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 important rule: There is no rule that relates the behavior of an expression in list context to its behavior in scalar context, or vice versa. It might do two totally different things. Each operator and function decides which sort of value it would be most appropriate to return in a scalar context. Some operators return the length of the list that would have been returned in list context. Some operators return the first value in the list. Some operators return the last value in the list. Some operators return a count of successful operations. In general, they do what you want, unless you want consistency.
An named array in scalar context is quite different from what would at first glance appear to be a list in scalar context. You can't get a list like (1,2,3) into being in scalar context, because the compiler knows the context at compile time. It would generate the scalar comma operator there, not the list construction version of the comma. That means it was never a list to start with.
In general, functions in Perl that serve as wrappers for system calls of the same name (like chown(2), fork(2), closedir(2), etc.) all return true when they succeed and undef otherwise, as is usually mentioned in the descriptions below. This is different from the C interfaces, which return -1 on failure. Exceptions to this rule are wait(), waitpid(), and syscall(). System calls also set the special $! variable on failure. Other functions do not, except accidentally.
* - sub was a keyword in perl4, but in perl5 it is an
operator, which can be used in expressions.
-r File is readable by effective uid/gid.
-w File is writable by effective uid/gid.
-x File is executable by effective uid/gid.
-o File is owned by effective uid.
-R File is readable by real uid/gid.
-W File is writable by real uid/gid.
-X File is executable by real uid/gid.
-O File is owned by real uid.
-e File exists.
-z File has zero size.
-s File has nonzero size (returns size).
-f File is a plain file.
-d File is a directory.
-l File is a symbolic link.
-p File is a named pipe (FIFO), or Filehandle is a pipe.
-S File is a socket.
-b File is a block special file.
-c File is a character special file.
-t Filehandle is opened to a tty.
-u File has setuid bit set.
-g File has setgid bit set.
-k File has sticky bit set.
-T File is a text file.
-B File is a binary file (opposite of -T).
-M Age of file in days when script started.
-A Same for access time.
-C Same for inode change time.
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, such as AFS access control lists. 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 strange characters (>30%) are found, it's a -B file, otherwise it's a -T file. Also, any file containing null in the first block is considered a binary file. If -T or -B is used on a filehandle, the current stdio buffer is examined rather than the first block. Both -T and -B return TRUE on a null file, or a file at EOF when testing a filehandle. Because you have to read a file to do the -T test, on most occasions you want to use a -f against the file first, as in next unless -f $file && -T $file.
If any of the file tests (or either the stat() or lstat() operators) are given the special filehandle consisting of a solitary underline, then the stat structure of the previous file test (or stat operator) is used, saving a system call. (This doesn't work with -t, and you need to remember that lstat() and -l will leave values in the stat structure for the symbolic link, not the real file.) Example:
print "Can do.\n" if -r $a || -w _ || -x _;
stat($filename);
print "Readable\n" if -r _;
print "Writable\n" if -w _;
print "Executable\n" if -x _;
print "Setuid\n" if -u _;
print "Setgid\n" if -g _;
print "Sticky\n" if -k _;
print "Text\n" if -T _;
print "Binary\n" if -B _;
For delays of finer granularity than one second, you may use Perl's syscall() interface to access setitimer(2) if your system supports it, or else see the select() entry elsewhere in this document. It is usually a mistake to intermix alarm() and sleep() calls.
If you want to use alarm() to time out a system call you need to use an eval()/die() pair. You can't rely on the alarm causing the system call to fail with $! set to EINTR because Perl sets up signal handlers to restart system calls on some systems. Using eval()/die() always works, modulo the caveats given in the section on Signals in the perlipc manpage.
eval {
local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
alarm $timeout;
$nread = sysread SOCKET, $buffer, $size;
alarm 0;
};
if ($@) {
die unless $@ eq "alarm\n"; # propagate unexpected errors
# timed out
}
else {
# didn't
}
For the tangent operation, you may use the POSIX::tan() function, or use the familiar relation:
sub tan { sin($_[0]) / cos($_[0]) }
($package, $filename, $line) = caller;With EXPR, it returns some extra information that the debugger uses to print a stack trace. The value of EXPR indicates how many call frames to go back before the current one.
($package, $filename, $line, $subroutine,
$hasargs, $wantarray, $evaltext, $is_require) = caller($i);
Here $subroutine may be "(eval)" if the frame is not a subroutine
call, but an eval(). In such a case additional elements $evaltext and
$is_require are set: $is_require is true if the frame is created by a
require or use statement, $evaltext contains the text of the
eval EXPR statement. In particular, for a eval BLOCK statement,
$filename is "(eval)", but $evaltext is undefined. (Note also that
each use statement creates a require frame inside an eval EXPR)
frame.
Furthermore, when called from within the DB package, caller returns more detailed information: it sets the list variable @DB::args to be the arguments with which the subroutine was invoked.
Be aware that the optimizer might have optimized call frames away before
caller() had a chance to get the information. That means that caller(N)
might not return information about the call frame you expect it do, for
N > 1. In particular, @DB::args might have information from the
previous time caller() was called.
$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 $mode, 'foo'; # this is best
while (<>) {
chomp; # avoid \n on last field
@array = split(/:/);
# ...
}
You can actually chomp anything that's an lvalue, including an assignment:
chomp($cwd = `pwd`);
chomp($answer = <STDIN>);
If you chomp a list, each element is chomped, and the total number of
characters removed is returned.
while (<>) {
chop; # avoid \n on last field
@array = split(/:/);
#...
