PERLFUNC
PERLFUNC
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 and list
contexts for its arguments. If it does both, the scalar arguments will
be first, and the list argument will follow. (Note that there can only
ever be one list argument.) For instance,
splice()
has three scalar
arguments followed by a list.
In the syntax descriptions that follow, list operators that expect a
list (and provide list context for the elements of the list) are shown
with LIST as an argument. Such a list may consist of any combination
of scalar arguments or list values; the list values will be included
in the list as if each individual element were interpolated at that
point in the list, forming a longer single-dimensional list value.
Elements of the LIST should be separated by commas.
Any function in the list below may be used either with or without
parentheses around its arguments. (The syntax descriptions omit the
parens.) If you use the parens, 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+3; # Prints 6.
print(1+2) + 3; # Prints 3.
print (1+2)+3; # Also prints 3!
print +(1+2)+3; # Prints 6.
print ((1+2)+3); # Prints 6.
If you run Perl with the
-w
switch it can warn you about this. For
example, the third line above produces:
print (...) interpreted as function at - line 1.
Useless use of integer addition in void context at - line 1.
For functions that can be used in either a scalar or list context,
non-abortive failure is generally indicated in a scalar context by
returning the undefined value, and in a list context by returning the
null list.
Remember the following rule:
*
THERE IS NO GENERAL RULE FOR CONVERTING A LIST INTO A SCALAR!
Each operator and function decides which sort of value it would be most.appropriate to return in a scalar context. Some operators return the
length of the list that would have been returned in a list context. Some
operators return the first value in the list. Some operators return the
last value in the list. Some operators return a count of successful
operations. In general, they do what you want, unless you want
consistency.
-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 non-zero size (returns size).
-f File is a plain file.
-d File is a directory.
-l File is a symbolic link.
-p File is a named pipe (FIFO).
-S File is a socket.
-b File is a block special file.
-c File is a character special file.
-t Filehandle is opened to a tty.
-u File has setuid bit set.
-g File has setgid bit set.
-k File has sticky bit set.
-T File is a text file.
-B File is a binary file (opposite of -T).
-M Age of file in days when script started.
-A Same for access time.
-C Same for inode change time.
The interpretation of the file permission operators -r , -R ,
-w
,
-W ,
-x
and
-X
is based solely on the mode of the file and the
uids and gids of the user. There may be other reasons you can't actually
read, write or execute the file. Also note that, for the superuser,
-r , -R ,
-w
and -W always return 1, and
-x
and
-X
return
1 if any execute bit is set in the mode. Scripts run by the superuser may
thus need to do a
stat()
in order to determine the actual mode of the
file, or temporarily set the uid to something else.
Example:
while () {
chop;
next unless -f $_; # ignore specials
...
}
Note that
-s/a/b/
does not do a negated substitution. Saying
-exp( $foo ) still works as expected, however--only single letters
following a minus are interpreted as file tests.
The
-T
and -B switches work as follows. The first block or so of the
file is examined for odd characters such as strange control codes or
characters with the high bit set. If too many odd characters (>30%)
are found, it's a -B file, otherwise it's a
-T
file. Also, any file
containing null in the first block is considered a binary file. If
-T
or -B is used on a filehandle, the current stdio buffer is examined
rather than the first block. Both
-T
and -B return TRUE on a null
file, or a file at EOF when testing a filehandle.
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
Returns the absolute value of its argument.
accept NEWSOCKET,GENERICSOCKET
Accepts an incoming socket connect, just as the accept(2) system call
does. Returns the packed address if it succeeded, FALSE otherwise.
See example in
the perlipc manpage
.
alarm SECONDS
Arranges to have a SIGALRM delivered to this process after the
specified number of seconds have elapsed. (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 sleeps of finer granularity than one second, you may use Perl's
syscall()
interface to access setitimer(2) if your system supports it,
or else see
select
below.
atan2 Y,X
Returns the arctangent of Y/X in the range -PI to PI.
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 example 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 Unix; in DOS, it may be imperative. If FILEHANDLE is an expression,
the value is taken as the name of the filehandle.
bless REF,PACKAGE
bless REF
This function tells the referenced object (passed as REF) that it is now
an object in PACKAGE--or the current package if no PACKAGE is specified,
which is the usual case. It returns the reference for convenience, since
a
bless()
is often the last thing in a constructor. See
the perlobj manpage
for
more about the blessing (and blessings) of objects.
caller EXPR
caller
Returns the context of the current subroutine call. In a scalar context,
returns TRUE if there is a caller, that is, if we're in a subroutine or
eval()
or
require()
, and FALSE otherwise. In a list context, returns
($package, $filename, $line) = caller;
With EXPR, it returns some extra information that the debugger uses to
print a stack trace. The value of EXPR indicates how many call frames
to go back before the current one.
($package, $filename, $line,
$subroutine, $hasargs, $wantargs) = caller($i);
Furthermore, when called from within the DB package, caller returns more
detailed information: it sets sets the list variable @DB:args to be the
arguments with which that subroutine was invoked.
chdir EXPR
Changes the working directory to EXPR, if possible. If EXPR is
omitted, changes to home directory. Returns TRUE upon success, FALSE
otherwise. See example under
die()
.
chmod LIST
Changes the permissions of a list of files. The first element of the
list must be the numerical mode. Returns the number of files
successfully changed.
$cnt = chmod 0755, 'foo', 'bar';
chmod 0755, @executables;
chomp VARIABLE
chomp LIST
chomp
This is a slightly safer version of chop (see below). It removes any
line ending that corresponds to the current value of
$/
(also known as
$INPUT_RECORD_SEPARATOR
in the English module). It returns the number
of characters removed. 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 = );
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 = );
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, $gid, @filenames;
Here's an example that looks up non-numeric uids in the passwd file:
print "User: ";
chop($user = );
print "Files: "
chop($pattern = );
($login,$pass,$uid,$gid) = getpwnam($user)
or die "$user not in passwd file";
@ary = ; # expand filenames
chown $uid, $gid, @ary;
chr NUMBER
Returns the character represented by that NUMBER in the character set.
