PERLFUNC(1)PERLFUNC(1)NAMEperlfunc - 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 the select() entry elsewhere in
this documentbelow.
atan2 Y,X
Returns the arctangent of Y/X in the range - to .
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 = <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, $gid, @filenames;
Here's an example that looks up non-numeric uids in the passwd
file:
print "User: ";
chop($user = <STDIN>);
print "Files: "
chop($pattern = <STDIN>);
($login,$pass,$uid,$gid) = getpwnam($user)
or die "$user not in passwd file";
@ary = <${pattern}>; # expand filenames
chown $uid, $gid, @ary;
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:
$pwd = (getpwuid($<))[1];
$salt = substr($pwd, 0, 2);
system "stty -echo";
print "Password: ";
chop($word = <STDIN>);
print "\n";
system "stty echo";
if (crypt($word, $salt) ne $pwd) {
die "Sorry...\n";
} else {
print "ok\n";
}
Of course, typing in your own password to whoever asks you for
it is unwise.
dbmclose ASSOC_ARRAY
[This function has been superseded by the untie() function.]
Breaks the binding between a DBM file and an associative array.
dbmopen ASSOC,DBNAME,MODE
[This function has been superseded by the tie() function.]
This binds a dbm(3) or ndbm(3) file to an associative array.
ASSOC is the name of the associative array. (Unlike normal
open, the first argument is NOT a filehandle, even though it
looks like one). DBNAME is the name of the database (without
the .dir or .pag extension). If the database does not exist,
it is created with protection specified by MODE (as modified by
the umask()). If your system only supports the older DBM
functions, you may perform only one dbmopen() in your program.
If your system has neither DBM nor ndbm, calling dbmopen()
produces a fatal error.
If you don't have write access to the DBM file, you can only
read associative array variables, not set them. If you want to
test whether you can write, either use file tests or try
setting a dummy array entry inside an eval(), which will trap
the error.
Note that functions such as keys() and values() may return huge
array values when used on large DBM files. You may prefer to
use the each() function to iterate over large DBM files.
Example:
# print out history file offsets
dbmopen(%HIST,'/usr/lib/news/history',0666);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
dbmclose(%HIST);
defined EXPR
Returns a boolean value saying whether the lvalue EXPR has a
real value or not. Many operations return the undefined value
under exceptional conditions, such as end of file,
uninitialized variable, system error and such. This function
allows you to distinguish between an undefined null scalar and
a defined null scalar with operations that might return a real
null string, such as referencing elements of an array. You may
also check to see if arrays or subroutines exist. Use of
defined on predefined variables is not guaranteed to produce
intuitive results.
When used on a hash array element, it tells you whether the
value is defined, not whether the key exists in the hash. Use
exists() for that.
Examples:
print if defined $switch{'D'};
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
eval '@foo = ()' if defined(@foo);
die "No XYZ package defined" unless defined %_XYZ;
sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
See also undef().
delete EXPR
Deletes the specified value from its hash array. Returns the
deleted value, or the undefined value if nothing was deleted.
Deleting from $ENV{} modifies the environment. Deleting from
an array 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 an associative array:
foreach $key (keys %ARRAY) {
delete $ARRAY{$key};
}
(But it would be faster to use the undef() command.) Note that
the EXPR can be arbitrarily complicated as long as the final
operation is a hash key lookup:
delete $ref->[$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 the section on Predefined Names in the
perlvar manpage). 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 = <STDIN>;
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] ⎪⎪ "Kilroy";
getpeername SOCKET
Returns the packed sockaddr address of other end of the SOCKET
connection.
# An internet sockaddr
$sockaddr = 'S n a4 x8';
$hersockaddr = getpeername(S);
($family, $port, $heraddr) = unpack($sockaddr,$hersockaddr);
getpgrp PID
Returns 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 getpgrp(2). If PID is omitted,
returns process group of current process.
