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perlfunc
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NAME
perlfunc - Perl builtin functions
DESCRIPTION
The functions in this section can serve as terms in an
expression. They fall into two major categories: list operators
and named unary operators. These differ in their precedence
relationship with a following comma. (See the precedence table
in the perlop manpage.) 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.
An 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',
`umask', `unlink', `utime'
Keywords related to the control flow of your perl program
`caller', `continue', `die', `do', `dump', `eval', `exit',
`goto', `last', `next', `redo', `return', `sub', `wantarray'
Keywords related to scoping
`caller', `import', `local', `my', `package', `use'
Miscellaneous functions
`defined', `dump', `eval', `formline', `local', `my',
`reset', `scalar', `undef', `wantarray'
Functions for processes and process groups
`alarm', `exec', `fork', `getpgrp', `getppid',
`getpriority', `kill', `pipe', `qx/STRING/', `setpgrp',
`setpriority', `sleep', `system', `times', `wait', `waitpid'
Keywords related to perl modules
`do', `import', `no', `package', `require', `use'
Keywords related to classes and object-orientedness
`bless', `dbmclose', `dbmopen', `package', `ref', `tie',
`tied', `untie', `use'
Low-level socket functions
`accept', `bind', `connect', `getpeername', `getsockname',
`getsockopt', `listen', `recv', `send', `setsockopt',
`shutdown', `socket', `socketpair'
System V interprocess communication functions
`msgctl', `msgget', `msgrcv', `msgsnd', `semctl', `semget',
`semop', `shmctl', `shmget', `shmread', `shmwrite'
Fetching user and group info
`endgrent', `endhostent', `endnetent', `endpwent',
`getgrent', `getgrgid', `getgrnam', `getlogin', `getpwent',
`getpwnam', `getpwuid', `setgrent', `setpwent'
Fetching network info
`endprotoent', `endservent', `gethostbyaddr',
`gethostbyname', `gethostent', `getnetbyaddr',
`getnetbyname', `getnetent', `getprotobyname',
`getprotobynumber', `getprotoent', `getservbyname',
`getservbyport', `getservent', `sethostent', `setnetent',
`setprotoent', `setservent'
Time-related functions
`gmtime', `localtime', `time', `times'
Functions new in perl5
`abs', `bless', `chomp', `chr', `exists', `formline',
`glob', `import', `lc', `lcfirst', `map', `my', `no',
`prototype', `qx', `qw', `readline', `readpipe', `ref',
`sub*', `sysopen', `tie', `tied', `uc', `ucfirst', `untie',
`use'
* - `sub' was a keyword in perl4, but in perl5 it is an
operator, which can be used in expressions.
Functions obsoleted in perl5
`dbmclose', `dbmopen'
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', `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 the perlport manpage 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.
-s File has nonzero size (returns size).
-f File is a plain file.
-d File is a directory.
-l File is a symbolic link.
-p File is a named pipe (FIFO), 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 a text file.
-B File is a binary file (opposite of -T).
-M Age of file in days when script started.
-A Same for access time.
-C Same for inode change time.
Example:
while (<>) {
chop;
next unless -f $_; # ignore specials
#...
}
The interpretation of the file permission operators `-
r', `-R', `-w', `-W', `-x', and `-X' is by default based
solely on the mode of the file and the uids and gids of
the user. There may be other reasons you can't actually
read, write, or execute the file. Such reasons may be
for example network filesystem access controls, ACLs
(access control lists), read-only filesystems, and
unrecognized executable formats.
Also note that, for the superuser on the local
filesystems, the `-r', `-R', `-w', and `-W' tests always
return 1, and `-x' and `-X' return 1 if any execute bit
is set in the mode. Scripts run by the superuser may
thus need to do a stat() to determine the actual mode of
the file, or temporarily set their effective uid to
something else.
Note that `-s/a/b/' does not do a negated substitution.
Saying `-exp($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 stdio buffer is examined rather
than the first block. Both `-T' and `-B' return TRUE on
a null file, or a file at EOF when testing a filehandle.
Because you have to read a file to do the `-T' test, on
most occasions you want to use a `-f' against the file
first, as in `next unless -f $file && -T $file'.
If any of the file tests (or either the `stat()' or
`lstat()' operators) are given the special filehandle
consisting of a solitary underline, then the stat
structure of the previous file test (or stat operator)
is used, saving a system call. (This doesn't work with
`-t', and you need to remember that lstat() and `-l'
will leave values in the stat structure for the symbolic
link, not the real file.) Example:
print "Can do.\n" if -r $a || -w _ || -x _;
stat($filename);
print "Readable\n" if -r _;
print "Writable\n" if -w _;
print "Executable\n" if -x _;
print "Setuid\n" if -u _;
print "Setgid\n" if -g _;
print "Sticky\n" if -k _;
print "Text\n" if -T _;
print "Binary\n" if -B _;
abs VALUE
abs Returns the absolute value of its argument. If VALUE is
omitted, uses `$_'.
accept NEWSOCKET,GENERICSOCKET
Accepts an incoming socket connect, just as the
accept(2) system call does. Returns the packed address
if it succeeded, FALSE otherwise. See the example in the
section on "Sockets: Client/Server Communication" in the
perlipc manpage.
alarm SECONDS
alarm Arranges to have a SIGALRM delivered to this process after
the specified number of seconds have elapsed. If SECONDS
is not specified, the value stored in `$_' is used. (On
some machines, unfortunately, the elapsed time may be up
to one second less than you specified because of how
seconds are counted.) Only one timer may be counting at
once. Each call disables the previous timer, and an
argument of `0' may be supplied to cancel the previous
timer without starting a new one. The returned value is
the amount of time remaining on the previous timer.
For delays of finer granularity than one second, you may
use Perl's four-arugment 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 may also prove useful.
It is usually a mistake to intermix `alarm()' and
`sleep()' calls.
If you want to use `alarm()' to time out a system call
you need to use an `eval()'/`die()' pair. You can't rely
on the alarm causing the system call to fail with `$!'
set to `EINTR' because Perl sets up signal handlers to
restart system calls on some systems. Using
`eval()'/`die()' always works, modulo the caveats given
in the section on "Signals" in the perlipc manpage.
eval {
local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
alarm $timeout;
$nread = sysread SOCKET, $buffer, $size;
alarm 0;
};
if ($@) {
die unless $@ eq "alarm\n"; # propagate unexpected errors
# timed out
}
else {
# didn't
}
atan2 Y,X
Returns the arctangent of Y/X in the range -PI to PI.
For the tangent operation, you may use the
`POSIX::tan()' function, or use the familiar relation:
sub tan { sin($_[0]) / cos($_[0]) }
bind SOCKET,NAME
Binds a network address to a socket, just as the bind
system call does. Returns TRUE if it succeeded, FALSE
otherwise. NAME should be a packed address of the
appropriate type for the socket. See the examples in the
section on "Sockets: Client/Server Communication" in the
perlipc manpage.
binmode FILEHANDLE
Arranges for the file to be read or written in "binary"
mode in operating systems that distinguish between
binary and text files. Files that are not in binary mode
have CR LF sequences translated to LF on input and LF
translated to CR LF on output. Binmode has no effect
under many sytems, but in MS-DOS and similarly archaic
systems, it may be imperative--otherwise your MS-DOS-
damaged C library may mangle your file. The key
distinction between systems that need `binmode()' and
those that don't is their text file formats. Systems
like Unix, MacOS, and Plan9 that delimit lines with a
single character, and that encode that character in C as
`"\n"', do not need `binmode()'. The rest may need it.
If FILEHANDLE is an expression, the value is taken as
the name of the filehandle.
If the system does care about it, using it when you
shouldn't is just as perilous as failing to use it when
you should. Fortunately for most of us, you can't go
wrong using binmode() on systems that don't care about
it, though.
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 the perltoot
manpage and the perlobj manpage 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 the section on "Perl Modules" in the perlmod
manpage.
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, $is_require) = caller($i);
Here `$subroutine' may be `"(eval)"' if the frame is not
a subroutine call, but an `eval()'. In such a case
additional elements `$evaltext' and `$is_require' are
set: `$is_require' is true if the frame is created by a
`require' or `use' statement, `$evaltext' contains the
text of the `eval EXPR' statement. In particular, for a
`eval BLOCK' statement, `$filename' is `"(eval)"', but
`$evaltext' is undefined. (Note also that each `use'
statement creates a `require' frame inside an `eval
EXPR') frame.
Furthermore, when called from within the DB package,
caller returns more detailed information: it sets the
list variable `@DB::args' to be the arguments with which
the subroutine was invoked.
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 user's home directory.
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* a string of octal digits: `0644' is okay,
`'0644'' is not. Returns the number of files
successfully changed. See also the "oct" entry in this
manpage, if all you have is a string.
$cnt = chmod 0755, 'foo', 'bar';
chmod 0755, @executables;
$mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to
# --w----r-T
$mode = '0644'; chmod oct($mode), 'foo'; # this is better
$mode = 0644; chmod $mode, 'foo'; # this is best
chomp VARIABLE
chomp LIST
chomp This safer version of the "chop" entry in this manpage
removes any trailing string that corresponds to the
current value of `$/' (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. If VARIABLE is omitted, it
chomps `$_'. Example:
while (<>) {
chomp; # avoid \n on last field
@array = split(/:/);
# ...
}
You can actually chomp anything that's an lvalue,
including an assignment:
chomp($cwd = `pwd`);
chomp($answer = <STDIN>);
If you chomp a list, each element is chomped, and the
total number of characters removed is returned.
chop VARIABLE
chop LIST
chop Chops off the last character of a string and returns the
character chopped. It's used primarily to remove the
newline from the end of an input record, but is much
more efficient than `s/\n//' because it neither scans
nor copies the string. If VARIABLE is omitted, chops
`$_'. Example:
while (<>) {
chop; # avoid \n on last field
@array = split(/:/);
#...
}
You can actually chop anything that's an lvalue,
including an assignment:
chop($cwd = `pwd`);
chop($answer = <STDIN>);
If you chop a list, each element is chopped. Only the
value of the last `chop()' is returned.
Note that `chop()' returns the last character. To return
all but the last character, use `substr($string, 0, -
1)'.
chown LIST
Changes the owner (and group) of a list of files. The
first two elements of the list must be the *NUMERICAL*
uid and gid, in that order. Returns the number of files
successfully changed.
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
Here's an example that looks up nonnumeric uids in the
passwd file:
print "User: ";
chop($user = <STDIN>);
print "Files: ";
chop($pattern = <STDIN>);
($login,$pass,$uid,$gid) = getpwnam($user)
or die "$user not in passwd file";
@ary = glob($pattern); # expand filenames
chown $uid, $gid, @ary;
On most systems, you are not allowed to change the
ownership of the file unless you're the superuser,
although you should be able to change the group to any
of your secondary groups. On insecure systems, these
restrictions may be relaxed, but this is not a portable
assumption.
chr NUMBER
chr Returns the character represented by that NUMBER in the
character set. For example, `chr(65)' is `"A"' in ASCII.
For the reverse, use the "ord" entry in this manpage.
If NUMBER is omitted, uses `$_'.
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
`$_'.
close FILEHANDLE
close Closes the file or pipe associated with the file handle,
returning TRUE only if stdio successfully flushes
buffers and closes the system file descriptor. Closes
the currently selected filehandle if the argument is
omitted.
You don't have to close FILEHANDLE if you are
immediately going to do another `open()' on it, because
`open()' will close it for you. (See `open()'.) However,
an explicit `close()' on an input file resets the line
counter (`$.'), 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 `$!' will be set to `0'.) Closing a pipe
also waits for the process executing on the pipe to
complete, in case you want to look at the output of the
pipe afterwards, and implicitly puts the exit status
value of that command into `$?'.
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: $!";
FILEHANDLE may be an expression whose value can be used
as an indirect filehandle, usually the real filehandle
name.
closedir DIRHANDLE
Closes a directory opened by `opendir()' and returns the
success of that system call.
DIRHANDLE may be an expression whose value can be used
as an indirect dirhandle, usually the real dirhandle
name.
connect SOCKET,NAME
Attempts to connect to a remote socket, just as the
connect system call does. Returns TRUE if it succeeded,
FALSE otherwise. NAME should be a packed address of the
appropriate type for the socket. See the examples in the
section on "Sockets: Client/Server Communication" in the
perlipc manpage.
continue BLOCK
Actually a flow control statement rather than a
function. If there is a `continue' BLOCK attached to a
BLOCK (typically in a `while' or `foreach'), it is
always executed just before the conditional is about to
be evaluated again, just like the third part of a `for'
loop in C. Thus it can be used to increment a loop
variable, even when the loop has been continued via the
`next' statement (which is similar to the C `continue'
statement).
`last', `next', or `redo' may appear within a `continue'
block. `last' and `redo' will behave as if they had been
executed within the main block. So will `next', but
since it will execute a `continue' block, it may be more
entertaining.
while (EXPR) {
### redo always comes here
do_something;
} continue {
### next always comes here
do_something_else;
# then back the top to re-check EXPR
}
### last always comes here
Omitting the `continue' section is semantically
equivalent to using an empty one, logically enough. In
that case, `next' goes directly back to check the
condition at the top of the loop.
cos EXPR
Returns the cosine of EXPR (expressed in radians). If
EXPR is omitted, takes cosine of `$_'.
For the inverse cosine operation, you may use the
`POSIX::acos()' function, or use this relation:
sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
crypt PLAINTEXT,SALT
Encrypts a string exactly like the crypt(3) function in
the C library (assuming that you actually have a version
there that has not been extirpated as a potential
munition). This can prove useful for checking the
password file for lousy passwords, amongst other things.
Only the guys wearing white hats should do this.
Note that `crypt()' is intended to be a one-way
function, much like breaking eggs to make an omelette.
There is no (known) corresponding decrypt function. As a
result, this function isn't all that useful for
cryptography. (For that, see your nearby CPAN mirror.)
When verifying an existing encrypted string you should
use the encrypted text as the salt (like `crypt($plain,
$crypted) eq $crypted'). This allows your code to work
with the standard `crypt()' and with more exotic
implementations. 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]').
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 $pwd) {
die "Sorry...\n";
} else {
print "ok\n";
}
Of course, typing in your own password to whoever asks
you for it is unwise.
dbmclose HASH
[This function has been largely superseded by the
`untie()' function.]
Breaks the binding between a DBM file and a hash.
dbmopen HASH,DBNAME,MODE
[This function has been largely superseded by the
`tie()' function.]
This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or
Berkeley DB file to a hash. HASH is the name of the
hash. (Unlike normal `open()', the first argument is
*NOT* a filehandle, even though it looks like one).
DBNAME is the name of the database (without the .dir or
.pag extension if any). If the database does not exist,
it is created with protection specified by MODE (as
modified by the `umask()'). If your system supports only
the older DBM functions, you may perform only one
`dbmopen()' in your program. In older versions of Perl,
if your system had neither DBM nor ndbm, calling
`dbmopen()' produced a fatal error; it now falls back to
sdbm(3).
If you don't have write access to the DBM file, you can
only read hash variables, not set them. If you want to
test whether you can write, either use file tests or try
setting a dummy hash entry inside an `eval()', which
will trap the error.
