec, but this may not be sup-
ported on some platforms (see perlport). To be safe, you may
need to set $| ($AUTOFLUSH in English) or call the "aut-
oflush()" method of "IO::Handle" on any open handles in order
to avoid lost output.
Note that "exec" will not call your "END" blocks, nor will it
call any "DESTROY" methods in your objects.
exists EXPR
Given an expression that specifies a hash element or array ele-
ment, returns true if the specified element in the hash or
array has ever been initialized, even if the corresponding
value is undefined. The element is not autovivified if it
doesn't exist.
print "Exists\n" if exists $hash{$key};
print "Defined\n" if defined $hash{$key};
print "True\n" if $hash{$key};
print "Exists\n" if exists $array[$index];
print "Defined\n" if defined $array[$index];
print "True\n" if $array[$index];
A hash or array element can be true only if it's defined, and
defined if it exists, but the reverse doesn't necessarily hold
true.
Given an expression that specifies the name of a subroutine,
returns true if the specified subroutine has ever been
declared, even if it is undefined. Mentioning a subroutine
name for exists or defined does not count as declaring it.
Note that a subroutine which does not exist may still be
callable: its package may have an "AUTOLOAD" method that makes
it spring into existence the first time that it is called --
see perlsub.
print "Exists\n" if exists &subroutine;
print "Defined\n" if defined &subroutine;
Note that the EXPR can be arbitrarily complicated as long as
the final operation is a hash or array key lookup or subroutine
name:
if (exists $ref->{A}->{B}->{$key}) { }
if (exists $hash{A}{B}{$key}) { }
if (exists $ref->{A}->{B}->[$ix]) { }
if (exists $hash{A}{B}[$ix]) { }
if (exists &{$ref->{A}{B}{$key}}) { }
Although the deepest nested array or hash will not spring into
existence just because its existence was tested, any interven-
ing ones will. Thus "$ref->{"A"}" and "$ref->{"A"}->{"B"}"
will spring into existence due to the existence test for the
$key element above. This happens anywhere the arrow operator
is used, including even:
undef $ref;
if (exists $ref->{"Some key"}) { }
print $ref; # prints HASH(0x80d3d5c)
This surprising autovivification in what does not at first--or
even second--glance appear to be an lvalue context may be fixed
in a future release.
See "Pseudo-hashes: Using an array as a hash" in perlref for
specifics on how exists() acts when used on a pseudo-hash.
Use of a subroutine call, rather than a subroutine name, as an
argument to exists() is an error.
exists ⊂ # OK
exists &sub(); # Error
exit EXPR
exit Evaluates EXPR and exits immediately with that value. Exam-
ple:
$ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
See also "die". If EXPR is omitted, exits with 0 status. The
only universally recognized values for EXPR are 0 for success
and 1 for error; other values are subject to interpretation
depending on the environment in which the Perl program is run-
ning. For example, exiting 69 (EX_UNAVAILABLE) from a sendmail
incoming-mail filter will cause the mailer to return the item
undelivered, but that's not true everywhere.
Don't use "exit" to abort a subroutine if there's any chance
that someone might want to trap whatever error happened. Use
"die" instead, which can be trapped by an "eval".
The exit() function does not always exit immediately. It calls
any defined "END" routines first, but these "END" routines may
not themselves abort the exit. Likewise any object destructors
that need to be called are called before the real exit. If
this is a problem, you can call "POSIX:_exit($status)" to avoid
END and destructor processing. See perlmod for details.
exp EXPR
exp Returns e (the natural logarithm base) to the power of EXPR.
If EXPR is omitted, gives "exp($_)".
fcntl FILEHANDLE,FUNCTION,SCALAR
Implements the fcntl(2) function. You'll probably have to say
use Fcntl;
first to get the correct constant definitions. Argument pro-
cessing and value return works just like "ioctl" below. For
example:
use Fcntl;
fcntl($filehandle, F_GETFL, $packed_return_buffer)
or die "can't fcntl F_GETFL: $!";
You don't have to check for "defined" on the return from
"fcntl". Like "ioctl", it maps a 0 return from the system call
into "0 but true" in Perl. This string is true in boolean con-
text and 0 in numeric context. It is also exempt from the nor-
mal -w warnings on improper numeric conversions.
Note that "fcntl" will produce a fatal error if used on a
machine that doesn't implement fcntl(2). See the Fcntl module
or your fcntl(2) manpage to learn what functions are available
on your system.