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 produce a
fatal error if used on a machine that doesn't implement
getpriority(2).
getpwnam NAME
getgrnam NAME
gethostbyname NAME
getnetbyname NAME
getprotobyname NAME
getpwuid UID
getgrgid GID
getservbyname NAME,PROTO
gethostbyaddr ADDR,ADDRTYPE
getnetbyaddr ADDR,ADDRTYPE
getprotobynumber NUMBER
getservbyport PORT,PROTO
getpwent
getgrent
gethostent
getnetent
getprotoent
getservent
setpwent
setgrent
sethostent STAYOPEN
setnetent STAYOPEN
setprotoent STAYOPEN
setservent STAYOPEN
endpwent
endgrent
endhostent
endnetent
endprotoent
endservent
These routines perform the same functions as their counterparts
in the system library. Within a list context, the return
values from the various get routines are as follows:
($name,$passwd,$uid,$gid,
$quota,$comment,$gcos,$dir,$shell) = getpw*
($name,$passwd,$gid,$members) = getgr*
($name,$aliases,$addrtype,$length,@addrs) = gethost*
($name,$aliases,$addrtype,$net) = getnet*
($name,$aliases,$proto) = getproto*
($name,$aliases,$port,$proto) = getserv*
(If the entry doesn't exist you get a null list.)
Within a scalar context, you get the name, unless the function
was a lookup by name, in which case you get the other thing,
whatever it is. (If the entry doesn't exist you get the
undefined value.) For example:
$uid = getpwnam
$name = getpwuid
$name = getpwent
$gid = getgrnam
$name = getgrgid
$name = getgrent
etc.
The $members value returned by getgr*() is a space separated
list of the login names of the members of the group.
For the gethost*() functions, if the h_errno variable is
supported in C, it will be returned to you via $? if the
function call fails. The @addrs value returned by a successful
call is a list of the raw addresses returned by the
corresponding system library call. In the Internet domain,
each address is four bytes long and you can unpack it by saying
something like:
($a,$b,$c,$d) = unpack('C4',$addr[0]);
getsockname SOCKET
Returns the packed sockaddr address of this end of the SOCKET
connection.
# An internet sockaddr
$sockaddr = 'S n a4 x8';
$mysockaddr = getsockname(S);
($family, $port, $myaddr) =
unpack($sockaddr,$mysockaddr);
getsockopt SOCKET,LEVEL,OPTNAME
Returns the socket option requested, or undefined if there is
an error.
glob EXPR
Returns the value of EXPR with filename expansions such as a
shell would do. This is the internal function implementing the
<*.*> operator.
gmtime EXPR
Converts a time as returned by the time function to a 9-element
array with the time localized for the Greenwich timezone.
Typically used as follows:
($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. If EXPR is omitted, does
gmtime(time()).
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. It can be used to go almost anywhere
else within the dynamic scope, including out of subroutines,
but it's usually better to use some other construct such as
last or die. The author of Perl has never felt the need to use
this form of goto (in Perl, that is--C is another matter).
The goto-EXPR form expects a label name, whose scope will be
resolved dynamically. This allows for computed gotos per
FORTRAN, but isn't necessarily recommended if you're optimizing
for maintainability:
goto ("FOO", "BAR", "GLARCH")[$i];
The goto-&NAME form is highly magical, and substitutes a call
to the named subroutine for the currently running subroutine.
This is used by AUTOLOAD subroutines that wish to load another
subroutine and then pretend that the other subroutine had been
called in the first place (except that any modifications to @_
in the current subroutine are propagated to the other
subroutine.) After the goto, not even caller() will be able to
tell that this routine was called first.
grep BLOCK LIST
grep EXPR,LIST
Evaluates the BLOCK or EXPR for each element of LIST (locally
setting $_ to each element) and returns the list value
consisting of those elements for which the expression evaluated
to TRUE. In a scalar context, returns the number of times the
expression was TRUE.
@foo = grep(!/^#/, @bar); # weed out comments
or equivalently,
@foo = grep {!/^#/} @bar; # weed out comments
Note that, since $_ 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.
hex EXPR
Returns the decimal value of EXPR interpreted as an hex string.
(To interpret strings that might start with 0 or 0x see oct().)