Note that functions such as `keys()' and `values()' may
return huge lists when used on large DBM files. You may
prefer to use the `each()' function to iterate over
large DBM files. Example:
# print out history file offsets
dbmopen(%HIST,'/usr/lib/news/history',0666);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
dbmclose(%HIST);
See also the AnyDBM_File manpage for a more general
description of the pros and cons of the various dbm
approaches, as well as the DB_File manpage 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()' to check whether a
subroutine exists, by saying `defined &func' without
parentheses. On the other hand, use of `defined()' upon
aggregates (hashes and arrays) is not guaranteed to
produce intuitive results, and should probably be
avoided.
When used on a hash element, it tells you whether the
value is defined, not whether the key exists in the
hash. Use the "exists" entry in this manpage 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 `$1' is defined, despite
the fact that it matched "nothing". But it didn't really
match nothing--rather, it matched something that
happened to be zero characters long. This is all very
above-board and honest. When a function returns an
undefined value, it's an admission that it couldn't give
you an honest answer. So you should use `defined()' only
when you're questioning the integrity of what you're
trying to do. At other times, a simple comparison to `0'
or `""' is what you want.
Currently, using `defined()' on an entire array or hash
reports whether memory for that aggregate has ever been
allocated. So an array you set to the empty list appears
undefined initially, and one that once was full and that
you then set to the empty list still appears defined.
You should instead use a simple test for size:
if (@an_array) { print "has array elements\n" }
if (%a_hash) { print "has hash members\n" }
Using `undef()' on these, however, does clear their
memory and then report them as not defined anymore, but
you shouldn't do that unless you don't plan to use them
again, because it saves time when you load them up again
to have memory already ready to be filled. The normal
way to free up space used by an aggregate is to assign
the empty list.
This counterintuitive behavior of `defined()' on
aggregates may be changed, fixed, or broken in a future
release of Perl.
See also the "undef" entry in this manpage, the "exists"
entry in this manpage, the "ref" entry in this manpage.
delete EXPR
Deletes the specified key(s) and their associated values
from a hash. For each key, returns the deleted value
associated with that key, or the undefined value if
there was no such key. Deleting from `$ENV{}' modifies
the environment. Deleting from a hash tied to a DBM file
deletes the entry from the DBM file. (But deleting from
a `tie()'d hash doesn't necessarily return anything.)
The following deletes all the values of a hash:
foreach $key (keys %HASH) {
delete $HASH{$key};
}
And so does this:
delete @HASH{keys %HASH}
But both of these are slower than just assigning the
empty list or undefining it:
%hash = (); # completely empty %hash
undef %hash; # forget %hash every existed
Note that the EXPR can be arbitrarily complicated as
long as the final operation is a hash element lookup or
hash slice:
delete $ref->[$x][$y]{$key};
delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
die LIST
Outside an `eval()', prints the value of LIST to
`STDERR' and exits with the current value of `$!'
(errno). If `$!' is `0', exits with the value of `($? >>
8)' (backtick `command` status). If `($? >> 8)' is `0',
exits with `255'. Inside an `eval(),' the error message
is stuffed into `$@' and the `eval()' is terminated with
the undefined value. This makes `die()' the way to raise
an exception.
Equivalent examples:
die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
If the value of EXPR does not end in a newline, the
current script line number and input line number (if
any) are also printed, and a newline is supplied. Note
that the "input line number" (also known as "chunk") is
subject to whatever notion of "line" happens to be
currently in effect, and is also available as the
special variable `$.'. See the section on "$/" in the
perlvar manpage and the section on "$." in the perlvar
manpage.
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 $@ =~ /Expected exception/;
If `$@' 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 arbitary 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
}
}
Since perl will stringify uncaught exception messages
before displaying them, you may want to overload
stringification operations on such custom exception
objects. See the overload manpage for details about
that.
You can arrange for a callback to be run just before the
`die()' does its deed, by setting the `$SIG{__DIE__}'
hook. The associated handler will be called with the
error text and can change the error message, if it sees
fit, by calling `die()' again. See the "$SIG{expr}"
entry in the perlvar manpage for details on setting
`%SIG' entries, and the section on "eval BLOCK" for some
examples.
Note that 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 $^S;
as the first line of the handler (see the "$^S" entry in
the perlvar manpage). Because this promotes 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 the perlsyn
manpage for alternative strategies.
do SUBROUTINE(LIST)
A deprecated form of subroutine call. See the perlsub
manpage.
do EXPR Uses the value of EXPR as a filename and executes the
contents of the file as a Perl script. Its primary use
is to include subroutines from a Perl subroutine
library.
do 'stat.pl';
is just like
scalar eval `cat stat.pl`;
except that it's more efficient and concise, keeps track
of the current filename for error messages, searches the
@INC libraries, and updates `%INC' if the file is found.
See the "Predefined Names" entry in the perlvar manpage
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 causes an immediate core dump. Primarily this is so
that you can use the undump program to turn your core
dump into an executable binary after having initialized
all your variables at the beginning of the program. When
the new binary is executed it will begin by executing a
`goto LABEL' (with all the restrictions that `goto'
suffers). Think of it as a goto with an intervening core
dump and reincarnation. If `LABEL' is omitted, restarts
the program from the top. WARNING: Any files opened at
the time of the dump will NOT be open any more when the
program is reincarnated, with possible resulting
confusion on the part of Perl. See also -u option in the
perlrun manpage.
Example:
#!/usr/bin/perl
require 'getopt.pl';
require 'stat.pl';
%days = (
'Sun' => 1,
'Mon' => 2,
'Tue' => 3,
'Wed' => 4,
'Thu' => 5,
'Fri' => 6,
'Sat' => 7,
);
dump QUICKSTART if $ARGV[0] eq '-d';
QUICKSTART:
Getopt('f');
This operator is largely obsolete, partly because it's
very hard to convert a core file into an executable, and
because the real perl-to-C compiler has superseded it.
each HASH
When called in list context, returns a 2-element list
consisting of the key and value for the next element of
a hash, so that you can iterate over it. When called in
scalar context, returns the key for only the "next"
element in the hash. (Note: Keys may be `"0"' or `""',
which are logically false; you may wish to avoid
constructs like `while ($k = each %foo) {}' for this
reason.)
Entries are returned in an apparently random order. 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.
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.
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.
Filetypes such as terminals may lose the end-of-file
condition if you do.
An `eof' without an argument uses the last file read as
argument. Using `eof()' with empty parentheses is very
different. It indicates the pseudo file formed of the
files listed on the command line, i.e., `eof()' is
reasonable to use inside a `while (<>)' loop to detect
the end of only the last file. Use `eof(ARGV)' or eof
without the parentheses to test *EACH* file in a while
(<>) loop. Examples:
# reset line numbering on each input file
while (<>) {
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 current file
print "--------------\n";
close(ARGV); # close or last; is needed if we
# are reading from the terminal
}
print;
}
Practical hint: you almost never need to use `eof' in
Perl, because the input operators return false values
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 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 `$_'. This form
is typically used to delay parsing and subsequent
execution of the text of EXPR until run time.
In the second form, the code within the BLOCK is parsed
only once--at the same time the code surrounding the
eval itself was parsed--and executed within the context
of the current Perl program. This form is typically used
to trap exceptions more efficiently than the first (see
below), while also providing the benefit of checking the
code within BLOCK at compile time.
The final semicolon, if any, may be omitted from the
value of EXPR or within the BLOCK.
In both forms, the value returned is the value of the
last expression evaluated inside the mini-program; a
return statement may be also used, just as with
subroutines. The expression providing the return value
is evaluated in void, scalar, or list context, depending
on the context of the eval itself. See the "wantarray"
entry in this manpage for more on how the evaluation
context can be determined.
If there is a syntax error or runtime error, or a
`die()' statement is executed, an undefined value is
returned by `eval()', and `$@' 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 `$SIG{__WARN__}'
facility. See the "warn" entry in this manpage and the
perlvar manpage.
Note that, because `eval()' traps otherwise-fatal
errors, it is useful for determining whether a
particular feature (such as `socket()' or `symlink()')
is implemented. It is also Perl's exception trapping
mechanism, where the die operator is used to raise
exceptions.
If the code to be executed doesn't vary, you may use the
eval-BLOCK form to trap run-time errors without
incurring the penalty of recompiling each time. The
error, if any, is still returned in `$@'. 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
counterintuive 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.
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. Note: `exec()' and `system()'
do not flush your output buffer, so you may need to set
`$|' to avoid lost output. Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";
If you don't really want to execute the first argument,
but want to lie to the program you are executing about
its own name, you can specify the program you actually
want to run as an "indirect object" (without a comma) in
front of the LIST. (This always forces interpretation of
the LIST as a multivalued list, even if there is only a
single scalar in the list.) Example:
$shell = '/bin/csh';
exec $shell '-sh'; # pretend it's a login shell
or, more directly,
exec {'/bin/csh'} '-sh'; # pretend it's a login shell
When the arguments get executed via the system shell,
results will be subject to its quirks and capabilities.
See the section on "`STRING`" in the perlop manpage for
details.
Using an indirect object with `exec()' or `system()' is
also more secure. This usage 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.
Note that `exec()' will not call your `END' blocks, nor
will it call any `DESTROY' methods in your objects.
exists EXPR
Returns TRUE if the specified hash key exists in its
hash array, even if the corresponding value is
undefined.
print "Exists\n" if exists $array{$key};
print "Defined\n" if defined $array{$key};
print "True\n" if $array{$key};
A hash element can be TRUE only if it's defined, and
defined if it exists, but the reverse doesn't
necessarily hold true.
Note that the EXPR can be arbitrarily complicated as
long as the final operation is a hash key lookup:
if (exists $ref->{A}->{B}->{$key}) { }
if (exists $hash{A}{B}{$key}) { }
Although the last element will not spring into existence
just because its existence was tested, intervening ones
will. Thus `$ref->{"A"}' and `$ref->{"A"}->{"B"}' will
spring into existence due to the existence test for a
$key element. 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.
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 the perlsub manpage 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 `fnctl()'. 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.
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";
}
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 the perlport manpage, 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 import them from the Fcntl module,
either individually, or as a group using the ':flock'
tag. LOCK_SH requests a shared lock, LOCK_EX requests an
exclusive lock, and LOCK_UN releases a previously
requested lock. If LOCK_NB is added to LOCK_SH or
LOCK_EX then `flock()' will return immediately rather
than blocking waiting for the lock (check the return
status to see if you got it).
To avoid the possibility of 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 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 the DB_File manpage 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.
Note: unflushed buffers remain unflushed in both
processes, which means you may need to set `$|'
($AUTOFLUSH in English) or call the `autoflush()' method
of `IO::Handle' to avoid duplicate output.
If you `fork()' without ever waiting on your children,
you will accumulate zombies. On some systems, you can
avoid this by setting `$SIG{CHLD}' to `"IGNORE"'. See
also the perlipc manpage 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 launced 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 the perlform manpage 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 the perlform
manpage) 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 the perlform manpage 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. 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 the "CPAN" entry
in the perlmodlib manpage.
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) = unpack_sockaddr_in($hersockaddr);
$herhostname = gethostbyaddr($iaddr, AF_INET);
$herstraddr = inet_ntoa($iaddr);
getpgrp PID
Returns the current process group for the specified PID.
Use a PID of `0' to get the current process group for
the current process. Will raise an exception if used on
a machine that doesn't implement getpgrp(2). If PID is
omitted, returns process group of current process. Note
that the POSIX version of `getpgrp()' does not accept a
PID argument, so only `PID==0' is truly portable.
getppid Returns the process id of the parent process.
getpriority WHICH,WHO
Returns the current priority for a process, a process
group, or a user. (See the getpriority(2) manpage.) 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.)
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.
Those that incorrectly implement a separate library call
are not 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 also provided in modules: `File::stat',
`Net::hostent', `Net::netent', `Net::protoent',
`Net::servent', `Time::gmtime', `Time::localtime', and
`User::grent'. These override the normal built-in,
replacing them with 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.
use Socket;
$mysockaddr = getsockname(SOCK);
($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
getsockopt SOCKET,LEVEL,OPTNAME
Returns the socket option requested, or undef if there
is an error.
glob EXPR
glob Returns the value of EXPR with filename expansions such as
the standard Unix shell /bin/csh would do. This is the
internal function implementing the `<*.c>' operator, but
you can use it directly. If EXPR is omitted, `$_' is
used. The `<*.c>' operator is discussed in more detail
in the section on "I/O Operators" in the perlop manpage.
gmtime EXPR
Converts a time as returned by the time function to a 9-
element array with the time localized for the standard
Greenwich time zone. Typically used as follows:
# 0 1 2 3 4 5 6 7 8
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
gmtime(time);
All array elements are numeric, and come straight out of
a struct tm. In particular this means that `$mon' has
the range `0..11' and `$wday' has the range `0..6' with
sunday as day `0'. Also, `$year' is the number of years
since 1900, that is, `$year' is `123' in year 2023,
*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?
If EXPR is omitted, does `gmtime(time())'.
In scalar context, returns the ctime(3) value:
$now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
Also see the `timegm()' function provided by the
`Time::Local' module, and the strftime(3) function
available via the POSIX module.
This scalar value is not locale dependent (see the
perllocale manpage), but is instead a Perl builtin. Also
see the `Time::Local' module, and the strftime(3) and
mktime(3) functions available via the POSIX module. To
get somewhat similar but locale dependent date strings,
set up your locale environment variables appropriately
(please see the perllocale manpage) and try for example:
use POSIX qw(strftime);
$now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
Note that the `%a' and `%b' escapes, which represent the
short forms of the day of the week and the month of the
year, may not necessarily be three characters wide in
all locales.
goto LABEL
goto EXPR
goto &NAME
The `goto-LABEL' form finds the statement labeled with
LABEL and resumes execution there. It may not be used to
go into any construct that requires initialization, such
as a subroutine or a `foreach' loop. It also can't be
used to go into a construct that is optimized away, or
to get out of a block or subroutine given to `sort()'.
It can be used to go almost anywhere else within the
dynamic scope, including out of subroutines, but it's
usually better to use some other construct such as
`last' or `die()'. The author of Perl has never felt the
need to use this form of `goto' (in Perl, that is--C is
another matter).
The `goto-EXPR' form expects a label name, whose scope
will be resolved dynamically. This allows for computed
`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 highly magical, and substitutes
a call to the named subroutine for the currently running
subroutine. This is used by `AUTOLOAD' subroutines that
wish to load another subroutine and then pretend that
the other subroutine had been called in the first place
(except that any modifications to `@_' in the current
subroutine are propagated to the other subroutine.)
After the `goto', not even `caller()' will be able to
tell that this routine was called first.
grep BLOCK LIST
grep EXPR,LIST
This is similar in spirit to, but not the same as,
grep(1) and its relatives. In particular, it is not
limited to using regular expressions.
Evaluates the BLOCK or EXPR for each element of LIST
(locally setting `$_' to each element) and returns the
list value consisting of those elements for which the
expression evaluated to TRUE. In scalar context, returns
the number of times the expression was TRUE.
