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 parentheses.) If you use the parentheses, the simple (but occasionally surprising) rule is this: It LOOKS like a function, therefore it IS a function, and precedence doesn't matter. Otherwise it's a list operator or unary operator, and precedence does matter. And whitespace between the function and left parenthesis doesn't count--so you need to be careful sometimes:
print 1+2+4; # Prints 7.
print(1+2) + 4; # Prints 3.
print (1+2)+4; # Also prints 3!
print +(1+2)+4; # Prints 7.
print ((1+2)+4); # Prints 7.
If you run Perl with the -w switch it can warn you about this. For example, the third line above produces:
print (...) interpreted as function at - line 1.
Useless use of integer addition in void context at - line 1.
For functions that can be used in either a scalar or list context, nonabortive failure is generally indicated in a scalar context by returning the undefined value, and in a list context by returning the null list.
Remember the following rule:
* - sub was a keyword in perl4, but in perl5 it is an operator which can be used in expressions.
-t, which tests STDIN. Unless otherwise documented, it returns 1 for TRUE and '' for FALSE, or the undefined value if the file doesn't exist. Despite the
funny names, precedence is the same as any other named unary operator, and
the argument may be parenthesized like any other unary operator. The
operator may be any of:
-r File is readable by effective uid/gid.
-w File is writable by effective uid/gid.
-x File is executable by effective uid/gid.
-o File is owned by effective uid.
-R File is readable by real uid/gid.
-W File is writable by real uid/gid.
-X File is executable by real uid/gid.
-O File is owned by real uid.
-e File exists.
-z File has zero size.
-s File has nonzero size (returns size).
-f File is a plain file.
-d File is a directory.
-l File is a symbolic link.
-p File is a named pipe (FIFO).
-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() 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.
Because you have to read a file to do the -T test, on most occasions you want to use a -f
against the file first, as in next unless -f $file && -T $file.
If any of the file tests (or either the stat() or
lstat() operators) are given the special filehandle consisting
of a solitary underline, then the stat structure of the previous file test
(or stat operator) is used, saving a system call. (This doesn't work with -t, and you need to remember that lstat() and -l will leave values in the stat structure for the symbolic link, not the real
file.) Example:
print "Can do.\n" if -r $a || -w _ || -x _;
stat($filename);
print "Readable\n" if -r _;
print "Writable\n" if -w _;
print "Executable\n" if -x _;
print "Setuid\n" if -u _;
print "Setgid\n" if -g _;
print "Sticky\n" if -k _;
print "Text\n" if -T _;
print "Binary\n" if -B _;
accept(2)
system call does. Returns the packed address if it succeeded, FALSE
otherwise. See example in Sockets: Client/Server Communication.
$_ is used. (On some machines, unfortunately, the
elapsed time may be up to one second less than you specified because of how
seconds are counted.) Only one timer may be counting at once. Each call
disables the previous timer, and an argument of 0 may be supplied to cancel
the previous timer without starting a new one. The returned value is the
amount of time remaining on the previous timer.
For delays of finer granularity than one second, you may use Perl's
syscall() interface to access setitimer(2) if
your system supports it, or else see select(). It is usually a mistake to intermix alarm() and
sleep() calls.
If you want to use alarm() to time out a system call you need
to use an eval/die pair. You can't rely on the alarm causing the system
call to fail with $! set to EINTR because Perl sets up signal handlers to
restart system calls on some systems. Using eval/die always works.
eval {
local $SIG{ALRM} = sub { die "alarm\n" }; # NB \n required
alarm $timeout;
$nread = sysread SOCKET, $buffer, $size;
alarm 0;
};
die if $@ && $@ ne "alarm\n"; # propagate errors
if ($@) {
# timed out
}
else {
# didn't
}
For the tangent operation, you may use the POSIX::tan() function, or use the familiar relation:
sub tan { sin($_[0]) / cos($_[0]) }
bless() is often the last thing in a
constructor. Always use the two-argument version if the function doing the
blessing might be inherited by a derived class. See the perlobj manpage for more about the blessing (and blessings) of objects.
eval() or require(), and the
undefined value 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, $wantarray, $evaltext, $is_require) = caller($i);
Here $subroutine may be "(eval)" if the frame is not a subroutine call, but an eval. In such a case additional elements $evaltext and
$is_require are set: $is_require is true if the
frame is created by a
require or use statement, $evaltext contains the text of the
eval EXPR statement. In particular, for a eval BLOCK statement, $filename is "(eval)", but $evaltext is undefined. (Note also that each use statement creates a require frame inside an eval EXPR) frame.
Furthermore, when called from within the DB package, caller returns more detailed information: it sets the list variable @DB::args to be the arguments with which the subroutine was invoked.
die().
0644 is okay, '0644' is not. Returns the number of files successfully changed. See also oct, if all you have is a string.
$cnt = chmod 0755, 'foo', 'bar';
chmod 0755, @executables;
$mode = '0644'; chmod $mode, 'foo'; # !!! sets mode to --w----r-T
$mode = '0644'; chmod oct($mode), 'foo'; # this is better
$mode = 0644; chmod $mode, 'foo'; # this is best
$/ (also known as $INPUT_RECORD_SEPARATOR in the English module). It returns the total number of characters removed from all its
arguments. It's often used to remove the newline from the end of an input
record when you're worried that the final record may be missing its
newline. When in paragraph mode ($/ = ""), it removes all trailing newlines from the string. If VARIABLE is
omitted, it chomps $_. Example:
while (<>) {
chomp; # avoid \n on last field
@array = split(/:/);
...
}
You can actually chomp anything that's an lvalue, including an assignment:
chomp($cwd = `pwd`);
chomp($answer = <STDIN>);
If you chomp a list, each element is chomped, and the total number of characters removed is returned.
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).
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
Here's an example that looks up nonnumeric uids in the passwd file:
print "User: ";
chop($user = <STDIN>);
print "Files: "
chop($pattern = <STDIN>);
($login,$pass,$uid,$gid) = getpwnam($user)
or die "$user not in passwd file";
@ary = <${pattern}>; # expand filenames
chown $uid, $gid, @ary;
On most systems, you are not allowed to change the ownership of the file unless you're the superuser, although you should be able to change the group to any of your secondary groups. On insecure systems, these restrictions may be relaxed, but this is not a portable assumption.
If NUMBER is omitted, uses $_.
open() on it, because open() will close it for
you. (See open().) However, an explicit close on an input file
resets the line counter ($.), while the implicit close done by
open() does not. 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.
opendir().
while or
foreach), it is always executed just before the conditional is about to be
evaluated again, just like the third part of a for loop in C. Thus it can be used to increment a loop variable, even when the
loop has been continued via the next statement (which is similar to the C continue
statement).
For the inverse cosine operation, you may use the POSIX::acos() function, or use this relation:
sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
crypt(3) function in the C
library (assuming that you actually have a version there that has not been
extirpated as a potential munition). This can prove useful for checking the
password file for lousy passwords, amongst other things. Only the guys
wearing white hats should do this.
Note that crypt is intended to be a one-way function, much like breaking eggs to make an omelette. There is no (known) corresponding decrypt function. As a result, this function isn't all that useful for cryptography. (For that, see your nearby CPAN mirror.)
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 $pwd) {
die "Sorry...\n";
} else {
print "ok\n";
}
Of course, typing in your own password to whomever asks you for it is unwise.
untie() function.]
Breaks the binding between a DBM file and a hash.
tie() function.]
This binds a dbm(3), ndbm(3),
sdbm(3), gdbm(), or Berkeley DB file to a hash.