}
You can actually chop anything that's an lvalue, including an assignment:
chop($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 "$user not in passwd file";
@ary = glob($pattern); # expand filenames
chown $uid, $gid, @ary;
On most systems, you are not allowed to change the ownership of the
file unless you're the superuser, although you should be able to change
the group to any of your secondary groups. On insecure systems, these
restrictions may be relaxed, but this is not a portable assumption.
If NUMBER is omitted, uses $_.
You don't have to close FILEHANDLE if you are immediately going to do another open() on it, because open() will close it for you. (See open().) However, an explicit close() on an input file resets the line counter ($.), while the implicit close done by open() does not.
If the file handle came from a piped open close() will additionally return FALSE if one of the other system calls involved fails or if the program exits with non-zero status. (If the only problem was that the program exited non-zero $! will be set to 0.) Also, closing a pipe waits 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 exit status value of the command into $?.
Example:
open(OUTPUT, '|sort >foo') # pipe to sort
or die "Can't start sort: $!";
#... # print stuff to output
close OUTPUT # wait for sort to finish
or warn $! ? "Error closing sort pipe: $!"
: "Exit status $? from sort";
open(INPUT, 'foo') # get sort's results
or die "Can't open 'foo' for input: $!";
FILEHANDLE may be an expression whose value can be used as an indirect
filehandle, usually the real filehandle name.
DIRHANDLE may be an expression whose value can be used as an indirect
dirhandle, usually the real dirhandle name.
last, next, or redo may appear within a continue block. last and redo will behave as if they had been executed within the main block. So will next, but since it will execute a continue block, it may be more entertaining.
while (EXPR) {
### redo always comes here
do_something;
} continue {
### next always comes here
do_something_else;
# then back the top to re-check EXPR
}
### last always comes here
Omitting the continue section is semantically equivalent to using an
empty one, logically enough. In that case, next goes directly back
to check the condition at the top of the loop.
For the inverse cosine operation, you may use the POSIX::acos() function, or use this relation:
sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
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 $pwd) {
die "Sorry...\n";
} else {
print "ok\n";
}
Of course, typing in your own password to whoever asks you
for it is unwise.
Breaks the binding between a DBM file and a hash.
This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a hash. HASH is the name of the hash. (Unlike normal open(), the first argument is NOT a filehandle, even though it looks like one). DBNAME is the name of the database (without the .dir or .pag extension if any). If the database does not exist, it is created with protection specified by MODE (as modified by the umask()). If your system supports only the older DBM functions, you may perform only one dbmopen() in your program. In older versions of Perl, if your system had neither DBM nor ndbm, calling dbmopen() produced a fatal error; it now falls back to sdbm(3).
If you don't have write access to the DBM file, you can only read hash variables, not set them. If you want to test whether you can write, either use file tests or try setting a dummy hash entry inside an eval(), which will trap the error.
Note that functions such as keys() and values() may return huge lists when used on large DBM files. You may prefer to use the each() function to iterate over large DBM files. Example:
# print out history file offsets
dbmopen(%HIST,'/usr/lib/news/history',0666);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
dbmclose(%HIST);
See also the AnyDBM_File manpage for a more general description of the pros and
cons of the various dbm approaches, as well as the DB_File manpage for a particularly
rich implementation.
Many operations return undef to indicate failure, end of file, system error, uninitialized variable, and other exceptional conditions. This function allows you to distinguish undef from other values. (A simple Boolean test will not distinguish among undef, zero, the empty string, and "0", which are all equally false.) Note that since undef is a valid scalar, its presence doesn't necessarily indicate an exceptional condition: pop() returns undef when its argument is an empty array, or when the element to return happens to be undef.
You may also use defined() to check whether a subroutine exists, by saying defined &func without parentheses. On the other hand, use of defined() upon aggregates (hashes and arrays) is not guaranteed to produce intuitive results, and should probably be avoided.
When used on a hash element, it tells you whether the value is defined, not whether the key exists in the hash. Use the exists entry elsewhere in this documentfor 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 shouldn't do that unless you don't
plan to use them again, because it saves time when you load them up
again to have memory already ready to be filled. The normal way to
free up space used by an aggregate is to assign the empty list.
This counterintuitive behavior of defined() on aggregates may be changed, fixed, or broken in a future release of Perl.
See also the undef, exists, and ref entries elsewhere in this document.
The following deletes all the values of a hash:
foreach $key (keys %HASH) {
delete $HASH{$key};
}
And so does this:
delete @HASH{keys %HASH}
(But both of these are slower than just assigning the empty list, or
using undef().) 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};
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().
If LIST is empty and $@ already contains a value (typically from a previous eval) that value is reused after appending "\t...propagated". This is useful for propagating exceptions:
eval { ... };
die unless $@ =~ /Expected exception/;
If $@ is empty then the string "Died" is used.
You can arrange for a callback to be run just before the die() does its deed, by setting the $SIG{__DIE__} hook. The associated handler will be called with the error text and can change the error message, if it sees fit, by calling die() again. See the section on $SIG{expr} in the perlvar manpage for details on setting %SIG entries, and the section on eval BLOCK for some examples.
Note that the $SIG{__DIE__} hook is called even inside eval()ed blocks/strings. If one wants the hook to do nothing in such situations, put
die @_ if $^S;as the first line of the handler (see the section on $^S in the perlvar manpage).
do 'stat.pl';is just like
scalar eval `cat stat.pl`;except that it's more efficient and concise, keeps track of the current filename for error messages, and searches all the -I libraries if the file isn't in the current directory (see also the @INC array in the section on Predefined Names in the perlvar manpage). It is also different in how code evaluated with do FILENAME doesn't see lexicals in the enclosing scope like eval STRING does. It's the same, however, in that it does reparse the file every time you call it, so you probably don't want to do this inside a loop.