@foo = grep(!/^#/, @bar); # weed out comments
or equivalently,
@foo = grep {!/^#/} @bar; # weed out comments
Note that, because `$_' is a reference into the list
value, it can be used to modify the elements of the
array. While this is useful and supported, it can cause
bizarre results if the LIST is not a named array.
Similarly, grep returns aliases into the original list,
much 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 the "map" entry in this manpage for an array
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 the "oct" entry in
this manpage.) If EXPR is omitted, uses `$_'.
print hex '0xAf'; # prints '175'
print hex 'aF'; # same
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 the "use()" entry in this
manpage, the perlmod manpage, and the Exporter manpage.
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 are useful for manipulating the values of
structures used by `ioctl()'. The following example sets
the erase character to DEL.
require 'ioctl.ph';
$getp = &TIOCGETP;
die "NO TIOCGETP" if $@ || !$getp;
$sgttyb_t = "ccccs"; # 4 chars and a short
if (ioctl(STDIN,$getp,$sgttyb)) {
@ary = unpack($sgttyb_t,$sgttyb);
$ary[2] = 127;
$sgttyb = pack($sgttyb_t,@ary);
ioctl(STDIN,&TIOCSETP,$sgttyb)
|| die "Can't ioctl: $!";
}
The return value of `ioctl()' (and `fcntl()') is as
follows:
if OS returns: then Perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
Thus Perl returns TRUE on success and FALSE on failure,
yet you can still easily determine the actual value
returned by the operating system:
$retval = ioctl(...) || -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);
See the "split" entry in this manpage.
keys HASH
Returns a list consisting of all the keys of the named
hash. (In a scalar context, returns the number of keys.)
The keys are returned in an apparently random order. 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). As
a side effect, it resets HASH's iterator.
Here is yet another way to print your environment:
@keys = keys %ENV;
@values = values %ENV;
while ($#keys >= 0) {
print pop(@keys), '=', pop(@values), "\n";
}
or how about sorted by key:
foreach $key (sort(keys %ENV)) {
print $key, '=', $ENV{$key}, "\n";
}
To sort 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 LIST
Sends a signal to a list of processes. The first element
of the list must be the signal to send. Returns the
number of processes successfully signaled.
$cnt = kill 1, $child1, $child2;
kill 9, @goners;
Unlike in the shell, in Perl if the *SIGNAL* is
negative, it kills process groups instead of processes.
(On System V, a negative *PROCESS* number will also kill
process groups, but that's not portable.) That means you
usually want to use positive not negative signals. You
may also use a signal name in quotes. See the section on
"Signals" in the perlipc manpage for details.
last LABEL
last The `last' command is like the `break' statement in C (as
used in loops); it immediately exits the loop in
question. If the LABEL is omitted, the command refers to
the innermost enclosing loop. The `continue' block, if
any, is not executed:
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
#...
}
`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.
See also the "continue" entry in this manpage for an
illustration of how `last', `next', and `redo' work.
lc EXPR
lc Returns an lowercased version of EXPR. This is the internal
function implementing the `\L' escape in double-quoted
strings. Respects current LC_CTYPE locale if `use
locale' in force. See the perllocale manpage.
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
the perllocale manpage.
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.
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 the section on "Sockets: Client/Server
Communication" in the perlipc manpage.
local EXPR
You really probably want to be using `my()' instead,
because `local()' isn't what most people think of as
"local". See the section on "Private Variables via my()"
in the perlsub manpage for details.
A local modifies the listed variables to be local to the
enclosing block, file, or eval. If more than one value
is listed, the list must be placed in parentheses. See
the section on "Temporary Values via local()" in the
perlsub manpage for details, including issues with tied
arrays and hashes.
localtime EXPR
Converts a time as returned by the time function to a 9-
element array with the time analyzed for the local time
zone. Typically used as follows:
# 0 1 2 3 4 5 6 7 8
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
localtime(time);
All array elements are numeric, and come straight out of
a struct tm. In particular this means that `$mon' has
the range `0..11' and `$wday' has the range `0..6' with
sunday as day `0'. Also, `$year' is the number of years
since 1900, that is, `$year' is `123' in year 2023, and
*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?
If EXPR is omitted, uses the current time
(`localtime(time)').
In scalar context, returns the ctime(3) value:
$now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
This scalar value is not locale dependent, see the
perllocale manpage, but instead a Perl builtin. Also see
the `Time::Local' module, and the strftime(3) and
mktime(3) function available via the POSIX module. To
get somewhat similar but locale dependent date strings,
set up your locale environment variables appropriately
(please see the perllocale manpage) and try for example:
use POSIX qw(strftime);
$now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
Note that the `%a' and `%b', the short forms of the day
of the week and the month of the year, may not
necessarily be three characters wide.
log EXPR
log Returns the natural logarithm (base *e*) of EXPR. If EXPR is
omitted, returns log of `$_'. To get the log of another
base, use basic algebra: The base-N log of a number is
is equal to the natural log of that number divided by
the natural log of N. For example:
sub log10 {
my $n = shift;
return log($n)/log(10);
}
See also the "exp" entry in this manpage for the inverse
operation.
lstat FILEHANDLE
lstat EXPR
lstat Does the same thing as the `stat()' function (including
setting the special `_' filehandle) but stats a symbolic
link instead of the file the symbolic link points to. If
symbolic links are unimplemented on your system, a
normal `stat()' is done.
If EXPR is omitted, stats `$_'.
m// The match operator. See the perlop manpage.
map BLOCK LIST
map EXPR,LIST
Evaluates the BLOCK or EXPR for each element of LIST
(locally setting `$_' to each element) and returns the
list value composed of the results of each such
evaluation. Evaluates BLOCK or EXPR in a list context,
so each element of LIST may produce zero, one, or more
elements in the returned value.
In scalar context, returns the total number of elements
so generated.
@chars = map(chr, @nums);
translates a list of numbers to the corresponding
characters. And
%hash = map { getkey($_) => $_ } @array;
is just a funny way to write
%hash = ();
foreach $_ (@array) {
$hash{getkey($_)} = $_;
}
Note that, because `$_' is a reference into the list
value, it can be used to modify the elements of the
array. While this is useful and supported, it can cause
bizarre results if the LIST is not a named array. Using
a regular `foreach' loop for this purpose would be
clearer in most cases. See also the "grep" entry in this
manpage for an array composed of those items of the
original list for which the BLOCK or EXPR evaluates to
true.
mkdir FILENAME,MODE
Creates the directory specified by FILENAME, with
permissions specified by MODE (as modified by `umask').
If it succeeds it returns TRUE, otherwise it returns
FALSE and sets `$!' (errno).
In general, it is better to create directories with
permissive MODEs, and let the user modify that with
their `umask', than it is to supply a restrictive MODE
and give the user no way to be more permissive. The
exceptions to this rule are when the file or directory
should be kept private (mail files, for instance). The
perlfunc(1) entry on `umask' discusses the choice of
MODE in more detail.
msgctl ID,CMD,ARG
Calls the System V IPC function msgctl(2). You'll
probably have to say
use IPC::SysV;
first to get the correct constant definitions. If CMD is
`IPC_STAT', then ARG must be a variable which will hold
the returned `msqid_ds' structure. Returns like
`ioctl()': the undefined value for error, "`0' but true"
for zero, or the actual return value otherwise. See also
`IPC::SysV' and `IPC::Semaphore::Msg' documentation.
msgget KEY,FLAGS
Calls the System V IPC function msgget(2). Returns the
message queue id, or the undefined value if there is an
error. See also `IPC::SysV' and `IPC::SysV::Msg'
documentation.
msgsnd ID,MSG,FLAGS
Calls the System V IPC function msgsnd to send the
message MSG to the message queue ID. MSG must begin with
the long integer message type, which may be created with
`pack("l", $type)'. Returns TRUE if successful, or FALSE
if there is an error. See also `IPC::SysV' and
`IPC::SysV::Msg' documentation.
msgrcv ID,VAR,SIZE,TYPE,FLAGS
Calls the System V IPC function msgrcv to receive a
message from message queue ID into variable VAR with a
maximum message size of SIZE. Note that if a message is
received, the message type will be the first thing in
VAR, and the maximum length of VAR is SIZE plus the size
of the message type. Returns TRUE if successful, or
FALSE if there is an error. See also `IPC::SysV' and
`IPC::SysV::Msg' documentation.
my EXPR A `my()' declares the listed variables to be local
(lexically) to the enclosing block, file, or `eval()'.
If more than one value is listed, the list must be
placed in parentheses. See the section on "Private
Variables via my()" in the perlsub manpage for details.
next LABEL
next The `next' command is like the `continue' statement in C; it
starts the next iteration of the loop:
LINE: while (<STDIN>) {
next LINE if /^#/; # discard comments
#...
}
Note that if there were a `continue' block on the above,
it would get executed even on discarded lines. If the
LABEL is omitted, the command refers to the innermost
enclosing loop.
`next' 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.
See also the "continue" entry in this manpage for an
illustration of how `last', `next', and `redo' work.
no Module LIST
See the the "use" entry in this manpage function, which
`no' is the opposite of.
oct EXPR
oct Interprets EXPR as an octal string and returns the
corresponding value. (If EXPR happens to start off with
`0x', interprets it as a hex string. If EXPR starts off
with `0b', it is interpreted as a binary string.) The
following will handle decimal, binary, octal, and hex in
the standard Perl or C notation:
$val = oct($val) if $val =~ /^0/;
If EXPR is omitted, uses `$_'. This function is commonly
used when a string such as `644' needs to be converted
into a file mode, for example. (Although perl will
automatically convert strings into numbers as needed,
this automatic conversion assumes base 10.)
open FILEHANDLE,EXPR
open FILEHANDLE
Opens the file whose filename is given by EXPR, and
associates it with FILEHANDLE. If FILEHANDLE is an
expression, its value is used as the name of the real
filehandle wanted. If EXPR is omitted, the scalar
variable of the same name as the FILEHANDLE contains the
filename. (Note that lexical variables--those declared
with `my()'--will not work for this purpose; so if
you're using `my()', specify EXPR in your call to open.)
See the perlopentut manpage for a kinder, gentler
explanation of opening files.
If the filename begins with `'<'' or nothing, the file
is opened for input. If the filename begins with `'>'',
the file is truncated and opened for output, being
created if necessary. If the filename begins with
`'>>'', the file is opened for appending, again being
created if necessary. You can put a `'+'' in front of
the `'>'' or `'<'' to indicate that you want both read
and write access to the file; thus `'+<'' is almost
always preferred for read/write updates--the `'+>'' mode
would clobber the file first. You can't usually use
either read-write mode for updating textfiles, since
they have variable length records. See the -i switch in
the perlrun manpage for a better approach. The file is
created with permissions of `0666' modified by the
process' `umask' value.
The prefix and the filename may be separated with
spaces. These various prefixes correspond to the
fopen(3) modes of `'r'', `'r+'', `'w'', `'w+'', `'a'',
and `'a+''.
If the filename begins with `'|'', the filename is
interpreted as a command to which output is to be piped,
and if the filename ends with a `'|'', the filename is
interpreted as a command which pipes output to us. See
the section on "Using open() for IPC" in the perlipc
manpage for more examples of this. (You are not allowed
to `open()' to a command that pipes both in *and* out,
but see the IPC::Open2 manpage, the IPC::Open3 manpage,
and the section on "Bidirectional Communication" in the
perlipc manpage for alternatives.)
Opening `'-'' opens STDIN and opening `'>-'' opens
STDOUT. Open returns nonzero upon success, the undefined
value otherwise. If the `open()' involved a pipe, the
return value happens to be the pid of the subprocess.
If you're unfortunate enough to be running Perl on a
system that distinguishes between text files and binary
files (modern operating systems don't care), then you
should check out the "binmode" entry in this manpage for
tips for dealing with this. The key distinction between
systems that need `binmode()' and those that don't is
their text file formats. Systems like Unix, MacOS, and
Plan9, which delimit lines with a single character, and
which encode that character in C as `"\n"', do not need
`binmode()'. The rest need it.
When opening a file, it's usually a bad idea to continue
normal execution if the request failed, so `open()' is
frequently used in connection with `die()'. Even if
`die()' won't do what you want (say, in a CGI script,
where you want to make a nicely formatted error message
(but there are modules that can help with that problem))
you should always check the return value from opening a
file. The infrequent exception is when working with an
unopened filehandle is actually what you want to do.
Examples:
$ARTICLE = 100;
open ARTICLE or die "Can't find article $ARTICLE: $!\n";
while (<ARTICLE>) {...
open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
# if the open fails, output is discarded
open(DBASE, '+<dbase.mine') # open for update
or die "Can't open 'dbase.mine' for update: $!";
open(ARTICLE, "caesar <$article |") # decrypt article
or die "Can't start caesar: $!";
open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
or die "Can't start sort: $!";
# process argument list of files along with any includes
foreach $file (@ARGV) {
process($file, 'fh00');
}
sub process {
my($filename, $input) = @_;
$input++; # this is a string increment
unless (open($input, $filename)) {
print STDERR "Can't open $filename: $!\n";
return;
}
local $_;
while (<$input>) { # note use of indirection
if (/^#include "(.*)"/) {
process($1, $input);
next;
}
#... # whatever
}
}
You may also, in the Bourne shell tradition, specify an
EXPR beginning with `'>&'', in which case the rest of
the string is interpreted as the name of a filehandle
(or file descriptor, if numeric) to be duped and opened.
You may use `&' after `>', `>>', `<', `+>', `+>>', and
`+<'. The mode you specify should match the mode of the
original filehandle. (Duping a filehandle does not take
into account any existing contents of stdio buffers.)
Here is a script that saves, redirects, and restores
STDOUT and STDERR:
#!/usr/bin/perl
open(OLDOUT, ">&STDOUT");
open(OLDERR, ">&STDERR");
open(STDOUT, ">foo.out") || die "Can't redirect stdout";
open(STDERR, ">&STDOUT") || die "Can't dup stdout";
select(STDERR); $| = 1; # make unbuffered
select(STDOUT); $| = 1; # make unbuffered
print STDOUT "stdout 1\n"; # this works for
print STDERR "stderr 1\n"; # subprocesses too
close(STDOUT);
close(STDERR);
open(STDOUT, ">&OLDOUT");
open(STDERR, ">&OLDERR");
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
If you specify `'<&=N'', where `N' is a number, then
Perl will do an equivalent of C's `fdopen()' of that
file descriptor; this is more parsimonious of file
descriptors. For example:
open(FILEHANDLE, "<&=$fd")
If you open a pipe on the command `'-'', i.e., either
`'|-'' or `'-|'', then there is an implicit fork done,
and the return value of open is the pid of the child
within the parent process, and `0' within the child
process. (Use `defined($pid)' to determine whether the
open was successful.) The filehandle behaves normally
for the parent, but i/o to that filehandle is piped
from/to the STDOUT/STDIN of the child process. In the
child process the filehandle isn't opened--i/o happens
from/to the new STDOUT or STDIN. Typically this is used
like the normal piped open when you want to exercise
more control over just how the pipe command gets
executed, such as when you are running setuid, and don't
want to have to scan shell commands for metacharacters.
The following pairs are more or less equivalent:
open(FOO, "|tr '[a-z]' '[A-Z]'");
open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
open(FOO, "cat -n '$file'|");
open(FOO, "-|") || exec 'cat', '-n', $file;
See the section on "Safe Pipe Opens" in the perlipc
manpage for more examples of this.
