 |
Index for
Section 1 |
|
 |
Alphabetical
listing for P |
|
 |
Bottom of
page |
|
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 following comma. (See
the precedence table in perlop.) List operators take more than one
argument, while unary operators can never take more than one argument.
Thus, a comma terminates the argument of a unary operator, but merely
separates the arguments of a list operator. A unary operator generally
provides a scalar context to its argument, while a list operator may
provide either scalar or list contexts for its arguments. If it does both,
the scalar arguments will be first, and the list argument will follow.
(Note that there can ever be only one such list argument.) For instance,
splice() has three scalar arguments followed by a list, whereas
gethostbyname() has four scalar arguments.
In the syntax descriptions that follow, list operators that expect a list
(and provide list context for the elements of the list) are shown with LIST
as an argument. Such a list may consist of any combination of scalar
arguments or list values; the list values will be included in the list as
if each individual element were interpolated at that point in the list,
forming a longer single-dimensional list value. Elements of the LIST
should be separated by commas.
Any function in the list below may be used either with or without
parentheses around its arguments. (The syntax descriptions omit the
parentheses.) If you use the parentheses, the simple (but occasionally
surprising) rule is this: It looks like a function, therefore it is a
function, and precedence doesn't matter. Otherwise it's a list operator or
unary operator, and precedence does matter. And whitespace between the
function and left parenthesis doesn't count--so you need to be careful
sometimes:
print 1+2+4; # Prints 7.
print(1+2) + 4; # Prints 3.
print (1+2)+4; # Also prints 3!
print +(1+2)+4; # Prints 7.
print ((1+2)+4); # Prints 7.
If you run Perl with the -w switch it can warn you about this. For
example, the third line above produces:
print (...) interpreted as function at - line 1.
Useless use of integer addition in void context at - line 1.
A few functions take no arguments at all, and therefore work as neither
unary nor list operators. These include such functions as "time" and
"endpwent". For example, "time+86_400" always means "time() + 86_400".
For functions that can be used in either a scalar or list context,
nonabortive failure is generally indicated in a scalar context by returning
the undefined value, and in a list context by returning the null list.
Remember the following important rule: There is no rule that relates the
behavior of an expression in list 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 successful operations. In
general, they do what you want, unless you want consistency.
A named array in scalar context is quite different from what would at first
glance appear to be a list in scalar context. You can't get a list like
"(1,2,3)" into being in scalar context, because the compiler knows the
context at compile time. It would generate the scalar comma operator
there, not the list construction version of the comma. That means it was
never a list to start with.
In general, functions in Perl that serve as wrappers for system calls of
the same name (like chown(2), fork(2), closedir(2), etc.) all return true
when they succeed and "undef" otherwise, as is usually mentioned in the
descriptions below. This is different from the C interfaces, which return
"-1" on failure. Exceptions to this rule are "wait", "waitpid", and
"syscall". System calls also set the special $! variable on failure.
Other functions do not, except accidentally.
Perl Functions by Category
Here are Perl's functions (including things that look like functions, like
some keywords and named operators) arranged by category. Some functions
appear in more than one place.
Functions for SCALARs or strings
"chomp", "chop", "chr", "crypt", "hex", "index", "lc", "lcfirst",
"length", "oct", "ord", "pack", "q/STRING/", "qq/STRING/", "reverse",
"rindex", "sprintf", "substr", "tr///", "uc", "ucfirst", "y///"
Regular expressions and pattern matching
"m//", "pos", "quotemeta", "s///", "split", "study", "qr//"
Numeric functions
"abs", "atan2", "cos", "exp", "hex", "int", "log", "oct", "rand",
"sin", "sqrt", "srand"
Functions for real @ARRAYs
"pop", "push", "shift", "splice", "unshift"
Functions for list data
"grep", "join", "map", "qw/STRING/", "reverse", "sort", "unpack"
Functions for real %HASHes
"delete", "each", "exists", "keys", "values"
Input and output functions
"binmode", "close", "closedir", "dbmclose", "dbmopen", "die", "eof",
"fileno", "flock", "format", "getc", "print", "printf", "read",
"readdir", "rewinddir", "seek", "seekdir", "select", "syscall",
"sysread", "sysseek", "syswrite", "tell", "telldir", "truncate",
"warn", "write"
Functions for fixed length data or records
"pack", "read", "syscall", "sysread", "syswrite", "unpack", "vec"
Functions for filehandles, files, or directories
"-X", "chdir", "chmod", "chown", "chroot", "fcntl", "glob", "ioctl",
"link", "lstat", "mkdir", "open", "opendir", "readlink", "rename",
"rmdir", "stat", "symlink", "sysopen", "umask", "unlink", "utime"
Keywords related to the control flow of your perl program
"caller", "continue", "die", "do", "dump", "eval", "exit", "goto",
"last", "next", "redo", "return", "sub", "wantarray"
Keywords related to scoping
"caller", "import", "local", "my", "our", "package", "use"
Miscellaneous functions
"defined", "dump", "eval", "formline", "local", "my", "our", "reset",
"scalar", "undef", "wantarray"
Functions for processes and process groups
"alarm", "exec", "fork", "getpgrp", "getppid", "getpriority", "kill",
"pipe", "qx/STRING/", "setpgrp", "setpriority", "sleep", "system",
"times", "wait", "waitpid"
Keywords related to perl modules
"do", "import", "no", "package", "require", "use"
Keywords related to classes and object-orientedness
"bless", "dbmclose", "dbmopen", "package", "ref", "tie", "tied",
"untie", "use"
Low-level socket functions
"accept", "bind", "connect", "getpeername", "getsockname",
"getsockopt", "listen", "recv", "send", "setsockopt", "shutdown",
"socket", "socketpair"
System V interprocess communication functions
"msgctl", "msgget", "msgrcv", "msgsnd", "semctl", "semget", "semop",
"shmctl", "shmget", "shmread", "shmwrite"
Fetching user and group info
"endgrent", "endhostent", "endnetent", "endpwent", "getgrent",
"getgrgid", "getgrnam", "getlogin", "getpwent", "getpwnam", "getpwuid",
"setgrent", "setpwent"
Fetching network info
"endprotoent", "endservent", "gethostbyaddr", "gethostbyname",
"gethostent", "getnetbyaddr", "getnetbyname", "getnetent",
"getprotobyname", "getprotobynumber", "getprotoent", "getservbyname",
"getservbyport", "getservent", "sethostent", "setnetent",
"setprotoent", "setservent"
Time-related functions
"gmtime", "localtime", "time", "times"
Functions new in perl5
"abs", "bless", "chomp", "chr", "exists", "formline", "glob", "import",
"lc", "lcfirst", "map", "my", "no", "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", "endprotoent",
"endpwent", "endservent", "exec", "fcntl", "flock", "fork", "getgrent",
"getgrgid", "gethostbyname", "gethostent", "getlogin", "getnetbyaddr",
"getnetbyname", "getnetent", "getppid", "getprgp", "getpriority",
"getprotobynumber", "getprotoent", "getpwent", "getpwnam", "getpwuid",
"getservbyport", "getservent", "getsockopt", "glob", "ioctl", "kill",
"link", "lstat", "msgctl", "msgget", "msgrcv", "msgsnd", "open", "pipe",
"readlink", "rename", "select", "semctl", "semget", "semop", "setgrent",
"sethostent", "setnetent", "setpgrp", "setpriority", "setprotoent",
"setpwent", "setservent", "setsockopt", "shmctl", "shmget", "shmread",
"shmwrite", "socket", "socketpair", "stat", "symlink", "syscall",
"sysopen", "system", "times", "truncate", "umask", "unlink", "utime",
"wait", "waitpid"
For more information about the portability of these functions, see perlport
and other available platform-specific documentation.