If do cannot read the file, it returns undef and sets $! to the error. If do can read the file but cannot compile it, it returns undef and sets an error message in $@. If the file is successfully compiled, do returns the value of the last expression evaluated.
Note that inclusion of library modules is better done with the use() and require() operators, which also do automatic error checking and raise an exception if there's a problem.
You might like to use do to read in a program configuration file. Manual error checking can be done this way:
# read in config files: system first, then user
for $file ("/share/prog/defaults.rc",
"$ENV{HOME}/.someprogrc") {
unless ($return = do $file) {
warn "couldn't parse $file: $@" if $@;
warn "couldn't do $file: $!" unless defined $return;
warn "couldn't run $file" unless $return;
}
}
Example:
#!/usr/bin/perl
require 'getopt.pl';
require 'stat.pl';
%days = (
'Sun' => 1,
'Mon' => 2,
'Tue' => 3,
'Wed' => 4,
'Thu' => 5,
'Fri' => 6,
'Sat' => 7,
);
dump QUICKSTART if $ARGV[0] eq '-d';
QUICKSTART:
Getopt('f');
This operator is largely obsolete, partly because it's very hard to
convert a core file into an executable, and because the real perl-to-C
compiler has superseded it.
Entries are returned in an apparently random order. When the hash is entirely read, a null array is returned in list context (which when assigned produces a FALSE (0) value), and undef in scalar context. The next call to each() after that will start iterating again. There is a single iterator for each hash, shared by all each(), keys(), and values() function calls in the program; it can be reset by reading all the elements from the hash, or by evaluating keys HASH or values HASH. If you add or delete elements of a hash while you're iterating over it, you may get entries skipped or duplicated, so don't.
The following prints out your environment like the printenv(1) program, only in a different order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}
See also keys() and values().
An eof without an argument uses the last file read as argument. Using eof() with empty parentheses is very different. It indicates the pseudo file formed of the files listed on the command line, i.e., eof() is reasonable to use inside a while (<>) loop to detect the end of only the last file. Use eof(ARGV) or eof without the parentheses to test EACH file in a while (<>) loop. Examples:
# reset line numbering on each input file
while (<>) {
next if /^\s*#/; # skip comments
print "$.\t$_";
} continue {
close ARGV if eof; # Not eof()!
}
# insert dashes just before last line of last file
while (<>) {
if (eof()) { # check for end of current file
print "--------------\n";
close(ARGV); # close or 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 false values when they run out of data, or if there
was an error.
In the second form, the code within the BLOCK is parsed only once---at the same time the code surrounding the eval itself was parsed---and executed within the context of the current Perl program. This form is typically used to trap exceptions more efficiently than the first (see below), while also providing the benefit of checking the code within BLOCK at compile time.
The final semicolon, if any, may be omitted from the value of EXPR or within the BLOCK.
In both forms, the value returned is the value of the last expression evaluated inside the mini-program; a return statement may be also used, just as with subroutines. The expression providing the return value is evaluated in void, scalar, or list context, depending on the context of the eval itself. See the wantarray entry elsewhere in this documentfor more on how the evaluation context can be determined.
If there is a syntax error or runtime error, or a die() statement is executed, an undefined value is returned by eval(), and $@ is set to the error message. If there was no error, $@ is guaranteed to be a null string. Beware that using eval() neither silences perl from printing warnings to STDERR, nor does it stuff the text of warning messages into $@. To do either of those, you have to use the $SIG{__WARN__} facility. See the warn entry elsewhere in this document and the perlvar manpage.
Note that, because eval() traps otherwise-fatal errors, it is useful for determining whether a particular feature (such as socket() or symlink()) is implemented. It is also Perl's exception trapping mechanism, where the die operator is used to raise exceptions.
If the code to be executed doesn't vary, you may use the eval-BLOCK form to trap run-time errors without incurring the penalty of recompiling each time. The error, if any, is still returned in $@. Examples:
# make divide-by-zero nonfatal
eval { $answer = $a / $b; }; warn $@ if $@;
# same thing, but less efficient
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
eval { $answer = }; # WRONG
# a run-time error
eval '$answer ='; # sets $@
When using the eval{} form as an exception trap in libraries, you may
wish not to trigger any __DIE__ hooks that user code may have
installed. You can use the local $SIG{__DIE__} construct for this
purpose, as shown in this example:
# a very private exception trap for divide-by-zero
eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
warn $@ if $@;
This is especially significant, given that __DIE__ hooks can call
die() again, which has the effect of changing their error messages:
# __DIE__ hooks may modify error messages
{
local $SIG{'__DIE__'} =
sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
eval { die "foo lives here" };
print $@ if $@; # prints "bar lives here"
}
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.
Since it's a common mistake to use exec() instead of system(), Perl warns you if there is a following statement which isn't die(), warn(), or exit() (if -w is set - but you always do that). If you really want to follow an exec() with some other statement, you can use one of these styles to avoid the warning:
exec ('foo') or print STDERR "couldn't exec foo: $!";
{ exec ('foo') }; print STDERR "couldn't exec foo: $!";
If there is more than one argument in LIST, or if LIST is an array
with more than one value, calls execvp(3) with the arguments in LIST.
If there is only one scalar argument or an array with one element in it,
the argument is checked for shell metacharacters, and if there are any,
the entire argument is passed to the system's command shell for parsing
(this is /bin/sh -c on Unix platforms, but varies on other platforms).