For example,
chr(65)
is "A" in ASCII.
chroot FILENAME
Does the same as the system call of that name. If you don't know what
it does, don't worry about it. If FILENAME is omitted, does chroot to
$_
.
close FILEHANDLE
Closes the file or pipe associated with the file handle, returning TRUE
only if stdio successfully flushes buffers and closes the system file
descriptor. You don't have to close FILEHANDLE if you are immediately
going to do another open on it, since open will close it for you. (See
open()
.) However, an explicit close on an input file resets the line
counter ($.), while the implicit close done by
open()
does not. Also,
closing a pipe will wait for the process executing on the pipe to
complete, in case you want to look at the output of the pipe
afterwards. Closing a pipe explicitly also puts the status value of
the command into
$?
. Example:
open(OUTPUT, '|sort >foo'); # pipe to sort
... # print stuff to output
close OUTPUT; # wait for sort to finish
open(INPUT, 'foo'); # get sort's results
FILEHANDLE may be an expression whose value gives the real filehandle name.
closedir DIRHANDLE
Closes a directory opened by
opendir()
.
connect SOCKET,NAME
Attempts to connect to a remote socket, just as the connect system call
does. Returns TRUE if it succeeded, FALSE otherwise. NAME should be a
packed address of the appropriate type for the socket. See example in
the perlipc manpage
.
cos EXPR
Returns the cosine of EXPR (expressed in radians). If EXPR is omitted
takes cosine of
$_
.
crypt PLAINTEXT,SALT
Encrypts a string exactly like the crypt(3) function in the C library.
Useful for checking the password file for lousy passwords, amongst
other things. Only the guys wearing white hats should do this.
Here's an example that makes sure that whoever runs this program knows
their own password:
$pwd = (getpwuid($[$x][$y]{$key};
die LIST
Outside of an
eval()
, prints the value of LIST to STDERR and exits with
the current value of $! (errno). If $! is 0, exits with the value of
($? >> 8) (backtick `command` status). If ($? >> 8) is 0,
exits with 255. Inside an
eval()
, the error message is stuffed into
$@
,
and the
eval()
is terminated with the undefined value.
Equivalent examples:
die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
If the value of EXPR does not end in a newline, the current script line
number and input line number (if any) are also printed, and a newline
is supplied. Hint: sometimes appending ", stopped" to your message
will cause it to make better sense when the string "at foo line 123" is
appended. Suppose you are running script "canasta".
die "/etc/games is no good";
die "/etc/games is no good, stopped";
produce, respectively
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
See also
exit()
and
warn()
.
do BLOCK
Not really a function. Returns the value of the last command in the
sequence of commands indicated by BLOCK. When modified by a loop
modifier, executes the BLOCK once before testing the loop condition.
(On other statements the loop modifiers test the conditional first.)
do SUBROUTINE(LIST)
A deprecated form of subroutine call. See
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
eval `cat stat.pl`;
except that it's more efficient, more concise, keeps track of the
current filename for error messages, and searches all the
-I
libraries if the file isn't in the current directory (see also the
@INC
array in
Predefined Names
). It's the same, however, in that it does
reparse the file every time you call it, so you probably don't want to
do this inside a loop.
Note that inclusion of library modules is better done with the
use()
and
require()
operators.
dump LABEL
This causes an immediate core dump. Primarily this is so that you can
use the undump program to turn your core dump into an executable binary
after having initialized all your variables at the beginning of the
program. When the new binary is executed it will begin by executing a
goto LABEL
(with all the restrictions that
goto
suffers). Think of
it as a goto with an intervening core dump and reincarnation. If LABEL
is omitted, restarts the program from the top. WARNING: any files
opened at the time of the dump will NOT be open any more when the
program is reincarnated, with possible resulting confusion on the part
of Perl. See also
-u
option in
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 $ARGV[0] eq '-d';
QUICKSTART:
Getopt('f');
each ASSOC_ARRAY
Returns a 2 element array consisting of the key and value for the next
value of an associative array, so that you can iterate over it.
Entries are returned in an apparently random order. When the array is
entirely read, a null array is returned (which when assigned produces a
FALSE (0) value). The next call to
each()
after that will start
iterating again. The iterator can be reset only by reading all the
elements from the array. You should not add elements to an array while
you're iterating over it. There is a single iterator for each
associative array, shared by all
each()
,
keys()
and
values()
function
calls in the program. The following prints out your environment like
the printenv(1) program, only in a different order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}
See also
keys()
and
values()
.
eof FILEHANDLE
eof
Returns 1 if the next read on FILEHANDLE will return end of file, or if
FILEHANDLE is not open. FILEHANDLE may be an expression whose value
gives the real filehandle name. (Note that this function actually
reads a character and then ungetc() s it, so it is not very useful in an
interactive context.) Do not read from a terminal file (or call
eof(FILEHANDLE)
on it) after end-of-file is reached. Filetypes such
as terminals may lose the end-of-file condition if you do.
An
eof
without an argument uses the last file read as argument.
Empty parentheses () may be used to indicate
the pseudo file formed of the files listed on the command line, i.e.
eof()
is reasonable to use inside a while <>) loop to detect the end
of only the last file. Use
eof(ARGV)
or eof without the parentheses to
test EACH file in a while <>) loop. Examples:
# reset line numbering on each input file
while () {
print "$.\t$_";
close(ARGV) if (eof); # Not eof().