Alphabetical Listing of Perl Functions
-X FILEHANDLE
-X EXPR
-X A file test, where X is one of the letters listed below. This
unary operator takes one argument, either a filename or a
filehandle, and tests the associated file to see if something is
true about it. If the argument is omitted, tests $_, except for
"-t", which tests STDIN. Unless otherwise documented, it returns 1
for true and '' for false, or the undefined value if the file
doesn't exist. Despite the funny names, precedence is the same as
any other named unary operator, and the argument may be
parenthesized like any other unary operator. The operator may be
any of:
-r File is readable by effective uid/gid.
-w File is writable by effective uid/gid.
-x File is executable by effective uid/gid.
-o File is owned by effective uid.
-R File is readable by real uid/gid.
-W File is writable by real uid/gid.
-X File is executable by real uid/gid.
-O File is owned by real uid.
-e File exists.
-z File has zero size (is empty).
-s File has nonzero size (returns size in bytes).
-f File is a plain file.
-d File is a directory.
-l File is a symbolic link.
-p File is a named pipe (FIFO), or Filehandle is a pipe.
-S File is a socket.
-b File is a block special file.
-c File is a character special file.
-t Filehandle is opened to a tty.
-u File has setuid bit set.
-g File has setgid bit set.
-k File has sticky bit set.
-T File is an ASCII text file (heuristic guess).
-B File is a "binary" file (opposite of -T).
-M Script start time minus file modification time, in days.
-A Same for access time.
-C Same for inode change time (Unix, may differ for other platforms)
Example:
while (<>) {
chomp;
next unless -f $_; # ignore specials
#...
}
The interpretation of the file permission operators "-r", "-R",
"-w", "-W", "-x", and "-X" is by default based solely on the mode
of the file and the uids and gids of the user. There may be other
reasons you can't actually read, write, or execute the file. Such
reasons may be for example network filesystem access controls, ACLs
(access control lists), read-only filesystems, and unrecognized
executable formats.
Also note that, for the superuser on the local filesystems, the
"-r", "-R", "-w", and "-W" tests always return 1, and "-x" and "-X"
return 1 if any execute bit is set in the mode. Scripts run by the
superuser may thus need to do a stat() to determine the actual mode
of the file, or temporarily set their effective uid to something
else.
If you are using ACLs, there is a pragma called "filetest" that may
produce more accurate results than the bare stat() mode bits. When
under the "use filetest 'access'" the above-mentioned filetests
will test whether the permission can (not) be granted using the
access() family of system calls. Also note that the "-x" and "-X"
may under this pragma return true even if there are no execute
permission bits set (nor any extra execute permission ACLs). This
strangeness is due to the underlying system calls' definitions.
Read the documentation for the "filetest" pragma for more
information.
Note that "-s/a/b/" does not do a negated substitution. Saying
"-exp($foo)" still works as expected, however--only single letters
following a minus are interpreted as file tests.
The "-T" and "-B" switches work as follows. The first block or so
of the file is examined for odd characters such as strange control
codes or characters with the high bit set. If too many strange
characters (>30%) are found, it's a "-B" file, otherwise it's a
"-T" file. Also, any file containing null in the first block is
considered a binary file. If "-T" or "-B" is used on a filehandle,
the current IO buffer is examined rather than the first block.
Both "-T" and "-B" return true on a null file, or a file at EOF
when testing a filehandle. Because you have to read a file to do
the "-T" test, on most occasions you want to use a "-f" against the
file first, as in "next unless -f $file && -T $file".
If any of the file tests (or either the "stat" or "lstat"
operators) are given the special filehandle consisting of a
solitary underline, then the stat structure of the previous file
test (or stat operator) is used, saving a system call. (This
doesn't work with "-t", and you need to remember that lstat() and
"-l" will leave values in the stat structure for the symbolic link,
not the real file.) (Also, if the stat buffer was filled by a
"lstat" call, "-T" and "-B" will reset it with the results of "stat
_"). Example:
print "Can do.\n" if -r $a || -w _ || -x _;
stat($filename);
print "Readable\n" if -r _;
print "Writable\n" if -w _;
print "Executable\n" if -x _;
print "Setuid\n" if -u _;
print "Setgid\n" if -g _;
print "Sticky\n" if -k _;
print "Text\n" if -T _;
print "Binary\n" if -B _;
abs VALUE
abs Returns the absolute value of its argument. If VALUE is omitted,
uses $_.
accept NEWSOCKET,GENERICSOCKET
Accepts an incoming socket connect, just as the accept(2) system
call does. Returns the packed address if it succeeded, false
otherwise. See the example in "Sockets: Client/Server
Communication" in perlipc.
On systems that support a close-on-exec flag on files, the flag
will be set for the newly opened file descriptor, as determined by
the value of $^F. See "$^F" in perlvar.
alarm SECONDS
alarm Arranges to have a SIGALRM delivered to this process after the
specified number of wallclock seconds have elapsed. If SECONDS is
not specified, the value stored in $_ is used. (On some machines,
unfortunately, the elapsed time may be up to one second less or
more than you specified because of how seconds are counted, and
process scheduling may delay the delivery of the signal even
further.)
Only one timer may be counting at once. Each call disables the
previous timer, and an argument of 0 may be supplied to cancel the
previous timer without starting a new one. The returned value is
the amount of time remaining on the previous timer.
For delays of finer granularity than one second, you may use Perl's
four-argument version of select() leaving the first three arguments
undefined, or you might be able to use the "syscall" interface to
access setitimer(2) if your system supports it. The Time::HiRes
module (from CPAN, and starting from Perl 5.8 part of the standard
distribution) may also prove useful.
It is usually a mistake to intermix "alarm" and "sleep" calls.
("sleep" may be internally implemented in your system with "alarm")
If you want to use "alarm" to time out a system call you need to
use an "eval"/"die" pair. You can't rely on the alarm causing the
system call to fail with $! set to "EINTR" because Perl sets up
signal handlers to restart system calls on some systems. Using
"eval"/"die" always works, modulo the caveats given in "Signals" in
perlipc.
eval {
local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
alarm $timeout;
$nread = sysread SOCKET, $buffer, $size;
alarm 0;
};
if ($@) {
die unless $@ eq "alarm\n"; # propagate unexpected errors
# timed out
}
else {
# didn't
}
For more information see perlipc.
atan2 Y,X
Returns the arctangent of Y/X in the range -PI to PI.
For the tangent operation, you may use the "Math::Trig::tan"
function, or use the familiar relation:
sub tan { sin($_[0]) / cos($_[0]) }
bind SOCKET,NAME
Binds a network address to a socket, just as the bind system call
does. Returns true if it succeeded, false otherwise. NAME should
be a packed address of the appropriate type for the socket. See
the examples in "Sockets: Client/Server Communication" in perlipc.
binmode FILEHANDLE, LAYER
binmode FILEHANDLE
Arranges for FILEHANDLE to be read or written in "binary" or "text"
mode on systems where the run-time libraries distinguish between
binary and text files. If FILEHANDLE is an expression, the value
is taken as the name of the filehandle. Returns true on success,
otherwise it returns "undef" and sets $! (errno).