If there are no shell metacharacters in the argument, it is split into
words and passed directly to execvp(), which is more efficient. Note:
exec() and system() do not flush your output buffer, so you may need to
set $| to avoid lost output. Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";
If you don't really want to execute the first argument, but want to lie
to the program you are executing about its own name, you can specify
the program you actually want to run as an ``indirect object'' (without a
comma) in front of the LIST. (This always forces interpretation of the
LIST as a multivalued list, even if there is only a single scalar in
the list.) Example:
$shell = '/bin/csh';
exec $shell '-sh'; # pretend it's a login shell
or, more directly,
exec {'/bin/csh'} '-sh'; # pretend it's a login shell
When the arguments get executed via the system shell, results will
be subject to its quirks and capabilities. See the section on `STRING` in the perlop manpage
for details.
Using an indirect object with exec() or system() is also more secure. This usage forces interpretation of the arguments as a multivalued list, even if the list had just one argument. That way you're safe from the shell expanding wildcards or splitting up words with whitespace in them.
@args = ( "echo surprise" );
system @args; # subject to shell escapes
# if @args == 1
system { $args[0] } @args; # safe even with one-arg list
The first version, the one without the indirect object, ran the echo
program, passing it "surprise" an argument. The second version
didn't---it tried to run a program literally called "echo surprise'',
didn't find it, and set $? to a non-zero value indicating failure.
Note that exec() will not call your END blocks, nor will it call
any DESTROY methods in your objects.
print "Exists\n" if exists $array{$key};
print "Defined\n" if defined $array{$key};
print "True\n" if $array{$key};
A hash element can be TRUE only if it's defined, and defined if
it exists, but the reverse doesn't necessarily hold true.
Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash key lookup:
if (exists $ref->{"A"}{"B"}{$key}) { ... }
Although the last element will not spring into existence just because its
existence was tested, intervening ones will. Thus $ref->{"A"}
$ref->{"B"} will spring into existence due to the existence
test for a $key element. This autovivification may be fixed in a later
release.
$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().
All END{} blocks are run at exit time. See the perlsub manpage for details.
use Fcntl;first to get the correct constant definitions. Argument processing and value return works just like ioctl() below. For example:
use Fcntl;
fcntl($filehandle, F_GETFL, $packed_return_buffer)
or die "can't fcntl F_GETFL: $!";
You don't have to check for defined() on the return from
fnctl(). Like ioctl(), it maps a 0 return from the system
call into ``0 but true'' in Perl. This string is true in
boolean context and 0 in numeric context. It is also
exempt from the normal -w warnings on improper numeric
conversions.
Note that fcntl() will produce a fatal error if used on a machine that
doesn't implement fcntl(2).
You can use this to find out whether two handles refer to the same underlying descriptor:
if (fileno(THIS) == fileno(THAT)) {
print "THIS and THAT are dups\n";
}
On many platforms (including most versions or clones of Unix), locks established by flock() are merely advisory. Such discretionary locks are more flexible, but offer fewer guarantees. This means that files locked with flock() may be modified by programs that do not also use flock(). Windows NT and OS/2 are among the platforms which enforce mandatory locking. See your local documentation for details.
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.
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, then 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.
format Something =
Test: @<<<<<<<< @||||| @>>>>>
$str, $%, '$' . int($num)
.
$str = "widget";
$num = $cost/$quantity;
$~ = 'Something';
write;
See the perlform manpage for many details and examples.
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
purporting POSIX compliance.
See also the Term::ReadKey module from your nearest CPAN site;
details on CPAN can be found on the CPAN entry in the perlmod manpage.
$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);
($name,$passwd,$uid,$gid,
$quota,$comment,$gcos,$dir,$shell,$expire) = getpw*
($name,$passwd,$gid,$members) = getgr*
($name,$aliases,$addrtype,$length,@addrs) = gethost*
($name,$aliases,$addrtype,$net) = getnet*
($name,$aliases,$proto) = getproto*
($name,$aliases,$port,$proto) = getserv*
(If the entry doesn't exist you get a null list.)
In scalar context, you get the name, unless the function was a lookup by name, in which case you get the other thing, whatever it is. (If the entry doesn't exist you get the undefined value.) For example:
$uid = getpwnam($name);
$name = getpwuid($num);
$name = getpwent();
$gid = getgrnam($name);
$name = getgrgid($num;
$name = getgrent();
#etc.
In getpw*() the fields $quota, $comment, and $expire are special
cases in the sense that in many systems they are unsupported. If the
$quota is unsupported, it is an empty scalar. If it is supported, it
usually encodes the disk quota. If the $comment field is unsupported,
it is an empty scalar. If it is supported it usually encodes some
administrative comment about the user. In some systems the $quota
field may be $change or $age, fields that have to do with password
aging. In some systems the $comment field may be $class. The $expire
field, if present, encodes the expiration period of the account or the
password. For the availability and the exact meaning of these fields
in your system, please consult your getpwnam(3) documentation and your
pwd.h file. You can also find out from within Perl which meaning
your $quota and $comment fields have and whether you have the $expire
field by using the Config module and the values d_pwquota, d_pwage,
d_pwchange, d_pwcomment, and d_pwexpire.
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]);
If you get tired of remembering which element of the return list contains
which return value, by-name interfaces are also provided in modules:
File::stat, Net::hostent, Net::netent, Net::protoent, Net::servent,
Time::gmtime, Time::localtime, and User::grent. These override the
normal built-in, replacing them with versions that return objects with
the appropriate names for each field. For example:
use File::stat; use User::pwent; $is_his = (stat($filename)->uid == pwent($whoever)->uid);Even though it looks like they're the same method calls (uid), they aren't, because a File::stat object is different from a User::pwent object.
use Socket;
$mysockaddr = getsockname(SOCK);
($port, $myaddr) = unpack_sockaddr_in($mysockaddr);
# 0 1 2 3 4 5 6 7 8
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
gmtime(time);
All array elements are numeric, and come straight out of a struct tm.