}
# insert dashes just before last line of last file
while () {
if (eof()) {
print "--------------\n";
close(ARGV); # close or break; is needed if we
# are reading from the terminal
}
print;
}
Practical hint: you almost never need to use
eof
in Perl, because the
input operators return undef when they run out of data.
eval EXPR
eval BLOCK
EXPR is parsed and executed as if it were a little Perl program. It
is executed in the context of the current Perl program, so that any
variable settings, subroutine or format definitions remain afterwards.
The value returned is the value of the last expression evaluated, or a
return statement may be used, just as with subroutines.
If there is a syntax error or runtime error, or a
die()
statement is
executed, an undefined value is returned by
eval()
, and
$@
is set to the
error message. If there was no error,
$@
is guaranteed to be a null
string. If EXPR is omitted, evaluates
$_
. The final semicolon, if
any, may be omitted from the expression.
Note that, since
eval()
traps otherwise-fatal errors, it is useful for
determining whether a particular feature (such as
dbmopen()
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 non-fatal
eval { $answer = $a / $b; }; warn $@ if $@;
# same thing, but less efficient
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
eval { $answer = };
# a run-time error
eval '$answer ='; # sets $@
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 at all. (Case 4 is preferred for purely visual reasons.) Case 5
is a place where normally you WOULD like to use double quotes, except
in that particular situation, you can just use symbolic references
instead, as in case 6.
exec LIST
The
exec()
function executes a system command AND NEVER RETURNS . Use
the
system()
function if you want it to return.
If there is more than one argument in LIST, or if LIST is an array with
more than one value, calls execvp(3) with the arguments in LIST. If
there is only one scalar argument, the argument is checked for shell
metacharacters. If there are any, the entire argument is passed to
/bin/sh -c for parsing. If there are none, the argument is split
into words and passed directly to execvp() , which is more efficient.
Note:
exec()
(and system(0) 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 multi-valued list, even if there is only a single scalar in
the list.) Example:
$shell = '/bin/csh';
exec $shell '-sh'; # pretend it's a login shell
or, more directly,
exec {'/bin/csh'} '-sh'; # pretend it's a login shell
exists EXPR
Returns TRUE if the specified hash key exists in its hash array, even
if the corresponding value is undefined.
print "Exists\n" if exists $array{$key};
print "Defined\n" if defined $array{$key};
print "True\n" if $array{$key};
A hash element can only be TRUE if it's defined, and defined if
it exists, but the reverse doesn't necessarily hold true.
Note that the EXPR can be arbitrarily complicated as long as the final
operation is a hash key lookup:
if (exists $ref->[$x][$y]{$key}) { ... }
exit EXPR
Evaluates EXPR and exits immediately with that value. (Actually, it
calls any defined END routines first, but the END routines may not
abort the exit. Likewise any object destructors that need to be called
are called before exit.) Example:
$ans = ;
exit 0 if $ans =~ /^[Xx]/;
See also
die()
. If EXPR is omitted, exits with 0 status.
exp EXPR
Returns e (the natural logarithm base) to the power of EXPR.
If EXPR is omitted, gives
exp(
$_
)
.
fcntl FILEHANDLE,FUNCTION,SCALAR
Implements the fcntl(2) function. You'll probably have to say
use Fcntl;
first to get the correct function definitions. Argument processing and
value return works just like
ioctl()
below. Note that
fcntl()
will produce
a fatal error if used on a machine that doesn't implement fcntl(2).
For example:
use Fcntl;
fcntl($filehandle, F_GETLK, $packed_return_buffer);
fileno FILEHANDLE
Returns the file descriptor for a filehandle. This is useful for
constructing bitmaps for
select()
. If FILEHANDLE is an expression, the
value is taken as the name of the filehandle.
flock FILEHANDLE,OPERATION
Calls flock(2) on FILEHANDLE. See the
flock
(2) manpage for
definition of OPERATION. Returns TRUE for success, FALSE on failure.
Will produce a fatal error if used on a machine that doesn't implement
flock(2). Here's a mailbox appender for BSD systems.
$LOCK_SH = 1;
$LOCK_EX = 2;
$LOCK_NB = 4;
$LOCK_UN = 8;
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();
Note that
flock()
can't lock things over the network. You need to do
locking with
fcntl()
for that.
fork
Does a fork(2) system call. Returns the child pid to the parent process
and 0 to the child process, or undef if the fork is unsuccessful.
Note: unflushed buffers remain unflushed in both processes, which means
you may need to set
$|
( $AUTOFLUSH in English) or call the
autoflush()
FileHandle method to avoid duplicate output.
If you
fork()
without ever waiting on your children, you will accumulate
zombies:
$SIG{'CHLD'} = sub { wait };
There's also the double-fork trick (error checking on
fork()
returns omitted);
unless ($pid = fork) {
unless (fork) {
exec "what you really wanna do";
die "no exec";
# ... or ...
some_perl_code_here;
exit 0;
}
exit 0;
}
waitpid($pid,0);
formline PICTURE, LIST
This is an internal function used by formats, 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
. 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, since an " @ "
character may be taken to mean the beginning of an array name.
formline()
always returns TRUE.
getc FILEHANDLE
getc
Returns the next character from the input file attached to FILEHANDLE,
or a null string at end of file. If FILEHANDLE is omitted, reads from STDIN.
getlogin
Returns the current login from /etc/utmp , if any. If null, use
getpwuid()
.
$login = getlogin || (getpwuid($= 0) {
print pop(@keys), '=', pop(@values), "\n";
}
or how about sorted by key:
foreach $key (sort(keys %ENV)) {
print $key, '=', $ENV{$key}, "\n";
}
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.
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 () {
last line if /^$/; # exit when done with header
...