HASH is the name of the hash. (Unlike normal open, the first argument is NOT a filehandle, even though it looks like one). DBNAME is the name of the
database (without the .dir or .pag extension if any). If the database does not exist, it is created with
protection specified by MODE (as modified by the umask()). If
your system supports only the older DBM functions, you may perform only one
dbmopen() in your program. In older versions of Perl, if your
system had neither DBM nor ndbm, calling dbmopen() produced a
fatal error; it now falls back to sdbm(3).
If you don't have write access to the DBM file, you can only read hash
variables, not set them. If you want to test whether you can write, either
use file tests or try setting a dummy hash entry inside an
eval(), which will trap the error.
Note that functions such as keys() and values()
may return huge array values when used on large DBM files. You may prefer
to use the each() function to iterate over large DBM files.
Example:
# print out history file offsets
dbmopen(%HIST,'/usr/lib/news/history',0666);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
dbmclose(%HIST);
See also the AnyDBM_File manpage for a more general description of the pros and cons of the various dbm approaches, as well as the DB_File manpage for a particularly rich implementation.
$_ will be checked.
Many operations return undef to indicate failure, end of file, system error, uninitialized variable, and
other exceptional conditions. This function allows you to distinguish undef from other values. (A simple Boolean test will not distinguish among
undef, zero, the empty string, and ``0'', which are all equally false.) Note
that since undef is a valid scalar, its presence doesn't necessarily indicate an exceptional condition: pop() returns undef when its argument is an empty array, or when the element to return happens to be undef.
You may also use defined() to check whether a subroutine
exists. On the other hand, use of defined() upon aggregates
(hashes and arrays) is not guaranteed to produce intuitive results, and
should probably be avoided.
When used on a hash element, it tells you whether the value is defined, not whether the key exists in the hash. Use exists for the latter purpose.
Examples:
print if defined $switch{'D'};
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
$debugging = 0 unless defined $debugging;
Note: Many folks tend to overuse defined(), and then are
surprised to discover that the number 0 and ``'' (the zero-length string)
are, in fact, defined values. For example, if you say
"ab" =~ /a(.*)b/;
the pattern match succeeds, and $1 is defined, despite the
fact that it matched ``nothing''. But it didn't really match
nothing--rather, it matched something that happened to be 0 characters
long. This is all very above-board and honest. When a function returns an
undefined value, it's an admission that it couldn't give you an honest
answer. So you should use defined() only when you're
questioning the integrity of what you're trying to do. At other times, a
simple comparison to 0 or ``'' is what you want.
Currently, using defined() on an entire array or hash reports
whether memory for that aggregate has ever been allocated. So an array you
set to the empty list appears undefined initially, and one that once was
full and that you then set to the empty list still appears defined. You
should instead use a simple test for size:
if (@an_array) { print "has array elements\n" }
if (%a_hash) { print "has hash members\n" }
Using undef() on these, however, does clear their memory and
then report them as not defined anymore, but you shoudln't do that unless
you don't plan to use them again, because it saves time when you load them
up again to have memory already ready to be filled.
This counterintuitive behaviour of defined() on aggregates may
be changed, fixed, or broken in a future release of Perl.
key(s) and their associated values from
a hash. For each key, returns the deleted value associated with that key,
or the undefined value if there was no such key. Deleting from $ENV{}
modifies the environment. Deleting from a hash tied to a DBM file deletes
the entry from the DBM file. (But deleting from a tie()d hash
doesn't necessarily return anything.)
The following deletes all the values of a hash:
foreach $key (keys %HASH) {
delete $HASH{$key};
}
And so does this:
delete @HASH{keys %HASH}
(But both of these are slower than the undef() command.) Note
that the EXPR can be arbitrarily complicated as long as the final operation
is a hash element lookup or hash slice:
delete $ref->[$x][$y]{$key};
delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
eval(), prints the value of LIST to STDERR and exits with the current value of $! (errno). If $! is 0, exits with the value of
($? >> 8) (backtick `command` status). If ($? >> 8)
is 0, exits with 255. Inside an eval(), the error message is
stuffed into
$@, and the eval() is terminated with the undefined value; this
makes die() the way to raise an exception.
Equivalent examples:
die "Can't cd to spool: $!\n" unless chdir '/usr/spool/news';
chdir '/usr/spool/news' or die "Can't cd to spool: $!\n"
If the value of EXPR does not end in a newline, the current script line number and input line number (if any) are also printed, and a newline is supplied. 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().
You can arrange for a callback to be called just before the
die() does its deed, by setting the $SIG{__DIE__} hook. The associated handler will be called with the error text and can
change the error message, if it sees fit, by calling die()
again. See the perlvar manpage for details on setting %SIG entries, and eval() for some examples.
do 'stat.pl';
is just like
eval `cat stat.pl`;
except that it's more efficient, more concise, keeps track of the current
filename for error messages, and searches all the -I
libraries if the file isn't in the current directory (see also the
@INC array in Predefined Names). It's the same, however, in that it does reparse the file every time you
call it, so you probably don't want to do this inside a loop.
Note that inclusion of library modules is better done with the
use() and require() operators, which also do
error checking and raise an exception if there's a problem.
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 $ARGV[0] eq '-d';
QUICKSTART:
Getopt('f');
while ($k = each %foo) {}
for this reason.)
Entries are returned in an apparently random order. When the hash is
entirely read, a null array is returned in list context (which when
assigned produces a FALSE (0) value), and undef is returned in a scalar context. The next call to each() after
that will start iterating again. There is a single iterator for each hash,
shared by all each(), keys(), and
values() function calls in the program; it can be reset by
reading all the elements from the hash, or by evaluating keys HASH or
values HASH. If you add or delete elements of a hash while you're iterating over it,
you may get entries skipped or duplicated, so don't.
The following prints out your environment like the printenv(1)
program, only in a different order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}
See also keys() and values().
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.
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. Beware
that using eval() neither silences perl from printing warnings
to STDERR, nor does it stuff the text of warning messages into $@. To do either of those, you have to use the $SIG{__WARN__} facility. See warn() and the perlvar manpage.
Note that, because eval() traps otherwise-fatal errors, it is
useful for determining whether a particular feature (such as
socket() or symlink()) is implemented. It is also
Perl's exception trapping mechanism, where the die operator is used to
raise exceptions.
If the code to be executed doesn't vary, you may use the eval-BLOCK form to
trap run-time errors without incurring the penalty of recompiling each
time. The error, if any, is still returned in $@. Examples:
# make divide-by-zero nonfatal
eval { $answer = $a / $b; }; warn $@ if $@;
# same thing, but less efficient
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
eval { $answer = };
# a run-time error
eval '$answer ='; # sets $@
When using the eval{} form as an exception trap in libraries, you may wish
not to trigger any __DIE__ hooks that user code may have installed. You can use the local $SIG{__DIE__} construct for this purpose, as shown in this example:
# a very private exception trap for divide-by-zero
eval { local $SIG{'__DIE__'}; $answer = $a / $b; }; warn $@ if $@;
This is especially significant, given that __DIE__ hooks can call die() again, which has the effect of changing
their error messages:
# __DIE__ hooks may modify error messages
{
local $SIG{'__DIE__'} = sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
eval { die "foo foofs here" };
print $@ if $@; # prints "bar barfs here"
}
With an eval(), you should be especially careful to remember
what's being looked at when:
eval $x; # CASE 1
eval "$x"; # CASE 2
eval '$x'; # CASE 3
eval { $x }; # CASE 4
eval "\$$x++" # CASE 5
$$x++; # CASE 6
Cases 1 and 2 above behave identically: they run the code contained in the
variable $x. (Although case 2 has misleading double quotes making the
reader wonder what else might be happening (nothing is).) Cases 3 and 4
likewise behave in the same way: they run the code '$x', which does nothing
but return the value of $x. (Case 4 is preferred for purely visual reasons, but it also has the
advantage of compiling at compile-time instead of at run-time.) Case 5 is a
place where normally you WOULD like to use double quotes, except that in this particular situation, you
can just use symbolic references instead, as in case 6.