Here's an example of setting a filehandle named "REMOTE" to be
non-blocking at the system level. You'll have to negotiate $|
on your own, though.
use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
$flags = fcntl(REMOTE, F_GETFL, 0)
or die "Can't get flags for the socket: $!\n";
$flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
or die "Can't set flags for the socket: $!\n";
fileno FILEHANDLE
Returns the file descriptor for a filehandle, or undefined if
the filehandle is not open. This is mainly useful for con-
structing bitmaps for "select" and low-level POSIX tty-handling
operations. If FILEHANDLE is an expression, the value is taken
as an indirect filehandle, generally its name.
You can use this to find out whether two handles refer to the
same underlying descriptor:
if (fileno(THIS) == fileno(THAT)) {
print "THIS and THAT are dups\n";
}
(Filehandles connected to memory objects via new features of
"open" may return undefined even though they are open.)
flock FILEHANDLE,OPERATION
Calls flock(2), or an emulation of it, on FILEHANDLE. Returns
true for success, false on failure. 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.
Two potentially non-obvious but traditional "flock" semantics
are that it waits indefinitely until the lock is granted, and
that its locks merely advisory. Such discretionary locks are
more flexible, but offer fewer guarantees. This means that
programs that do not also use "flock" may modify files locked
with "flock". See perlport, your port's specific documenta-
tion, or your system-specific local manpages for details. It's
best to assume traditional behavior if you're writing portable
programs. (But if you're not, you should as always feel per-
fectly free to write for your own system's idiosyncrasies
(sometimes called "features"). Slavish adherence to portabil-
ity concerns shouldn't get in the way of your getting your job
done.)
OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly com-
bined with LOCK_NB. These constants are traditionally valued
1, 2, 8 and 4, but you can use the symbolic names if you import
them from the Fcntl module, either individually, or as a group
using the ':flock' tag. LOCK_SH requests a shared lock,
LOCK_EX requests an exclusive lock, and LOCK_UN releases a pre-
viously requested lock. If LOCK_NB is bitwise-or'ed with
LOCK_SH or LOCK_EX then "flock" will return immediately rather
than blocking waiting for the lock (check the return status to
see if you got it).
To avoid the possibility of miscoordination, Perl now flushes
FILEHANDLE before locking or unlocking it.
Note that the emulation built with lockf(3) doesn't provide
shared locks, and it requires that FILEHANDLE be open with
write intent. These are the semantics that lockf(3) imple-
ments. Most if not all systems implement lockf(3) in terms of
fcntl(2) locking, though, so the differing semantics shouldn't
bite too many people.
Note that the fcntl(2) emulation of flock(3) requires that
FILEHANDLE be open with read intent to use LOCK_SH and requires
that it be open with write intent to use LOCK_EX.
Note also that some versions of "flock" cannot lock things over
the network; you would need to use the more system-specific
"fcntl" for that. If you like you can force Perl to ignore
your system's flock(2) function, and so provide its own
fcntl(2)-based emulation, by passing the switch "-Ud_flock" to
the 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();
On systems that support a real flock(), locks are inherited
across fork() calls, whereas those that must resort to the more
capricious fcntl() function lose the locks, making it harder to
write servers.
See also DB_File for other flock() examples.
fork Does a fork(2) system call to create a new process running the
same program at the same point. It returns the child pid to
the parent process, 0 to the child process, or "undef" if the
fork is unsuccessful. File descriptors (and sometimes locks on
those descriptors) are shared, while everything else is copied.
On most systems supporting fork(), great care has gone into
making it extremely efficient (for example, using copy-on-write
technology on data pages), making it the dominant paradigm for
multitasking over the last few decades.
Beginning with v5.6.0, Perl will attempt to flush all files
opened for output before forking the child process, but this
may not be supported on some platforms (see perlport). To be
safe, you may need to set $| ($AUTOFLUSH in English) or call
the "autoflush()" method of "IO::Handle" on any open handles in
order to avoid duplicate output.
If you "fork" without ever waiting on your children, you will
accumulate zombies. On some systems, you can avoid this by
setting $SIG{CHLD} to "IGNORE". See also perlipc for more
examples of forking and reaping moribund children.