}
lc EXPR
Returns an lowercased version of EXPR. This is the internal function
implementing the \L escape in double-quoted strings.
lcfirst EXPR
Returns the value of EXPR with the first character lowercased. This is
the internal function implementing the \l escape in double-quoted strings.
length EXPR
Returns the length in characters of the value of EXPR. If EXPR is
omitted, returns length of
$_
.
link OLDFILE,NEWFILE
Creates a new filename linked to the old filename. Returns 1 for
success, 0 otherwise.
listen SOCKET,QUEUESIZE
Does the same thing that the listen system call does. Returns TRUE if
it succeeded, FALSE otherwise. See example in
the perlipc manpage
.
local EXPR
In general, you should be using "my" instead of "local", because it's
faster and safer. Format variables often use "local" though, as
do other variables whose current value must be visible to called
subroutines. This is known as dynamic scoping. Lexical scoping is
done with "my", which works more like C's auto declarations.
A local modifies the listed variables to be local to the enclosing block,
subroutine, eval or "do". If more than one value is listed, the list
must be placed in parens. All the listed elements must be legal
lvalues. This operator works by saving the current values of those
variables in LIST on a hidden stack and restoring them upon exiting the
block, subroutine or eval. This means that called subroutines can also
reference the local variable, but not the global one. The LIST may be
assigned to if desired, which allows you to initialize your local
variables. (If no initializer is given for a particular variable, it
is created with an undefined value.) Commonly this is used to name the
parameters to a subroutine. Examples:
sub RANGEVAL {
local($min, $max, $thunk) = @_;
local $result = '';
local $i;
# Presumably $thunk makes reference to $i
for ($i = $min; $i < $max; $i++) {
$result .= eval $thunk;
}
$result;
}
if ($sw eq '-v') {
# init local array with global array
local @ARGV = @ARGV;
unshift(@ARGV,'echo');
system @ARGV;
}
# @ARGV restored
# temporarily add to digits associative array
if ($base12) {
# (NOTE: not claiming this is efficient!)
local(%digits) = (%digits,'t',10,'e',11);
parse_num();
}
Note that
local()
is a run-time command, and so gets executed every
time through a loop. In Perl 4 it used more stack storage each
time until the loop was exited. Perl 5 reclaims the space each time
through, but it's still more efficient to declare your variables
outside the loop.
A local is simply a modifier on an lvalue expression.
When you assign to a localized EXPR, the local doesn't change whether
EXPR is viewed as a scalar or an array. So
local($foo) = ;
local @FOO = ;
both supply a list context to the righthand side, while
local $foo = ;
supplies a scalar context.
localtime EXPR
Converts a time as returned by the time function to a 9-element array
with the time analyzed for the local timezone. Typically used as
follows:
($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. If EXPR is omitted, does localtime(time).
In a scalar context, prints out the ctime(3) value:
$now_string = localtime; # e.g. "Thu Oct 13 04:54:34 1994"
See also timelocal and the strftime(3) function available
via the POSIX modulie.
log EXPR
Returns logarithm (base e ) of EXPR. If EXPR is omitted, returns log
of
$_
.
lstat FILEHANDLE
lstat EXPR
Does the same thing as the
stat()
function, but stats a symbolic link
instead of the file the symbolic link points to. If symbolic links are
unimplemented on your system, a normal
stat()
is done.
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.
@chars = map(chr, @nums);
translates a list of numbers to the corresponding characters. And
%hash = map {&key($_), $_} @array;
is just a funny way to write
%hash = ();
foreach $_ (@array) {
$hash{&key($_)} = $_;
}
mkdir FILENAME,MODE
Creates the directory specified by FILENAME, with permissions specified
by MODE (as modified by umask). If it succeeds it returns 1, otherwise
it returns 0 and sets $! (errno).
msgctl ID,CMD,ARG
Calls the System V IPC function msgctl. If CMD is &IPC_STAT, then ARG
must be a variable which will hold the returned msqid_ds structure.
Returns like ioctl: the undefined value for error, "0 but true" for
zero, or the actual return value otherwise.
msgget KEY,FLAGS
Calls the System V IPC function msgget. Returns the message queue id,
or the undefined value if there is an error.
msgsnd ID,MSG,FLAGS
Calls the System V IPC function msgsnd to send the message MSG to the
message queue ID. MSG must begin with the long integer message type,
which may be created with
pack("L", $type )
. Returns TRUE if
successful, or FALSE if there is an error.
msgrcv ID,VAR,SIZE,TYPE,FLAGS
Calls the System V IPC function msgrcv to receive a message from
message queue ID into variable VAR with a maximum message size of
SIZE. Note that if a message is received, the message type will be the
first thing in VAR, and the maximum length of VAR is SIZE plus the size
of the message type. Returns TRUE if successful, or FALSE if there is
an error.
my EXPR
A "my" declares the listed variables to be local (lexically) to the
enclosing block, subroutine, eval or "do". If more than one value is
listed, the list must be placed in parens. All the listed elements
must be legal lvalues. Only alphanumeric identifiers may be lexically
scoped--magical builtins like $/ must be localized with "local"
instead. In particular, you're not allowed to say
my $_; # Illegal.
Unlike the "local" declaration, variables declared with "my"
are totally hidden from the outside world, including any called
subroutines (even if it's the same subroutine--every call gets its own
copy).
(An
eval()
, however, can see the lexical variables of the scope it is
being evaluated in so long as the names aren't hidden by declarations within
the
eval()
itself. See
the perlref manpage
.)