NOTE: On any operation that may do a fork, any unflushed
buffers remain unflushed in both processes, which means
you may need to set `$|' to avoid duplicate output. 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 the "$^F" entry in
the perlvar manpage.
Closing any piped filehandle causes the parent process
to wait for the child to finish, and returns the status
value in `$?'.
The filename passed to open will have leading and
trailing whitespace deleted, and the normal redirection
characters honored. This property, known as "magic
open", can often be used to good effect. A user could
specify a filename of "rsh cat file |", or you could
change certain filenames as needed:
$filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
open(FH, $filename) or die "Can't open $filename: $!";
However, to open a file with arbitrary weird characters
in it, it's necessary to protect any leading and
trailing whitespace:
$file =~ s#^(\s)#./$1#;
open(FOO, "< $file\0");
If you want a "real" C `open()' (see the open(2) manpage
on your system), then you should use the `sysopen()'
function, which involves no such magic. This is another
way to protect your filenames from interpretation. For
example:
use IO::Handle;
sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
or die "sysopen $path: $!";
$oldfh = select(HANDLE); $| = 1; select($oldfh);
print HANDLE "stuff $$\n");
seek(HANDLE, 0, 0);
print "File contains: ", <HANDLE>;
Using the constructor from the `IO::Handle' package (or
one of its subclasses, such as `IO::File' or
`IO::Socket'), you can generate anonymous filehandles
that have the scope of whatever variables hold
references to them, and automatically close whenever and
however you leave that scope:
use IO::File;
#...
sub read_myfile_munged {
my $ALL = shift;
my $handle = new IO::File;
open($handle, "myfile") or die "myfile: $!";
$first = <$handle>
or return (); # Automatically closed here.
mung $first or die "mung failed"; # Or here.
return $first, <$handle> if $ALL; # Or here.
$first; # Or here.
}
See the "seek" entry in this manpage for some details
about mixing reading and writing.
opendir DIRHANDLE,EXPR
Opens a directory named EXPR for processing by
`readdir()', `telldir()', `seekdir()', `rewinddir()',
and `closedir()'. Returns TRUE if successful. DIRHANDLEs
have their own namespace separate from FILEHANDLEs.
ord EXPR
ord Returns the numeric ascii value of the first character of
EXPR. If EXPR is omitted, uses `$_'. For the reverse,
see the "chr" entry in this manpage.
pack TEMPLATE,LIST
Takes an array or list of values and packs it into a
binary structure, returning the string containing the
structure. The TEMPLATE is a sequence of characters that
give the order and type of values, as follows:
a A string with arbitrary binary data, will be null padded.
A An ascii string, will be space padded.
Z A null terminated (asciz) string, will be null padded.
b A bit string (ascending bit order, like vec()).
B A bit string (descending bit order).
h A hex string (low nybble first).
H A hex string (high nybble first).
c A signed char value.
C An unsigned char value.
s A signed short value.
S An unsigned short value.
(This 'short' is _exactly_ 16 bits, which may differ from
what a local C compiler calls 'short'.)
i A signed integer value.
I An unsigned integer value.
(This 'integer' is _at least_ 32 bits wide. Its exact
size depends on what a local C compiler calls 'int',
and may even be larger than the 'long' described in
the next item.)
l A signed long value.
L An unsigned long value.
(This 'long' is _exactly_ 32 bits, which may differ from
what a local C compiler calls 'long'.)
n A short in "network" (big-endian) order.
N A long in "network" (big-endian) order.
v A short in "VAX" (little-endian) order.
V A long in "VAX" (little-endian) order.
(These 'shorts' and 'longs' are _exactly_ 16 bits and
_exactly_ 32 bits, respectively.)
q A signed quad (64-bit) value.
Q An unsigned quad value.
(Available only if your system supports 64-bit integer values
_and_ if Perl has been compiled to support those.
Causes a fatal error otherwise.)
f A single-precision float in the native format.
d A double-precision float in the native format.
p A pointer to a null-terminated string.
P A pointer to a structure (fixed-length string).
u A uuencoded string.
w A BER compressed integer. Its bytes represent an unsigned
integer in base 128, most significant digit first, with as
few digits as possible. Bit eight (the high bit) is set
on each byte except the last.
x A null byte.
X Back up a byte.
@ Null fill to absolute position.
The following rules apply:
* Each letter may optionally be followed by a number
giving a repeat count. With all types except
`"a"', `"A"', `"Z"', `"b"', `"B"', `"h"', `"H"',
and `"P"' the pack function will gobble up that
many values from the LIST. A `*' for the repeat
count means to use however many items are left.
* The `"a"', `"A"', and `"Z"' types gobble just one
value, but pack it as a string of length count,
padding with nulls or spaces as necessary. When
unpacking, `"A"' strips trailing spaces and
nulls, `"Z"' strips everything after the first
null, and `"a"' returns data verbatim.
* Likewise, the `"b"' and `"B"' fields pack a string
that many bits long.
* The `"h"' and `"H"' fields pack a string that many
nybbles long.
* The `"p"' type packs a pointer to a null-terminated
string. You are responsible for ensuring the
string is not a temporary value (which can
potentially get deallocated before you get
around to using the packed result). The `"P"'
type packs a pointer to a structure of the size
indicated by the length. A NULL pointer is
created if the corresponding value for `"p"' or
`"P"' is `undef'.
* The integer formats `"s"', `"S"', `"i"', `"I"',
`"l"', and `"L"' are inherently non-portable
between processors and operating systems because
they obey the native byteorder and endianness.
For example a 4-byte integer 0x87654321
(2271560481 decimal) be ordered natively
(arranged in and handled by the CPU registers)
into bytes as
0x12 0x34 0x56 0x78 # little-endian
0x78 0x56 0x34 0x12 # big-endian
Basically, the Intel, Alpha, and VAX CPUs and
little-endian, while everybody else, for example
Motorola m68k/88k, PPC, Sparc, HP PA, Power, and
Cray are big-endian. MIPS can be either: Digital
used it in little-endian mode, SGI uses it in
big-endian mode.
The names `big-endian' and `little-endian' are
joking references to the classic "Gulliver's
Travels" (via the paper "On Holy Wars and a Plea
for Peace" by Danny Cohen, USC/ISI IEN 137,
April 1, 1980) and the egg-eating habits of the
lilliputs.
Some systems may even have weird byte orders
such as
0x56 0x78 0x12 0x34
0x34 0x12 0x78 0x56
You can see your system's preference with
print join(" ", map { sprintf "%#02x", $_ }
unpack("C*",pack("L",0x12345678))), "\n";
The byteorder on the platform where Perl was
built is also available via the Config manpage:
use Config;
print $Config{byteorder}, "\n";
Byteorders `'1234'' and `'12345678'' are little-
endian, `'4321'' and `'87654321'' are big-
endian.
If you want portable packed integers use the
formats `"n"', `"N"', `"v"', and `"V"', their
byte endianness and size is known.
* Real numbers (floats and doubles) are in the native
machine format only; due to the multiplicity of
floating formats around, and the lack of a
standard "network" representation, no facility
for interchange has been made. This means that
packed floating point data written on one
machine may not be readable on another - even if
both use IEEE floating point arithmetic (as the
endian-ness of the memory representation is not
part of the IEEE spec).
Note that Perl uses doubles internally for all
numeric calculation, and converting from double
into float and thence back to double again will
lose precision (i.e., `unpack("f", pack("f",
$foo)') will not in general equal `$foo').
Examples:
$foo = pack("CCCC",65,66,67,68);
# foo eq "ABCD"
$foo = pack("C4",65,66,67,68);
# same thing
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
$foo = pack("s2",1,2);
# "\1\0\2\0" on little-endian
# "\0\1\0\2" on big-endian
$foo = pack("a4","abcd","x","y","z");
# "abcd"
$foo = pack("aaaa","abcd","x","y","z");
# "axyz"
$foo = pack("a14","abcdefg");
# "abcdefg\0\0\0\0\0\0\0"
$foo = pack("i9pl", gmtime);
# a real struct tm (on my system anyway)
$utmp_template = "Z8 Z8 Z16 L";
$utmp = pack($utmp_template, @utmp1);
# a struct utmp (BSDish)
@utmp2 = unpack($utmp_template, $utmp);
# "@utmp1" eq "@utmp2"
sub bintodec {
unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}
The same template may generally also be used in
unpack().
package
package NAMESPACE
Declares the compilation unit as being in the given
namespace. The scope of the package declaration is from
the declaration itself through the end of the enclosing
block, file, or eval (the same as the `my()' operator).
All further unqualified dynamic identifiers will be in
this namespace. A package statement affects only dynamic
variables--including those you've used `local()' on--but
*not* lexical variables, which are created with `my()'.
Typically it would be the first declaration in a file to
be included by the `require' or `use' operator. You can
switch into a package in more than one place; it merely
influences which symbol table is used by the compiler
for the rest of that block. You can refer to variables
and filehandles in other packages by prefixing the
identifier with the package name and a double colon:
`$Package::Variable'. If the package name is null, the
`main' package as assumed. That is, `$::sail' is
equivalent to `$main::sail' (as well as to `$main'sail',
still seen in older code).
If NAMESPACE is omitted, then there is no current
package, and all identifiers must be fully qualified or
lexicals. This is stricter than `use strict', since it
also extends to function names.
See the section on "Packages" in the perlmod manpage for
more information about packages, modules, and classes.
See the perlsub manpage for other scoping issues.
pipe READHANDLE,WRITEHANDLE
Opens a pair of connected pipes like the corresponding
system call. Note that if you set up a loop of piped
processes, deadlock can occur unless you are very
careful. In addition, note that Perl's pipes use stdio
buffering, so you may need to set `$|' to flush your
WRITEHANDLE after each command, depending on the
application.
See the IPC::Open2 manpage, the IPC::Open3 manpage, and
the section on "Bidirectional Communication" in the
perlipc manpage for examples of such things.
On systems that support a close-on-exec flag on files,
the flag will be set for the newly opened file
descriptors as determined by the value of $^F. See the
"$^F" entry in the perlvar manpage.
pop ARRAY
pop Pops and returns the last value of the array, shortening the
array by one element. Has a similar effect to
$tmp = $ARRAY[$#ARRAY--];
If there are no elements in the array, returns the
undefined value. If ARRAY is omitted, pops the `@ARGV'
array in the main program, and the `@_' array in
subroutines, just like `shift()'.
pos SCALAR
pos Returns the offset of where the last `m//g' search left off
for the variable is in question (`$_' is used when the
variable is not specified). May be modified to change
that offset. Such modification will also influence the
`\G' zero-width assertion in regular expressions. See
the perlre manpage and the perlop manpage.
print FILEHANDLE LIST
print LIST
print Prints a string or a comma-separated list of strings.
Returns TRUE if successful. FILEHANDLE may be a scalar
variable name, in which case the variable contains the
name of or a reference to the filehandle, thus
introducing one level of indirection. (NOTE: If
FILEHANDLE is a variable and the next token is a term,
it may be misinterpreted as an operator unless you
interpose a `+' or put parentheses around the
arguments.) If FILEHANDLE is omitted, prints by default
to standard output (or to the last selected output
channel--see the "select" entry in this manpage). If
LIST is also omitted, prints `$_' to the currently
selected output channel. To set the default output
channel to something other than STDOUT use the select
operation. Note that, because print takes a LIST,
anything in the LIST is evaluated in list context, and
any subroutine that you call will have one or more of
its expressions evaluated in list context. Also be
careful not to follow the print keyword with a left
parenthesis unless you want the corresponding right
parenthesis to terminate the arguments to the print--
interpose a `+' or put parentheses around all the
arguments.
Note that if you're storing FILEHANDLES in an array or
other expression, you will have to use a block returning
its value instead:
print { $files[$i] } "stuff\n";
print { $OK ? STDOUT : STDERR } "stuff\n";
printf FILEHANDLE FORMAT, LIST
printf FORMAT, LIST
Equivalent to `print FILEHANDLE sprintf(FORMAT, LIST)',
except that `$\' (the output record separator) is not
appended. The first argument of the list will be
interpreted as the `printf()' format. If `use locale' is
in effect, the character used for the decimal point in
formatted real numbers is affected by the LC_NUMERIC
locale. See the perllocale manpage.
Don't fall into the trap of using a `printf()' when a
simple `print()' would do. The `print()' is more
efficient and less error prone.
prototype FUNCTION
Returns the prototype of a function as a string (or
`undef' if the function has no prototype). FUNCTION is a
reference to, or the name of, the function whose
prototype you want to retrieve.
If FUNCTION is a string starting with `CORE::', the rest
is taken as a name for Perl builtin. If the builtin is
not *overridable* (such as `qw//') or its arguments
cannot be expressed by a prototype (such as `system()')
returns `undef' because the builtin does not really
behave like a Perl function. Otherwise, the string
describing the equivalent prototype is returned.
push ARRAY,LIST
Treats ARRAY as a stack, and pushes the values of LIST
onto the end of ARRAY. The length of ARRAY increases by
the length of LIST. Has the same effect as
for $value (LIST) {
$ARRAY[++$#ARRAY] = $value;
}
but is more efficient. Returns the new number of
elements in the array.
q/STRING/
qq/STRING/
qr/STRING/
qx/STRING/
qw/STRING/
Generalized quotes. See the section on "Regexp Quote-
Like Operators" in the perlop manpage.
quotemeta EXPR
quotemeta
Returns the value of EXPR with all non-alphanumeric
characters backslashed. (That is, all characters not
matching `/[A-Za-z_0-9]/' will be preceded by a
backslash in the returned string, regardless of any
locale settings.) This is the internal function
implementing the `\Q' escape in double-quoted strings.
If EXPR is omitted, uses `$_'.
rand EXPR
rand Returns a random fractional number greater than or equal to
`0' and less than the value of EXPR. (EXPR should be
positive.) If EXPR is omitted, the value `1' is used.
Automatically calls `srand()' unless `srand()' has
already been called. See also `srand()'.
(Note: If your rand function consistently returns
numbers that are too large or too small, then your
version of Perl was probably compiled with the wrong
number of RANDBITS.)
read FILEHANDLE,SCALAR,LENGTH,OFFSET
read FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH bytes of data into variable
SCALAR from the specified FILEHANDLE. Returns the number
of bytes actually read, `0' at end of file, or undef if
there was an error. SCALAR will be grown or shrunk to
the length actually read. An OFFSET may be specified to
place the read data at some other place than the
beginning of the string. This call is actually
implemented in terms of stdio's fread(3) call. To get a
true read(2) system call, see `sysread()'.
readdir DIRHANDLE
Returns the next directory entry for a directory opened
by `opendir()'. If used in list context, returns all the
rest of the entries in the directory. If there are no
more entries, returns an undefined value in scalar
context or a null list in list context.