Note that if your forked child inherits system file descriptors
like STDIN and STDOUT that are actually connected by a pipe or
socket, even if you exit, then the remote server (such as, say,
a CGI script or a backgrounded job launched from a remote
shell) won't think you're done. You should reopen those to
/dev/null if it's any issue.
format Declare a picture format for use by the "write" function. For
example:
format Something =
Test: @<<<<<<<< @||||| @>>>>>
$str, $%, '$' . int($num)
.
$str = "widget";
$num = $cost/$quantity;
$~ = 'Something';
write;
See perlform for many details and examples.
formline PICTURE,LIST
This is an internal function used by "format"s, though you may
call it, too. It formats (see perlform) a list of values
according to the contents of PICTURE, placing the output into
the format output accumulator, $^A (or $ACCUMULATOR in
English). Eventually, when a "write" is done, the contents of
$^A are written to some filehandle. You could also read $^A
and then set $^A back to "". Note that a format typically does
one "formline" per line of form, but the "formline" function
itself doesn't care how many newlines are embedded in the PIC-
TURE. This means that the "~" and "~~" tokens will treat the
entire PICTURE as a single line. You may therefore need to use
multiple formlines to implement a single record format, just
like the format compiler.
Be careful if you put double quotes around the picture, because
an "@" character may be taken to mean the beginning of an array
name. "formline" always returns true. See perlform for other
examples.
getc FILEHANDLE
getc Returns the next character from the input file attached to
FILEHANDLE, or the undefined value at end of file, or if there
was an error (in the latter case $! is set). If FILEHANDLE is
omitted, reads from STDIN. This is not particularly efficient.
However, it cannot be used by itself to fetch single characters
without waiting for the user to hit enter. For that, try some-
thing more like:
if ($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 sys-
tems purporting POSIX compliance. See also the "Term::ReadKey"
module from your nearest CPAN site; details on CPAN can be
found on "CPAN" in perlmodlib.
getlogin
This implements the C library function of the same name, which
on most systems returns the current login from /etc/utmp, if
any. If null, use "getpwuid".
$login = getlogin || getpwuid($<) || "Kilroy";
Do not consider "getlogin" for authentication: it is not as
secure as "getpwuid".
getpeername SOCKET
Returns the packed sockaddr address of other end of the SOCKET
connection.
use Socket;
$hersockaddr = getpeername(SOCK);
($port, $iaddr) = sockaddr_in($hersockaddr);
$herhostname = gethostbyaddr($iaddr, AF_INET);
$herstraddr = inet_ntoa($iaddr);
getpgrp PID
Returns the current process group for the specified PID. Use a
PID of 0 to get the current process group for the current
process. Will raise an exception if used on a machine that
doesn't implement getpgrp(2). If PID is omitted, returns
process group of current process. Note that the POSIX version
of "getpgrp" does not accept a PID argument, so only "PID==0"
is truly portable.
getppid Returns the process id of the parent process.
Note for Linux users: on Linux, the C functions "getpid()" and
"getppid()" return different values from different threads. In
order to be portable, this behavior is not reflected by the
perl-level function "getppid()", that returns a consistent
value across threads. If you want to call the underlying "getp-
pid()", you may use the CPAN module "Linux::Pid".
getpriority WHICH,WHO
Returns the current priority for a process, a process group, or
a user. (See getpriority(2).) Will raise a fatal exception if
used on a machine that doesn't implement getpriority(2).
getpwnam NAME
getgrnam NAME
gethostbyname NAME
getnetbyname NAME
getprotobyname NAME
getpwuid UID
getgrgid GID
getservbyname NAME,PROTO
gethostbyaddr ADDR,ADDRTYPE
getnetbyaddr ADDR,ADDRTYPE
getprotobynumber NUMBER
getservbyport PORT,PROTO
getpwent
getgrent
gethostent
getnetent
getprotoent
getservent
setpwent
setgrent
sethostent STAYOPEN
setnetent STAYOPEN
setprotoent STAYOPEN
setservent STAYOPEN
endpwent
endgrent
endhostent
endnetent
endprotoent
endservent
These routines perform the same functions as their counterparts
in the system library. In list context, the return values from
the various get routines are as follows:
($name,$passwd,$uid,$gid,
$quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
($name,$passwd,$gid,$members) = getgr*
($name,$aliases,$addrtype,$length,@addrs) = gethost*
($name,$aliases,$addrtype,$net) = getnet*
($name,$aliases,$proto) = getproto*
($name,$aliases,$port,$proto) = getserv*
(If the entry doesn't exist you get a null list.)