exec() function executes a system command AND NEVER RETURNS, unless the command does not exist and is executed directly instead of via /bin/sh -c (see below). Use system() instead of exec() 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() do not flush your output
buffer, so you may need to set $| to avoid lost output. Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";
If you don't really want to execute the first argument, but want to lie to the program you are executing about its own name, you can specify the program you actually want to run as an ``indirect object'' (without a comma) in front of the LIST. (This always forces interpretation of the LIST as a multivalued list, even if there is only a single scalar in the list.) Example:
$shell = '/bin/csh';
exec $shell '-sh'; # pretend it's a login shell
or, more directly,
exec {'/bin/csh'} '-sh'; # pretend it's a login shell
print "Exists\n" if exists $array{$key};
print "Defined\n" if defined $array{$key};
print "True\n" if $array{$key};
A hash element can be TRUE only if it's defined, and defined if it exists, but the reverse doesn't necessarily hold true.
Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash key lookup:
if (exists $ref->[$x][$y]{$key}) { ... }
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. The
only universally portable values for EXPR are 0 for success and 1 for
error; all other values are subject to unpredictable interpretation
depending on the environment in which the Perl program is running.
You shouldn't use exit() to abort a subroutine if there's any
chance that someone might want to trap whatever error happened. Use
die() instead, which can be trapped by an eval().
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);
select(). If FILEHANDLE is an
expression, the value is taken as the name of the filehandle.
flock(2), or an emulation of it, on FILEHANDLE. Returns
TRUE for success, FALSE on failure. Produces a fatal error if used on a
machine that doesn't implement flock(2), fcntl(2)
locking, or lockf(3). flock() is Perl's portable
file locking interface, although it locks only entire files, not records.
OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but you
can use the symbolic names if import them from the Fcntl module, either
individually, or as a group using the ':flock' tag. LOCK_SH requests a
shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN releases a
previously requested lock. If LOCK_NB is added to LOCK_SH or LOCK_EX then
flock() will return immediately rather than blocking waiting
for the lock (check the return status to see if you got it).
To avoid the possibility of mis-coordination, Perl flushes FILEHANDLE before (un)locking it.
Note that the emulation built with lockf(3) doesn't provide
shared locks, and it requires that FILEHANDLE be open with write intent.
These are the semantics that lockf(3) implements. Most (all?)
systems implement lockf(3) in terms of fcntl(2)
locking, though, so the differing semantics shouldn't bite too many people.
Note also that some versions of flock() cannot lock things
over the network; you would need to use the more system-specific
fcntl() for that. If you like you can force Perl to ignore
your system's flock(2) function, and so provide its own
fcntl(2)-based emulation, by passing the switch -Ud_flock to the Configure program when you configure perl.
Here's a mailbox appender for BSD systems.
use Fcntl ':flock'; # import LOCK_* constants
sub lock {
flock(MBOX,LOCK_EX);
# and, in case someone appended
# while we were waiting...
seek(MBOX, 0, 2);
}
sub unlock {
flock(MBOX,LOCK_UN);
}
open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
or die "Can't open mailbox: $!";
lock();
print MBOX $msg,"\n\n";
unlock();
See also the DB_File manpage for other flock() examples.
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() method of
IO::Handle 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);
See also the perlipc manpage for more examples of forking and reaping moribund children.
Note that if your forked child inherits system file descriptors like STDIN and STDOUT that are actually connected by a pipe or socket, even if you exit, the remote server (such as, say, httpd or rsh) won't think you're done. You should reopen those to /dev/null if it's any issue.
write() function. For
example:
format Something =
Test: @<<<<<<<< @||||| @>>>>>
$str, $%, '$' . int($num)
.
$str = "widget";
$num = $cost/$quantity;
$~ = 'Something';
write;
See the perlform manpage for many details and examples.
$^A (or $ACCUMULATOR in English). Eventually, when a
write() is done, the contents of
$^A are written to some filehandle, but you could also read $^A
yourself and then set $^A back to ``''. Note that a format typically does one formline()
per line of form, but the formline() function itself doesn't
care how many newlines are embedded in the PICTURE. This means that the ~ and ~~ tokens will treat the entire PICTURE as a single line. You may therefore
need to use multiple formlines to implement a single record format, just
like the format compiler.
Be careful if you put double quotes around the picture, because an ``@'' character may be taken to mean the beginning of an array name.
formline() always returns TRUE. See the perlform manpage for other examples.
if ($BSD_STYLE) {
system "stty cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system "stty", '-icanon', 'eol', "\001";
}
$key = getc(STDIN);
if ($BSD_STYLE) {
system "stty -cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system "stty", 'icanon', 'eol', '^@'; # ASCII null
}
print "\n";
Determination of whether $BSD_STYLE should be set is left as
an exercise to the reader.
The POSIX::getattr() function can do this more portably on systems alleging
POSIX compliance. See also the Term::ReadKey module from your nearest CPAN site; details on CPAN can be found on CPAN.
getpwuid().
$login = getlogin || getpwuid($<) || "Kilroy";
Do not consider getlogin() for authentication: it is not as
secure as getpwuid().
use Socket;
$hersockaddr = getpeername(SOCK);
($port, $iaddr) = unpack_sockaddr_in($hersockaddr);
$herhostname = gethostbyaddr($iaddr, AF_INET);
$herstraddr = inet_ntoa($iaddr);
getpgrp(2). If PID is omitted, returns process group of
current process. Note that the POSIX version of getpgrp() does
not accept a PID argument, so only PID==0 is truly portable.
getpriority(2).
($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]);
use Socket;
$mysockaddr = getsockname(SOCK);
($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
<*.c>
operator, but you can use it directly. If EXPR is omitted, $_
is used. The <*.c> operator is discussed in more detail in
I/O Operators.
# 0 1 2 3 4 5 6 7 8
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
gmtime(time);
All array elements are numeric, and come straight out of a struct tm. In
particular this means that $mon has the range 0..11 and
$wday has the range 0..6 with sunday as day 0. Also,
$year is the number of years since 1900, not simply the last two digits of the year.
If EXPR is omitted, does gmtime(time()).
In a scalar context, returns the ctime(3) value:
$now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"
Also see the timegm() function provided by the Time::Local
module, and the strftime(3) function available via the POSIX
module.
sort(). It can be used to go
almost anywhere else within the dynamic scope, including out of
subroutines, but it's usually better to use some other construct such as
last or die. The author of Perl has never felt the need to use this form of
goto (in Perl, that is--C is another matter).
The goto-EXPR form expects a label name, whose scope will be resolved dynamically. This allows for computed 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(1) and
its relatives. In particular, it is not limited to using regular
expressions.
Evaluates the BLOCK or EXPR for each element of LIST (locally setting
$_ to each element) and returns the list value consisting of
those elements for which the expression evaluated to TRUE. In 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, because $_ is a reference into the list value, it
can be used to modify the elements of the array. While this is useful and
supported, it can cause bizarre results if the LIST is not a named array.
Similarly, grep returns aliases into the original list, much like the way
that Foreach Loops's index variable aliases the list elements. That is, modifying an element
of a list returned by grep actually modifies the element in the original
list.
print hex '0xAf'; # prints '175'
print hex 'aF'; # same
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 use(), the perlmod manpage, and the Exporter manpage.