If you're planning to filetest the return values out of
a `readdir()', you'd better prepend the directory in
question. Otherwise, because we didn't `chdir()' there,
it would have been testing the wrong file.
opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
@dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
closedir DIR;
readline EXPR
Reads from the filehandle whose typeglob is contained in
EXPR. In scalar context, each call reads and returns the
next line, until end-of-file is reached, whereupon the
subsequent call returns undef. In list context, reads
until end-of-file is reached and returns a list of
lines. Note that the notion of "line" used here is
however you may have defined it with `$/' or
`$INPUT_RECORD_SEPARATOR'). See the section on "$/" in
the perlvar manpage.
When `$/' is set to `undef', when readline() is in
scalar context (i.e. file slurp mode), and when an empty
file is read, it returns `''' the first time, followed
by `undef' subsequently.
This is the internal function implementing the `<EXPR>'
operator, but you can use it directly. The `<EXPR>'
operator is discussed in more detail in the section on
"I/O Operators" in the perlop manpage.
$line = <STDIN>;
$line = readline(*STDIN); # same thing
readlink EXPR
readlink
Returns the value of a symbolic link, if symbolic links
are implemented. If not, gives a fatal error. If there
is some system error, returns the undefined value and
sets `$!' (errno). If EXPR is omitted, uses `$_'.
readpipe EXPR
EXPR is executed as a system command. The collected
standard output of the command is returned. In scalar
context, it comes back as a single (potentially multi-
line) string. In list context, returns a list of lines
(however you've defined lines with `$/' or
`$INPUT_RECORD_SEPARATOR'). This is the internal
function implementing the `qx/EXPR/' operator, but you
can use it directly. The `qx/EXPR/' operator is
discussed in more detail in the section on "I/O
Operators" in the perlop manpage.
recv SOCKET,SCALAR,LENGTH,FLAGS
Receives a message on a socket. Attempts to receive
LENGTH bytes of data into variable SCALAR from the
specified SOCKET filehandle. Actually does a C
`recvfrom()', so that it can return the address of the
sender. Returns the undefined value if there's an error.
SCALAR will be grown or shrunk to the length actually
read. Takes the same flags as the system call of the
same name. See the section on "UDP: Message Passing" in
the perlipc manpage for examples.
redo LABEL
redo The `redo' command restarts the loop block without
evaluating the conditional again. The `continue' block,
if any, is not executed. If the LABEL is omitted, the
command refers to the innermost enclosing loop. This
command is normally used by programs that want to lie to
themselves about what was just input:
# a simpleminded Pascal comment stripper
# (warning: assumes no { or } in strings)
LINE: while (<STDIN>) {
while (s|({.*}.*){.*}|$1 |) {}
s|{.*}| |;
if (s|{.*| |) {
$front = $_;
while (<STDIN>) {
if (/}/) { # end of comment?
s|^|$front\{|;
redo LINE;
}
}
}
print;
}
`redo' cannot be used to retry a block which returns a
value such as `eval {}', `sub {}' or `do {}', and should
not be used to exit a grep() or map() operation.
See also the "continue" entry in this manpage for an
illustration of how `last', `next', and `redo' work.
ref EXPR
ref Returns a TRUE value if EXPR is a reference, FALSE
otherwise. If EXPR is not specified, `$_' will be used.
The value returned depends on the type of thing the
reference is a reference to. Builtin types include:
REF
SCALAR
ARRAY
HASH
CODE
GLOB
If the referenced object has been blessed into a
package, then that package name is returned instead. You
can think of `ref()' as a `typeof()' operator.
if (ref($r) eq "HASH") {
print "r is a reference to a hash.\n";
}
unless (ref($r)) {
print "r is not a reference at all.\n";
}
if (UNIVERSAL::isa($r, "HASH")) { # for subclassing
print "r is a reference to something that isa hash.\n";
}
See also the perlref manpage.
rename OLDNAME,NEWNAME
Changes the name of a file. Returns `1' for success, `0'
otherwise. Behavior of this function varies wildly
depending on your system implementation. For example, it
will usually not work across file system boundaries,
even though the system *mv* command sometimes
compensates for this. Other restrictions include whether
it works on directories, open files, or pre-existing
files. Check the perlport manpage and either the
rename(2) manpage or equivalent system documentation for
details.
require EXPR
require Demands some semantics specified by EXPR, or by `$_' if EXPR
is not supplied. If EXPR is numeric, demands that the
current version of Perl (`$]' or $PERL_VERSION) be equal
or greater than EXPR.
Otherwise, demands that a library file be included if it
hasn't already been included. The file is included via
the do-FILE mechanism, which is essentially just a
variety of `eval()'. Has semantics similar to the
following subroutine:
sub require {
my($filename) = @_;
return 1 if $INC{$filename};
my($realfilename,$result);
ITER: {
foreach $prefix (@INC) {
$realfilename = "$prefix/$filename";
if (-f $realfilename) {
$result = do $realfilename;
last ITER;
}
}
die "Can't find $filename in \@INC";
}
die $@ if $@;
die "$filename did not return true value" unless $result;
$INC{$filename} = $realfilename;
return $result;
}
Note that the file will not be included twice under the
same specified name. The file must return TRUE as the
last statement to indicate successful execution of any
initialization code, so it's customary to end such a
file with "`1;'" unless you're sure it'll return TRUE
otherwise. But it's better just to put the "`1;'", in
case you add more statements.
If EXPR is a bareword, the require assumes a ".pm"
extension and replaces "::" with "/" in the filename for
you, to make it easy to load standard modules. This form
of loading of modules does not risk altering your
namespace.
In other words, if you try this:
require Foo::Bar; # a splendid bareword
The require function will actually look for the
"Foo/Bar.pm" file in the directories specified in the
`@INC' array.
But if you try this:
$class = 'Foo::Bar';
require $class; # $class is not a bareword
#or
require "Foo::Bar"; # not a bareword because of the ""
The require function will look for the "Foo::Bar" file
in the @INC array and will complain about not finding
"Foo::Bar" there. In this case you can do:
eval "require $class";
For a yet-more-powerful import facility, see the "use"
entry in this manpage and the perlmod manpage.
reset EXPR
reset Generally used in a `continue' block at the end of a loop to
clear variables and reset `??' searches so that they
work again. The expression is interpreted as a list of
single characters (hyphens allowed for ranges). All
variables and arrays beginning with one of those letters
are reset to their pristine state. If the expression is
omitted, one-match searches (`?pattern?') are reset to
match again. Resets only variables or searches in the
current package. Always returns 1. Examples:
reset 'X'; # reset all X variables
reset 'a-z'; # reset lower case variables
reset; # just reset ?one-time? searches
Resetting `"A-Z"' is not recommended because you'll wipe
out your `@ARGV' and `@INC' arrays and your `%ENV' hash.
Resets only package variables--lexical variables are
unaffected, but they clean themselves up on scope exit
anyway, so you'll probably want to use them instead. See
the "my" entry in this manpage.
return EXPR
return Returns from a subroutine, `eval()', or `do FILE' with the
value given in EXPR. Evaluation of EXPR may be in list,
scalar, or void context, depending on how the return
value will be used, and the context may vary from one
execution to the next (see `wantarray()'). If no EXPR is
given, returns an empty list in list context, the
undefined value in scalar context, and (of course)
nothing at all in a void context.
(Note that in the absence of a explicit `return', a
subroutine, eval, or do FILE will automatically return
the value of the last expression evaluated.)
reverse LIST
In list context, returns a list value consisting of the
elements of LIST in the opposite order. In scalar
context, concatenates the elements of LIST and returns a
string value with all characters in the opposite order.
print reverse <>; # line tac, last line first
undef $/; # for efficiency of <>
print scalar reverse <>; # character tac, last line tsrif
This operator is also handy for inverting a hash,
although there are some caveats. If a value is
duplicated in the original hash, only one of those can
be represented as a key in the inverted hash. Also, this
has to unwind one hash and build a whole new one, which
may take some time on a large hash, such as from a DBM
file.
%by_name = reverse %by_address; # Invert the hash
rewinddir DIRHANDLE
Sets the current position to the beginning of the
directory for the `readdir()' routine on DIRHANDLE.
rindex STR,SUBSTR,POSITION
rindex STR,SUBSTR
Works just like index() except that it returns the
position of the LAST occurrence of SUBSTR in STR. If
POSITION is specified, returns the last occurrence at or
before that position.
rmdir FILENAME
rmdir Deletes the directory specified by FILENAME if that
directory is empty. If it succeeds it returns TRUE,
otherwise it returns FALSE and sets `$!' (errno). If
FILENAME is omitted, uses `$_'.
s/// The substitution operator. See the perlop manpage.
scalar EXPR
Forces EXPR to be interpreted in scalar context and
returns the value of EXPR.
@counts = ( scalar @a, scalar @b, scalar @c );
There is no equivalent operator to force an expression
to be interpolated in list context because in practice,
this is never needed. If you really wanted to do so,
however, you could use the construction `@{[ (some
expression) ]}', but usually a simple `(some
expression)' suffices.
Since `scalar' is a unary operator, if you accidentally
use for EXPR a parenthesized list, this behaves as a
scalar comma expression, evaluating all but the last
element in void context and returning the final element
evaluated in scalar context. This is seldom what you
want.
The following single statement:
print uc(scalar(&foo,$bar)),$baz;
is the moral equivalent of these two:
&foo;
print(uc($bar),$baz);
See the perlop manpage for more details on unary
operators and the comma operator.
seek FILEHANDLE,POSITION,WHENCE
Sets FILEHANDLE's position, just like the `fseek()' call
of `stdio()'. FILEHANDLE may be an expression whose
value gives the name of the filehandle. The values for
WHENCE are `0' to set the new position to POSITION, `1'
to set it to the current position plus POSITION, and `2'
to set it to EOF plus POSITION (typically negative). For
WHENCE you may use the constants `SEEK_SET', `SEEK_CUR',
and `SEEK_END' from either the `IO::Seekable' or the
POSIX module. Returns `1' upon success, `0' otherwise.
If you want to position file for `sysread()' or
`syswrite()', don't use `seek()' -- buffering makes its
effect on the file's system position unpredictable and
non-portable. Use `sysseek()' instead.
Due to the rules and rigors of ANSI C, on some systems
you have to do a seek whenever you switch between
reading and writing. Amongst other things, this may have
the effect of calling stdio's clearerr(3). A WHENCE of
`1' (`SEEK_CUR') is useful for not moving the file
position:
seek(TEST,0,1);
This is also useful for applications emulating `tail -
f'. Once you hit EOF on your read, and then sleep for a
while, you might have to stick in a seek() to reset
things. The `seek()' doesn't change the current
position, but it *does* clear the end-of-file condition
on the handle, so that the next `<FILE>' makes Perl try
again to read something. We hope.
If that doesn't work (some stdios are particularly
cantankerous), then you may need something more like
this:
for (;;) {
for ($curpos = tell(FILE); $_ = <FILE>;
$curpos = tell(FILE)) {
# search for some stuff and put it into files
}
sleep($for_a_while);
seek(FILE, $curpos, 0);
}
seekdir DIRHANDLE,POS
Sets the current position for the `readdir()' routine on
DIRHANDLE. POS must be a value returned by `telldir()'.
Has the same caveats about possible directory compaction
as the corresponding system library routine.
select FILEHANDLE
select Returns the currently selected filehandle. Sets the current
default filehandle for output, if FILEHANDLE is
supplied. This has two effects: first, a `write()' or a
`print()' without a filehandle will default to this
FILEHANDLE. Second, references to variables related to
output will refer to this output channel. For example,
if you have to set the top of form format for more than
one output channel, you might do the following:
select(REPORT1);
$^ = 'report1_top';
select(REPORT2);
$^ = 'report2_top';
FILEHANDLE may be an expression whose value gives the
name of the actual filehandle. Thus:
$oldfh = select(STDERR); $| = 1; select($oldfh);
Some programmers may prefer to think of filehandles as
objects with methods, preferring to write the last
example as:
use IO::Handle;
STDERR->autoflush(1);
select RBITS,WBITS,EBITS,TIMEOUT
This calls the select(2) system call with the bit masks
specified, which can be constructed using `fileno()' and
`vec()', along these lines:
$rin = $win = $ein = '';
vec($rin,fileno(STDIN),1) = 1;
vec($win,fileno(STDOUT),1) = 1;
$ein = $rin | $win;
If you want to select on many filehandles you might wish
to write a subroutine:
sub fhbits {
my(@fhlist) = split(' ',$_[0]);
my($bits);
for (@fhlist) {
vec($bits,fileno($_),1) = 1;
}
$bits;
}
$rin = fhbits('STDIN TTY SOCK');
The usual idiom is:
($nfound,$timeleft) =
select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
or to block until something becomes ready just do this
$nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
Most systems do not bother to return anything useful in
`$timeleft', so calling select() in scalar context just
returns `$nfound'.
Any of the bit masks can also be undef. The timeout, if
specified, is in seconds, which may be fractional. Note:
not all implementations are capable of returning
the`$timeleft'. If not, they always return `$timeleft'
equal to the supplied `$timeout'.
You can effect a sleep of 250 milliseconds this way:
select(undef, undef, undef, 0.25);
WARNING: One should not attempt to mix buffered I/O
(like `read()' or <FH>) with `select()', except as
permitted by POSIX, and even then only on POSIX systems.
You have to use `sysread()' instead.
semctl ID,SEMNUM,CMD,ARG
Calls the System V IPC function `semctl()'. You'll
probably have to say
use IPC::SysV;
first to get the correct constant definitions. If CMD is
IPC_STAT or GETALL, then ARG must be a variable which
will hold the returned semid_ds structure or semaphore
value array. Returns like `ioctl()': the undefined value
for error, "`0' but true" for zero, or the actual return
value otherwise. See also `IPC::SysV' and
`IPC::Semaphore' documentation.
semget KEY,NSEMS,FLAGS
Calls the System V IPC function semget. Returns the
semaphore id, or the undefined value if there is an
error. See also `IPC::SysV' and `IPC::SysV::Semaphore'
documentation.
semop KEY,OPSTRING
Calls the System V IPC function semop to perform
semaphore operations such as signaling and waiting.
OPSTRING must be a packed array of semop structures.
Each semop structure can be generated with `pack("sss",
$semnum, $semop, $semflag)'. The number of semaphore
operations is implied by the length of OPSTRING. Returns
TRUE if successful, or FALSE if there is an error. As an
example, the following code waits on semaphore `$semnum'
of semaphore id `$semid':
$semop = pack("sss", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid, $semop);
To signal the semaphore, replace `-1' with `1'. See also
`IPC::SysV' and `IPC::SysV::Semaphore' documentation.
send SOCKET,MSG,FLAGS,TO
send SOCKET,MSG,FLAGS
Sends a message on a socket. Takes the same flags as the
system call of the same name. On unconnected sockets you
must specify a destination to send TO, in which case it
does a C `sendto()'. Returns the number of characters
sent, or the undefined value if there is an error. The C
system call sendmsg(2) is currently unimplemented. See
the section on "UDP: Message Passing" in the perlipc
manpage for examples.
setpgrp PID,PGRP
Sets the current process group for the specified PID,
`0' for the current process. Will produce a fatal error
if used on a machine that doesn't implement setpgrp(2).