The exact meaning of the $gcos field varies but it usually con-
tains the real name of the user (as opposed to the login name)
and other information pertaining to the user. Beware, however,
that in many system users are able to change this information
and therefore it cannot be trusted and therefore the $gcos is
tainted (see perlsec). The $passwd and $shell, user's
encrypted password and login shell, are also tainted, because
of the same reason.
In scalar context, you get the name, unless the function was a
lookup by name, in which case you get the other thing, whatever
it is. (If the entry doesn't exist you get the undefined
value.) For example:
$uid = getpwnam($name);
$name = getpwuid($num);
$name = getpwent();
$gid = getgrnam($name);
$name = getgrgid($num);
$name = getgrent();
#etc.
In getpw*() the fields $quota, $comment, and $expire are spe-
cial cases in the sense that in many systems they are unsup-
ported. If the $quota is unsupported, it is an empty scalar.
If it is supported, it usually encodes the disk quota. If the
$comment field is unsupported, it is an empty scalar. If it is
supported it usually encodes some administrative comment about
the user. In some systems the $quota field may be $change or
$age, fields that have to do with password aging. In some sys-
tems the $comment field may be $class. The $expire field, if
present, encodes the expiration period of the account or the
password. For the availability and the exact meaning of these
fields in your system, please consult your getpwnam(3) documen-
tation and your pwd.h file. You can also find out from within
Perl what your $quota and $comment fields mean and whether you
have the $expire field by using the "Config" module and the
values "d_pwquota", "d_pwage", "d_pwchange", "d_pwcomment", and
"d_pwexpire". Shadow password files are only supported if your
vendor has implemented them in the intuitive fashion that call-
ing the regular C library routines gets the shadow versions if
you're running under privilege or if there exists the shadow(3)
functions as found in System V (this includes Solaris and
Linux.) Those systems that implement a proprietary shadow
password facility are unlikely to be supported.
The $members value returned by getgr*() is a space separated
list of the login names of the members of the group.
For the gethost*() functions, if the "h_errno" variable is sup-
ported in C, it will be returned to you via $? if the function
call fails. The @addrs value returned by a successful call is
a list of the raw addresses returned by the corresponding sys-
tem library call. In the Internet domain, each address is four
bytes long and you can unpack it by saying something like:
($a,$b,$c,$d) = unpack('C4',$addr[0]);
The Socket library makes this slightly easier:
use Socket;
$iaddr = inet_aton("127.1"); # or whatever address
$name = gethostbyaddr($iaddr, AF_INET);
# or going the other way
$straddr = inet_ntoa($iaddr);
If you get tired of remembering which element of the return
list contains which return value, by-name interfaces are pro-
vided in standard modules: "File::stat", "Net::hostent",
"Net::netent", "Net::protoent", "Net::servent", "Time::gmtime",
"Time::localtime", and "User::grent". These override the nor-
mal built-ins, supplying versions that return objects with the
appropriate names for each field. For example:
use File::stat;
use User::pwent;
$is_his = (stat($filename)->uid == pwent($whoever)->uid);
Even though it looks like they're the same method calls (uid),
they aren't, because a "File::stat" object is different from a
"User::pwent" object.
getsockname SOCKET
Returns the packed sockaddr address of this end of the SOCKET
connection, in case you don't know the address because you have
several different IPs that the connection might have come in
on.
use Socket;
$mysockaddr = getsockname(SOCK);
($port, $myaddr) = sockaddr_in($mysockaddr);
printf "Connect to %s [%s]\n",
scalar gethostbyaddr($myaddr, AF_INET),
inet_ntoa($myaddr);
getsockopt SOCKET,LEVEL,OPTNAME
Queries the option named OPTNAME associated with SOCKET at a
given LEVEL. Options may exist at multiple protocol levels
depending on the socket type, but at least the uppermost socket
level SOL_SOCKET (defined in the "Socket" module) will exist.
To query options at another level the protocol number of the
appropriate protocol controlling the option should be supplied.
For example, to indicate that an option is to be interpreted by
the TCP protocol, LEVEL should be set to the protocol number of
TCP, which you can get using getprotobyname.
The call returns a packed string representing the requested
socket option, or "undef" if there is an error (the error rea-
son will be in $!). What exactly is in the packed string
depends in the LEVEL and OPTNAME, consult your system documen-
tation for details. A very common case however is that the
option is an integer, in which case the result will be a packed
integer which you can decode using unpack with the "i" (or "I")
format.