The EXPR may be assigned to if desired, which allows you to initialize
your variables. (If no initializer is given for a particular
variable, it is created with an undefined value.) Commonly this is
used to name the parameters to a subroutine. Examples:
sub RANGEVAL {
my($min, $max, $thunk) = @_;
my $result = '';
my $i;
# Presumably $thunk makes reference to $i
for ($i = $min; $i ", the file is opened for output. If the
filename begins with >>", the file is opened for appending. (You can
put a '+' in front of the '>' or <' to indicate that you want both
read and write access to the file.) 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 command which pipes input to us. (You may not have a command that
pipes both in and out.) Opening '-' opens STDIN and opening '>-'
opens STDOUT. Open returns non-zero upon success, the undefined
value otherwise. If the open involved a pipe, the return value happens
to be the pid of the subprocess. Examples:
$ARTICLE = 100;
open ARTICLE or die "Can't find article $ARTICLE: $!\n";
while () {...
open(LOG, >>/usr/spool/news/twitlog'); # (log is reserved)
open(article, "caesar /tmp/Tmp$$"); # $$ is our process id
# process argument list of files along with any includes
foreach $file (@ARGV) {
process($file, 'fh00');
}
sub process {
local($filename, $input) = @_;
$input++; # this is a string increment
unless (open($input, $filename)) {
print STDERR "Can't open $filename: $!\n";
return;
}
while () { # note use of indirection
if (/^#include "(.*)"/) {
process($1, $input);
next;
}
... # whatever
}
}
You may also, in the Bourne shell tradition, specify an EXPR beginning
with ">&", in which case the rest of the string is interpreted as the
name of a filehandle (or file descriptor, if numeric) which is to be
duped and opened. You may use & after >,>>,<, +>, >> and <. The
mode you specify should match the mode of the original filehandle.
Here is a script that saves, redirects, and restores STDOUT and
STDERR:
#!/usr/bin/perl
open(SAVEOUT, ">&STDOUT");
open(SAVEERR, ">&STDERR");
open(STDOUT, ">foo.out") || die "Can't redirect stdout";
open(STDERR, ">&STDOUT") || die "Can't dup stdout";
select(STDERR); $| = 1; # make unbuffered
select(STDOUT); $| = 1; # make unbuffered
print STDOUT "stdout 1\n"; # this works for
print STDERR "stderr 1\n"; # subprocesses too
close(STDOUT);
close(STDERR);
open(STDOUT, ">&SAVEOUT");
open(STDERR, ">&SAVEERR");
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
If you specify <&=N", where N is a number, then Perl will do an
equivalent of C's fdopen() of that file descriptor. For example:
open(FILEHANDLE, "autoflush(1);
select RBITS,WBITS,EBITS,TIMEOUT
This calls the select system(2) call with the bitmasks specified, which
can be constructed using
fileno()
and
vec()
, along these lines:
$rin = $win = $ein = '';
vec($rin,fileno(STDIN),1) = 1;
vec($win,fileno(STDOUT),1) = 1;
$ein = $rin | $win;
If you want to select on many filehandles you might wish to write a
subroutine:
sub fhbits {
local(@fhlist) = split(' ',$_[0]);
local($bits);
for (@fhlist) {
vec($bits,fileno($_),1) = 1;
}
$bits;
}
$rin = &fhbits('STDIN TTY SOCK');
The usual idiom is:
($nfound,$timeleft) =
select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
or to block until something becomes ready:
$nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
Any of the bitmasks 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 250 microsecond sleep this way:
select(undef, undef, undef, 0.25);
semctl ID,SEMNUM,CMD,ARG
Calls the System V IPC function semctl. If CMD is &IPC_STAT or
&GETALL, then ARG must be a variable which will hold the returned
semid_ds structure or semaphore value array. Returns like ioctl: the
undefined value for error, "0 but true" for zero, or the actual return
value otherwise.
semget KEY,NSEMS,FLAGS
Calls the System V IPC function semget. Returns the semaphore id, or
the undefined value if there is an error.
semop KEY,OPSTRING
Calls the System V IPC function semop to perform semaphore operations
such as signaling and waiting. OPSTRING must be a packed array of
semop structures. Each semop structure can be generated with
pack("sss", $semnum , $semop , $semflag )
. The number of semaphore
operations is implied by the length of OPSTRING. Returns TRUE if
successful, or FALSE if there is an error. As an example, the
following code waits on semaphore $semnum of semaphore id $semid:
$semop = pack("sss", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid, $semop);
To signal the semaphore, replace "-1" with "1".
send SOCKET,MSG,FLAGS,TO
send SOCKET,MSG,FLAGS
Sends a message on a socket. Takes the same flags as the system call
of the same name. On unconnected sockets you must specify a
destination to send TO, in which case it does a C sendto() . Returns
the number of characters sent, or the undefined value if there is an
error.
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).
setpriority WHICH,WHO,PRIORITY
Sets the current priority for a process, a process group, or a user.
(See setpriority(2).) Will produce a fatal error if used on a machine
that doesn't implement setpriority(2).
setsockopt SOCKET,LEVEL,OPTNAME,OPTVAL
Sets the socket option requested. Returns undefined if there is an
error. OPTVAL may be specified as undef if you don't want to pass an
argument.
shift ARRAY
shift
Shifts the first value of the array off and returns it, shortening the
array by 1 and moving everything down. If there are no elements in the
array, returns the undefined value. If ARRAY is omitted, shifts the
@ARGV
array in the main program, and the @_ array in subroutines.
(This is determined lexically.) See also
unshift()
,
push()
, and
pop()
.
Shift() and
unshift()
do the same thing to the left end of an array
that
push()
and
pop()
do to the right end.
shmctl ID,CMD,ARG
Calls the System V IPC function shmctl. If CMD is &IPC_STAT, then ARG
must be a variable which will hold the returned shmid_ds structure.