$[
variable to--but don't do that). If the substring is not found, returns one
less than the base, ordinarily -1.
ioctl(2) function. You'll probably have to say
require "ioctl.ph"; # probably in /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, but it's
nontrivial.) SCALAR will be read and/or written depending on the
FUNCTION--a pointer to the string value of SCALAR will be passed as the
third argument of the actual ioctl call. (If SCALAR has no string value but
does have a numeric value, that value will be passed rather than a pointer
to the string value. To guarantee this to be TRUE, add a 0 to the scalar
before using it.) The pack() and unpack()
functions are useful for manipulating the values of structures used by
ioctl(). The following example sets the erase character to
DEL.
require 'ioctl.ph';
$getp = &TIOCGETP;
die "NO TIOCGETP" if $@ || !$getp;
$sgttyb_t = "ccccs"; # 4 chars and a short
if (ioctl(STDIN,$getp,$sgttyb)) {
@ary = unpack($sgttyb_t,$sgttyb);
$ary[2] = 127;
$sgttyb = pack($sgttyb_t,@ary);
ioctl(STDIN,&TIOCSETP,$sgttyb)
|| die "Can't ioctl: $!";
}
The return value of ioctl (and fcntl) is as follows:
if OS returns: then Perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
Thus Perl returns TRUE on success and FALSE on failure, yet you can still easily determine the actual value returned by the operating system:
($retval = ioctl(...)) || ($retval = -1);
printf "System returned %d\n", $retval;
$_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
See split.
values() or each() function produces (given that
the hash has not been modified). As a side effect, it resets HASH's
iterator.
Here is yet another way to print your environment:
@keys = keys %ENV;
@values = values %ENV;
while ($#keys >= 0) {
print pop(@keys), '=', pop(@values), "\n";
}
or how about sorted by key:
foreach $key (sort(keys %ENV)) {
print $key, '=', $ENV{$key}, "\n";
}
To sort an array by value, you'll need to use a sort function. Here's a descending numeric sort of a hash by its values:
foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash)) {
printf "%4d %s\n", $hash{$key}, $key;
}
As an lvalue keys allows you to increase the number of hash buckets allocated for the given hash. This can gain you a measure of efficiency if you know the hash is going to get big. (This is similar to pre-extending an array by assigning a larger number to $#array.) If you say
keys %hash = 200;
then %hash will have at least 200 buckets allocated for it. These buckets will be
retained even if you do %hash = (), use undef
%hash if you want to free the storage while %hash is still in scope. You can't shrink the number of buckets allocated for the
hash using
keys in this way (but you needn't worry about doing this by accident, as trying
has no effect).
$cnt = kill 1, $child1, $child2;
kill 9, @goners;
Unlike in the shell, in Perl if the SIGNAL is negative, it kills process groups instead of processes. (On System V, a negative PROCESS number will also kill process groups, but that's not portable.) That means you usually want to use positive not negative signals. You may also use a signal name in quotes. See Signals for details.
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
...
}
use locale in force. See the perllocale manpage.
If EXPR is omitted, uses $_.
use locale in force. See the perllocale manpage.
If EXPR is omitted, uses $_.
But you really probably want to be using my() instead, because
local() isn't what most people think of as ``local''). See Private Variables via my() for details.
# 0 1 2 3 4 5 6 7 8
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
localtime(time);
All array elements are numeric, and come straight out of a struct tm. In
particular this means that $mon has the range 0..11 and
$wday has the range 0..6 with sunday as day 0. Also,
$year is the number of years since 1900, that is,
$year is 123 in year 2023.
If EXPR is omitted, uses the current time (localtime(time)).
In a scalar context, returns the ctime(3) value:
$now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
Also see the Time::Local module, and the strftime(3) and
mktime(3) function available via the POSIX module.
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.
If EXPR is omitted, stats $_.
$_ 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 { getkey($_) => $_ } @array;
is just a funny way to write
%hash = ();
foreach $_ (@array) {
$hash{getkey($_)} = $_;
}
$! (errno).
msgctl(2). 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(2). Returns the message
queue id, or the undefined value if there is an error.
do/require/use'd file. If more than one value is listed, the list must be placed in
parentheses. See
Private Variables via my() for details.
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.
$val = oct($val) if $val =~ /^0/;
If EXPR is omitted, uses $_. This function is commonly used when a string such as ``644'' needs to be converted into a file mode, for example. (Although perl will automatically convert strings into numbers as needed, this automatic conversion assumes base 10.)
If the filename begins with '<' or nothing, the file is opened for
input. If the filename begins with '>', the file is truncated and opened
for output. 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; thus '+<' is
almost always preferred for read/write updates--the '+>' mode would
clobber the file first. The prefix and the filename may be separated with
spaces. These various prefixes correspond to the fopen(3)
modes of 'r', 'r+', 'w', 'w+', 'a', and 'a+'.
If the filename begins with ``|'', the filename is interpreted as a command
to which output is to be piped, and if the filename ends with a ``|'', the
filename is interpreted See Using open() for IPC for more examples of this. as command which pipes input to us. (You may not
have a raw open() to a command that pipes both in and out, but see
Open2, Open3, and Bidirectional Communication
for alternatives.)
Opening '-' opens STDIN and opening '>-' opens STDOUT. Open returns nonzero upon success, the undefined value otherwise. If the open involved a pipe, the return value happens to be the pid of the subprocess.
If you're unfortunate enough to be running Perl on a system that distinguishes between text files and binary files (modern operating systems don't care), then you should check out binmode for tips for dealing with this. The key distinction between systems that need binmode and those that don't is their text file formats. Systems like Unix and Plan9 that delimit lines with a single character, and that encode that character in C as '\n', do not need binmode. The rest need it.
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(DBASE, '+<dbase.mine'); # open for update
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. (Duping a filehandle does not take into account any existing contents of stdio buffers.) Here is a script that saves, redirects, and restores STDOUT and STDERR:
#!/usr/bin/perl
open(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; this is more parsimonious of
file descriptors. For example:
open(FILEHANDLE, "<&=$fd")
If you open a pipe on the command ``-'', i.e., either ``|-'' or ``-|'', then there is an implicit fork done, and the return value of open is the pid of the child within the parent process, and 0 within the child process. (Use defined($pid) to determine whether the open was successful.) The filehandle behaves normally for the parent, but i/o to that filehandle is piped from/to the STDOUT/STDIN of the child process. In the child process the filehandle isn't opened--i/o happens from/to the new STDOUT or STDIN. Typically this is used like the normal piped open when you want to exercise more control over just how the pipe command gets executed, such as when you are running setuid, and don't want to have to scan shell commands for metacharacters. The following pairs are more or less equivalent:
open(FOO, "|tr '[a-z]' '[A-Z]'");
open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
open(FOO, "cat -n '$file'|");
open(FOO, "-|") || exec 'cat', '-n', $file;
See Safe Pipe Opens for more examples of this.
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.
Closing any piped filehandle causes the parent process to wait for the
child to finish, and returns the status value in $?.
Using the constructor from the IO::Handle package (or one of its subclasses, such as IO::File or IO::Socket), you can generate anonymous filehandles which have the scope of whatever variables hold references to them, and automatically close whenever and however you leave that scope:
use IO::File;
...
sub read_myfile_munged {
my $ALL = shift;
my $handle = new IO::File;
open($handle, "myfile") or die "myfile: $!";
$first = <$handle>
or return (); # Automatically closed here.
mung $first or die "mung failed"; # Or here.
return $first, <$handle> if $ALL; # Or here.
$first; # Or here.
}
The filename that is passed to open will have leading and trailing whitespace deleted. 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");
If you want a ``real'' C open() (see open(2) on your system), then you should use the sysopen() function.