An example testing if Nagle's algorithm is turned on on a
socket:
use Socket qw(:all);
defined(my $tcp = getprotobyname("tcp"))
or die "Could not determine the protocol number for tcp";
# my $tcp = IPPROTO_TCP; # Alternative
my $packed = getsockopt($socket, $tcp, TCP_NODELAY)
or die "Could not query TCP_NODELAY socket option: $!";
my $nodelay = unpack("I", $packed);
print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
glob EXPR
glob In list context, returns a (possibly empty) list of filename
expansions on the value of EXPR such as the standard Unix shell
/bin/csh would do. In scalar context, glob iterates through
such filename expansions, returning undef when the list is
exhausted. This is the internal function implementing the
"<*.c>" operator, but you can use it directly. If EXPR is omit-
ted, $_ is used. The "<*.c>" operator is discussed in more
detail in "I/O Operators" in perlop.
Beginning with v5.6.0, this operator is implemented using the
standard "File::Glob" extension. See File::Glob for details.
gmtime EXPR
gmtime Converts a time as returned by the time function to an 9-ele-
ment list with the time localized for the standard Greenwich
time zone. Typically used as follows:
# 0 1 2 3 4 5 6 7 8
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
gmtime(time);
All list elements are numeric, and come straight out of the C
`struct tm'. $sec, $min, and $hour are the seconds, minutes,
and hours of the specified time. $mday is the day of the
month, and $mon is the month itself, in the range 0..11 with 0
indicating January and 11 indicating December. $year is the
number of years since 1900. That is, $year is 123 in year
2023. $wday is the day of the week, with 0 indicating Sunday
and 3 indicating Wednesday. $yday is the day of the year, in
the range 0..364 (or 0..365 in leap years). $isdst is always
0.
Note that the $year element is not simply the last two digits
of the year. If you assume it is then you create non-Y2K-com-
pliant programs--and you wouldn't want to do that, would you?
The proper way to get a complete 4-digit year is simply:
$year += 1900;
And to get the last two digits of the year (e.g., '01' in 2001)
do:
$year = sprintf("%02d", $year % 100);
If EXPR is omitted, "gmtime()" uses the current time
("gmtime(time)").
In scalar context, "gmtime()" returns the ctime(3) value:
$now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
If you need local time instead of GMT use the "localtime"
builtin. See also the "timegm" function provided by the
"Time::Local" module, and the strftime(3) and mktime(3) func-
tions available via the POSIX module.
This scalar value is not locale dependent (see perllocale), but
is instead a Perl builtin. To get somewhat similar but locale
dependent date strings, see the example in "localtime".
See "gmtime" in perlport for portability concerns.
goto LABEL
goto EXPR
goto &NAME
The "goto-LABEL" form finds the statement labeled with LABEL
and resumes execution there. It may not be used to go into any
construct that requires initialization, such as a subroutine or
a "foreach" loop. It also can't be used to go into a construct
that is optimized away, or to get out of a block or subroutine
given to "sort". It can be used to go almost anywhere else
within the dynamic scope, including out of subroutines, but
it's usually better to use some other construct such as "last"
or "die". The author of Perl has never felt the need to use
this form of "goto" (in Perl, that is--C is another matter).
(The difference being that C does not offer named loops com-
bined with loop control. Perl does, and this replaces most
structured uses of "goto" in other languages.)
The "goto-EXPR" form expects a label name, whose scope will be
resolved dynamically. This allows for computed "goto"s per
FORTRAN, but isn't necessarily recommended if you're optimizing
for maintainability:
goto ("FOO", "BAR", "GLARCH")[$i];
The "goto-&NAME" form is quite different from the other forms
of "goto". In fact, it isn't a goto in the normal sense at
all, and doesn't have the stigma associated with other gotos.
Instead, it exits the current subroutine (losing any changes
set by local()) and immediately calls in its place the named
subroutine using the current value of @_. This is used by
"AUTOLOAD" subroutines that wish to load another subroutine and
then pretend that the other subroutine had been called in the
first place (except that any modifications to @_ in the current
subroutine are propagated to the other subroutine.) After the
"goto", not even "caller" will be able to tell that this rou-
tine was called first.