On some systems (in general, DOS and Windows-based systems)
binmode() is necessary when you're not 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 multiple
directives. The directives alter the behaviour of the file handle.
When LAYER is present using binmode on text file makes sense.
If LAYER is omitted or specified as ":raw" the filehandle is made
suitable for passing binary data. This includes turning off
possible CRLF translation and marking it as bytes (as opposed to
Unicode 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 documentation...
To mark FILEHANDLE as UTF-8, use ":utf8".
In general, binmode() should be called after open() but before any
I/O is done on the filehandle. Calling binmode() will normally
flush any pending buffered output data (and perhaps pending input
data) on the handle. An exception to this is the ":encoding" layer
that changes the default character encoding of the handle, see
open. The ":encoding" layer sometimes needs to be called in
mid-stream, and it doesn't flush the stream. The ":encoding" also
implicitly pushes on top of itself the ":utf8" layer because
internally Perl will operate on UTF-8 encoded Unicode characters.
The operating system, device drivers, C libraries, and Perl run-
time system all work together to let the programmer treat a single
character ("\n") as the line terminator, irrespective of the
external representation. On many operating systems, the native
text file representation matches the internal representation, but
on some platforms the external representation of "\n" is made up of
more than one character.
Mac OS, all variants of Unix, and Stream_LF files on VMS use a
single character to end each line in the external representation of
text (even though that single character is CARRIAGE RETURN on Mac
OS and LINE FEED on Unix and most VMS files). In other systems like
OS/2, DOS and the various flavors of MS-Windows your program sees a
"\n" as a simple "\cJ", but what's stored in text files are the two
characters "\cM\cJ". That means that, if you don't use binmode()
on these systems, "\cM\cJ" sequences on disk will be converted to
"\n" on input, and any "\n" in your program will be converted back
to "\cM\cJ" on output. This is what you want for text files, but
it can be disastrous for binary files.
Another consequence of using binmode() (on some systems) is that
special end-of-file markers will be seen as part of the data
stream. For systems from the Microsoft family this means that if
your binary data contains "\cZ", the I/O subsystem will regard it
as the end of the file, unless you use binmode().
binmode() is not only important for readline() and print()
operations, but also when using read(), seek(), sysread(),
syswrite() and tell() (see perlport for more details). See the $/
and "$\" variables in perlvar for how to manually set your input
and output line-termination sequences.
bless REF,CLASSNAME
bless REF
This function tells the thingy referenced by REF that it is now an
object in the CLASSNAME package. If CLASSNAME is omitted, the
current package is used. Because a "bless" is often the last thing
in a constructor, it returns the reference for convenience. Always
use the two-argument version if the function doing the blessing
might be inherited by a derived class. See perltoot 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, so to
prevent confusion, you may wish to avoid such package names as
well. Make sure that CLASSNAME is a true value.
See "Perl Modules" in perlmod.
caller EXPR
caller Returns the context of the current subroutine call. In scalar
context, returns the caller's package name if there is a caller,
that is, if we're in a subroutine or "eval" or "require", and the
undefined value otherwise. In list context, returns
($package, $filename, $line) = caller;
With EXPR, it returns some extra information that the debugger uses
to print a stack trace. The value of EXPR indicates how many call
frames to go back before the current one.
($package, $filename, $line, $subroutine, $hasargs,
$wantarray, $evaltext, $is_require, $hints, $bitmask) = caller($i);
Here $subroutine may be "(eval)" if the frame is not a subroutine
call, but an "eval". In such a case additional elements $evaltext
and $is_require are set: $is_require is true if the frame is
created by a "require" or "use" statement, $evaltext contains the
text of the "eval EXPR" statement. In particular, for an "eval
BLOCK" statement, $filename is "(eval)", but $evaltext is
undefined. (Note also that each "use" statement creates a
"require" frame inside an "eval EXPR" frame.) $subroutine may also
be "(unknown)" if this particular subroutine happens to have been
deleted from the symbol table. $hasargs is true if a new instance
of @_ was set up for the frame. $hints and $bitmask contain
pragmatic hints that the caller was compiled with. The $hints and
$bitmask values are subject to change between versions of Perl, and
are not meant for external use.
Furthermore, when called from within the DB package, caller returns
more detailed information: it sets the list variable @DB::args to
be the arguments with which the subroutine was invoked.
Be aware that the optimizer might have optimized call frames away
before "caller" had a chance to get the information. That means
that caller(N) might not return information about the call frame
you expect it do, for "N > 1". In particular, @DB::args might have
information from the previous time "caller" was called.
chdir EXPR
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".
chmod LIST
Changes the permissions of a list of files. The first element of
the list must be the numerical mode, which should probably be an
octal number, and which definitely should not be a string of octal
digits: 0644 is okay, '0644' is not. Returns the number of files
successfully changed. See also "oct", if all you have is a string.
$cnt = chmod 0755, 'foo', 'bar';
chmod 0755, @executables;
$mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
# --w----r-T
$mode = '0644'; chmod oct($mode), 'foo'; # this is better
$mode = 0644; chmod $mode, 'foo'; # this is best
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 anything.
If VARIABLE is omitted, it chomps $_. Example:
while (<>) {
chomp; # avoid \n on last field
@array = split(/:/);
# ...
}
If VARIABLE is a hash, it chomps the hash's values, but not its
keys.
You can actually chomp anything that's an lvalue, including an
assignment:
chomp($cwd = `pwd`);
chomp($answer = <STDIN>);
If you chomp a list, each element is chomped, and the total number
of characters removed is returned.
If the "encoding" pragma is in scope then the lengths returned are
calculated from the length of $/ in Unicode characters, which is
not always the same as the length of $/ in the native encoding.
Note that parentheses are necessary when you're chomping anything
that is not a simple variable. This is because "chomp $cwd =
`pwd`;" is interpreted as "(chomp $cwd) = `pwd`;", rather than as
"chomp( $cwd = `pwd` )" which you might expect. Similarly, "chomp
$a, $b" is interpreted as "chomp($a), $b" rather than as "chomp($a,
$b)".
chop VARIABLE
chop( LIST )
chop Chops off the last character of a string and returns the character
chopped. It is much more efficient than "s/.$//s" because it
neither scans nor copies the string. If VARIABLE is omitted, chops
$_. If VARIABLE is a hash, it chops the hash's values, but not its
keys.
You can actually chop anything that's an lvalue, including an
assignment.
If you chop a list, each element is chopped. Only the value of the
last "chop" is returned.
Note that "chop" returns the last character. To return all but the
last character, use "substr($string, 0, -1)".
See also "chomp".
chown LIST
Changes the owner (and group) of a list of files. The first two
elements of the list must be the numeric uid and gid, in that
order. A value of -1 in either position is interpreted by most
systems to leave that value unchanged. Returns the number of files
successfully changed.
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
Here's an example that looks up nonnumeric uids in the passwd file:
print "User: ";
chomp($user = <STDIN>);
print "Files: ";
chomp($pattern = <STDIN>);
($login,$pass,$uid,$gid) = getpwnam($user)
or die "$user not in passwd file";
@ary = glob($pattern); # expand filenames
chown $uid, $gid, @ary;
On most systems, you are not allowed to change the ownership of the
file unless you're the superuser, although you should be able to
change the group to any of your secondary groups. On insecure
systems, these restrictions may be relaxed, but this is not a
portable assumption. On POSIX systems, you can detect this
condition this way:
use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
$can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
chr NUMBER
chr Returns the character represented by that NUMBER in the character
set. For example, "chr(65)" is "A" in either ASCII or Unicode, and
chr(0x263a) is a Unicode smiley face. Note that characters from
128 to 255 (inclusive) are by default not encoded in UTF-8 Unicode
for backward compatibility reasons (but see encoding).