In particular this means that $mon has the range 0..11 and $wday has
the range 0..6 with sunday as day 0. Also, $year is the number of
years since 1900, that is, $year is 123 in year 2023, not simply the last two digits of the year.
If EXPR is omitted, does gmtime(time()).
In scalar context, returns the ctime(3) value:
$now_string = gmtime; # e.g., "Thu Oct 13 04:54:34 1994"Also see the timegm() function provided by the Time::Local module, and the strftime(3) function available via the POSIX module.
This scalar value is not locale dependent, see the perllocale manpage, but instead a Perl builtin. Also see the Time::Local module, and the strftime(3) and mktime(3) function available via the POSIX module. To get somewhat similar but locale dependent date strings, set up your locale environment variables appropriately (please see the perllocale manpage) and try for example:
use POSIX qw(strftime);
$now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
Note that the %a and %b, the short forms of the day of the week
and the month of the year, may not necessarily be three characters wide.
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.
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 commentsor 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 a for loop's index variable aliases the list
elements. That is, modifying an element of a list returned by grep
(for example, in a foreach, map() or another grep())
actually modifies the element in the original list.
See also the map entry elsewhere in this documentfor an array composed of the results of the BLOCK or EXPR.
print hex '0xAf'; # prints '175'
print hex 'aF'; # same
require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.phfirst to get the correct function definitions. If ioctl.ph doesn't exist or doesn't have the correct definitions you'll have to roll your own, based on your C header files such as <sys/ioctl.h>. (There is a Perl script called h2ph that comes with the Perl kit that may help you in this, but it's nontrivial.) SCALAR will be read and/or written depending on the FUNCTION---a pointer to the string value of SCALAR will be passed as the third argument of the actual ioctl() call. (If SCALAR has no string value but does have a numeric value, that value will be passed rather than a pointer to the string value. To guarantee this to be TRUE, add a 0 to the scalar before using it.) The pack() and unpack() functions are useful for manipulating the values of structures used by ioctl(). The following example sets the erase character to DEL.
require 'ioctl.ph';
$getp = &TIOCGETP;
die "NO TIOCGETP" if $@ || !$getp;
$sgttyb_t = "ccccs"; # 4 chars and a short
if (ioctl(STDIN,$getp,$sgttyb)) {
@ary = unpack($sgttyb_t,$sgttyb);
$ary[2] = 127;
$sgttyb = pack($sgttyb_t,@ary);
ioctl(STDIN,&TIOCSETP,$sgttyb)
|| die "Can't ioctl: $!";
}
The return value of ioctl() (and fcntl()) is as follows:
if OS returns: then Perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
Thus Perl returns TRUE on success and FALSE on failure, yet you can
still easily determine the actual value returned by the operating
system:
($retval = ioctl(...)) || ($retval = -1);
printf "System returned %d\n", $retval;
The special string ``0 but true'' is excempt from -w complaints
about improper numeric conversions.
$_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
See the split entry elsewhere in this document.
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---256 of them, in fact, since it rounds up to the next power of two. These buckets will be retained even if you do %hash = (), use undef %hash if you want to free the storage while %hash is still in scope. You can't shrink the number of buckets allocated for the hash using keys() in this way (but you needn't worry about doing this by accident, as trying has no effect).
$cnt = kill 1, $child1, $child2;
kill 9, @goners;
Unlike in the shell, in Perl if the SIGNAL is negative, it kills
process groups instead of processes. (On System V, a negative PROCESS
number will also kill process groups, but that's not portable.) That
means you usually want to use positive not negative signals. You may also
use a signal name in quotes. See the section on Signals in the perlipc manpage for details.
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
#...
}
See also the continue entry elsewhere in this documentfor an illustration of how last, next, and
redo work.
If EXPR is omitted, uses $_.
If EXPR is omitted, uses $_.
You really probably want to be using my() instead, because local() isn't
what most people think of as ``local''. See the section on Private Variables
via my() in the perlsub manpage for details.
# 0 1 2 3 4 5 6 7 8
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
localtime(time);
All array elements are numeric, and come straight out of a struct tm.
In particular this means that $mon has the range 0..11 and $wday has
the range 0..6 with sunday as day 0. Also, $year is the number of
years since 1900, that is, $year is 123 in year 2023, and not simply the last two digits of the year.
If EXPR is omitted, uses the current time (localtime(time)).
In scalar context, returns the ctime(3) value:
$now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"This scalar value is not locale dependent, see the perllocale manpage, but instead a Perl builtin. Also see the Time::Local module, and the strftime(3) and mktime(3) function available via the POSIX module. To get somewhat similar but locale dependent date strings, set up your locale environment variables appropriately (please see the perllocale manpage) and try for example:
use POSIX qw(strftime);
$now_string = strftime "%a %b %e %H:%M:%S %Y", localtime;
Note that the %a and %b, the short forms of the day of the week
and the month of the year, may not necessarily be three characters wide.
If EXPR is omitted, stats $_.
@chars = map(chr, @nums);translates a list of numbers to the corresponding characters. And
%hash = map { getkey($_) => $_ } @array;
is just a funny way to write
%hash = ();
foreach $_ (@array) {
$hash{getkey($_)} = $_;
}
Note that, because $_ is a reference into the list value, it can be used
to modify the elements of the array. While this is useful and
supported, it can cause bizarre results if the LIST is not a named
array. See also the grep entry elsewhere in this documentfor an array composed of those items of the
original list for which the BLOCK or EXPR evaluates to true.
use IPC::SysV;first to get the correct constant definitions. If CMD is IPC_STAT, then ARG must be a variable which will hold the returned msqid_ds structure. Returns like ioctl(): the undefined value for error, ``0 but true'' for zero, or the actual return value otherwise. See also IPC::SysV and IPC::Semaphore::Msg documentation.