If the arguments are omitted, it defaults to `0,0'. Note
that the POSIX version of `setpgrp()' does not accept
any arguments, so only `setpgrp(0,0)' is portable. See
also `POSIX::setsid()'.
setpriority WHICH,WHO,PRIORITY
Sets the current priority for a process, a process
group, or a user. (See setpriority(2).) Will produce a
fatal error if used on a machine that doesn't implement
setpriority(2).
setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
Sets the socket option requested. Returns undefined if
there is an error. OPTVAL may be specified as `undef' if
you don't want to pass an argument.
shift ARRAY
shift Shifts the first value of the array off and returns it,
shortening the array by 1 and moving everything down. If
there are no elements in the array, returns the
undefined value. If ARRAY is omitted, shifts the `@_'
array within the lexical scope of subroutines and
formats, and the `@ARGV' array at file scopes or within
the lexical scopes established by the `eval ''', `BEGIN
{}', `END {}', and `INIT {}' constructs. See also
`unshift()', `push()', and `pop()'. `Shift()' and
`unshift()' do the same thing to the left end of an
array that `pop()' and `push()' do to the right end.
shmctl ID,CMD,ARG
Calls the System V IPC function shmctl. You'll probably
have to say
use IPC::SysV;
first to get the correct constant definitions. If CMD is
`IPC_STAT', then ARG must be a variable which will hold
the returned `shmid_ds' structure. Returns like ioctl:
the undefined value for error, "`0' but true" for zero,
or the actual return value otherwise. See also
`IPC::SysV' documentation.
shmget KEY,SIZE,FLAGS
Calls the System V IPC function shmget. Returns the
shared memory segment id, or the undefined value if
there is an error. See also `IPC::SysV' documentation.
shmread ID,VAR,POS,SIZE
shmwrite ID,STRING,POS,SIZE
Reads or writes the System V shared memory segment ID
starting at position POS for size SIZE by attaching to
it, copying in/out, and detaching from it. When reading,
VAR must be a variable that will hold the data read.
When writing, if STRING is too long, only SIZE bytes are
used; if STRING is too short, nulls are written to fill
out SIZE bytes. Return TRUE if successful, or FALSE if
there is an error. See also `IPC::SysV' documentation
and the `IPC::Shareable' module from CPAN.
shutdown SOCKET,HOW
Shuts down a socket connection in the manner indicated
by HOW, which has the same interpretation as in the
system call of the same name.
shutdown(SOCKET, 0); # I/we have stopped reading data
shutdown(SOCKET, 1); # I/we have stopped writing data
shutdown(SOCKET, 2); # I/we have stopped using this socket
This is useful with sockets when you want to tell the
other side you're done writing but not done reading, or
vice versa. It's also a more insistent form of close
because it also disables the filedescriptor in any
forked copies in other processes.
sin EXPR
sin Returns the sine of EXPR (expressed in radians). If EXPR is
omitted, returns sine of `$_'.
For the inverse sine operation, you may use the
`POSIX::asin()' function, or use this relation:
sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
sleep EXPR
sleep Causes the script to sleep for EXPR seconds, or forever if
no EXPR. May be interrupted if the process receives a
signal such as `SIGALRM'. Returns the number of seconds
actually slept. You probably cannot mix `alarm()' and
`sleep()' calls, because `sleep()' is often implemented
using `alarm()'.
On some older systems, it may sleep up to a full second
less than what you requested, depending on how it counts
seconds. Most modern systems always sleep the full
amount. They may appear to sleep longer than that,
however, because your process might not be scheduled
right away in a busy multitasking system.
For delays of finer granularity than one second, you may
use Perl's `syscall()' interface to access setitimer(2)
if your system supports it, or else see the "select"
entry in this manpage above.
See also the POSIX module's `sigpause()' function.
socket SOCKET,DOMAIN,TYPE,PROTOCOL
Opens a socket of the specified kind and attaches it to
filehandle SOCKET. DOMAIN, TYPE, and PROTOCOL are
specified the same as for the system call of the same
name. You should "`use Socket;'" first to get the proper
definitions imported. See the examples in the section on
"Sockets: Client/Server Communication" in the perlipc
manpage.
socketpair SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL
Creates an unnamed pair of sockets in the specified
domain, of the specified type. DOMAIN, TYPE, and
PROTOCOL are specified the same as for the system call
of the same name. If unimplemented, yields a fatal
error. Returns TRUE if successful.
Some systems defined `pipe()' in terms of
`socketpair()', in which a call to `pipe(Rdr, Wtr)' is
essentially:
use Socket;
socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
shutdown(Rdr, 1); # no more writing for reader
shutdown(Wtr, 0); # no more reading for writer
See the perlipc manpage for an example of socketpair
use.
sort SUBNAME LIST
sort BLOCK LIST
sort LIST
Sorts the LIST and returns the sorted list value. If
SUBNAME or BLOCK is omitted, `sort()'s in standard
string comparison order. If SUBNAME is specified, it
gives the name of a subroutine that returns an integer
less than, equal to, or greater than `0', depending on
how the elements of the array are to be ordered. (The
`<=>' and `cmp' operators are extremely useful in such
routines.) SUBNAME may be a scalar variable name
(unsubscripted), in which case the value provides the
name of (or a reference to) the actual subroutine to
use. In place of a SUBNAME, you can provide a BLOCK as
an anonymous, in-line sort subroutine.
In the interests of efficiency the normal calling code
for subroutines is bypassed, with the following effects:
the subroutine may not be a recursive subroutine, and
the two elements to be compared are passed into the
subroutine not via `@_' but as the package global
variables `$a' and `$b' (see example below). They are
passed by reference, so don't modify `$a' and `$b'. And
don't try to declare them as lexicals either.
You also cannot exit out of the sort block or subroutine
using any of the loop control operators described in the
perlsyn manpage or with `goto()'.
When `use locale' is in effect, `sort LIST' sorts LIST
according to the current collation locale. See the
perllocale manpage.
Examples:
# sort lexically
@articles = sort @files;
# same thing, but with explicit sort routine
@articles = sort {$a cmp $b} @files;
# now case-insensitively
@articles = sort {uc($a) cmp uc($b)} @files;
# same thing in reversed order
@articles = sort {$b cmp $a} @files;
# sort numerically ascending
@articles = sort {$a <=> $b} @files;
# sort numerically descending
@articles = sort {$b <=> $a} @files;
# sort using explicit subroutine name
sub byage {
$age{$a} <=> $age{$b}; # presuming numeric
}
@sortedclass = sort byage @class;
# this sorts the %age hash by value instead of key
# using an in-line function
@eldest = sort { $age{$b} <=> $age{$a} } keys %age;
sub backwards { $b cmp $a; }
@harry = ('dog','cat','x','Cain','Abel');
@george = ('gone','chased','yz','Punished','Axed');
print sort @harry;
# prints AbelCaincatdogx
print sort backwards @harry;
# prints xdogcatCainAbel
print sort @george, 'to', @harry;
# prints AbelAxedCainPunishedcatchaseddoggonetoxyz
# inefficiently sort by descending numeric compare using
# the first integer after the first = sign, or the
# whole record case-insensitively otherwise
@new = sort {
($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
||
uc($a) cmp uc($b)
} @old;
# same thing, but much more efficiently;
# we'll build auxiliary indices instead
# for speed
@nums = @caps = ();
for (@old) {
push @nums, /=(\d+)/;
push @caps, uc($_);
}
@new = @old[ sort {
$nums[$b] <=> $nums[$a]
||
$caps[$a] cmp $caps[$b]
} 0..$#old
];
# same thing using a Schwartzian Transform (no temps)
@new = map { $_->[0] }
sort { $b->[1] <=> $a->[1]
||
$a->[2] cmp $b->[2]
} map { [$_, /=(\d+)/, uc($_)] } @old;
If you're using strict, you *MUST NOT* declare `$a' and
`$b' as lexicals. They are package globals. That means
if you're in the `main' package, it's
@articles = sort {$main::b <=> $main::a} @files;
or just
@articles = sort {$::b <=> $::a} @files;
but if you're in the `FooPack' package, it's
@articles = sort {$FooPack::b <=> $FooPack::a} @files;
The comparison function is required to behave. If it
returns inconsistent results (sometimes saying `$x[1]'
is less than `$x[2]' and sometimes saying the opposite,
for example) the results are not well-defined.
splice ARRAY,OFFSET,LENGTH,LIST
splice ARRAY,OFFSET,LENGTH
splice ARRAY,OFFSET
Removes the elements designated by OFFSET and LENGTH
from an array, and replaces them with the elements of
LIST, if any. In list context, returns the elements
removed from the array. In scalar context, returns the
last element removed, or `undef' if no elements are
removed. The array grows or shrinks as necessary. If
OFFSET is negative then it start that far from the end
of the array. If LENGTH is omitted, removes everything
from OFFSET onward. If LENGTH is negative, leave that
many elements off the end of the array. The following
equivalences hold (assuming `$[ == 0'):
push(@a,$x,$y) splice(@a,@a,0,$x,$y)
pop(@a) splice(@a,-1)
shift(@a) splice(@a,0,1)
unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
$a[$x] = $y splice(@a,$x,1,$y)
Example, assuming array lengths are passed before
arrays:
sub aeq { # compare two list values
my(@a) = splice(@_,0,shift);
my(@b) = splice(@_,0,shift);
return 0 unless @a == @b; # same len?
while (@a) {
return 0 if pop(@a) ne pop(@b);
}
return 1;
}
if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
split /PATTERN/,EXPR,LIMIT
split /PATTERN/,EXPR
split /PATTERN/
split Splits a string into an array of strings, and returns it. By
default, empty leading fields are preserved, and empty
trailing ones are deleted.
If not in list context, returns the number of fields
found and splits into the `@_' array. (In list context,
you can force the split into `@_' by using `??' as the
pattern delimiters, but it still returns the list
value.) The use of implicit split to `@_' is deprecated,
however, because it clobbers your subroutine arguments.
If EXPR is omitted, splits the `$_' string. If PATTERN
is also omitted, splits on whitespace (after skipping
any leading whitespace). Anything matching PATTERN is
taken to be a delimiter separating the fields. (Note
that the delimiter may be longer than one character.)
If LIMIT is specified and positive, splits into no more
than that many fields (though it may split into fewer).
If LIMIT is unspecified or zero, trailing null fields
are stripped (which potential users of `pop()' would do
well to remember). If LIMIT is negative, it is treated
as if an arbitrarily large LIMIT had been specified.
A pattern matching the null string (not to be confused
with a null pattern `//', which is just one member of
the set of patterns matching a null string) will split
the value of EXPR into separate characters at each point
it matches that way. For example:
print join(':', split(/ */, 'hi there'));
produces the output 'h:i:t:h:e:r:e'.
The LIMIT parameter can be used to split a line
partially
($login, $passwd, $remainder) = split(/:/, $_, 3);
When assigning to a list, if LIMIT is omitted, Perl
supplies a LIMIT one larger than the number of variables
in the list, to avoid unnecessary work. For the list
above LIMIT would have been 4 by default. In time
critical applications it behooves you not to split into
more fields than you really need.
If the PATTERN contains parentheses, additional array
elements are created from each matching substring in the
delimiter.
split(/([,-])/, "1-10,20", 3);
produces the list value
(1, '-', 10, ',', 20)
If you had the entire header of a normal Unix email
message in `$header', you could split it up into fields
and their values this way:
$header =~ s/\n\s+/ /g; # fix continuation lines
%hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
The pattern `/PATTERN/' may be replaced with an
expression to specify patterns that vary at runtime. (To
do runtime compilation only once, use `/$variable/o'.)
As a special case, specifying a PATTERN of space (`' '')
will split on white space just as `split()' with no
arguments does. Thus, `split(' ')' can be used to
emulate awk's default behavior, whereas `split(/ /)'
will give you as many null initial fields as there are
leading spaces. A `split()' on `/\s+/' is like a
`split(' ')' except that any leading whitespace produces
a null first field. A `split()' with no arguments really
does a `split(' ', $_)' internally.
Example:
open(PASSWD, '/etc/passwd');
while (<PASSWD>) {
($login, $passwd, $uid, $gid,
$gcos, $home, $shell) = split(/:/);
#...
}
(Note that `$shell' above will still have a newline on
it. See the "chop" entry in this manpage, the "chomp"
entry in this manpage, and the "join" entry in this
manpage.)
sprintf FORMAT, LIST
Returns a string formatted by the usual `printf()'
conventions of the C library function `sprintf()'. See
the sprintf(3) manpage or the printf(3) manpage on your
system for an explanation of the general principles.
Perl does its own `sprintf()' formatting -- it emulates
the C function `sprintf()', but it doesn't use it
(except for floating-point numbers, and even then only
the standard modifiers are allowed). As a result, any
non-standard extensions in your local `sprintf()' are
not available from Perl.
Perl's `sprintf()' permits the following universally-
known conversions:
%% a percent sign
%c a character with the given number
%s a string
%d a signed integer, in decimal
%u an unsigned integer, in decimal
%o an unsigned integer, in octal
%x an unsigned integer, in hexadecimal
%e a floating-point number, in scientific notation
%f a floating-point number, in fixed decimal notation
%g a floating-point number, in %e or %f notation
In addition, Perl permits the following widely-supported
conversions:
%X like %x, but using upper-case letters
%E like %e, but using an upper-case "E"
%G like %g, but with an upper-case "E" (if applicable)
%p a pointer (outputs the Perl value's address in hexadecimal)
%n special: *stores* the number of characters output so far
into the next variable in the parameter list
Finally, for backward (and we do mean "backward")
compatibility, Perl permits these unnecessary but
widely-supported conversions:
%i a synonym for %d
%D a synonym for %ld
%U a synonym for %lu
%O a synonym for %lo
%F a synonym for %f
Perl permits the following universally-known flags
between the `%' and the conversion letter:
space prefix positive number with a space
+ prefix positive number with a plus sign
- left-justify within the field
0 use zeros, not spaces, to right-justify
# prefix non-zero octal with "0", non-zero hex with "0x"
number minimum field width
.number "precision": digits after decimal point for
floating-point, max length for string, minimum length
for integer
l interpret integer as C type "long" or "unsigned long"
h interpret integer as C type "short" or "unsigned short"
There is also one Perl-specific flag:
V interpret integer as Perl's standard integer type
Where a number would appear in the flags, an asterisk
("`*'") may be used instead, in which case Perl uses the
next item in the parameter list as the given number
(that is, as the field width or precision). If a field
width obtained through "`*'" is negative, it has the
same effect as the "`-'" flag: left-justification.
If `use locale' is in effect, the character used for the
decimal point in formatted real numbers is affected by
the LC_NUMERIC locale. See the perllocale manpage.
sqrt EXPR
sqrt Return the square root of EXPR. If EXPR is omitted, returns
square root of `$_'. Only works on non-negative
operands, unless you've loaded the standard
Math::Complex module.
use Math::Complex;
print sqrt(-2); # prints 1.4142135623731i
srand EXPR
srand Sets the random number seed for the `rand()' operator. If
EXPR is omitted, uses a semi-random value supplied by
the kernel (if it supports the /dev/urandom device) or
based on the current time and process ID, among other
things. In versions of Perl prior to 5.004 the default
seed was just the current `time()'. This isn't a
particularly good seed, so many old programs supply
their own seed value (often `time ^ $$' or `time ^ ($$ +
($$ << 15))'), but that isn't necessary any more.