This is another way to protect your filenames from interpretation. For
example:
use IO::Handle;
sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL, 0700)
or die "sysopen $path: $!";
HANDLE->autoflush(1);
HANDLE->print("stuff $$\n");
seek(HANDLE, 0, 0);
print "File contains: ", <HANDLE>;
See seek() for some details about mixing reading and writing.
readdir(),
telldir(), seekdir(), rewinddir(),
and closedir(). Returns TRUE if successful. DIRHANDLEs have
their own namespace separate from FILEHANDLEs.
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.
(This 'short' is _exactly_ 16 bits, which may differ from
what a local C compiler calls 'short'.)
i A signed integer value.
I An unsigned integer value.
(This 'integer' is _at_least_ 32 bits wide. Its exact size
depends on what a local C compiler calls 'int', and may
even be larger than the 'long' described in the next item.)
l A signed long value.
L An unsigned long value.
(This 'long' is _exactly_ 32 bits, which may differ from
what a local C compiler calls 'long'.)
n A short in "network" (big-endian) order.
N A long in "network" (big-endian) order.
v A short in "VAX" (little-endian) order.
V A long in "VAX" (little-endian) order.
(These 'shorts' and 'longs' are _exactly_ 16 bits and
_exactly_ 32 bits, respectively.)
f A single-precision float in the native format.
d A double-precision float in the native format.
p A pointer to a null-terminated string.
P A pointer to a structure (fixed-length string).
u A uuencoded string.
w A BER compressed integer. Its bytes represent an unsigned
integer in base 128, most significant digit first, with as few
digits as possible. Bit eight (the high bit) is set on each
byte except the last.
x A null byte.
X Back up a byte.
@ Null fill to absolute position.
Each letter may optionally be followed by a number which gives a repeat
count. With all types except ``a'', ``A'', ``b'', ``B'', ``h'', ``H'', and
``P'' the pack function will gobble up that many values from the LIST. A *
for the repeat count means to use however many items are left. The ``a''
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.
local() operator).
All further unqualified dynamic identifiers will be in this namespace. A
package statement affects only dynamic variables--including those you've
used local() on--but not lexical variables created with my(). Typically it would be the
first declaration in a file to be included by the require
or use operator. You can switch into a package in more than one place; it
influences merely which symbol table is used by the compiler for the rest
of that block. You can refer to variables and filehandles in other packages
by prefixing the identifier with the package name and a double colon: $Package::Variable. If the package name is null, the main
package as assumed. That is, $::sail is equivalent to $main::sail.
See Packages for more information about packages, modules, and classes. See the perlsub manpage for other scoping issues.
$| to flush your WRITEHANDLE after each command, depending on the application.
See Open2, Open3, and Bidirectional Communication for examples of such things.
$tmp = $ARRAY[$#ARRAY--];
If there are no elements in the array, returns the undefined value. If
ARRAY is omitted, pops the @ARGV array in the main program,
and the @_ array in subroutines, just like
shift().
m//g search left off for the variable is in question ($_ is used when the
variable is not specified). May be modified to change that offset. Such
modification will also influence the \G zero-width assertion in regular expressions. See the perlre manpage and
the perlop manpage.
$_ 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 parentheses around all the arguments.
Note that if you're storing FILEHANDLES in an array or other expression, you will have to use a block returning its value instead:
print { $files[$i] } "stuff\n";
print { $OK ? STDOUT : STDERR } "stuff\n";
print FILEHANDLE sprintf(FORMAT, LIST). The first argument of the list will be interpreted as the printf format.
If use locale is in effect, the character used for the decimal point in formatted real
numbers is affected by the LC_NUMERIC locale. See the perllocale manpage.
Don't fall into the trap of using a printf() when a simple
print() would do. The print() is more efficient,
and less error prone.
for $value (LIST) {
$ARRAY[++$#ARRAY] = $value;
}
but is more efficient. Returns the new number of elements in the array.
/[A-Za-z_0-9]/ will be preceded by a backslash in the returned string, regardless of any
locale settings.) This is the internal function implementing the \Q escape
in double-quoted strings.
If EXPR is omitted, uses $_.
srand() unless srand() has
already been called. See also srand().
(Note: If your rand function consistently returns numbers that are too large or too small, then your version of Perl was probably compiled with the wrong number of RANDBITS.)
sysread().
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.
If you're planning to filetest the return values out of a
readdir(), you'd better prepend the directory in question.
Otherwise, because we didn't chdir() there, it would have been
testing the wrong file.
opendir(DIR, $some_dir) || die "can't opendir $some_dir: $!";
@dots = grep { /^\./ && -f "$some_dir/$_" } readdir(DIR);
closedir DIR;
$! (errno). If EXPR is omitted, uses $_.
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. See UDP: Message Passing for examples.
# 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;
}
$_ will be used. The value returned depends on
the type of thing the reference is a reference to. Builtin types include:
REF
SCALAR
ARRAY
HASH
CODE
GLOB
If the referenced object has been blessed into a package, then that package
name is returned instead. You can think of ref() as a
typeof() operator.
if (ref($r) eq "HASH") {
print "r is a reference to a hash.\n";
}
if (!ref ($r) {
print "r is not a reference at all.\n";
}
See also the perlref manpage.
$_ if EXPR is
not supplied. If EXPR is numeric, demands that the current version of Perl
($] 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 bareword, the require assumes a ``.pm'' extension and replaces ``::'' with ``/'' in the filename for you, to make it easy to load standard modules. This form of loading of modules does not risk altering your namespace.
For a yet-more-powerful import facility, see use and the perlmod manpage.
reset 'X'; # reset all X variables
reset 'a-z'; # reset lower case variables
reset; # just reset ?? searches
Resetting ``A-Z'' is not recommended because you'll wipe out your ARGV and ENV arrays. Resets only package variables--lexical variables are unaffected, but they clean themselves up on scope exit anyway, so you'll probably want to use them instead. See my.
eval(), or do FILE with the value
of the given EXPR. Evaluation of EXPR may be in a list, scalar, or void
context, depending on how the return value will be used, and the context
may vary from one execution to the next (see wantarray()). If
no EXPR is given, returns an empty list in a list context, an undefined
value in a scalar context, or nothing in a void context.
(Note that in the absence of a return, a subroutine, eval, or do FILE will automatically return the value of the last expression evaluated.)
print reverse <>; # line tac, last line first
undef $/; # for efficiency of <>
print scalar reverse <>; # byte tac, last line tsrif
This operator is also handy for inverting a hash, although there are some caveats. If a value is duplicated in the original hash, only one of those can be represented as a key in the inverted hash. Also, this has to unwind one hash and build a whole new one, which may take some time on a large hash.
%by_name = reverse %by_address; # Invert the hash
readdir() routine on DIRHANDLE.
$! (errno). If FILENAME is omitted, uses $_.
@counts = ( scalar @a, scalar @b, scalar @c );
There is no equivalent operator to force an expression to be interpolated
in a list context because it's in practice never needed. If you really
wanted to do so, however, you could use the construction @{[ (some expression) ]}, but usually a simple
(some expression) suffices.
fseek() call of
stdio. FILEHANDLE may be an expression whose value gives the name of the
filehandle. The values for WHENCE are 0 to set the new position to
POSITION, 1 to set it to the current position plus POSITION, and 2 to set
it to EOF plus POSITION (typically negative). For WHENCE you may use the
constants SEEK_SET, SEEK_CUR, and SEEK_END from either the IO::Seekable or
the POSIX module. Returns 1 upon success, 0 otherwise.
If you want to position file for sysread() or
syswrite(), don't use seek() -- buffering makes
its effect on the file's system position unpredictable and non-portable.