If NUMBER is omitted, uses $_.
For the reverse, use "ord".
Note that under the "bytes" pragma the NUMBER is masked to the low
eight bits.
See perlunicode and encoding for more about Unicode.
chroot FILENAME
chroot This function works like the system call by the same name: it makes
the named directory the new root directory for all further
pathnames that begin with a "/" by your process and all its
children. (It doesn't change your current working directory, which
is unaffected.) For security reasons, this call is restricted to
the superuser. If FILENAME is omitted, does a "chroot" to $_.
close FILEHANDLE
close Closes the file or pipe associated with the file handle, returning
true only if IO buffers are successfully flushed and closes the
system file descriptor. Closes the currently selected filehandle
if the argument is omitted.
You don't have to close FILEHANDLE if you are immediately going to
do another "open" on it, because "open" will close it for you.
(See "open".) However, an explicit "close" on an input file resets
the line counter ($.), while the implicit close done by "open" does
not.
If the file handle came from a piped open, "close" will
additionally return false if one of the other system calls involved
fails, or if the program exits with non-zero status. (If the only
problem was that the program exited non-zero, $! will be set to 0.)
Closing a pipe also waits for the process executing on the pipe to
complete, in case you want to look at the output of the pipe
afterwards, and implicitly puts the exit status value of that
command into $?.
Prematurely closing the read end of a pipe (i.e. before the process
writing to it at the other end has closed it) will result in a
SIGPIPE being delivered to the writer. If the other end can't
handle that, be sure to read all the data before closing the pipe.
Example:
open(OUTPUT, '|sort >foo') # pipe to sort
or die "Can't start sort: $!";
#... # print stuff to output
close OUTPUT # wait for sort to finish
or warn $! ? "Error closing sort pipe: $!"
: "Exit status $? from sort";
open(INPUT, 'foo') # get sort's results
or die "Can't open 'foo' for input: $!";
FILEHANDLE may be an expression whose value can be used as an
indirect filehandle, usually the real 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 otherwise. NAME
should be a packed address of the appropriate type for the socket.
See the examples in "Sockets: Client/Server Communication" in
perlipc.
continue BLOCK
Actually a flow control statement rather than a function. If there
is a "continue" BLOCK attached to a BLOCK (typically in a "while"
or "foreach"), it is always executed just before the conditional is
about to be evaluated again, just like the third part of a "for"
loop in C. Thus it can be used to increment a loop variable, even
when the loop has been continued via the "next" statement (which is
similar to the C "continue" statement).
"last", "next", or "redo" may appear within a "continue" block.
"last" and "redo" will behave as if they had been executed within
the main block. So will "next", but since it will execute a
"continue" block, it may be more entertaining.
while (EXPR) {
### redo always comes here
do_something;
} continue {
### next always comes here
do_something_else;
# then back the top to re-check EXPR
}
### last always comes here
Omitting the "continue" section is semantically equivalent to using
an empty one, logically enough. In that case, "next" goes directly
back to check the condition at the top of the loop.
cos EXPR
cos Returns the cosine of EXPR (expressed in radians). If EXPR is
omitted, takes cosine of $_.
For the inverse cosine operation, you may use the
"Math::Trig::acos()" function, or use this relation:
sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
crypt PLAINTEXT,SALT
Encrypts a string exactly like the crypt(3) function in the C
library (assuming that you actually have a version there that has
not been extirpated as a potential munition). This can prove
useful for checking the password file for lousy passwords, amongst
other things. Only the guys wearing white hats should do this.
Note that crypt is intended to be a one-way function, much like
breaking eggs to make an omelette. There is no (known)
corresponding decrypt function (in other words, the crypt() is a
one-way hash function). As a result, this function isn't all that
useful for cryptography. (For that, see your nearby CPAN mirror.)
When verifying an existing encrypted string you should use the
encrypted text as the salt (like "crypt($plain, $crypted) eq
$crypted"). 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 encrypted string 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 encrypted string mattered, but
alternative hashing schemes (like MD5), higher level security
schemes (like C2), and implementations on non-UNIX platforms may
produce different strings.
When choosing a new salt create a random two character string whose
characters come from the set "[./0-9A-Za-z]" (like "join '', ('.',
'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]"). This set of
characters is just a recommendation; the characters allowed in the
salt depend solely on your system's crypt library, and Perl can't
restrict what salts "crypt()" accepts.
Here's an example that makes sure that whoever runs this program
knows their own 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 encrypting large quantities of
data, not least of all because you can't get the information back.
Look at the by-module/Crypt and by-module/PGP directories on your
favorite CPAN mirror for a slew of potentially useful modules.
If using crypt() on a Unicode string (which potentially has
characters with codepoints above 255), Perl tries to make sense of
the situation by trying to downgrade (a copy of the string) the
string back to an eight-bit byte string before calling crypt() (on
that copy). If that works, good. If not, crypt() dies with "Wide
character in crypt".
dbmclose HASH
[This function has been largely superseded by the "untie"
function.]
Breaks the binding between a DBM file and a hash.
dbmopen HASH,DBNAME,MASK
[This function has been largely superseded by the "tie" function.]
This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file
to a hash. HASH is the name of the hash. (Unlike normal "open",
the first argument is not a filehandle, even though it looks like
one). DBNAME is the name of the database (without the .dir or .pag
extension if any). If the database does not exist, it is created
with protection specified by MASK (as modified by the "umask"). If
your system supports only the older DBM functions, you may perform
only one "dbmopen" in your program. In older versions of Perl, if
your system had neither DBM nor ndbm, calling "dbmopen" produced a
fatal error; it now falls back to sdbm(3).
If you don't have write access to the DBM file, you can only read
hash variables, not set them. If you want to test whether you can
write, either use file tests or try setting a dummy hash entry
inside an "eval", which will trap the error.
Note that functions such as "keys" and "values" may return huge
lists when used on large DBM files. You may prefer to use the
"each" function to iterate over large DBM files. Example:
# print out history file offsets
dbmopen(%HIST,'/usr/lib/news/history',0666);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
dbmclose(%HIST);
See also AnyDBM_File for a more general description of the pros and
cons of the various dbm approaches, as well as DB_File for a
particularly rich implementation.
You can control which DBM library you use by loading that library
before you call dbmopen():
use DB_File;
dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
or die "Can't open netscape history file: $!";
defined EXPR
defined Returns a Boolean value telling whether EXPR has a value other than
the undefined value "undef". If EXPR is not present, $_ will be
checked.
Many operations return "undef" to indicate failure, end of file,
system error, uninitialized variable, and other exceptional
conditions. This function allows you to distinguish "undef" from
other values. (A simple Boolean test will not distinguish among
"undef", zero, the empty string, and "0", which are all equally
false.) Note that since "undef" is a valid scalar, its presence
doesn't necessarily indicate an exceptional condition: "pop"
returns "undef" when its argument is an empty array, or when the
element to return happens to be "undef".