If EXPR is omitted, uses $_.
import There is no built-in import() function. It is merely an
ordinary method subroutine defined (or inherited) by modules
that wish to export names to another module. The use()
function calls the import() method for the package used. See
also the use entry elsewhere in this documentthe perlmod
manpage.
index STR,SUBSTR,POSITION
index STR,SUBSTR
Returns the position of the first occurrence of SUBSTR in STR
at or after POSITION. If POSITION is omitted, starts searching
from the beginning of the string. The return value is based at
0, or whatever you've set the $[ variable to. If the substring
is not found, returns one less than the base, ordinarily -1.
int EXPR
Returns the integer portion of EXPR. If EXPR is omitted, uses
$_.
ioctl FILEHANDLE,FUNCTION,SCALAR
Implements the ioctl(2) function. You'll probably have to say
require "ioctl.ph"; # probably /usr/local/lib/perl/ioctl.ph
first to get the correct function definitions. If ioctl.ph
doesn't exist or doesn't have the correct definitions you'll
have to roll your own, based on your C header files such as
<sys/ioctl.h>. (There is a Perl script called h2ph that comes
with the Perl kit which may help you in this.) 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';
$sgttyb_t = "ccccs"; # 4 chars and a short
if (ioctl(STDIN,$TIOCGETP,$sgttyb)) {
@ary = unpack($sgttyb_t,$sgttyb);
$ary[2] = 127;
$sgttyb = pack($sgttyb_t,@ary);
ioctl(STDIN,$TIOCSETP,$sgttyb)
⎪⎪ die "Can't ioctl: $!";
}
The return value of ioctl (and fcntl) is as follows:
if OS returns: then Perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
Thus Perl returns TRUE on success and FALSE on failure, yet you
can still easily determine the actual value returned by the
operating system:
($retval = ioctl(...)) ⎪⎪ ($retval = -1);
printf "System returned %d\n", $retval;
join EXPR,LIST
Joins the separate strings of LIST or ARRAY into a single
string with fields separated by the value of EXPR, and returns
the string. Example:
$_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
See the split entry in the perlfunc manpage.
keys ASSOC_ARRAY
Returns a normal array consisting of all the keys of the named
associative array. (In a scalar context, returns the number of
keys.) The keys are returned in an apparently random order,
but it is the same order as either the values() or each()
function produces (given that the associative array has not
been modified). 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";
}
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 (<STDIN>) {
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) = <STDIN>;
local @FOO = <STDIN>;
both supply a list context to the righthand side, while
local $foo = <STDIN>;
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 the timelocal entry in the perlmod manpage 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 < $max; $i++) {
$result .= eval $thunk;
}
$result;
}
if ($sw eq '-v') {
# init my array with global array
my @ARGV = @ARGV;
unshift(@ARGV,'echo');
system @ARGV;
}
# Outer @ARGV again visible
The "my" is simply a modifier on something you might assign to.
So when you do assign to the EXPR, the "my" doesn't change
whether EXPR is viewed as a scalar or an array. So
my ($foo) = <STDIN>;
my @FOO = <STDIN>;
both supply a list context to the righthand side, while
my $foo = <STDIN>;
supplies a scalar context. But the following only declares one
variable:
my $foo, $bar = 1;
That has the same effect as
my $foo;
$bar = 1;
The declared variable is not introduced (is not visible) until
after the current statement. Thus,
my $x = $x;
can be used to initialize the new $x with the value of the old
$x, and the expression
my $x = 123 and $x == 123
is false unless the old $x happened to have the value 123.
Some users may wish to encourage the use of lexically scoped
variables. As an aid to catching implicit references to
package variables, if you say
use strict 'vars';
then any variable reference from there to the end of the
enclosing block must either refer to a lexical variable, or
must be fully qualified with the package name. A compilation
error results otherwise. An inner block may countermand this
with "no strict 'vars'".
next LABEL
next The next command is like the continue statement in C; it starts
the next iteration of the loop:
line: while (<STDIN>) {
next line if /^#/; # discard comments
...
}
Note that if there were a continue block on the above, it would
get executed even on discarded lines. If the LABEL is omitted,
the command refers to the innermost enclosing loop.
no Module LIST
See the "use" function, which "no" is the opposite of.
oct EXPR
Returns the decimal value of EXPR interpreted as an octal
string. (If EXPR happens to start off with 0x, interprets it
as a hex string instead.) The following will handle decimal,
octal, and hex in the standard Perl or C notation:
$val = oct($val) if $val =~ /^0/;
If EXPR is omitted, uses $_.