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.
See also the continue entry elsewhere in this documentfor an illustration of how last, next, and
redo work.
$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, being created if necessary. If the filename begins with '>>', the file is opened for appending, again being created if necessary. You can put a '+' in front of the '>' or '<' to indicate that you want both read and write access to the file; thus '+<' is almost always preferred for read/write updates---the '+>' mode would clobber the file first. You can't usually use either read-write mode for updating textfiles, since they have variable length records. See the -i switch in the perlrun manpage for a better approach.
The 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 the section on Using open() for IPC in the perlipc manpage for more examples of this. (You are not allowed to open() to a command that pipes both in and out, but see the IPC::Open2 manpage, the IPC::Open3 manpage, and the section on Bidirectional Communication in the perlipc manpage for alternatives.)
Opening '-' opens STDIN and opening '>-' opens STDOUT. Open returns nonzero upon success, the undefined value otherwise. If the open() involved a pipe, the return value happens to be the pid of the subprocess.
If you're unfortunate enough to be running Perl on a system that distinguishes between text files and binary files (modern operating systems don't care), then you should check out the binmode entry elsewhere in this documentfor tips for dealing with this. The key distinction between systems that need binmode() and those that don't is their text file formats. Systems like Unix, MacOS, and Plan9, which delimit lines with a single character, and which encode that character in C as "\n", do not need binmode(). The rest need it.
When opening a file, it's usually a bad idea to continue normal execution if the request failed, so open() is frequently used in connection with die(). Even if die() won't do what you want (say, in a CGI script, where you want to make a nicely formatted error message (but there are modules that can help with that problem)) you should always check the return value from opening a file. The infrequent exception is when working with an unopened filehandle is actually what you want to do.
Examples:
$ARTICLE = 100;
open ARTICLE or die "Can't find article $ARTICLE: $!\n";
while (<ARTICLE>) {...
open(LOG, '>>/usr/spool/news/twitlog'); # (log is reserved)
# if the open fails, output is discarded
open(DBASE, '+<dbase.mine') # open for update
or die "Can't open 'dbase.mine' for update: $!";
open(ARTICLE, "caesar <$article |") # decrypt article
or die "Can't start caesar: $!";
open(EXTRACT, "|sort >/tmp/Tmp$$") # $$ is our process id
or die "Can't start sort: $!";
# process argument list of files along with any includes
foreach $file (@ARGV) {
process($file, 'fh00');
}
sub process {
my($filename, $input) = @_;
$input++; # this is a string increment
unless (open($input, $filename)) {
print STDERR "Can't open $filename: $!\n";
return;
}
local $_;
while (<$input>) { # note use of indirection
if (/^#include "(.*)"/) {
process($1, $input);
next;
}
#... # whatever
}
}
You may also, in the Bourne shell tradition, specify an EXPR beginning
with '>&', in which case the rest of the string is interpreted as the
name of a filehandle (or file descriptor, if numeric) to be
duped and opened. You may use & after >, >>, <, +>,
+>>, and +<. The
mode you specify should match the mode of the original filehandle.
(Duping a filehandle does not take into account any existing contents of
stdio buffers.)
Here is a script that saves, redirects, and restores STDOUT and
STDERR:
#!/usr/bin/perl
open(OLDOUT, ">&STDOUT");
open(OLDERR, ">&STDERR");
open(STDOUT, ">foo.out") || die "Can't redirect stdout";
open(STDERR, ">&STDOUT") || die "Can't dup stdout";
select(STDERR); $| = 1; # make unbuffered
select(STDOUT); $| = 1; # make unbuffered
print STDOUT "stdout 1\n"; # this works for
print STDERR "stderr 1\n"; # subprocesses too
close(STDOUT);
close(STDERR);
open(STDOUT, ">&OLDOUT");
open(STDERR, ">&OLDERR");
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
If you specify '<&=N', where N is a number, then Perl will do an
equivalent of C's fdopen() of that file descriptor; this is more
parsimonious of file descriptors. For example:
open(FILEHANDLE, "<&=$fd")If you open a pipe on the command '-', i.e., either '|-' or '-|', then there is an implicit fork done, and the return value of open is the pid of the child within the parent process, and 0 within the child process. (Use defined($pid) to determine whether the open was successful.) The filehandle behaves normally for the parent, but i/o to that filehandle is piped from/to the STDOUT/STDIN of the child process. In the child process the filehandle isn't opened---i/o happens from/to the new STDOUT or STDIN. Typically this is used like the normal piped open when you want to exercise more control over just how the pipe command gets executed, such as when you are running setuid, and don't want to have to scan shell commands for metacharacters. The following pairs are more or less equivalent:
open(FOO, "|tr '[a-z]' '[A-Z]'");
open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]';
open(FOO, "cat -n '$file'|");
open(FOO, "-|") || exec 'cat', '-n', $file;
See the section on Safe Pipe Opens in the perlipc manpage for more examples of this.
NOTE: On any operation that may do a fork, any unflushed buffers remain unflushed in both processes, which means you may need to set $| to avoid duplicate output.
Closing any piped filehandle causes the parent process to wait for the child to finish, and returns the status value in $?.