Returns like ioctl: the undefined value for error, "0 but true" for
zero, or the actual return value otherwise.
shmget KEY,SIZE,FLAGS
Calls the System V IPC function shmget. Returns the shared memory
segment id, or the undefined value if there is an error.
shmread ID,VAR,POS,SIZE
shmwrite ID,STRING,POS,SIZE
Reads or writes the System V shared memory segment ID starting at
position POS for size SIZE by attaching to it, copying in/out, and
detaching from it. When reading, VAR must be a variable which will
hold the data read. When writing, if STRING is too long, only SIZE
bytes are used; if STRING is too short, nulls are written to fill out
SIZE bytes. Return TRUE if successful, or FALSE if there is an error.
shutdown SOCKET,HOW
Shuts down a socket connection in the manner indicated by HOW, which
has the same interpretation as in the system call of the same name.
sin EXPR
Returns the sine of EXPR (expressed in radians). If EXPR is omitted,
returns sine of
$_
.
sleep EXPR
sleep
Causes the script to sleep for EXPR seconds, or forever if no EXPR.
May be interrupted by sending the process a SIGALRM. Returns the
number of seconds actually slept. You probably cannot mix
alarm()
and
sleep()
calls, since
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.
socket SOCKET,DOMAIN,TYPE,PROTOCOL
Opens a socket of the specified kind and attaches it to filehandle
SOCKET. DOMAIN, TYPE and PROTOCOL are specified the same as for the
system call of the same name. You should "use Socket;" first to get
the proper definitions imported. See the example in
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.
sort SUBNAME LIST
sort BLOCK LIST
sort LIST
Sorts the LIST and returns the sorted list value. Nonexistent values
of arrays are stripped out. If SUBNAME or BLOCK is omitted, sorts
in standard string comparison order. If SUBNAME is specified, it
gives the name of a subroutine that returns an integer less than, equal
to, or greater than 0, depending on how the elements of the array are
to be ordered. (The<=> and cmp operators are extremely useful in such
routines.) SUBNAME may be a scalar variable name, in which case the
value provides the name of the subroutine to use. In place of a
SUBNAME, you can provide a BLOCK as an anonymous, in-line sort
subroutine.
In the interests of efficiency the normal calling code for subroutines
is bypassed, with the following effects: the subroutine may not be a
recursive subroutine, and the two elements to be compared are passed
into the subroutine not via @_ but as $a and $b (see example below).
They are passed by reference, so don't modify $a and $b .
Examples:
# sort lexically
@articles = sort @files;
# same thing, but with explicit sort routine
@articles = sort {$a cmp $b} @files;
# same thing in reversed order
@articles = sort {$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 integers
}
@sortedclass = sort byage @class;
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
splice ARRAY,OFFSET,LENGTH,LIST
splice ARRAY,OFFSET,LENGTH
splice ARRAY,OFFSET
Removes the elements designated by OFFSET and LENGTH from an array, and
replaces them with the elements of LIST, if any. Returns the elements
removed from the array. The array grows or shrinks as necessary. If
LENGTH is omitted, removes everything from OFFSET onward. The
following equivalencies hold (assuming $[ == 0):
push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
pop(@a) splice(@a,-1)
shift(@a) splice(@a,0,1)
unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
$a[$x] = $y splice(@a,$x,1,$y);
Example, assuming array lengths are passed before arrays:
sub aeq { # compare two list values
local(@a) = splice(@_,0,shift);
local(@b) = splice(@_,0,shift);
return 0 unless @a == @b; # same len?
while (@a) {
return 0 if pop(@a) ne pop(@b);
}
return 1;
}
if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
split /PATTERN/,EXPR,LIMIT
split /PATTERN/,EXPR
split /PATTERN/
split
Splits a string into an array of strings, and returns it.
If not in a list context, returns the number of fields found and splits into
the @_ array. (In a list context, you can force the split into @_ by
using ?? as the pattern delimiters, but it still returns the array
value.) The use of implicit split to @_ is deprecated, however.
If EXPR is omitted, splits the
$_
string. If PATTERN is also omitted,
splits on whitespace (after skipping any leading whitespace).
Anything matching PATTERN is taken
to be a delimiter separating the fields. (Note that the delimiter may
be longer than one character.) If LIMIT is specified and is not
negative, splits into no more than that many fields (though it may
split into fewer). If LIMIT is unspecified, trailing null fields are
stripped (which potential users of
pop()
would do well to remember).
If LIMIT is negative, it is treated as if an arbitrarily large LIMIT
had been specified.
A pattern matching the null string (not to be confused with
a null pattern // , which is just one member of the set of patterns
matching a null string) will split the value of EXPR into separate
characters at each point it matches that way. For example:
print join(':', split(/ */, 'hi there'));
produces the output 'h:i:t:h:e:r:e'.
The LIMIT parameter can be used to partially split a line
($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");
produces the list value
(1, '-', 10, ',', 20)
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(' ',
$_
)
internally.
Example:
open(passwd, '/etc/passwd');
while () {
($login, $passwd, $uid, $gid, $gcos,
$home, $shell) = split(/:/);
...
}
(Note that $shell above will still have a newline on it. See
chop
,
chomp
, and
join
.)
sprintf FORMAT,LIST
Returns a string formatted by the usual printf conventions of the C
language. (The * character for an indirectly specified length is not
supported, but you can get the same effect by interpolating a variable
into the pattern.)
sqrt EXPR
Return the square root of EXPR. If EXPR is omitted, returns square
root of
$_
.
srand EXPR
Sets the random number seed for the
rand
operator. If EXPR is
omitted, does
srand(time)
. Of course, you'd need something much more
random than that for cryptographic purposes, since it's easy to guess
the current time. Checksumming the compressed output of rapidly
changing operating system status programs is the usual method.
Examples are posted regularly to comp.security.unix.
stat FILEHANDLE
stat EXPR
Returns a 13-element array giving the status info for a file, either the
file opened via FILEHANDLE, or named by EXPR. Returns a null list if
the stat fails. Typically used as follows:
($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
$atime,$mtime,$ctime,$blksize,$blocks)
= stat($filename);
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 only works on machines for which the device number is negative under NFS.)
study SCALAR
study
Takes extra time to study SCALAR (
$_
if unspecified) in anticipation of
doing many pattern matches on the string before it is next modified.