You may also use "defined(&func)" to check whether subroutine &func
has ever been defined. The return value is unaffected by any
forward declarations of &func. Note that a subroutine which is not
defined may still be callable: its package may have an "AUTOLOAD"
method that makes it spring into existence the first time that it
is called -- see perlsub.
Use of "defined" on aggregates (hashes and arrays) is deprecated.
It used to report whether memory for that aggregate has ever been
allocated. This behavior may disappear in future versions of Perl.
You should instead use a simple test for size:
if (@an_array) { print "has array elements\n" }
if (%a_hash) { print "has hash members\n" }
When used on a hash element, it tells you whether the value is
defined, not whether the key exists in the hash. Use "exists" for
the latter purpose.
Examples:
print if defined $switch{'D'};
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
$debugging = 0 unless defined $debugging;
Note: Many folks tend to overuse "defined", and then are surprised
to discover that the number 0 and "" (the zero-length string) are,
in fact, defined values. For example, if you say
"ab" =~ /a(.*)b/;
The pattern match succeeds, and $1 is defined, despite the fact
that it matched "nothing". But it didn't really match nothing--
rather, it matched something that happened to be zero characters
long. This is all very above-board and honest. When a function
returns an undefined value, it's an admission that it couldn't give
you an honest answer. So you should use "defined" only when you're
questioning the integrity of what you're trying to do. At other
times, a simple comparison to 0 or "" is what you want.
See also "undef", "exists", "ref".
delete EXPR
Given an expression that specifies a hash element, array element,
hash slice, or array slice, deletes the specified element(s) from
the hash or array. In the case of an array, if the array elements
happen to be at the end, the size of the array will shrink to the
highest element that tests true for exists() (or 0 if no such
element exists).
Returns a list with the same number of elements as the number of
elements for which deletion was attempted. Each element of that
list consists of either the value of the element deleted, or the
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. Note that
deleting array elements in the middle of an array will not shift
the index of the ones after them down--use splice() for that. See
"exists".
The following (inefficiently) deletes all the values of %HASH and
@ARRAY:
foreach $key (keys %HASH) {
delete $HASH{$key};
}
foreach $index (0 .. $#ARRAY) {
delete $ARRAY[$index];
}
And so do these:
delete @HASH{keys %HASH};
delete @ARRAY[0 .. $#ARRAY];
But both of these are slower than just assigning the empty list or
undefining %HASH or @ARRAY:
%HASH = (); # completely empty %HASH
undef %HASH; # forget %HASH ever existed
@ARRAY = (); # completely empty @ARRAY
undef @ARRAY; # forget @ARRAY ever existed
Note that the EXPR can be arbitrarily complicated as long as the
final operation is a hash element, array element, hash slice, or
array slice lookup:
delete $ref->[$x][$y]{$key};
delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
delete $ref->[$x][$y][$index];
delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
die LIST
Outside an "eval", prints the value of LIST to "STDERR" and exits
with the current value of $! (errno). If $! is 0, exits with the
value of "($? >> 8)" (backtick `command` status). If "($? >> 8)"
is 0, exits with 255. Inside an "eval()," the error message is
stuffed into $@ and the "eval" is terminated with the undefined
value. This makes "die" the way to raise an exception.
Equivalent examples:
die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
If the last element of LIST does not end in a newline, the current
script line number and input line 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 available as the special
variable $.. See "$/" in perlvar and "$." in perlvar.
Hint: sometimes appending ", stopped" to your message will cause it
to make better sense when the string "at foo line 123" is appended.
Suppose you are running script "canasta".
die "/etc/games is no good";
die "/etc/games is no good, stopped";
produce, respectively
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
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
$@. ie. as if "$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };"
were called.
If $@ is empty then the string "Died" is used.
die() can also be called with a reference argument. If this
happens to be trapped within an eval(), $@ contains the reference.
This behavior permits a more elaborate exception handling
implementation using objects that maintain arbitrary state about
the nature of the exception. Such a scheme is sometimes preferable
to matching particular string values of $@ using regular
expressions. Here's an example:
eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
if ($@) {
if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
# handle Some::Module::Exception
}
else {
# handle all other possible exceptions
}
}
Because perl will stringify uncaught exception messages before
displaying them, you may want to overload stringification
operations on such custom exception objects. See overload for
details about that.
You can arrange for a callback to be run just before the "die" does
its deed, by setting the $SIG{__DIE__} hook. The associated
handler will be called with the error text and can change the error
message, if it sees fit, by calling "die" again. See "$SIG{expr}"
in perlvar for details on setting %SIG entries, and "eval BLOCK"
for some examples. Although this feature was meant 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 counterintuitive
behavior may be fixed in a future release.
do BLOCK
Not really a function. Returns the value of the last command in
the sequence of commands indicated by BLOCK. When modified by a
loop modifier, executes the BLOCK once before testing the loop
condition. (On other statements the loop modifiers test the
conditional first.)
"do BLOCK" does not count as a loop, so the loop control statements
"next", "last", or "redo" cannot be used to leave or restart the
block. See perlsyn for alternative strategies.
do SUBROUTINE(LIST)
A deprecated form of subroutine call. See perlsub.
do EXPR Uses the value of EXPR as a filename and executes the contents of
the file as a Perl script.
do 'stat.pl';
is just like
eval `cat stat.pl`;
except that it's more efficient and concise, keeps track of the
current filename for error messages, searches the @INC directories,
and updates %INC if the file is found. See "Predefined Names" in
perlvar for these variables. It also differs in that code
evaluated with "do FILENAME" cannot see lexicals in the enclosing
scope; "eval STRING" does. It's the same, however, in that it does
reparse the file every time you call it, so you probably don't want
to do this inside a loop.
If "do" cannot read the file, it returns undef and sets $! to the
error. If "do" can read the file but cannot compile it, it returns
undef and sets an error message in $@. If the file is
successfully 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, consider
generating bytecode or native C code as described in perlcc. If
you're just trying to accelerate a CGI script, consider using the
"mod_perl" extension to Apache, or the CPAN module, CGI::Fast. You
might also consider autoloading or selfloading, which at least make
your program appear to run faster.
each HASH
When called in list context, returns a 2-element list consisting of
the key and value for the next 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 (unmodified) hash.
Since Perl 5.8.1 the ordering is different even between different
runs of Perl for security reasons (see "Algorithmic Complexity
Attacks" in perlsec).
When the hash is entirely read, a null array is returned in list
context (which when assigned produces a false (0) value), and
"undef" in scalar context. The next call to "each" after that will
start iterating again. There is a single iterator for each hash,
shared by all "each", "keys", and "values" function calls in the
program; it can be reset by reading all the elements from the hash,
or by evaluating "keys HASH" or "values HASH". If you add or
delete elements of a hash while you're iterating over it, you may
get entries skipped or duplicated, so don't. Exception: It is
always safe to delete the item most recently returned by "each()",
which means that the following code will work:
while (($key, $value) = each %hash) {
print $key, "\n";
delete $hash{$key}; # This is safe
}
The following prints out your environment like the printenv(1)
program, only in a different order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}
See also "keys", "values" and "sort".
eof FILEHANDLE
eof ()
eof Returns 1 if the next read on FILEHANDLE will return end of file,
or if FILEHANDLE is not open. 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
In the first form, the return value of EXPR is parsed and executed
as if it were a little Perl program. The value of the expression
(which is itself determined within scalar context) is first parsed,
and if there weren't any errors, executed in the lexical context of
the current Perl program, so that any 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 statement
may be also used, just as with subroutines. The expression
providing the return value is evaluated in void, scalar, or list
context, depending on the context of the eval itself. See
"wantarray" for more on how the evaluation context can be
determined.