The filename passed to open will have leading and trailing whitespace deleted, and the normal redirection characters honored. This property, known as ``magic open'', can often be used to good effect. A user could specify a filename of "rsh cat file |'', or you could change certain filenames as needed:
$filename =~ s/(.*\.gz)\s*$/gzip -dc < $1|/;
open(FH, $filename) or die "Can't open $filename: $!";
However, to open a file with arbitrary weird characters in it, it's
necessary to protect any leading and trailing whitespace:
$file =~ s#^(\s)#./$1#;
open(FOO, "< $file\0");
If you want a ``real'' C open() (see the open(2) manpage on your system), then you
should use the sysopen() function, which involves no such magic. This is
another way to protect your filenames from interpretation. For example:
use IO::Handle;
sysopen(HANDLE, $path, O_RDWR|O_CREAT|O_EXCL)
or die "sysopen $path: $!";
$oldfh = select(HANDLE); $| = 1; select($oldfh);
print HANDLE "stuff $$\n");
seek(HANDLE, 0, 0);
print "File contains: ", <HANDLE>;
Using the constructor from the IO::Handle package (or one of its
subclasses, such as IO::File or IO::Socket), you can generate anonymous
filehandles that have the scope of whatever variables hold references to
them, and automatically close whenever and however you leave that scope:
use IO::File;
#...
sub read_myfile_munged {
my $ALL = shift;
my $handle = new IO::File;
open($handle, "myfile") or die "myfile: $!";
$first = <$handle>
or return (); # Automatically closed here.
mung $first or die "mung failed"; # Or here.
return $first, <$handle> if $ALL; # Or here.
$first; # Or here.
}
See the seek() entry elsewhere in this documentfor some details about mixing reading and writing.
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 giving 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" type packs a pointer to a null-
terminated string. You are responsible for ensuring the string is not a
temporary value (which can potentially get deallocated before you get
around to using the packed result). The "P" packs a pointer to a structure
of the size indicated by the length. A NULL pointer is created if the
corresponding value for "p" or "P" is undef.
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.
If NAMESPACE is omitted, then there is no current package, and all identifiers must be fully qualified or lexicals. This is stricter than use strict, since it also extends to function names.
See the section on Packages in the perlmod manpage for more information about packages, modules,
and classes. See the perlsub manpage for other scoping issues.
See the IPC::Open2 manpage, the IPC::Open3 manpage, and the section on Bidirectional Communication in the perlipc manpage
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().
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";
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.
If FUNCTION is a string starting with CORE::, the rest is taken as
a name for Perl builtin. If builtin is not overridable (such as
qw//) or its arguments cannot be expressed by a prototype (such as
system()) - in other words, the builtin does not behave like a Perl
function - returns undef. Otherwise, the string describing the
equivalent prototype is returned.
for $value (LIST) {
$ARRAY[++$#ARRAY] = $value;
}
but is more efficient. Returns the new number of elements in the array.
If EXPR is omitted, uses $_.
(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.)
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;
$line = <STDIN>;
$line = readline(*STDIN); # same thing
# 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;
}
See also the continue entry elsewhere in this documentfor an illustration of how last, next, and
redo work.
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.
Otherwise, demands that a library file be included if it hasn't already been included. The file is included via the do-FILE mechanism, which is essentially just a variety of eval(). Has semantics similar to the following subroutine:
sub require {
my($filename) = @_;
return 1 if $INC{$filename};
my($realfilename,$result);
ITER: {
foreach $prefix (@INC) {
$realfilename = "$prefix/$filename";
if (-f $realfilename) {
$result = do $realfilename;
last ITER;
}
}
die "Can't find $filename in \@INC";
}
die $@ if $@;
die "$filename did not return true value" unless $result;
$INC{$filename} = $realfilename;
return $result;
}
Note that the file will not be included twice under the same specified
name. The file must return TRUE as the last statement to indicate
successful execution of any initialization code, so it's customary to
end such a file with ``1;'' unless you're sure it'll return TRUE
otherwise. But it's better just to put the ``1;'', in case you add more
statements.
If EXPR is a bareword, the require assumes a ``.pm'' extension and replaces ``::'' with ``/'' in the filename for you, to make it easy to load standard modules. This form of loading of modules does not risk altering your namespace.
In other words, if you try this:
require Foo::Bar; # a splendid barewordThe require function will actually look for the ``Foo/Bar.pm'' file in the directories specified in the @INC array.
But if you try this:
$class = 'Foo::Bar';
require $class; # $class is not a bareword
#or
require "Foo::Bar"; # not a bareword because of the ""
The require function will look for the ``Foo::Bar'' file in the @INC array and
will complain about not finding ``Foo::Bar'' there. In this case you can do:
eval "require $class";For a yet-more-powerful import facility, see the use entry elsewhere in this document 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 @INC arrays and your %ENV hash. Resets only package variables---lexical variables
are unaffected, but they clean themselves up on scope exit anyway,
so you'll probably want to use them instead. See the my entry elsewhere in this document.
(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 $/; # for efficiency of <>
print scalar reverse <>; # byte tac, last line tsrif
This operator is also handy for inverting a hash, although there are some
caveats. If a value is duplicated in the original hash, only one of those
can be represented as a key in the inverted hash. Also, this has to
unwind one hash and build a whole new one, which may take some time
on a large hash.
%by_name = reverse %by_address; # Invert the hash
@counts = ( scalar @a, scalar @b, scalar @c );There is no equivalent operator to force an expression to be interpolated in list context because it's in practice never needed. If you really wanted to do so, however, you could use the construction @{[ (some expression) ]}, but usually a simple (some expression) suffices.
If you want to position file for sysread() or syswrite(), don't use seek() -- buffering makes its effect on the file's system position unpredictable and non-portable. Use sysseek() instead.
On some systems you have to do a seek whenever you switch between reading and writing. Amongst other things, this may have the effect of calling stdio's clearerr(3). A WHENCE of 1 (SEEK_CUR) is useful for not moving the file position:
seek(TEST,0,1);This is also useful for applications emulating tail -f. Once you hit EOF on your read, and then sleep for a while, you might have to stick in a seek() to reset things. The seek() doesn't change the current position, but it does clear the end-of-file condition on the handle, so that the next <FILE> makes Perl try again to read something. We hope.