This may or may not save time, depending on the nature and number of
patterns you are searching on, and on the distribution of character
frequencies in the string to be searched--you probably want to compare
runtimes with and without it to see which runs faster. Those loops
which scan for many short constant strings (including the constant
parts of more complex patterns) will benefit most. You may have only
one study active at a time--if you study a different scalar the first
is "unstudied". (The way study works is this: a linked list of every
character in the string to be searched is made, so we know, for
example, where all the 'k' characters are. From each search string,
the rarest character is selected, based on some static frequency tables
constructed from some C programs and English text. Only those places
that contain this "rarest" character are examined.)
For example, here is a loop which inserts index producing entries
before any line containing a certain pattern:
while () {
study;
print ".IX foo\n" if /\bfoo\b/;
print ".IX bar\n" if /\bbar\b/;
print ".IX blurfl\n" if /\bblurfl\b/;
...
print;
}
In searching for /\bfoo\b/, only those locations in
$_
that contain "f"
will be looked at, because "f" is rarer than "o". In general, this is
a big win except in pathological cases. The only question is whether
it saves you more time than it took to build the linked list in the
first place.
Note that if you have to look for strings that you don't know till
runtime, you can build an entire loop as a string and eval that to
avoid recompiling all your patterns all the time. Together with
undefining $/ to input entire files as one record, this can be very
fast, often faster than specialized programs like fgrep(1). The following
scans a list of files ( @files ) for a list of words ( @words ), and prints
out the names of those files that contain a match:
$search = 'while () { study;';
foreach $word (@words) {
$search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
}
$search .= "}";
@ARGV = @files;
undef $/;
eval $search; # this screams
$/ = "\n"; # put back to normal input delim
foreach $file (sort keys(%seen)) {
print $file, "\n";
}
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. If OFFSET is negative, starts
that far from the end of the string. If LEN is omitted, returns
everything to the end of the string. If LEN is negative, leaves that
many characters off the end of the string.
You can use the
substr()
function
as an lvalue, in which case EXPR must be an lvalue. If you assign
something shorter than LEN, the string will shrink, and if you assign
something longer than LEN, the string will grow to accommodate it. To
keep the string the same length you may need to pad or chop your value
using
sprintf()
.
symlink OLDFILE,NEWFILE
Creates a new filename symbolically linked to the old filename.
Returns 1 for success, 0 otherwise. On systems that don't support
symbolic links, produces a fatal error at run time. To check for that,
use eval:
$symlink_exists = (eval 'symlink("","");', $@ eq '');
syscall LIST
Calls the system call specified as the first element of the list,
passing the remaining elements as arguments to the system call. If
unimplemented, produces a fatal error. The arguments are interpreted
as follows: if a given argument is numeric, the argument is passed as
an int. If not, the pointer to the string value is passed. You are
responsible to make sure a string is pre-extended long enough to
receive any result that might be written into a string. If your
integer arguments are not literals and have never been interpreted in a
numeric context, you may need to add 0 to them to force them to look
like numbers.
require 'syscall.ph'; # may need to run h2ph
syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
Note that Perl only supports passing of up to 14 arguments to your system call,
which in practice should usually suffice.
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 may cause confusion.
Returns the number of bytes actually read, or undef if there was an
error. SCALAR will be grown or shrunk to the length actually read. An
OFFSET may be specified to place the read data at some other place than
the beginning of the string.
system LIST
Does exactly the same thing as "exec LIST" except that a fork is done
first, and the parent process waits for the child process to complete.
Note that argument processing varies depending on the number of
arguments. The return value is the exit status of the program as
returned by the
wait()
call. To get the actual exit value divide by
256. See also
exec
.
syswrite FILEHANDLE,SCALAR,LENGTH,OFFSET
syswrite FILEHANDLE,SCALAR,LENGTH
Attempts to write LENGTH bytes of data from variable SCALAR to the
specified FILEHANDLE, using the system call write(2). It bypasses
stdio, so mixing this with prints may cause confusion. Returns the
number of bytes actually written, or undef if there was an error. An
OFFSET may be specified to place the read data at some other place than
the beginning of the string.
tell FILEHANDLE
tell
Returns the current file position for FILEHANDLE. FILEHANDLE may be an
expression whose value gives the name of the actual filehandle. If
FILEHANDLE is omitted, assumes the file last read.
telldir DIRHANDLE
Returns the current position of the
readdir()
routines on DIRHANDLE.
Value may be given to
seekdir()
to access a particular location in a
directory. Has the same caveats about possible directory compaction as
the corresponding system library routine.
tie VARIABLE,PACKAGENAME,LIST
This function binds a variable to a package that will provide the
implementation for the variable. VARIABLE is the name of the variable to
be enchanted. PACKAGENAME is the name of a package implementing objects
of correct type. Any additional arguments are passed to the "new" method
of the package (meaning TIESCALAR, TIEARRAY, or TIEHASH). Typically these
are arguments such as might be passed to the dbm_open() function of C.
Note that functions such as
keys()
and
values()
may return huge array
values when used on large objects, like DBM files. You may prefer to
use the
each()
function to iterate over such. Example:
# print out history file offsets
tie(%HIST, NDBM_File, '/usr/lib/news/history', 1, 0);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
untie(%HIST);
A package implementing an associative array should have the following
methods:
TIEHASH objectname, LIST
DESTROY this
FETCH this, key
STORE this, key, value
DELETE this, key
EXISTS this, key
FIRSTKEY this
NEXTKEY this, lastkey
A package implementing an ordinary array should have the following methods:
TIEARRAY objectname, LIST
DESTROY this
FETCH this, key
STORE this, key, value
[others TBD]
A package implementing a scalar should have the following methods:
TIESCALAR objectname, LIST
DESTROY this
FETCH this,
STORE this, value
time
Returns the number of non-leap seconds since 00:00:00 UTC, January 1,
1970. Suitable for feeding to
gmtime()
and
localtime()
.
times
Returns a four-element array giving the user and system times, in
seconds, for this process and the children of this process.