In fact, it's usually not necessary to call `srand()' at
all, because if it is not called explicitly, it is
called implicitly at the first use of the `rand()'
operator. However, this was not the case in version of
Perl before 5.004, so if your script will run under
older Perl versions, it should call `srand()'.
Note that you need something much more random than the
default seed for cryptographic purposes. Checksumming
the compressed output of one or more rapidly changing
operating system status programs is the usual method.
For example:
srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
If you're particularly concerned with this, see the
`Math::TrulyRandom' module in CPAN.
Do *not* call `srand()' multiple times in your program
unless you know exactly what you're doing and why you're
doing it. The point of the function is to "seed" the
`rand()' function so that `rand()' can produce a
different sequence each time you run your program. Just
do it once at the top of your program, or you *won't*
get random numbers out of `rand()'!
Frequently called programs (like CGI scripts) that
simply use
time ^ $$
for a seed can fall prey to the mathematical property
that
a^b == (a+1)^(b+1)
one-third of the time. So don't do that.
stat FILEHANDLE
stat EXPR
stat Returns a 13-element list giving the status info for a file,
either the file opened via FILEHANDLE, or named by EXPR.
If EXPR is omitted, it stats `$_'. Returns a null list
if the stat fails. Typically used as follows:
($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
$atime,$mtime,$ctime,$blksize,$blocks)
= stat($filename);
Not all fields are supported on all filesystem types.
Here are the meaning of the fields:
0 dev device number of filesystem
1 ino inode number
2 mode file mode (type and permissions)
3 nlink number of (hard) links to the file
4 uid numeric user ID of file's owner
5 gid numeric group ID of file's owner
6 rdev the device identifier (special files only)
7 size total size of file, in bytes
8 atime last access time since the epoch
9 mtime last modify time since the epoch
10 ctime inode change time (NOT creation time!) since the epoch
11 blksize preferred block size for file system I/O
12 blocks actual number of blocks allocated
(The epoch was at 00:00 January 1, 1970 GMT.)
If stat is passed the special filehandle consisting of
an underline, no stat is done, but the current contents
of the stat structure from the last stat or filetest are
returned. Example:
if (-x $file && (($d) = stat(_)) && $d < 0) {
print "$file is executable NFS file\n";
}
(This works on machines only for which the device number
is negative under NFS.)
Because the mode contains both the file type and its
permissions, you should mask off the file type portion
and (s)printf using a `"%o"' if you want to see the real
permissions.
$mode = (stat($filename))[2];
printf "Permissions are %04o\n", $mode & 07777;
In scalar context, `stat()' returns a boolean value
indicating success or failure, and, if successful, sets
the information associated with the special filehandle
`_'.
The File::stat module provides a convenient, by-name
access mechanism:
use File::stat;
$sb = stat($filename);
printf "File is %s, size is %s, perm %04o, mtime %s\n",
$filename, $sb->size, $sb->mode & 07777,
scalar localtime $sb->mtime;
study SCALAR
study Takes extra time to study SCALAR (`$_' if unspecified) in
anticipation of doing many pattern matches on the string
before it is next modified. This may or may not save
time, depending on the nature and number of patterns you
are searching on, and on the distribution of character
frequencies in the string to be searched -- you probably
want to compare run times with and without it to see
which runs faster. Those loops which scan for many short
constant strings (including the constant parts of more
complex patterns) will benefit most. You may have only
one `study()' active at a time -- if you study a
different scalar the first is "unstudied". (The way
`study()' works is this: a linked list of every
character in the string to be searched is made, so we
know, for example, where all the `'k'' characters are.
From each search string, the rarest character is
selected, based on some static frequency tables
constructed from some C programs and English text. Only
those places that contain this "rarest" character are
examined.)
For example, here is a loop that inserts index producing
entries before any line containing a certain pattern:
while (<>) {
study;
print ".IX foo\n" if /\bfoo\b/;
print ".IX bar\n" if /\bbar\b/;
print ".IX blurfl\n" if /\bblurfl\b/;
# ...
print;
}
In searching for `/\bfoo\b/', only those locations in
`$_' that contain `"f"' will be looked at, because `"f"'
is rarer than `"o"'. In general, this is a big win
except in pathological cases. The only question is
whether it saves you more time than it took to build the
linked list in the first place.
Note that if you have to look for strings that you don't
know till runtime, you can build an entire loop as a
string and `eval()' that to avoid recompiling all your
patterns all the time. Together with undefining `$/' to
input entire files as one record, this can be very fast,
often faster than specialized programs like fgrep(1).
The following scans a list of files (`@files') for a
list of words (`@words'), and prints out the names of
those files that contain a match:
$search = 'while (<>) { study;';
foreach $word (@words) {
$search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
}
$search .= "}";
@ARGV = @files;
undef $/;
eval $search; # this screams
$/ = "\n"; # put back to normal input delimiter
foreach $file (sort keys(%seen)) {
print $file, "\n";
}
sub BLOCK
sub NAME
sub NAME BLOCK
This is subroutine definition, not a real function *per
se*. With just a NAME (and possibly prototypes), it's
just a forward declaration. Without a NAME, it's an
anonymous function declaration, and does actually return
a value: the CODE ref of the closure you just created.
See the perlsub manpage and the perlref manpage for
details.
substr EXPR,OFFSET,LEN,REPLACEMENT
substr EXPR,OFFSET,LEN
substr EXPR,OFFSET
Extracts a substring out of EXPR and returns it. First
character is at offset `0', or whatever you've set `$['
to (but don't do that). If OFFSET is negative (or more
precisely, less than `$['), starts that far from the end
of the string. If LEN is omitted, returns everything to
the end of the string. If LEN is negative, leaves that
many characters off the end of the string.
If you specify a substring that is partly outside the
string, the part within the string is returned. If the
substring is totally outside the string a warning is
produced.
You can use the substr() function as an lvalue, in which
case EXPR must itself be an lvalue. If you assign
something shorter than LEN, the string will shrink, and
if you assign something longer than LEN, the string will
grow to accommodate it. To keep the string the same
length you may need to pad or chop your value using
`sprintf()'.
An alternative to using substr() as an lvalue is to
specify the replacement string as the 4th argument. This
allows you to replace parts of the EXPR and return what
was there before in one operation, just as you can with
splice().
symlink OLDFILE,NEWFILE
Creates a new filename symbolically linked to the old
filename. Returns `1' for success, `0' otherwise. On
systems that don't support symbolic links, produces a
fatal error at run time. To check for that, use eval:
$symlink_exists = eval { symlink("",""); 1 };
syscall LIST
Calls the system call specified as the first element of
the list, passing the remaining elements as arguments to
the system call. If unimplemented, produces a fatal
error. The arguments are interpreted as follows: if a
given argument is numeric, the argument is passed as an
int. If not, the pointer to the string value is passed.
You are responsible to make sure a string is pre-
extended long enough to receive any result that might be
written into a string. You can't use a string literal
(or other read-only string) as an argument to
`syscall()' because Perl has to assume that any string
pointer might be written through. If your integer
arguments are not literals and have never been
interpreted in a numeric context, you may need to add
`0' to them to force them to look like numbers. This
emulates the `syswrite()' function (or vice versa):
require 'syscall.ph'; # may need to run h2ph
$s = "hi there\n";
syscall(&SYS_write, fileno(STDOUT), $s, length $s);
Note that Perl supports passing of up to only 14
arguments to your system call, which in practice should
usually suffice.
Syscall returns whatever value returned by the system
call it calls. If the system call fails, `syscall()'
returns `-1' and sets `$!' (errno). Note that some
system calls can legitimately return `-1'. The proper
way to handle such calls is to assign `$!=0;' before the
call and check the value of `$!' if syscall returns `-
1'.
There's a problem with `syscall(&SYS_pipe)': it returns
the file number of the read end of the pipe it creates.
There is no way to retrieve the file number of the other
end. You can avoid this problem by using `pipe()'
instead.
sysopen FILEHANDLE,FILENAME,MODE
sysopen FILEHANDLE,FILENAME,MODE,PERMS
Opens the file whose filename is given by FILENAME, and
associates it with FILEHANDLE. If FILEHANDLE is an
expression, its value is used as the name of the real
filehandle wanted. This function calls the underlying
operating system's `open()' function with the parameters
FILENAME, MODE, PERMS.
The possible values and flag bits of the MODE parameter
are system-dependent; they are available via the
standard module `Fcntl'. For historical reasons, some
values work on almost every system supported by perl:
zero means read-only, one means write-only, and two
means read/write. We know that these values do *not*
work under OS/390 & VM/ESA Unix and on the Macintosh;
you probably don't want to use them in new code.
If the file named by FILENAME does not exist and the
`open()' call creates it (typically because MODE
includes the `O_CREAT' flag), then the value of PERMS
specifies the permissions of the newly created file. If
you omit the PERMS argument to `sysopen()', Perl uses
the octal value `0666'. These permission values need to
be in octal, and are modified by your process's current
`umask'.
You should seldom if ever use `0644' as argument to
`sysopen()', because that takes away the user's option
to have a more permissive umask. Better to omit it. See
the perlfunc(1) entry on `umask' for more on this.
See the perlopentut manpage for a kinder, gentler
explanation of opening files.
sysread FILEHANDLE,SCALAR,LENGTH,OFFSET
sysread FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH bytes of data into variable
SCALAR from the specified FILEHANDLE, using the system
call read(2). It bypasses stdio, so mixing this with
other kinds of reads, `print()', `write()', `seek()',
`tell()', or `eof()' can cause confusion because stdio
usually buffers data. Returns the number of bytes
actually read, `0' at end of file, or undef if there was
an error. SCALAR will be grown or shrunk so that the
last byte actually read is the last byte of the scalar
after the read.
An OFFSET may be specified to place the read data at
some place in the string other than the beginning. A
negative OFFSET specifies placement at that many bytes
counting backwards from the end of the string. A
positive OFFSET greater than the length of SCALAR
results in the string being padded to the required size
with `"\0"' bytes before the result of the read is
appended.
There is no syseof() function, which is ok, since eof()
doesn't work very well on device files (like ttys)
anyway. Use sysread() and check for a return value for 0
to decide whether you're done.
sysseek FILEHANDLE,POSITION,WHENCE
Sets FILEHANDLE's system position using the system call
lseek(2). It bypasses stdio, so mixing this with reads
(other than `sysread()'), `print()', `write()',
`seek()', `tell()', or `eof()' may cause confusion.
FILEHANDLE may be an expression whose value gives the
name of the filehandle. The values for WHENCE are `0' to
set the new position to POSITION, `1' to set the it to
the current position plus POSITION, and `2' to set it to
EOF plus POSITION (typically negative). For WHENCE, you
may use the constants `SEEK_SET', `SEEK_CUR', and
`SEEK_END' from either the `IO::Seekable' or the POSIX
module.
Returns the new position, or the undefined value on
failure. A position of zero is returned as the string
"`0' but true"; thus `sysseek()' returns TRUE on success
and FALSE on failure, yet you can still easily determine
the new position.
system LIST
system PROGRAM LIST
Does exactly the same thing as "`exec LIST'", except
that a fork is done first, and the parent process waits
for the child process to complete. Note that argument
processing varies depending on the number of arguments.
If there is more than one argument in LIST, or if LIST
is an array with more than one value, starts the program
given by the first element of the list with arguments
given by the rest of the list. If there is only one
scalar argument, 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.
The return value is the exit status of the program as
returned by the `wait()' call. To get the actual exit
value divide by 256. See also the "exec" entry in this
manpage. This is *NOT* what you want to use to capture
the output from a command, for that you should use
merely backticks or `qx//', as described in the section
on "`STRING`" in the perlop manpage.
Like `exec()', `system()' allows you to lie to a program
about its name if you use the "`system PROGRAM LIST'"
syntax. Again, see the "exec" entry in this manpage.
Because `system()' and backticks block `SIGINT' and
`SIGQUIT', killing the program they're running doesn't
actually interrupt your program.
@args = ("command", "arg1", "arg2");
system(@args) == 0
or die "system @args failed: $?"
You can check all the failure possibilities by
inspecting `$?' like this:
$exit_value = $? >> 8;
$signal_num = $? & 127;
$dumped_core = $? & 128;
When the arguments get executed via the system shell,
results and return codes will be subject to its quirks
and capabilities. See the section on "`STRING`" in the
perlop manpage and the "exec" entry in this manpage for
details.
syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
syswrite FILEHANDLE,SCALAR,LENGTH
syswrite FILEHANDLE,SCALAR
Attempts to write LENGTH bytes of data from variable
SCALAR to the specified FILEHANDLE, using the system
call write(2). If LENGTH is not specified, writes whole
SCALAR. It bypasses stdio, so mixing this with reads
(other than `sysread())', `print()', `write()',
`seek()', `tell()', or `eof()' may cause confusion
because stdio usually buffers data. Returns the number
of bytes actually written, or `undef' if there was an
error. If the LENGTH is greater than the available data
in the SCALAR after the OFFSET, only as much data as is
available will be written.
An OFFSET may be specified to write the data from some
part of the string other than the beginning. A negative
OFFSET specifies writing that many bytes counting
backwards from the end of the string. In the case the
SCALAR is empty you can use OFFSET but only zero offset.
tell FILEHANDLE
tell Returns the current position for FILEHANDLE. FILEHANDLE may
be an expression whose value gives the name of the
actual filehandle. If FILEHANDLE is omitted, assumes the
file last read.
There is no `systell()' function. Use `sysseek(FH, 0,
1)' for that.
telldir DIRHANDLE
Returns the current position of the `readdir()' routines
on DIRHANDLE. Value may be given to `seekdir()' to
access a particular location in a directory. Has the
same caveats about possible directory compaction as the
corresponding system library routine.
tie VARIABLE,CLASSNAME,LIST
This function binds a variable to a package class that
will provide the implementation for the variable.
VARIABLE is the name of the variable to be enchanted.
CLASSNAME is the name of a class implementing objects of
correct type. Any additional arguments are passed to the
"`new()'" method of the class (meaning `TIESCALAR',
`TIEHANDLE', `TIEARRAY', or `TIEHASH'). Typically these
are arguments such as might be passed to the
`dbm_open()' function of C. The object returned by the
"`new()'" method is also returned by the `tie()'
function, which would be useful if you want to access
other methods in CLASSNAME.
Note that functions such as `keys()' and `values()' may
return huge lists when used on large objects, like DBM
files. You may prefer to use the `each()' function to
iterate over such. Example:
# print out history file offsets
use NDBM_File;
tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
untie(%HIST);
A class implementing a hash should have the following
methods:
TIEHASH classname, LIST
FETCH this, key
STORE this, key, value
DELETE this, key
CLEAR this
EXISTS this, key
FIRSTKEY this
NEXTKEY this, lastkey
DESTROY this
A class implementing an ordinary array should have the
following methods:
TIEARRAY classname, LIST
FETCH this, key
STORE this, key, value
FETCHSIZE this
STORESIZE this, count
CLEAR this
PUSH this, LIST
POP this
SHIFT this
UNSHIFT this, LIST
SPLICE this, offset, length, LIST
EXTEND this, count
DESTROY this
A class implementing a file handle should have the
following methods:
TIEHANDLE classname, LIST
READ this, scalar, length, offset
READLINE this
GETC this
WRITE this, scalar, length, offset
PRINT this, LIST
PRINTF this, format, LIST
CLOSE this
DESTROY this
A class implementing a scalar should have the following
methods:
TIESCALAR classname, LIST
FETCH this,
STORE this, value
DESTROY this
Not all methods indicated above need be implemented. See
the perltie manpage, the Tie::Hash manpage, the
Tie::Array manpage, the Tie::Scalar manpage, and the
Tie::Handle manpage.