NAME needn't be the name of a subroutine; it can be a scalar
variable containing a code reference, or a block that evaluates
to a code reference.
grep BLOCK LIST
grep EXPR,LIST
This is similar in spirit to, but not the same as, grep(1) and
its relatives. In particular, it is not limited to using regu-
lar expressions.
Evaluates the BLOCK or EXPR for each element of LIST (locally
setting $_ to each element) and returns the list value consist-
ing of those elements for which the expression evaluated to
true. In scalar context, returns the number of times the
expression was true.
@foo = grep(!/^#/, @bar); # weed out comments
or equivalently,
@foo = grep {!/^#/} @bar; # weed out comments
Note that $_ is an alias to the list value, so it can be used
to modify the elements of the LIST. While this is useful and
supported, it can cause bizarre results if the elements of LIST
are not variables. Similarly, grep returns aliases into the
original list, much as a for loop's index variable aliases the
list elements. That is, modifying an element of a list
returned by grep (for example, in a "foreach", "map" or another
"grep") actually modifies the element in the original list.
This is usually something to be avoided when writing clear
code.
See also "map" for a list composed of the results of the BLOCK
or EXPR.
hex EXPR
hex Interprets EXPR as a hex string and returns the corresponding
value. (To convert strings that might start with either 0,
"0x", or "0b", see "oct".) If EXPR is omitted, uses $_.
print hex '0xAf'; # prints '175'
print hex 'aF'; # same
Hex strings may only represent integers. Strings that would
cause integer overflow trigger a warning. Leading whitespace
is not stripped, unlike oct(). To present something as hex,
look into "printf", "sprintf", or "unpack".
import LIST
There is no builtin "import" function. It is just an ordinary
method (subroutine) defined (or inherited) by modules that wish
to export names to another module. The "use" function calls
the "import" method for the package used. See also "use",
perlmod, and Exporter.
index STR,SUBSTR,POSITION
index STR,SUBSTR
The index function searches for one string within another, but
without the wildcard-like behavior of a full regular-expression
pattern match. It returns the position of the first occurrence
of SUBSTR in STR at or after POSITION. If POSITION is omitted,
starts searching from the beginning of the string. POSITION
before the beginning of the string or after its end is treated
as if it were the beginning or the end, respectively. POSITION
and the return value are based at 0 (or whatever you've set the
$[ variable to--but don't do that). If the substring is not
found, "index" returns one less than the base, ordinarily "-1".
int EXPR
int Returns the integer portion of EXPR. If EXPR is omitted, uses
$_. You should not use this function for rounding: one because
it truncates towards 0, and two because machine representations
of floating point numbers can sometimes produce counterintu-
itive results. For example, "int(-6.725/0.025)" produces -268
rather than the correct -269; that's because it's really more
like -268.99999999999994315658 instead. Usually, the
"sprintf", "printf", or the "POSIX::floor" and "POSIX::ceil"
functions will serve you better than will int().
ioctl FILEHANDLE,FUNCTION,SCALAR
Implements the ioctl(2) function. You'll probably first have
to say
require "sys/ioctl.ph"; # probably in $Config{archlib}/sys/ioctl.ph
to get the correct function definitions. If sys/ioctl.ph
doesn't exist or doesn't have the correct 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 that may help you in this, but it's nontriv-
ial.) SCALAR will be read and/or written depending on the
FUNCTION--a pointer to the string value of SCALAR will be
passed as the third argument of the actual "ioctl" call. (If
SCALAR has no string value but does have a numeric value, that
value will be passed rather than a pointer to the string value.
To guarantee this to be true, add a 0 to the scalar before
using it.) The "pack" and "unpack" functions may be needed to
manipulate the values of structures used by "ioctl".
The return value of "ioctl" (and "fcntl") is as follows:
if OS returns: then Perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
Thus Perl returns true on success and false on failure, yet you
can still easily determine the actual value returned by the
operating system:
$retval = ioctl(...) || -1;
printf "System returned %d\n", $retval;
The special string "0 but true" is exempt from -w complaints
about improper numeric conversions.
join EXPR,LIST
Joins the separate strings of LIST into a single string with
fields separated by the value of EXPR, and returns that new
string. Example:
$rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
Beware that unlike "split", "join" doesn't take a pattern as
its first argument. Compare "split".
keys HASH
Returns a list consisting of all the keys of the named hash.
(In scalar context, returns the number of keys.)