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. If the filename begins with "<" or
nothing, the file is opened for input. If the filename begins
with ">", 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 (<ARTICLE>) {...
open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
open(article, "caesar <$article ⎪"); # decrypt article
open(extract, "⎪sort >/tmp/Tmp$$"); # $$ is our process id
# process argument list of files along with any includes
foreach $file (@ARGV) {
process($file, 'fh00');
}
sub process {
local($filename, $input) = @_;
$input++; # this is a string increment
unless (open($input, $filename)) {
print STDERR "Can't open $filename: $!\n";
return;
}
while (<$input>) { # note use of indirection
if (/^#include "(.*)"/) {
process($1, $input);
next;
}
... # whatever
}
}
You may also, in the Bourne shell tradition, specify an EXPR
beginning with ">&", in which case the rest of the string is
interpreted as the name of a filehandle (or file descriptor, if
numeric) which is to be duped and opened. You may use & after
>, >>, <, +>, +>> and +<. The mode you specify should match
the mode of the original filehandle. 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); $⎪ = 1; # make unbuffered
print STDOUT "stdout 1\n"; # this works for
print STDERR "stderr 1\n"; # subprocesses too
close(STDOUT);
close(STDERR);
open(STDOUT, ">&SAVEOUT");
open(STDERR, ">&SAVEERR");
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
If you specify "<&=N", where N is a number, then Perl will do
an equivalent of C's fdopen() of that file descriptor. For
example:
open(FILEHANDLE, "<&=$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;
Explicitly closing any piped filehandle causes the parent
process to wait for the child to finish, and returns the status
value in $?. Note: on any operation which may do a fork,
unflushed buffers remain unflushed in both processes, which
means you may need to set $⎪ to avoid duplicate output.
The filename that is passed to open will have leading and
trailing whitespace deleted. In order to open a file with
arbitrary weird characters in it, it's necessary to protect any
leading and trailing whitespace thusly:
$file =~ s#^(\s)#./$1#;
open(FOO, "< $file\0");
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
Returns the numeric ascii value of the first character of EXPR.
If EXPR is omitted, uses $_.
pack TEMPLATE,LIST
Takes an array or list of values and packs it into a binary
structure, returning the string containing the structure. The
TEMPLATE is a sequence of characters that give the order and
type of values, as follows:
A An ascii string, will be space padded.
a An ascii string, will be null padded.
b A bit string (ascending bit order, like vec()).
B A bit string (descending bit order).
h A hex string (low nybble first).
H A hex string (high nybble first).
c A signed char value.
C An unsigned char value.
s A signed short value.
S An unsigned short value.
i A signed integer value.
I An unsigned integer value.
l A signed long value.
L An unsigned long value.
n A short in "network" order.
N A long in "network" order.
v A short in "VAX" (little-endian) order.
V A long in "VAX" (little-endian) order.
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.
x A null byte.
X Back up a byte.
@ Null fill to absolute position.
Each letter may optionally be followed by a number which gives
a repeat count. With all types except "a", "A", "b", "B", "h"
and "H", and "P" the pack function will gobble up that many
values from the LIST. A * for the repeat count means to use
however many items are left. The "a" and "A" types gobble just
one value, but pack it as a string of length count, padding
with nulls or spaces as necessary. (When unpacking, "A" strips
trailing spaces and nulls, but "a" does not.) Likewise, the
"b" and "B" fields pack a string that many bits long. The "h"
and "H" fields pack a string that many nybbles long. The "P"
packs a pointer to a structure of the size indicated by the
length. Real numbers (floats and doubles) are in the native
machine format only; due to the multiplicity of floating
formats around, and the lack of a standard "network"
representation, no facility for interchange has been made.
This means that packed floating point data written on one
machine may not be readable on another - even if both use IEEE
floating point arithmetic (as the endian-ness of the memory
representation is not part of the IEEE spec). Note that Perl
uses doubles internally for all numeric calculation, and
converting from double into float and thence back to double
again will lose precision (i.e. unpack("f", pack("f", $foo))
will not in general equal $foo).
Examples:
$foo = pack("cccc",65,66,67,68);
# foo eq "ABCD"
$foo = pack("c4",65,66,67,68);
# same thing
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
$foo = pack("s2",1,2);
# "\1\0\2\0" on little-endian
# "\0\1\0\2" on big-endian
$foo = pack("a4","abcd","x","y","z");
# "abcd"
$foo = pack("aaaa","abcd","x","y","z");
# "axyz"
$foo = pack("a14","abcdefg");
# "abcdefg\0\0\0\0\0\0\0"
$foo = pack("i9pl", gmtime);
# a real struct tm (on my system anyway)
sub bintodec {
unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}
The same template may generally also be used in the unpack
function.