Unlike `dbmopen()', the `tie()' function will not use or
require a module for you--you need to do that explicitly
yourself. See the DB_File manpage or the Config module
for interesting `tie()' implementations.
For further details see the perltie manpage, the section
on "tied VARIABLE".
tied VARIABLE
Returns a reference to the object underlying VARIABLE
(the same value that was originally returned by the
`tie()' call that bound the variable to a package.)
Returns the undefined value if VARIABLE isn't tied to a
package.
time Returns the number of non-leap seconds since whatever time
the system considers to be the epoch (that's 00:00:00,
January 1, 1904 for MacOS, and 00:00:00 UTC, January 1,
1970 for most other systems). Suitable for feeding to
`gmtime()' and `localtime()'.
times Returns a four-element list giving the user and system
times, in seconds, for this process and the children of
this process.
($user,$system,$cuser,$csystem) = times;
tr/// The transliteration operator. Same as `y///'. See the perlop
manpage.
truncate FILEHANDLE,LENGTH
truncate EXPR,LENGTH
Truncates the file opened on FILEHANDLE, or named by
EXPR, to the specified length. Produces a fatal error if
truncate isn't implemented on your system. Returns TRUE
if successful, the undefined value otherwise.
uc EXPR
uc Returns an uppercased version of EXPR. This is the internal
function implementing the `\U' escape in double-quoted
strings. Respects current LC_CTYPE locale if `use
locale' in force. See the perllocale manpage. (It does
not attempt to do titlecase mapping on initial letters.
See `ucfirst()' for that.)
If EXPR is omitted, uses `$_'.
ucfirst EXPR
ucfirst Returns the value of EXPR with the first character in
uppercase. This is the internal function implementing
the `\u' escape in double-quoted strings. Respects
current LC_CTYPE locale if `use locale' in force. See
the perllocale manpage.
If EXPR is omitted, uses `$_'.
umask EXPR
umask Sets the umask for the process to EXPR and returns the
previous value. If EXPR is omitted, merely returns the
current umask.
The Unix permission `rwxr-x---' is represented as three
sets of three bits, or three octal digits: `0750' (the
leading 0 indicates octal and isn't one of the digits).
The `umask' value is such a number representing disabled
permissions bits. The permission (or "mode") values you
pass `mkdir' or `sysopen' are modified by your umask, so
even if you tell `sysopen' to create a file with
permissions `0777', if your umask is `0022' then the
file will actually be created with permissions `0755'.
If your `umask' were `0027' (group can't write; others
can't read, write, or execute), then passing `sysopen()'
`0666' would create a file with mode `0640' (`0666 &~
027' is `0640').
Here's some advice: supply a creation mode of `0666' for
regular files (in `sysopen()') and one of `0777' for
directories (in `mkdir()') and executable files. This
gives users the freedom of choice: if they want
protected files, they might choose process umasks of
`022', `027', or even the particularly antisocial mask
of `077'. Programs should rarely if ever make policy
decisions better left to the user. The exception to this
is when writing files that should be kept private: mail
files, web browser cookies, *.rhosts* files, and so on.
If umask(2) is not implemented on your system and you
are trying to restrict access for *yourself* (i.e.,
(EXPR & 0700) > 0), produces a fatal error at run time.
If umask(2) is not implemented and you are not trying to
restrict access for yourself, returns `undef'.
Remember that a umask is a number, usually given in
octal; it is *not* a string of octal digits. See also
the "oct" entry in this manpage, if all you have is a
string.
undef EXPR
undef Undefines the value of EXPR, which must be an lvalue. Use
only on a scalar value, an array (using "`@'"), a hash
(using "`%'"), a subroutine (using "`&'"), or a typeglob
(using "<*>"). (Saying `undef $hash{$key}' will probably
not do what you expect on most predefined variables or
DBM list values, so don't do that; see the delete
manpage.) Always returns the undefined value. You can
omit the EXPR, in which case nothing is undefined, but
you still get an undefined value that you could, for
instance, return from a subroutine, assign to a variable
or pass as a parameter. Examples:
undef $foo;
undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
undef @ary;
undef %hash;
undef &mysub;
undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
select undef, undef, undef, 0.25;
($a, $b, undef, $c) = &foo; # Ignore third value returned
Note that this is a unary operator, not a list operator.
unlink LIST
unlink Deletes a list of files. Returns the number of files
successfully deleted.
$cnt = unlink 'a', 'b', 'c';
unlink @goners;
unlink <*.bak>;
Note: `unlink()' will not delete directories unless you
are superuser and the -U flag is supplied to Perl. Even
if these conditions are met, be warned that unlinking a
directory can inflict damage on your filesystem. Use
`rmdir()' instead.
If LIST is omitted, uses `$_'.
unpack TEMPLATE,EXPR
`Unpack()' does the reverse of `pack()': it takes a
string representing a structure and expands it out into
a list value, returning the array value. (In scalar
context, it returns merely the first value produced.)
The TEMPLATE has the same format as in the `pack()'
function. Here's a subroutine that does substring:
sub substr {
my($what,$where,$howmuch) = @_;
unpack("x$where a$howmuch", $what);
}
and then there's
sub ordinal { unpack("c",$_[0]); } # same as ord()
In addition, you may prefix a field with a %<number> to
indicate that you want a <number>-bit checksum of the
items instead of the items themselves. Default is a 16-
bit checksum. For example, the following computes the
same number as the System V sum program:
while (<>) {
$checksum += unpack("%32C*", $_);
}
$checksum %= 65535;
The following efficiently counts the number of set bits
in a bit vector:
$setbits = unpack("%32b*", $selectmask);
See the "pack" entry in this manpage for more examples.
untie VARIABLE
Breaks the binding between a variable and a package.
(See `tie()'.)
unshift ARRAY,LIST
Does the opposite of a `shift()'. Or the opposite of a
`push()', depending on how you look at it. Prepends list
to the front of the array, and returns the new number of
elements in the array.
unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
Note the LIST is prepended whole, not one element at a
time, so the prepended elements stay in the same order.
Use `reverse()' to do the reverse.
use Module LIST
use Module
use Module VERSION LIST
use VERSION
Imports some semantics into the current package from the
named module, generally by aliasing certain subroutine
or variable names into your package. It is exactly
equivalent to
BEGIN { require Module; import Module LIST; }
except that Module *must* be a bareword.
If the first argument to `use' is a number, it is
treated as a version number instead of a module name. If
the version of the Perl interpreter is less than
VERSION, then an error message is printed and Perl exits
immediately. This is often useful if you need to check
the current Perl version before `use'ing library modules
that have changed in incompatible ways from older
versions of Perl. (We try not to do this more than we
have to.)
The `BEGIN' forces the `require' and `import()' to
happen at compile time. The `require' makes sure the
module is loaded into memory if it hasn't been yet. The
`import()' is not a builtin--it's just an ordinary
static method call into the "`Module'" package to tell
the module to import the list of features back into the
current package. The module can implement its `import()'
method any way it likes, though most modules just choose
to derive their `import()' method via inheritance from
the `Exporter' class that is defined in the `Exporter'
module. See the Exporter manpage. If no `import()'
method can be found then the error is currently silently
ignored. This may change to a fatal error in a future
version.
If you don't want your namespace altered, explicitly
supply an empty list:
use Module ();
That is exactly equivalent to
BEGIN { require Module }
If the VERSION argument is present between Module and
LIST, then the `use' will call the VERSION method in
class Module with the given version as an argument. The
default VERSION method, inherited from the Universal
class, croaks if the given version is larger than the
value of the variable `$Module::VERSION'. (Note that
there is not a comma after VERSION!)
Because this is a wide-open interface, pragmas (compiler
directives) are also implemented this way. Currently
implemented pragmas are:
use integer;
use diagnostics;
use sigtrap qw(SEGV BUS);
use strict qw(subs vars refs);
use subs qw(afunc blurfl);
Some of these these pseudo-modules import semantics into
the current block scope (like `strict' or `integer',
unlike ordinary modules, which import symbols into the
current package (which are effective through the end of
the file).
There's a corresponding "`no'" command that unimports
meanings imported by `use', i.e., it calls `unimport
Module LIST' instead of `import()'.
no integer;
no strict 'refs';
If no `unimport()' method can be found the call fails
with a fatal error.
See the perlmod manpage for a list of standard modules
and pragmas.
utime LIST
Changes the access and modification times on each file
of a list of files. The first two elements of the list
must be the NUMERICAL access and modification times, in
that order. Returns the number of files successfully
changed. The inode modification time of each file is set
to the current time. This code has the same effect as
the "`touch'" command if the files already exist:
#!/usr/bin/perl
$now = time;
utime $now, $now, @ARGV;
values HASH
Returns a list consisting of all the values of the named
hash. (In a scalar context, returns the number of
values.) The values are returned in an apparently random
order. The actual random order is subject to change in
future versions of perl, but it is guaranteed to be the
same order as either the `keys()' or `each()' function
would produce on the same (unmodified) hash.
Note that you cannot modify the values of a hash this
way, because the returned list is just a copy. You need
to use a hash slice for that, since it's lvaluable in a
way that values() is not.
for (values %hash) { s/foo/bar/g } # FAILS!
for (@hash{keys %hash}) { s/foo/bar/g } # ok
As a side effect, calling values() resets the HASH's
internal iterator. See also `keys()', `each()', and
`sort()'.
vec EXPR,OFFSET,BITS
Treats the string in EXPR as a vector of unsigned
integers, and returns the value of the bit field
specified by OFFSET. BITS specifies the number of bits
that are reserved for each entry in the bit vector. This
must be a power of two from 1 to 32. `vec()' may also be
assigned to, in which case parentheses are needed to
give the expression the correct precedence as in
vec($image, $max_x * $x + $y, 8) = 3;
Vectors created with `vec()' can also be manipulated
with the logical operators `|', `&', and `^', which will
assume a bit vector operation is desired when both
operands are strings. See the section on "Bitwise String
Operators" in the perlop manpage.
The following code will build up an ASCII string saying
`'PerlPerlPerl''. The comments show the string after
each step. Note that this code works in the same way on
big-endian or little-endian machines.
my $foo = '';
vec($foo, 0, 32) = 0x5065726C; # 'Perl'
vec($foo, 2, 16) = 0x5065; # 'PerlPe'
vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
vec($foo, 21, 4) = 7; # 'PerlPerlPer'
# 'r' is "\x72"
vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
# 'l' is "\x6c"
To transform a bit vector into a string or array of 0's
and 1's, use these:
$bits = unpack("b*", $vector);
@bits = split(//, unpack("b*", $vector));
If you know the exact length in bits, it can be used in
place of the `*'.
wait Behaves like the wait(2) system call on your system: it
waits for a child process to terminate and returns the
pid of the deceased process, or `-1' if there are no
child processes. The status is rketurned in `$?'. Note
that a return value of `-1' could mean that child
processes are being automatically reaped, as described
in the perlipc manpage.
waitpid PID,FLAGS
Waits for a particular child process to terminate and
returns the pid of the deceased process, or `-1' if
there is no such child process. On some systems, a value
of 0 indicates that there are processes still running.
The status is returned in `$?'. If you say
use POSIX ":sys_wait_h";
#...
do {
$kid = waitpid(-1,&WNOHANG);
} until $kid == -1;
then you can do a non-blocking wait for all pending
zombie processes. Non-blocking wait is available on
machines supporting either the waitpid(2) or wait4(2)
system calls. However, waiting for a particular pid with
FLAGS of `0' is implemented everywhere. (Perl emulates
the system call by remembering the status values of
processes that have exited but have not been harvested
by the Perl script yet.)
Note that on some systems, a return value of `-1' could
mean that child processes are being automatically
reaped. See the perlipc manpage for details, and for
other examples.
wantarray
Returns TRUE if the context of the currently executing
subroutine is looking for a list value. Returns FALSE if
the context is looking for a scalar. Returns the
undefined value if the context is looking for no value
(void context).
return unless defined wantarray; # don't bother doing more
my @a = complex_calculation();
return wantarray ? @a : "@a";
warn LIST
Produces a message on STDERR just like `die()', but
doesn't exit or throw an exception.
If LIST is empty and `$@' already contains a value
(typically from a previous eval) that value is used
after appending `"\t...caught"' to `$@'. This is useful
for staying almost, but not entirely similar to `die()'.
If `$@' is empty then the string `"Warning: Something's
wrong"' is used.
No message is printed if there is a `$SIG{__WARN__}'
handler installed. It is the handler's responsibility to
deal with the message as it sees fit (like, for
instance, converting it into a `die()'). Most handlers
must therefore make arrangements to actually display the
warnings that they are not prepared to deal with, by
calling `warn()' again in the handler. Note that this is
quite safe and will not produce an endless loop, since
`__WARN__' hooks are not called from inside one.
You will find this behavior is slightly different from
that of `$SIG{__DIE__}' handlers (which don't suppress
the error text, but can instead call `die()' again to
change it).
Using a `__WARN__' handler provides a powerful way to
silence all warnings (even the so-called mandatory
ones). An example:
# wipe out *all* compile-time warnings
BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
my $foo = 10;
my $foo = 20; # no warning about duplicate my $foo,
# but hey, you asked for it!
# no compile-time or run-time warnings before here
$DOWARN = 1;
# run-time warnings enabled after here
warn "\$foo is alive and $foo!"; # does show up
See the perlvar manpage for details on setting `%SIG'
entries, and for more examples. See the Carp module for
other kinds of warnings using its carp() and cluck()
functions.
write FILEHANDLE
write EXPR
write Writes a formatted record (possibly multi-line) to the
specified FILEHANDLE, using the format associated with
that file. By default the format for a file is the one
having the same name as the filehandle, but the format
for the current output channel (see the `select()'
function) may be set explicitly by assigning the name of
the format to the `$~' variable.
Top of form processing is handled automatically: if
there is insufficient room on the current page for the
formatted record, the page is advanced by writing a form
feed, a special top-of-page format is used to format the
new page header, and then the record is written. By
default the top-of-page format is the name of the
filehandle with "_TOP" appended, but it may be
dynamically set to the format of your choice by
assigning the name to the `$^' variable while the
filehandle is selected. The number of lines remaining on
the current page is in variable `$-', which can be set
to `0' to force a new page.
If FILEHANDLE is unspecified, output goes to the current
default output channel, which starts out as STDOUT but
may be changed by the `select()' operator. If the
FILEHANDLE is an EXPR, then the expression is evaluated
and the resulting string is used to look up the name of
the FILEHANDLE at run time. For more on formats, see the
perlform manpage.
Note that write is *NOT* the opposite of `read()'.
Unfortunately.
y/// The transliteration operator. Same as `tr///'. See the
perlop manpage.