Use sysseek() instead.
On some systems you have to do a seek whenever you switch between reading
and writing. Amongst other things, this may have the effect of calling
stdio's clearerr(3). A WHENCE of 1 (SEEK_CUR) is useful for
not moving the file position:
seek(TEST,0,1);
This is also useful for applications emulating tail -f. Once you hit EOF on your read, and then sleep for a while, you might have
to stick in a seek() to reset things. The seek()
doesn't change the current position, but it does clear the end-of-file condition on the handle, so that the next <FILE> makes Perl try again to read something. We hope.
If that doesn't work (some stdios are particularly cantankerous), then you may need something more like this:
for (;;) {
for ($curpos = tell(FILE); $_ = <FILE>; $curpos = tell(FILE)) {
# search for some stuff and put it into files
}
sleep($for_a_while);
seek(FILE, $curpos, 0);
}
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(REPORT1);
$^ = 'report1_top';
select(REPORT2);
$^ = 'report2_top';
FILEHANDLE may be an expression whose value gives the name of the actual filehandle. Thus:
$oldfh = select(STDERR); $| = 1; select($oldfh);
Some programmers may prefer to think of filehandles as objects with methods, preferring to write the last example as:
use IO::Handle;
STDERR->autoflush(1);
select(2) system call with the bit masks
specified, which can be constructed using fileno() and
vec(), along these lines:
$rin = $win = $ein = '';
vec($rin,fileno(STDIN),1) = 1;
vec($win,fileno(STDOUT),1) = 1;
$ein = $rin | $win;
If you want to select on many filehandles you might wish to write a subroutine:
sub fhbits {
local(@fhlist) = split(' ',$_[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 just do this
$nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);
Most systems do not bother to return anything useful in $timeleft, so
calling select() in a scalar context just returns $nfound.
Any of the bit masks can also be undef. The timeout, if specified, is in
seconds, which may be fractional. Note: not all implementations are capable
of returning the $timeleft. If not, they always return
$timeleft equal to the supplied $timeout.
You can effect a sleep of 250 milliseconds this way:
select(undef, undef, undef, 0.25);
WARNING: Do not attempt to mix buffered I/O (like read() or <FH>) with select(). You have to use sysread()
instead.
&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.
$semnum of semaphore id
$semid:
$semop = pack("sss", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid, $semop);
To signal the semaphore, replace ``-1'' with ``1''.
sendto(). Returns the number of
characters sent, or the undefined value if there is an error. See UDP: Message Passing for examples.
setpgrp(2). If the arguments are omitted, it
defaults to 0,0. Note that the POSIX version of setpgrp() does
not accept any arguments, so only setpgrp 0,0 is portable.
setpriority(2).) Will produce a fatal error if used on a
machine that doesn't implement setpriority(2).
@ARGV array in the main program, and the @_ array
in subroutines. (This is determined lexically.) See also
unshift(), push(), and pop().
Shift() and unshift() do the same thing to the
left end of an array that pop() and push() do to
the right end.
For the inverse sine operation, you may use the POSIX::asin() function, or use this relation:
sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
alarm() and
sleep() calls, because sleep() is often
implemented using alarm().
On some older systems, it may sleep up to a full second less than what you requested, depending on how it counts seconds. Most modern systems always sleep the full amount.
For delays of finer granularity than one second, you may use Perl's
syscall() interface to access setitimer(2) if
your system supports it, or else see select() below.
See also the POSIX module's sigpause() function.
<=> 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 the package global variables
$a and $b (see example below). They are passed by
reference, so don't modify $a and $b. And don't try to declare
them as lexicals either.
You also cannot exit out of the sort block or subroutine using any of the
loop control operators described in the perlsyn manpage or with goto().
When use locale is in effect, sort LIST sorts LIST according to the current collation locale. See the perllocale manpage.
Examples:
# sort lexically
@articles = sort @files;
# same thing, but with explicit sort routine
@articles = sort {$a cmp $b} @files;
# now case-insensitively
@articles = sort {uc($a) cmp uc($b)} @files;
# same thing in reversed order
@articles = sort {$b cmp $a} @files;
# sort numerically ascending
@articles = sort {$a <=> $b} @files;
# sort numerically descending
@articles = sort {$b <=> $a} @files;
# sort using explicit subroutine name
sub byage {
$age{$a} <=> $age{$b}; # presuming numeric
}
@sortedclass = sort byage @class;
# this sorts the %age hash by value instead of key
# using an in-line function
@eldest = sort { $age{$b} <=> $age{$a} } keys %age;
sub backwards { $b cmp $a; }
@harry = ('dog','cat','x','Cain','Abel');
@george = ('gone','chased','yz','Punished','Axed');
print sort @harry;
# prints AbelCaincatdogx
print sort backwards @harry;
# prints xdogcatCainAbel
print sort @george, 'to', @harry;
# prints AbelAxedCainPunishedcatchaseddoggonetoxyz
# inefficiently sort by descending numeric compare using
# the first integer after the first = sign, or the
# whole record case-insensitively otherwise
@new = sort {
($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
||
uc($a) cmp uc($b)
} @old;
# same thing, but much more efficiently;
# we'll build auxiliary indices instead
# for speed
@nums = @caps = ();
for (@old) {
push @nums, /=(\d+)/;
push @caps, uc($_);
}
@new = @old[ sort {
$nums[$b] <=> $nums[$a]
||
$caps[$a] cmp $caps[$b]
} 0..$#old
];
# same thing using a Schwartzian Transform (no temps)
@new = map { $_->[0] }
sort { $b->[1] <=> $a->[1]
||
$a->[2] cmp $b->[2]
} map { [$_, /=(\d+)/, uc($_)] } @old;
If you're using strict, you MUST NOT declare $a and $b as lexicals. They are package
globals. That means if you're in the main package, it's
@articles = sort {$main::b <=> $main::a} @files;
or just
@articles = sort {$::b <=> $::a} @files;
but if you're in the FooPack package, it's
@articles = sort {$FooPack::b <=> $FooPack::a} @files;
The comparison function is required to behave. If it returns inconsistent
results (sometimes saying $x[1] is less than $x[2] and sometimes saying the
opposite, for example) the Perl interpreter will probably crash and dump
core. This is entirely due to and dependent upon your system's
qsort(3) library routine; this routine often avoids sanity
checks in the interest of speed.
$[ == 0):
push(@a,$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)) { ... }
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 $_ string. If PATTERN is also
omitted, splits on whitespace (after skipping any leading whitespace).
Anything matching PATTERN is taken to be a delimiter separating the fields.
(Note that the delimiter may be longer than one character.) If LIMIT is
specified and is not negative, splits into no more than that many fields
(though it may split into fewer). If LIMIT is unspecified, trailing null
fields are stripped (which potential users of pop() would do
well to remember). If LIMIT is negative, it is treated as if an arbitrarily
large LIMIT had been specified.
A pattern matching the null string (not to be confused with a null pattern //, which is just one member of the set of patterns matching a null string)
will split the value of EXPR into separate characters at each point it
matches that way. For example:
print join(':', split(/ */, 'hi there'));
produces the output 'h:i:t:h:e:r:e'.
The LIMIT parameter can be used to split a line partially
($login, $passwd, $remainder) = split(/:/, $_, 3);
When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT one larger than the number of variables in the list, to avoid unnecessary work. For the list above LIMIT would have been 4 by default. In time critical applications it behooves you not to split into more fields than you really need.