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.
pop ARRAY
Pops and returns the last value of the array, shortening the
array by 1. Has a similar effect to
$tmp = $ARRAY[$#ARRAY--];
If there are no elements in the array, returns the undefined
value.
pos SCALAR
Returns the offset of where the last m//g search left off for
the variable in question. May be modified to change that
offset.
print FILEHANDLE LIST
print LIST
print Prints a string or a comma-separated list of strings. Returns
non-zero if successful. FILEHANDLE may be a scalar variable
name, in which case the variable contains the name of 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 parens around the arguments.) If FILEHANDLE is omitted,
prints by default to standard output (or to the last selected
output channel--see select()). If LIST is also omitted, prints
$_ to STDOUT. To set the default output channel to something
other than STDOUT use the select operation. Note that, because
print takes a LIST, anything in the LIST is evaluated in a list
context, and any subroutine that you call will have one or more
of its expressions evaluated in a list context. Also be
careful not to follow the print keyword with a left parenthesis
unless you want the corresponding right parenthesis to
terminate the arguments to the print--interpose a + or put
parens around all the arguments.
printf FILEHANDLE LIST
printf LIST
Equivalent to a "print FILEHANDLE sprintf(LIST)". The first
argument of the list will be interpreted as the printf format.
push ARRAY,LIST
Treats ARRAY as a stack, and pushes the values of LIST onto the
end of ARRAY. The length of ARRAY increases by the length of
LIST. Has the same effect as
for $value (LIST) {
$ARRAY[++$#ARRAY] = $value;
}
but is more efficient. Returns the new number of elements in
the array.
q/STRING/
qq/STRING/
qx/STRING/
qw/STRING/
Generalized quotes. See the perlop manpage.
quotemeta EXPR
Returns the value of EXPR with with all regular expression
metacharacters backslashed. This is the internal function
implementing the \Q escape in double-quoted strings.
rand EXPR
rand Returns a random fractional number between 0 and the value of
EXPR. (EXPR should be positive.) If EXPR is omitted, returns
a value between 0 and 1. This function produces repeatable
sequences unless srand() is invoked. 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. As a
workaround, you can usually multiply EXPR by the correct power
of 2 to get the range you want. This will make your script
unportable, however. It's better to recompile if you can.)
read FILEHANDLE,SCALAR,LENGTH,OFFSET
read FILEHANDLE,SCALAR,LENGTH
Attempts to read LENGTH bytes of data into variable SCALAR from
the specified FILEHANDLE. Returns the number of bytes actually
read, or undef if there was an error. SCALAR will be grown or
shrunk to the length actually read. An OFFSET may be specified
to place the read data at some other place than the beginning
of the string. This call is actually implemented in terms of
stdio's fread call. To get a true read system call, see
sysread().
readdir DIRHANDLE
Returns the next directory entry for a directory opened by
opendir(). If used in a list context, returns all the rest of
the entries in the directory. If there are no more entries,
returns an undefined value in a scalar context or a null list
in a list context.
readlink EXPR
Returns the value of a symbolic link, if symbolic links are
implemented. If not, gives a fatal error. If there is some
system error, returns the undefined value and sets $! (errno).
If EXPR is omitted, uses $_.
recv SOCKET,SCALAR,LEN,FLAGS
Receives a message on a socket. Attempts to receive LENGTH
bytes of data into variable SCALAR from the specified SOCKET
filehandle. Actually does a C recvfrom(), so that it can
returns the address of the sender. Returns the undefined value
if there's an error. SCALAR will be grown or shrunk to the
length actually read. Takes the same flags as the system call
of the same name.
redo LABEL
redo The redo command restarts the loop block without evaluating the
conditional again. The continue block, if any, is not
executed. If the LABEL is omitted, the command refers to the
innermost enclosing loop. This command is normally used by
programs that want to lie to themselves about what was just
input:
# a simpleminded Pascal comment stripper
# (warning: assumes no { or } in strings)
line: while (<STDIN>) {
while (s⎪({.*}.*){.*}⎪$1 ⎪) {}
s⎪{.*}⎪ ⎪;
if (s⎪{.*⎪ ⎪) {
$front = $_;
while (<STDIN>) {
if (/}/) { # end of comment?
s⎪^⎪$front{⎪;
redo line;
}
}
}
print;
}
ref EXPR
Returns a TRUE value if EXPR is a reference, FALSE otherwise.