The keys are returned in an apparently random order. The
actual random order is subject to change in future versions of
perl, but it is guaranteed to be the same order as either the
"values" or "each" function produces (given that the hash has
not been modified). Since Perl 5.8.1 the ordering is different
even between different runs of Perl for security reasons (see
"Algorithmic Complexity Attacks" in perlsec).
As a side effect, calling keys() resets the HASH's internal
iterator (see "each"). In particular, calling keys() in void
context resets the iterator with no other overhead.
Here is yet another way to print your environment:
@keys = keys %ENV;
@values = values %ENV;
while (@keys) {
print pop(@keys), '=', pop(@values), "\n";
}
or how about sorted by key:
foreach $key (sort(keys %ENV)) {
print $key, '=', $ENV{$key}, "\n";
}
The returned values are copies of the original keys in the
hash, so modifying them will not affect the original hash.
Compare "values".
To sort a hash by value, you'll need to use a "sort" function.
Here's a descending numeric sort of a hash by its values:
foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
printf "%4d %s\n", $hash{$key}, $key;
}
As an lvalue "keys" allows you to increase the number of hash
buckets allocated for the given hash. This can gain you a mea-
sure of efficiency if you know the hash is going to get big.
(This is similar to pre-extending an array by assigning a
larger number to $#array.) If you say
keys %hash = 200;
then %hash will have at least 200 buckets allocated for it--256
of them, in fact, since it rounds up to the next power of two.
These buckets will be retained even if you do "%hash = ()", use
"undef %hash" if you want to free the storage while %hash is
still in scope. You can't shrink the number of buckets allo-
cated for the hash using "keys" in this way (but you needn't
worry about doing this by accident, as trying has no effect).
See also "each", "values" and "sort".
kill SIGNAL, LIST
Sends a signal to a list of processes. Returns the number of
processes successfully signaled (which is not necessarily the
same as the number actually killed).
$cnt = kill 1, $child1, $child2;
kill 9, @goners;
If SIGNAL is zero, no signal is sent to the process. This is a
useful way to check that a child process is alive and hasn't
changed its UID. See perlport for notes on the portability of
this construct.
Unlike in the shell, if SIGNAL is negative, it kills process
groups instead of processes. (On System V, a negative PROCESS
number will also kill process groups, but that's not portable.)
That means you usually want to use positive not negative sig-
nals. You may also use a signal name in quotes.
See "Signals" in perlipc for more details.
last LABEL
last The "last" command is like the "break" statement in C (as used
in loops); it immediately exits the loop in question. If the
LABEL is omitted, the command refers to the innermost enclosing
loop. The "continue" block, if any, is not executed:
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
#...
}
"last" cannot be used to exit a block which returns a value
such as "eval {}", "sub {}" or "do {}", and should not be used
to exit a grep() or map() operation.
Note that a block by itself is semantically identical to a loop
that executes once. Thus "last" can be used to effect an early
exit out of such a block.
See also "continue" for an illustration of how "last", "next",
and "redo" work.
lc EXPR
lc Returns a lowercased version of EXPR. This is the internal
function implementing the "\L" escape in double-quoted strings.
Respects current LC_CTYPE locale if "use locale" in force. See
perllocale and perlunicode for more details about locale and
Unicode support.
If EXPR is omitted, uses $_.
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 and perlunicode for more
details about locale and Unicode support.
If EXPR is omitted, uses $_.
length EXPR
length Returns the length in characters of the value of EXPR. If EXPR
is omitted, returns length of $_. Note that this cannot be
used on an entire array or hash to find out how many elements
these have. For that, use "scalar @array" and "scalar keys
%hash" respectively.
Note the characters: if the EXPR is in Unicode, you will get
the number of characters, not the number of bytes. To get the
length in bytes, use "do { use bytes; length(EXPR) }", see
bytes.
link OLDFILE,NEWFILE
Creates a new filename linked to the old filename. Returns
true for success, false otherwise.
listen SOCKET,QUEUESIZE
Does the same thing that the listen system call does. Returns
true if it succeeded, false otherwise. See the example in
"Sockets: Client/Server Communication" in perlipc.
local EXPR
You really probably want to be using "my" instead, because
"local" isn't what most people think of as "local". See "Pri-
vate Variables via my()" in perlsub for details.
A local modifies the listed variables to be local to the
enclosing block, file, or eval. If more than one value is
listed, the list must be placed in parentheses. See "Tempora | | |
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