If there is a syntax error or runtime error, or a "die" statement
is executed, an undefined value is returned by "eval", and $@ is
set to the error message. If there was no error, $@ is guaranteed
to be a null string. Beware that using "eval" neither silences
perl from printing warnings to STDERR, nor does it stuff the text
of warning messages into $@. To do either of those, you have to
use the $SIG{__WARN__} facility, or turn off warnings inside the
BLOCK or EXPR using "no warnings 'all'". See "warn", perlvar,
warnings and perllexwarn.
Note that, because "eval" traps otherwise-fatal errors, it is
useful for determining whether a particular feature (such as
"socket" or "symlink") is implemented. It is also Perl's exception
trapping mechanism, where the die operator is used to raise
exceptions.
If the code to be executed doesn't vary, you may use the eval-BLOCK
form to trap run-time errors without incurring the penalty of
recompiling each time. The error, if any, is still returned in $@.
Examples:
# make divide-by-zero nonfatal
eval { $answer = $a / $b; }; warn $@ if $@;
# same thing, but less efficient
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
eval { $answer = }; # WRONG
# a run-time error
eval '$answer ='; # sets $@
Due to the current arguably broken state of "__DIE__" hooks, when
using the "eval{}" form as an exception trap in libraries, you may
wish not to trigger any "__DIE__" hooks that user code may have
installed. You can use the "local $SIG{__DIE__}" construct for
this purpose, as shown in this example:
# a very private exception trap for divide-by-zero
eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
warn $@ if $@;
This is especially significant, given that "__DIE__" hooks can call
"die" again, which has the effect of changing their error messages:
# __DIE__ hooks may modify error messages
{
local $SIG{'__DIE__'} =
sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
eval { die "foo lives here" };
print $@ if $@; # prints "bar lives here"
}
Because this promotes action at a distance, this counterintuitive
behavior may be fixed in a future release.
With an "eval", you should be especially careful to remember what's
being looked at when:
eval $x; # CASE 1
eval "$x"; # CASE 2
eval '$x'; # CASE 3
eval { $x }; # CASE 4
eval "\$$x++"; # CASE 5
$$x++; # CASE 6
Cases 1 and 2 above behave identically: they run the code contained
in the variable $x. (Although case 2 has misleading double quotes
making the reader wonder what else might be happening (nothing
is).) Cases 3 and 4 likewise behave in the same way: they run the
code '$x', which does nothing but return the value of $x. (Case 4
is preferred for purely visual reasons, but it also has the
advantage of compiling at compile-time instead of at run-time.)
Case 5 is a place where normally you would like to use double
quotes, except that in this particular situation, you can just use
symbolic references instead, as in case 6.
"eval BLOCK" does not count as a loop, so the loop control
statements "next", "last", or "redo" cannot be used to leave or
restart the block.
Note that as a very special case, an "eval ''" executed within the
"DB" package doesn't see the usual surrounding lexical scope, but
rather the scope of the first non-DB piece of code that called it.
You don't normally need to worry about this unless you are writing
a Perl debugger.
exec LIST
exec PROGRAM LIST
The "exec" function executes a system command and never returns--
use "system" instead of "exec" if you want it to return. It fails
and returns false only if the command does not exist and it is
executed directly instead of via your system's command shell (see
below).
Since it's a common mistake to use "exec" instead of "system", Perl
warns you if there is a following statement which isn't "die",
"warn", or "exit" (if "-w" is set - but you always do that). If
you really want to follow an "exec" with some other statement, you
can use one of these styles to avoid the warning:
exec ('foo') or print STDERR "couldn't exec foo: $!";
{ exec ('foo') }; print STDERR "couldn't exec foo: $!";
If there is more than one argument in LIST, or if LIST is an array
with more than one value, calls execvp(3) with the arguments in
LIST. If there is only one scalar argument or an array with one
element in it, the argument is checked for shell metacharacters,
and if there are any, the entire argument is passed to the system's
command shell for parsing (this is "/bin/sh -c" on Unix platforms,
but varies on other platforms). If there are no shell
metacharacters in the argument, it is split into words and passed
directly to "execvp", which is more efficient. Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";
If you don't really want to execute the first argument, but want to
lie to the program you are executing about its own name, you can
specify the program you actually want to run as an "indirect
object" (without a comma) in front of the LIST. (This always
forces interpretation of the LIST as a multivalued list, even if
there is only a single scalar in the list.) Example:
$shell = '/bin/csh';
exec $shell '-sh'; # pretend it's a login shell
or, more directly,
exec {'/bin/csh'} '-sh'; # pretend it's a login shell
When the arguments get executed via the system shell, results will
be subject to its quirks and 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 supported on some
platforms (see perlport). To be safe, you may need to set $|
($AUTOFLUSH in English) or call the "autoflush()" method of
"IO::Handle" on any open handles in order to avoid lost 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 element,
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 intervening
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
Evaluates EXPR and exits immediately with that value. Example:
$ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
See also "die". If EXPR is omitted, exits with 0 status. The only
universally recognized values for EXPR are 0 for success and 1 for
error; other values are subject to interpretation depending on the
environment in which the Perl program is running. For example,
exiting 69 (EX_UNAVAILABLE) from a sendmail incoming-mail filter
will cause the mailer to return the item undelivered, but that's
not true everywhere.
Don't use "exit" to abort a subroutine if there's any chance that
someone might want to trap whatever error happened. Use "die"
instead, which can be trapped by an "eval".
The exit() function does not always exit immediately. It calls any
defined "END" routines first, but these "END" routines may not
themselves abort the exit. Likewise any object destructors that
need to be called are called before the real exit. If this is a
problem, you can call "POSIX:_exit($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($_)".
fcntl FILEHANDLE,FUNCTION,SCALAR
Implements the fcntl(2) function. You'll probably have to say
use Fcntl;
first to get the correct constant definitions. Argument processing
and value return works just like "ioctl" below. For example:
use Fcntl;
fcntl($filehandle, F_GETFL, $packed_return_buffer)
or die "can't fcntl F_GETFL: $!";
You don't have to check for "defined" on the return from "fcntl".
Like "ioctl", it maps a 0 return from the system call into "0 but
true" in Perl. This string is true in boolean context and 0 in
numeric context. It is also exempt from the normal -w warnings on
improper numeric conversions.
Note that "fcntl" will produce a fatal error if used on a machine
that doesn't implement fcntl(2). See the Fcntl module or your
fcntl(2) manpage to learn what functions are available on your
system.
Here's an example of setting a filehandle named "REMOTE" to be
non-blocking at the system level. You'll have to negotiate $| on
your own, though.
use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
$flags = fcntl(REMOTE, F_GETFL, 0)
or die "Can't get flags for the socket: $!\n";
$flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
or die "Can't set flags for the socket: $!\n";
fileno FILEHANDLE
Returns the file descriptor for a filehandle, or undefined if the
filehandle is not open. This is mainly useful for constructing
bitmaps for "select" and low-level POSIX tty-handling operations.
If FILEHANDLE is an expression, the value is taken as an indirect
filehandle, generally its name.