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($r) eq "HASH") {
print "r is a reference to an associative array.\n";
}
if (!ref ($r) {
print "r is not a reference at all.\n";
}
See also the perlref manpage.
rename OLDNAME,NEWNAME
Changes the name of a file. Returns 1 for success, 0
otherwise. Will not work across filesystem boundaries.
require EXPR
require Demands some semantics specified by EXPR, or by $_ if EXPR is
not supplied. If EXPR is numeric, demands that the current
version of Perl ($] or $PERL_VERSION) be equal or greater than
EXPR.
Otherwise, demands that a library file be included if it hasn't
already been included. The file is included via the do-FILE
mechanism, which is essentially just a variety of eval(). Has
semantics similar to the following subroutine:
sub require {
local($filename) = @_;
return 1 if $INC{$filename};
local($realfilename,$result);
ITER: {
foreach $prefix (@INC) {
$realfilename = "$prefix/$filename";
if (-f $realfilename) {
$result = do $realfilename;
last ITER;
}
}
die "Can't find $filename in \@INC";
}
die $@ if $@;
die "$filename did not return true value" unless $result;
$INC{$filename} = $realfilename;
$result;
}
Note that the file will not be included twice under the same
specified name. The file must return TRUE as the last
statement to indicate successful execution of any
initialization code, so it's customary to end such a file with
"1;" unless you're sure it'll return TRUE otherwise. But it's
better just to put the "1;", in case you add more statements.
If EXPR is a bare word, the require assumes a ".pm" extension
for you, to make it easy to load standard modules. This form
of loading of modules does not risk altering your namespace.
For a yet-more-powerful import facility, see the the use()
entry elsewhere in this documentthe 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. Only resets variables or
searches in the current package. Always returns 1. Examples:
reset 'X'; # reset all X variables
reset 'a-z'; # reset lower case variables
reset; # just reset ?? searches
Resetting "A-Z" is not recommended since you'll wipe out your
ARGV and ENV arrays. Only resets package variables--lexical
variables are unaffected, but they clean themselves up on scope
exit anyway, so anymore you probably want to use them instead.
See the my entry elsewhere in this document.
return LIST
Returns from a subroutine or eval with the value specified.
(Note that in the absence of a return a subroutine or eval will
automatically return the value of the last expression
evaluated.)
reverse LIST
In a list context, returns a list value consisting of the
elements of LIST in the opposite order. In a scalar context,
returns a string value consisting of the bytes of the first
element of LIST in the opposite order.
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
Deletes the directory specified by FILENAME if it is empty. If
it succeeds it returns 1, otherwise it returns 0 and sets $!
(errno). If FILENAME is omitted, uses $_.
s/// The substitution operator. See the perlop manpage.
scalar EXPR
Forces EXPR to be interpreted in a scalar context and returns
the value of EXPR.
seek FILEHANDLE,POSITION,WHENCE
Randomly positions the file pointer for FILEHANDLE, 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 file pointer to POSITION, 1 to set the
it to current plus POSITION, and 2 to set it to EOF plus
offset. You may use the values SEEK_SET, SEEK_CUR, and
SEEK_END for this is using the POSIX module. Returns 1 upon
success, 0 otherwise.
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);
With Perl 5, filehandles are objects with methods, and the last
example is preferably written
use FileHandle;
STDERR->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(' ', $_)
internally.
Example:
open(passwd, '/etc/passwd');
while (<passwd>) {
($login, $passwd, $uid, $gid, $gcos,
$home, $shell) = split(/:/);
...
}
(Note that $shell above will still have a newline on it. See
the chop, chomp, and join entries elsewhere in this document.)
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 the exec
entry elsewhere in this document.
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.
$cnt = unlink 'a', 'b', 'c';
unlink @goners;
unlink <*.bak>;
Note: unlink will not delete directories unless you are
superuser and the -U flag is supplied to Perl. Even if these
conditions are met, be warned that unlinking a directory can
inflict damage on your filesystem. Use rmdir instead.
unpack TEMPLATE,EXPR
Unpack does the reverse of pack: it takes a string representing
a structure and expands it out into a list value, returning the
array value. (In a scalar context, it merely returns the first
value produced.) The TEMPLATE has the same format as in the
pack function. Here's a subroutine that does substring:
sub substr {
local($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 the section on tr/// in the
perlop manpage.
3rd Berkeley DistributionPERLFUNC(1)