($user,$system,$cuser,$csystem) = times;
tr///
The translation operator. 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.
uc EXPR
Returns an uppercased version of EXPR. This is the internal function
implementing the \U escape in double-quoted strings.
ucfirst EXPR
Returns the value of EXPR with the first character uppercased. This is
the internal function implementing the \u escape in double-quoted strings.
umask EXPR
umask
Sets the umask for the process and returns the old one. If EXPR is
omitted, merely returns current umask.
undef EXPR
undef
Undefines the value of EXPR, which must be an lvalue. Use only on a
scalar value, an entire array, or a subroutine name (using "&"). (Using
undef()
will probably not do what you expect on most predefined variables or
DBM list values, so don't do that.) Always returns the undefined value. You can omit
the EXPR, in which case nothing is undefined, but you still get an
undefined value that you could, for instance, return from a
subroutine. Examples:
undef $foo;
undef $bar{'blurfl'};
undef @ary;
undef %assoc;
undef &mysub;
return (wantarray ? () : undef) if $they_blew_it;
unlink LIST
Deletes a list of files. Returns the number of files successfully
deleted.
$cnt = unlink 'a', 'b', 'c';
unlink @goners;
unlink ;
Note: unlink will not delete directories unless you are superuser and
the
-U
flag is supplied to Perl. Even if these conditions are
met, be warned that unlinking a directory can inflict damage on your
filesystem. Use rmdir instead.
unpack TEMPLATE,EXPR
Unpack does the reverse of pack: it takes a string representing a
structure and expands it out into a list value, returning the array
value. (In a scalar context, it merely returns the first value
produced.) The TEMPLATE has the same format as in the pack function.
Here's a subroutine that does substring:
sub substr {
local($what,$where,$howmuch) = @_;
unpack("x$where a$howmuch", $what);
}
and then there's
sub ordinal { unpack("c",$_[0]); } # same as ord()
In addition, you may prefix a field with a <number> to indicate that
you want a<number>-bit checksum of the items instead of the items
themselves. Default is a 16-bit checksum. For example, the following
computes the same number as the System V sum program:
while () {
$checksum += unpack("%16C*", $_);
}
$checksum %= 65536;
The following efficiently counts the number of set bits in a bit vector:
$setbits = unpack("%32b*", $selectmask);
untie VARIABLE
Breaks the binding between a variable and a package. (See
tie()
.)
unshift ARRAY,LIST
Does the opposite of a
shift
. Or the opposite of a
push
,
depending on how you look at it. Prepends list to the front of the
array, and returns the new number of elements in the array.
unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
Note the LIST is prepended whole, not one element at a time, so the
prepended elements stay in the same order. Use reverse to do the
reverse.
use Module LIST
use Module
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; }
If you don't want your namespace altered, use require instead.
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.
Because this is a wide-open interface, pragmas (compiler directives)
are also implemented this way. Currently implemented pragmas are:
use integer;
use sigtrap qw(SEGV BUS);
use strict qw(subs vars refs);
use subs qw(afunc blurfl);
These pseudomodules import semantics into the current block scope, unlike
ordinary modules, which import symbols into the current package (which are
effective through the end of the file).
There's a corresponding "no" command that unimports meanings imported
by use.
no integer;
no strict 'refs';
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. Example of a "touch" command:
#!/usr/bin/perl
$now = time;
utime $now, $now, @ARGV;
values ASSOC_ARRAY
Returns a normal array consisting of all the values of the named
associative array. (In a scalar context, returns the number of
values.) The values are returned in an apparently random order, but it
is the same order as either the
keys()
or
each()
function would produce
on the same array. See also
keys()
and
each()
.
vec EXPR,OFFSET,BITS
Treats a string as a vector of unsigned integers, and returns the value
of the bitfield specified. May also be assigned to. BITS must be a
power of two from 1 to 32.
Vectors created with
vec()
can also be manipulated with the logical
operators |, & and ^, which will assume a bit vector operation is
desired when both operands are strings.
To transform a bit vector into a string or array of 0's and 1's, use these:
$bits = unpack("b*", $vector);
@bits = split(//, unpack("b*", $vector));
If you know the exact length in bits, it can be used in place of the *.
wait
Waits for a child process to terminate and returns the pid of the
deceased process, or -1 if there are no child processes. The status is
returned in $?.
waitpid PID,FLAGS
Waits for a particular child process to terminate and returns the pid
of the deceased process, or -1 if there is no such child process. The
status is returned in $?. If you say
use POSIX "wait_h";
...
waitpid(-1,&WNOHANG);
then you can do a non-blocking wait for any process. Non-blocking wait
is only available on machines supporting either the waitpid(2) or
wait4(2) system calls. However, waiting for a particular pid with
FLAGS of 0 is implemented everywhere. (Perl emulates the system call
by remembering the status values of processes that have exited but have
not been harvested by the Perl script yet.)
wantarray
Returns TRUE if the context of the currently executing subroutine is
looking for a list value. Returns FALSE if the context is looking
for a scalar.
return wantarray ? () : undef;
warn LIST
Produces a message on STDERR just like
die()
, but doesn't exit or
throw an exception.
write FILEHANDLE
write EXPR
write
Writes a formatted record (possibly multi-line) to the specified file,
using the format associated with that file. By default the format for
a file is the one having the same name is 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 translation operator. See
tr///
.
.
| 2 | Vermeer
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