You can use this to find out whether two handles refer to the same
underlying descriptor:
if (fileno(THIS) == fileno(THAT)) {
print "THIS and THAT are dups\n";
}
(Filehandles connected to memory objects via new features of "open"
may return undefined even though they are open.)
flock FILEHANDLE,OPERATION
Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
for success, false on failure. 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 files locked with
"flock" may be modified by programs that do not also use "flock".
See perlport, your port's specific documentation, or your system-
specific local manpages for details. It's best to assume
traditional behavior if you're writing portable programs. (But if
you're not, you should as always feel perfectly free to write for
your own system's idiosyncrasies (sometimes called "features").
Slavish adherence to portability concerns shouldn't get in the way
of your getting your job done.)
OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined
with LOCK_NB. These constants are traditionally valued 1, 2, 8 and
4, but you can use the symbolic names if you import them from the
Fcntl module, either individually, or as a group using the ':flock'
tag. LOCK_SH requests a shared lock, LOCK_EX requests an exclusive
lock, and LOCK_UN releases a previously requested lock. If LOCK_NB
is bitwise-or'ed with LOCK_SH or LOCK_EX then "flock" will return
immediately rather than blocking waiting for the lock (check the
return status to see if you got it).
To avoid the possibility of miscoordination, Perl now flushes
FILEHANDLE before locking or unlocking it.
Note that the emulation built with lockf(3) doesn't provide shared
locks, and it requires that FILEHANDLE be open with write intent.
These are the semantics that lockf(3) implements. Most if not all
systems implement lockf(3) in terms of fcntl(2) locking, though, so
the differing semantics shouldn't bite too many people.
Note that the fcntl(2) emulation of flock(3) requires that
FILEHANDLE be open with read intent to use LOCK_SH and requires
that it be open with write intent to use LOCK_EX.
Note also that some versions of "flock" cannot lock things over the
network; you would need to use the more system-specific "fcntl" for
that. If you like you can force Perl to ignore your system's
flock(2) function, and so provide its own fcntl(2)-based emulation,
by passing the switch "-Ud_flock" to the 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, but you could also read $^A yourself and then set $^A
back to "". Note that a format typically does one "formline" per
line of form, but the "formline" function itself doesn't care how
many newlines are embedded in the PICTURE. This means that the "~"
and "~~" tokens will treat the entire PICTURE as a single line.
You may therefore need to use multiple formlines to implement a
single record format, just like the format compiler.
Be careful if you put double quotes around the picture, because an
"@" character may be taken to mean the beginning of an array name.
"formline" always returns true. See perlform for other examples.
getc FILEHANDLE
getc Returns the next character from the input file attached to
FILEHANDLE, or the undefined value at end of file, or if there was
an error (in the latter case $! is set). If FILEHANDLE is omitted,
reads from STDIN. This is not particularly efficient. However, it
cannot be used by itself to fetch single characters without waiting
for the user to hit enter. For that, try something more like:
if ($BSD_STYLE) {
system "stty cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system "stty", '-icanon', 'eol', "\001";
}
$key = getc(STDIN);
if ($BSD_STYLE) {
system "stty -cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system "stty", 'icanon', 'eol', '^@'; # ASCII null
}
print "\n";
Determination of whether $BSD_STYLE should be set is left as an
exercise to the reader.
The "POSIX::getattr" function can do this more portably on systems
purporting POSIX compliance. See also the "Term::ReadKey" module
from your nearest CPAN site; details on CPAN can be found on "CPAN"
in perlmodlib.
getlogin
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 "getppid()", you may
use the CPAN module "Linux::Pid".
getpriority WHICH,WHO
Returns the current priority for a process, a process group, or a
user. (See getpriority(2).) Will raise a fatal exception if used
on a machine that doesn't implement getpriority(2).
getpwnam NAME
getgrnam NAME
gethostbyname NAME
getnetbyname NAME
getprotobyname NAME
getpwuid UID
getgrgid GID
getservbyname NAME,PROTO
gethostbyaddr ADDR,ADDRTYPE
getnetbyaddr ADDR,ADDRTYPE
getprotobynumber NUMBER
getservbyport PORT,PROTO
getpwent
getgrent
gethostent
getnetent
getprotoent
getservent
setpwent
setgrent
sethostent STAYOPEN
setnetent STAYOPEN
setprotoent STAYOPEN
setservent STAYOPEN
endpwent
endgrent
endhostent
endnetent
endprotoent
endservent
These routines perform the same functions as their counterparts in
the system library. In list context, the return values from the
various get routines are as follows:
($name,$passwd,$uid,$gid,
$quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
($name,$passwd,$gid,$members) = getgr*
($name,$aliases,$addrtype,$length,@addrs) = gethost*
($name,$aliases,$addrtype,$net) = getnet*
($name,$aliases,$proto) = getproto*
($name,$aliases,$port,$proto) = getserv*
(If the entry doesn't exist you get a null list.)
The exact meaning of the $gcos field varies but it usually contains
the real name of the user (as opposed to the login name) and other
information pertaining to the user. Beware, however, that in many
system users are able to change this information and therefore it
cannot be trusted and therefore the $gcos is tainted (see perlsec).
The $passwd and $shell, user's encrypted password and login shell,
are also tainted, because of the same reason.
In scalar context, you get the name, unless the function was a
lookup by name, in which case you get the other thing, whatever it
is. (If the entry doesn't exist you get the undefined value.) For
example:
$uid = getpwnam($name);
$name = getpwuid($num);
$name = getpwent();
$gid = getgrnam($name);
$name = getgrgid($num);
$name = getgrent();
#etc.
In getpw*() the fields $quota, $comment, and $expire are special
cases in the sense that in many systems they are unsupported. If
the $quota is unsupported, it is an empty scalar. If it is
supported, it 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 systems 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) documentation and your pwd.h file.
You can also find out from within Perl what your $quota and
$comment fields mean and whether you have the $expire field by
using the "Config" module and the 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 calling the regular C library routines gets
the shadow versions if you're running under privilege or if there
exists the shadow(3) functions as found in System V ( this includes
Solaris and Linux.) Those systems which implement a proprietary
shadow password facility are unlikely to be supported.
The $members value returned by getgr*() is a space separated list
of the login names of the members of the group.
For the gethost*() functions, if the "h_errno" variable is
supported in C, it will be returned to you via $? if the function
call fails. The @addrs value returned by a successful call is a
list of the raw addresses returned by the corresponding system
library call. In the Internet domain, each address is four bytes
long and you can unpack it by saying something like:
($a,$b,$c,$d) = unpack('C4',$addr[0]);
The Socket library makes this slightly easier:
use Socket;
$iaddr = inet_aton("127.1"); # or whatever address
$name = gethostbyaddr($iaddr, AF_INET);
# or going the other way
$straddr = inet_ntoa($iaddr);
If you get tired of remembering which element of the return list
contains which return value, by-name interfaces are provided in
standard modules: "File::stat", "Net::hostent", "Net::netent",
"Net::protoent", "Net::servent", "Time::gmtime", "Time::localtime",
and "User::grent". These override the normal built-ins, supplying
versions that return objects with the appropriate names for each
field. For example:
use File::stat;
use User::pwent;
$is_his = (stat($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 reason will be
in $!). What exactly is in the packed string depends in the LEVEL
and OPTNAME, consult your system documentation for details. A very
common case however is that the option is an integer, in which case
the result will be an 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 omitted, $_ is used. The
"<*.c>" operator is discussed in more detail in "I/O Operators" in
perlop.