If the PATTERN contains parentheses, additional array elements are created from each matching substring in the delimiter.
split(/([,-])/, "1-10,20", 3);
produces the list value
(1, '-', 10, ',', 20)
If you had the entire header of a normal Unix email message in $header, you could split it up into fields and their values this way:
$header =~ s/\n\s+/ /g; # fix continuation lines
%hdrs = (UNIX_FROM => split /^(.*?):\s*/m, $header);
The pattern /PATTERN/ may be replaced with an expression to specify patterns that vary at
runtime. (To do runtime compilation only once, use /$variable/o.)
As a special case, specifying a PATTERN of space (' ') will split on white space just as split with no arguments does. Thus,
split(' ') can be used to emulate awk's default behavior, whereas split(/ /)
will give you as many null initial fields as there are leading spaces. A
split on /\s+/ is like a split(' ') except that any leading
whitespace produces a null first field. A split with no arguments really
does a split(' ', $_) internally.
Example:
open(passwd, '/etc/passwd');
while (<passwd>) {
($login, $passwd, $uid, $gid, $gcos,
$home, $shell) = split(/:/);
...
}
(Note that $shell above will still have a newline on it. See chop,
chomp, and join.)
sprintf(). See sprintf(3) or printf(3)
on your system for an explanation of the general principles.
Perl does all of its own sprintf() formatting -- it emulates
the C function sprintf(), but it doesn't use it (except for
floating-point numbers, and even then only the standard modifiers are
allowed). As a result, any non-standard extensions in your local
sprintf() are not available from Perl.
Perl's sprintf() permits the following universally-known
conversions:
%% a percent sign %c a character with the given number %s a string %d a signed integer, in decimal %u an unsigned integer, in decimal %o an unsigned integer, in octal %x an unsigned integer, in hexadecimal %e a floating-point number, in scientific notation %f a floating-point number, in fixed decimal notation %g a floating-point number, in %e or %f notation
In addition, Perl permits the following widely-supported conversions:
%X like %x, but using upper-case letters
%E like %e, but using an upper-case "E"
%G like %g, but with an upper-case "E" (if applicable)
%p a pointer (outputs the Perl value's address in hexadecimal)
%n special: *stores* the number of characters output so far
into the next variable in the parameter list
Finally, for backward (and we do mean ``backward'') compatibility, Perl permits these unnecessary but widely-supported conversions:
%i a synonym for %d %D a synonym for %ld %U a synonym for %lu %O a synonym for %lo %F a synonym for %f
Perl permits the following universally-known flags between the %
and the conversion letter:
space prefix positive number with a space
+ prefix positive number with a plus sign
- left-justify within the field
0 use zeros, not spaces, to right-justify
# prefix octal with "0", hex with "0x"
number minimum field width
.number "precision": digits after decimal point for floating-point,
max length for string, minimum length for integer
l interpret integer as C type "long" or "unsigned long"
h interpret integer as C type "short" or "unsigned short"
There is also one Perl-specific flag:
V interpret integer as Perl's standard integer type
Where a number would appear in the flags, an asterisk (``*'') may be used instead, in which case Perl uses the next item in the parameter list as the given number (that is, as the field width or precision). If a field width obtained through ``*'' is negative, it has the same effect as the '-' flag: left-justification.
If use locale is in effect, the character used for the decimal point in formatted real
numbers is affected by the LC_NUMERIC locale. See the perllocale manpage.
time(). This isn't a
particularly good seed, so many old programs supply their own seed value
(often time ^ $$ or
time ^ ($$ + ($$ << 15))), but that isn't necessary any more.
In fact, it's usually not necessary to call srand() at all,
because if it is not called explicitly, it is called implicitly at the
first use of the rand operator. However, this was not the case in version of Perl before 5.004,
so if your script will run under older Perl versions, it should call
srand().
Note that you need something much more random than the default seed for cryptographic purposes. Checksumming the compressed output of one or more rapidly changing operating system status programs is the usual method. For example:
srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
If you're particularly concerned with this, see the Math::TrulyRandom module in CPAN.
Do not call srand() multiple times in your program unless you know
exactly what you're doing and why you're doing it. The point of the
function is to ``seed'' the rand() function so that
rand() can produce a different sequence each time you run your
program. Just do it once at the top of your program, or you won't get random numbers out of rand()!
Frequently called programs (like CGI scripts) that simply use
time ^ $$
for a seed can fall prey to the mathematical property that
a^b == (a+1)^(b+1)
one-third of the time. So don't do that.
($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
$atime,$mtime,$ctime,$blksize,$blocks)
= stat($filename);
Not all fields are supported on all filesystem types. Here are the meaning of the fields:
0 dev device number of filesystem 1 ino inode number 2 mode file mode (type and permissions) 3 nlink number of (hard) links to the file 4 uid numeric user ID of file's owner 5 gid numeric group ID of file's owner 6 rdev the device identifier (special files only) 7 size total size of file, in bytes 8 atime last access time since the epoch 9 mtime last modify time since the epoch 10 ctime inode change time (NOT creation time!) since the epoch 11 blksize preferred block size for file system I/O 12 blocks actual number of blocks allocated
(The epoch was at 00:00 January 1, 1970 GMT.)
If stat is passed the special filehandle consisting of an underline, no stat is done, but the current contents of the stat structure from the last stat or filetest are returned. Example:
if (-x $file && (($d) = stat(_)) && $d < 0) {
print "$file is executable NFS file\n";
}
(This works on machines only for which the device number is negative under NFS.)
$_ if unspecified) in anticipation of doing many pattern matches on the string
before it is next modified. This may or may not save time, depending on the
nature and number of patterns you are searching on, and on the distribution
of character frequencies in the string to be searched -- you probably want
to compare run times with and without it to see which runs faster. Those
loops which scan for many short constant strings (including the constant
parts of more complex patterns) will benefit most. You may have only one
study active at a time -- if you study a different scalar the first is
``unstudied''. (The way study works is this: a linked list of every
character in the string to be searched is made, so we know, for example,
where all the 'k' characters are. From each search string, the rarest
character is selected, based on some static frequency tables constructed
from some C programs and English text. Only those places that contain this
``rarest'' character are examined.)
For example, here is a loop 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 delimiter
foreach $file (sort keys(%seen)) {
print $file, "\n";
}
$[ to (but don't do that). 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_exists = (eval {symlink("","")};, $@ eq '');
require 'syscall.ph'; # may need to run h2ph
syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
Note that Perl supports passing of up to only 14 arguments to your system call, which in practice should usually suffice.
The possible values and flag bits of the MODE parameter are
system-dependent; they are available via the standard module Fcntl. However, for historical reasons, some values are universal: zero means
read-only, one means write-only, and two means read/write.
If the file named by FILENAME does not exist and the open call creates it (typically because MODE includes the O_CREAT flag), then the value of PERMS specifies the permissions of the newly created file. If PERMS is omitted, the default value is 0666, which allows read and write for all. This default is reasonable: see umask.
The IO::File module provides a more object-oriented approach, if you're into that kind of thing.
read(2). It
bypasses stdio, so mixing this with other kinds of reads,
print(), write(), seek(), or
tell() can cause confusion. Returns the number of bytes
actually read, or undef if there was an error. SCALAR will be grown or
shrunk so that the last byte actually read is the last byte of the scalar
after the read.
An OFFSET may be specified to place the read data at some place in the string other than the beginning. A negative OFFSET specifies placement at that many bytes counting backwards from the end of the string. A positive OFFSET greater than the length of SCALAR results in the string being padded to the required size with ``\0'' bytes before the result of the read is appended.
lseek(2). It bypasses stdio, so mixing this with reads (other
than sysread()), print(), write(),
seek(), or tell() may cause confusion. FILEHANDLE
may be an expression whose value gives the name of the filehandle. The
values for WHENCE are 0 to set the new position to POSITION, 1 to set the
it to the current position plus POSITION, and 2 to set it to EOF plus
POSITION (typically negative). For WHENCE, you may use the constants
SEEK_SET, SEEK_CUR, and SEEK_END from either the IO::Seekable or the POSIX
module.
Returns the new position, or the undefined value on failure. A position of
zero is returned as the string ``0 but true''; thus sysseek()
returns TRUE on success and FALSE on failure, yet you can still easily
determine the new position.
wait() call. To get the actual exit value divide by 256. See
also exec. This is NOT what you want to use to capture the output from a command, for that you
should use merely backticks or qx//, as described in `STRING`.
Because system() and backticks block SIGINT and SIGQUIT,
killing the program they're running doesn't actually interrupt your
program.
@args = ("command", "arg1", "arg2");
system(@args) == 0
or die "system @args failed: $?"
Here's a more elaborate example of analysing the return value from
system() on a Unix system to check for all possibilities,
including for signals and core dumps.
$rc = 0xffff & system @args;
printf "system(%s) returned %#04x: ", "@args", $rc;
if ($rc == 0) {
print "ran with normal exit\n";
}
elsif ($rc == 0xff00) {
print "command failed: $!\n";
}
elsif ($rc > 0x80) {
$rc >>= 8;
print "ran with non-zero exit status $rc\n";
}
else {
print "ran with ";
if ($rc & 0x80) {
$rc &= ~0x80;
print "core dump from ";
}
print "signal $rc\n"
}
$ok = ($rc != 0);
write(2). It
bypasses stdio, so mixing this with reads (other than
sysread()), print(), write(),
seek(), or tell() may cause confusion. Returns
the number of bytes actually written, or undef if there was an error. If
the length is greater than the available data, only as much data as is
available will be written.
An OFFSET may be specified to write the data from some part of the string other than the beginning. A negative OFFSET specifies writing that many bytes counting backwards from the end of the string.
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.
dbm_open() function
of C. The object returned by the ``new'' method is also returned by the
tie() function, which would be useful if you want to access
other methods in CLASSNAME.
Note that functions such as keys() and values()
may return huge 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
use NDBM_File;
tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
untie(%HIST);
A class implementing a hash should have the following methods:
TIEHASH classname, LIST
DESTROY this
FETCH this, key
STORE this, key, value
DELETE this, key
EXISTS this, key
FIRSTKEY this
NEXTKEY this, lastkey
A class implementing an ordinary array should have the following methods:
TIEARRAY classname, LIST
DESTROY this
FETCH this, key
STORE this, key, value
[others TBD]
A class implementing a scalar should have the following methods:
TIESCALAR classname, LIST
DESTROY this
FETCH this,
STORE this, value
Unlike dbmopen(), the tie() function will not use
or require a module for you--you need to do that explicitly yourself. See the DB_File manpage
or the Config module for interesting tie() implementations.
tie() call which bound the
variable to a package.) Returns the undefined value if VARIABLE isn't tied
to a package.
gmtime() and localtime().
($user,$system,$cuser,$csystem) = times;
use locale in force. See the perllocale manpage.
If EXPR is omitted, uses $_.
use locale in force. See the perllocale manpage.
If EXPR is omitted, uses $_.
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, assign to a variable or pass as a
parameter. Examples:
undef $foo;
undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
undef @ary;
undef %hash;
undef &mysub;
return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
select undef, undef, undef, 0.25;
($a, $b, undef, $c) = &foo; # Ignore third value returned
$cnt = unlink 'a', 'b', 'c';
unlink @goners;
unlink <*.bak>;
Note: unlink will not delete directories unless you are superuser and the -U flag is supplied to Perl. Even if these conditions are met, be warned that unlinking a directory can inflict damage on your filesystem. Use rmdir instead.
If LIST is omitted, uses $_.
sub substr {
local($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);
tie().)
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.
BEGIN { require Module; import Module LIST; }
except that Module must be a bareword.
If the first argument to use is a number, it is treated as a version number instead of a module name. If the version of the Perl interpreter is less than VERSION, then an error message is printed and Perl exits immediately. This is often useful if you need to check the current Perl version before useing library modules which have changed in incompatible ways from older versions of Perl. (We try not to do this more than we have to.)
The BEGIN forces the require and import to happen at compile time. The require makes sure the module is loaded into memory if it hasn't been yet. The import is not a builtin--it's just an ordinary static method call into the ``Module'' package to tell the module to import the list of features back into the current package. The module can implement its import method any way it likes, though most modules just choose to derive their import method via inheritance from the Exporter class that is defined in the Exporter module. See the Exporter manpage. If no import method can be found then the error is currently silently ignored. This may change to a fatal error in a future version.
If you don't want your namespace altered, explicitly supply an empty list:
use Module ();
That is exactly equivalent to
BEGIN { require Module; }
If the VERSION argument is present between Module and LIST, then the use will call the VERSION method in class Module with the given version as an argument. The default VERSION method, inherited from the Universal class, croaks if the given version is larger than the value of the variable $Module::VERSION. (Note that there is not a comma after VERSION!)
Because this is a wide-open interface, pragmas (compiler directives) are also implemented this way. Currently implemented pragmas are:
use integer;
use diagnostics;
use sigtrap qw(SEGV BUS);
use strict qw(subs vars refs);
use subs qw(afunc blurfl);
These pseudo-modules 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, i.e., it calls unimport Module LIST instead of import.
no integer;
no strict 'refs';
If no unimport method can be found the call fails with a fatal error.
See the perlmod manpage for a list of standard modules and pragmas.
#!/usr/bin/perl
$now = time;
utime $now, $now, @ARGV;
keys() or each() function would produce on the
same hash. As a side effect, it resets HASH's iterator. See also
keys(), each(), and sort().
vec() may also be assigned to, in
which case parentheses are needed to give the expression the correct
precedence as in
vec($image, $max_x * $x + $y, 8) = 3;
Vectors created with vec() can also be manipulated with the
logical operators |, &, and ^, which will assume a bit vector operation
is desired when both operands are strings.
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 *.
$?.
$?. If you say
use POSIX ":sys_wait_h";
...
waitpid(-1,&WNOHANG);
then you can do a non-blocking wait for any process. Non-blocking wait is
available on machines supporting either the waitpid(2) or
wait4(2) system calls. However, waiting for a particular pid
with FLAGS of 0 is implemented everywhere. (Perl emulates the system call
by remembering the status values of processes that have exited but have not
been harvested by the Perl script yet.)
return unless defined wantarray; # don't bother doing more
my @a = complex_calculation();
return wantarray ? @a : "@a";
die(), but doesn't exit
or throw an exception.
No message is printed if there is a $SIG{__WARN__} handler installed. It is the handler's responsibility to deal with the
message as it sees fit (like, for instance, converting it into a
die()). Most handlers must therefore make arrangements to
actually display the warnings that they are not prepared to deal with, by
calling warn() again in the handler. Note that this is quite
safe and will not produce an endless loop, since __WARN__ hooks are not called from inside one.
You will find this behavior is slightly different from that of
$SIG{__DIE__} handlers (which don't suppress the error text, but can instead call
die() again to change it).
Using a __WARN__ handler provides a powerful way to silence all warnings (even the so-called
mandatory ones). An example:
# wipe out *all* compile-time warnings
BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
my $foo = 10;
my $foo = 20; # no warning about duplicate my $foo,
# but hey, you asked for it!
# no compile-time or run-time warnings before here
$DOWARN = 1;
# run-time warnings enabled after here
warn "\$foo is alive and $foo!"; # does show up
See the perlvar manpage for details on setting %SIG entries, and for more examples.
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.