Beginning with v5.6.0, this operator is implemented using the
standard "File::Glob" extension. See File::Glob for details.
gmtime EXPR
Converts a time as returned by the time function to an 8-element
list with the time localized for the standard Greenwich time zone.
Typically used as follows:
# 0 1 2 3 4 5 6 7
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
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.)
Note that the $year element is not simply the last two digits of
the year. If you assume it is, then you create non-Y2K-compliant
programs--and you wouldn't want to do that, would you?
The proper way to get a complete 4-digit year is simply:
$year += 1900;
And to get the last two digits of the year (e.g., '01' in 2001) do:
$year = sprintf("%02d", $year % 100);
If EXPR is omitted, "gmtime()" uses the current time
("gmtime(time)").
In scalar context, "gmtime()" returns the ctime(3) value:
$now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
If you need local time instead of GMT use the "localtime" builtin.
See also the "timegm" function provided by the "Time::Local"
module, and the strftime(3) and mktime(3) functions available via
the POSIX module.
This scalar value is not locale dependent (see perllocale), but is
instead a Perl builtin. To get somewhat similar but locale
dependent date strings, see the example in "localtime".
See "gmtime" in perlport for portability concerns.
goto LABEL
goto EXPR
goto &NAME
The "goto-LABEL" form finds the statement labeled with LABEL and
resumes execution there. It may not be used to go into any
construct that requires initialization, such as a subroutine or a
"foreach" loop. It also can't be used to go into a construct that
is optimized away, or to get out of a block or subroutine given to
"sort". It can be used to go almost anywhere else within the
dynamic scope, including out of subroutines, but it's usually
better to use some other construct such as "last" or "die". The
author of Perl has never felt the need to use this form of "goto"
(in Perl, that is--C is another matter). (The difference being
that C does not offer named loops combined with loop control. Perl
does, and this replaces most structured uses of "goto" in other
languages.)
The "goto-EXPR" form expects a label name, whose scope will be
resolved dynamically. This allows for computed "goto"s per
FORTRAN, but isn't necessarily recommended if you're optimizing for
maintainability:
goto ("FOO", "BAR", "GLARCH")[$i];
The "goto-&NAME" form is quite different from the other forms of
"goto". In fact, it isn't a goto in the normal sense at all, and
doesn't have the stigma associated with other gotos. Instead, it
exits the 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 routine was called first.
NAME needn't be the name of a subroutine; it can be a scalar
variable containing a code reference, or a block which evaluates to
a code reference.
grep BLOCK LIST
grep EXPR,LIST
This is similar in spirit to, but not the same as, grep(1) and its
relatives. In particular, it is not limited to using regular
expressions.
Evaluates the BLOCK or EXPR for each element of LIST (locally
setting $_ to each element) and returns the list value consisting
of those elements for which the expression evaluated to true. In
scalar context, returns the number of times the expression was
true.
@foo = grep(!/^#/, @bar); # weed out comments
or equivalently,
@foo = grep {!/^#/} @bar; # weed out comments
Note that $_ is an alias to the list value, so it can be used to
modify the elements of the LIST. While this is useful and
supported, it can cause bizarre results if the elements of LIST are
not variables. Similarly, grep returns aliases into the original
list, much as a for loop's index variable aliases the list
elements. That is, modifying an element of a list returned by grep
(for example, in a "foreach", "map" or another "grep") actually
modifies the element in the original list. This is usually
something to be avoided when writing clear code.
See also "map" for a list composed of the results of the BLOCK or
EXPR.
hex EXPR
hex Interprets EXPR as a hex string and returns the corresponding
value. (To convert strings that might start with either 0, "0x",
or "0b", see "oct".) If EXPR is omitted, uses $_.
print hex '0xAf'; # prints '175'
print hex 'aF'; # same
Hex strings may only represent integers. Strings that would cause
integer overflow trigger a warning. Leading whitespace is not
stripped, unlike oct(). To present something as hex, look into
"printf", "sprintf", or "unpack".
import 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. The return value is
based at 0 (or whatever you've set the $[ variable to--but don't do
that). If the substring is not found, returns one less than the
base, ordinarily "-1".
int EXPR
int Returns the integer portion of EXPR. If EXPR is omitted, uses $_.
You should not use this function for rounding: one because it
truncates towards 0, and two because machine representations of
floating point numbers can sometimes produce counterintuitive
results. For example, "int(-6.725/0.025)" produces -268 rather
than the correct -269; that's because it's really more like
-268.99999999999994315658 instead. Usually, the "sprintf",
"printf", or the "POSIX::floor" and "POSIX::ceil" functions will
serve you better than will int().
ioctl FILEHANDLE,FUNCTION,SCALAR
Implements the ioctl(2) function. You'll probably first have to
say
require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
to get the correct function definitions. If ioctl.ph doesn't exist
or doesn't have the correct definitions you'll have to roll your
own, based on your C header files such as <sys/ioctl.h>. (There is
a Perl script called h2ph that comes with the Perl kit that may
help you in this, but it's nontrivial.) SCALAR will be read and/or
written depending on the FUNCTION--a pointer to the string value of
SCALAR will be passed as the third argument of the actual "ioctl"
call. (If SCALAR has no string value but does have a numeric
value, that value will be passed rather than a pointer to the
string value. To guarantee this to be true, add a 0 to the scalar
before using it.) The "pack" and "unpack" functions may be needed
to manipulate the values of structures used by "ioctl".
The return value of "ioctl" (and "fcntl") is as follows:
if OS returns: then Perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
Thus Perl returns true on success and false on failure, yet you can
still easily determine the actual value returned by the operating
system:
$retval = ioctl(...) || -1;
printf "System returned %d\n", $retval;
The special string "0 but true" is exempt from -w complaints about
improper numeric conversions.
join EXPR,LIST
Joins the separate strings of LIST into a single string with fields
separated by the value of EXPR, and returns that new string.
Example:
$rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
Beware that unlike "split", "join" doesn't take a pattern as its
first argument. Compare "split".
keys HASH
Returns a list consisting of all the keys of the named hash. (In
scalar context, returns the number of keys.)
The keys are returned in an apparently random order. The actual
random order is subject to change in future versions of perl, but
it is guaranteed to be the same order as either the "values" or
"each" 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 measure
of efficiency if you know the hash is going to get big. (This is
similar to pre-extending an array by assigning a larger number to
$#array.) If you say
keys %hash = 200;
then %hash will have at least 200 buckets allocated for it--256 of
them, in fact, since it rounds up to the next power of two. These
buckets will be retained even if you do "%hash = ()", use "undef
%hash" if you want to free the storage while %hash is still in
scope. You can't shrink the number of buckets allocated 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, $child2;
kill 9, @goners;
If SIGNAL is zero, no signal is sent to the process. This is a
useful way to check that a child process is alive and hasn't
changed its UID. See perlport for notes on the portability of this
construct.
Unlike in the shell, if SIGNAL is negative, it kills process groups
instead of processes. (On System V, a negative PROCESS number will
also kill process groups, but that's not portable.) That means you
usually want to use positive not negative signals. You may also
use a signal name in quotes.
See "Signals" in perlipc for more details.
last LABEL
last The "last" command is like the "break" statement in C (as used in
loops); it immediately exits the loop in question. If the LABEL is
omitted, the command refers to the innermost enclosing loop. The
"continue" block, if any, is not executed:
LINE: while (<STDIN>) {
last LINE if /^$/; # 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 "Private Variables
via my()" in perlsub for details.
A loc