If that doesn't work (some stdios are particularly cantankerous), then you may need something more like this:
for (;;) {
for ($curpos = tell(FILE); $_ = <FILE>;
$curpos = tell(FILE)) {
# search for some stuff and put it into files
}
sleep($for_a_while);
seek(FILE, $curpos, 0);
}
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);
$rin = $win = $ein = '';
vec($rin,fileno(STDIN),1) = 1;
vec($win,fileno(STDOUT),1) = 1;
$ein = $rin | $win;
If you want to select on many filehandles you might wish to write a
subroutine:
sub fhbits {
my(@fhlist) = split(' ',$_[0]);
my($bits);
for (@fhlist) {
vec($bits,fileno($_),1) = 1;
}
$bits;
}
$rin = fhbits('STDIN TTY SOCK');
The usual idiom is:
($nfound,$timeleft) =
select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
or to block until something becomes ready just do this
$nfound = select($rout=$rin, $wout=$win, $eout=$ein, undef);Most systems do not bother to return anything useful in $timeleft, so calling select() in scalar context just returns $nfound.
Any of the bit masks can also be undef. The timeout, if specified, is in seconds, which may be fractional. Note: not all implementations are capable of returning the$timeleft. If not, they always return $timeleft equal to the supplied $timeout.
You can effect a sleep of 250 milliseconds this way:
select(undef, undef, undef, 0.25);WARNING: One should not attempt to mix buffered I/O (like read() or <FH>) with select(), except as permitted by POSIX, and even then only on POSIX systems. You have to use sysread() instead.
use IPC::SysV;first to get the correct constant definitions. If CMD is IPC_STAT or GETALL, then ARG must be a variable which will hold the returned semid_ds structure or semaphore value array. Returns like ioctl(): the undefined value for error, ``0 but true'' for zero, or the actual return value otherwise. See also IPC::SysV and IPC::Semaphore documentation.
$semop = pack("sss", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid, $semop);
To signal the semaphore, replace -1 with 1. See also IPC::SysV
and IPC::SysV::Semaphore documentation.
use IPC::SysV;first to get the correct constant definitions. If CMD is IPC_STAT, then ARG must be a variable which will hold the returned shmid_ds structure. Returns like ioctl: the undefined value for error, ``0 but true'' for zero, or the actual return value otherwise. See also IPC::SysV documentation.
shutdown(SOCKET, 0); # I/we have stopped reading data
shutdown(SOCKET, 1); # I/we have stopped writing data
shutdown(SOCKET, 2); # I/we have stopped using this socket
This is useful with sockets when you want to tell the other
side you're done writing but not done reading, or vice versa.
It's also a more insistent form of close because it also
disables the filedescriptor in any forked copies in other
processes.
For the inverse sine operation, you may use the POSIX::asin() function, or use this relation:
sub asin { atan2($_[0], sqrt(1 - $_[0] * $_[0])) }
On some older systems, it may sleep up to a full second less than what you requested, depending on how it counts seconds. Most modern systems always sleep the full amount. They may appear to sleep longer than that, however, because your process might not be scheduled right away in a busy multitasking system.
For delays of finer granularity than one second, you may use Perl's syscall() interface to access setitimer(2) if your system supports it, or else see the select() entry elsewhere in this documentabove.
See also the POSIX module's sigpause() function.
Some systems defined pipe() in terms of socketpair(), in which a call to pipe(Rdr, Wtr) is essentially:
use Socket;
socketpair(Rdr, Wtr, AF_UNIX, SOCK_STREAM, PF_UNSPEC);
shutdown(Rdr, 1); # no more writing for reader
shutdown(Wtr, 0); # no more reading for writer
See the perlipc manpage for an example of socketpair use.
In the interests of efficiency the normal calling code for subroutines is bypassed, with the following effects: the subroutine may not be a recursive subroutine, and the two elements to be compared are passed into the subroutine not via @_ but as the package global variables $a and $b (see example below). They are passed by reference, so don't modify $a and $b. And don't try to declare them as lexicals either.
You also cannot exit out of the sort block or subroutine using any of the loop control operators described in the perlsyn manpage or with goto().
When use locale is in effect, sort LIST sorts LIST according to the current collation locale. See the perllocale manpage.
Examples:
# sort lexically
@articles = sort @files;
# same thing, but with explicit sort routine
@articles = sort {$a cmp $b} @files;
# now case-insensitively
@articles = sort {uc($a) cmp uc($b)} @files;
# same thing in reversed order
@articles = sort {$b cmp $a} @files;
# sort numerically ascending
@articles = sort {$a <=> $b} @files;
# sort numerically descending
@articles = sort {$b <=> $a} @files;
# sort using explicit subroutine name
sub byage {
$age{$a} <=> $age{$b}; # presuming numeric
}
@sortedclass = sort byage @class;
# this sorts the %age hash by value instead of key
# using an in-line function
@eldest = sort { $age{$b} <=> $age{$a} } keys %age;
sub backwards { $b cmp $a; }
@harry = ('dog','cat','x','Cain','Abel');
@george = ('gone','chased','yz','Punished','Axed');
print sort @harry;
# prints AbelCaincatdogx
print sort backwards @harry;
# prints xdogcatCainAbel
print sort @george, 'to', @harry;
# prints AbelAxedCainPunishedcatchaseddoggonetoxyz
# inefficiently sort by descending numeric compare using
# the first integer after the first = sign, or the
# whole record case-insensitively otherwise
@new = sort {
($b =~ /=(\d+)/)[0] <=> ($a =~ /=(\d+)/)[0]
||
uc($a) cmp uc($b)
} @old;
# same thing, but much more efficiently;
# we'll build auxiliary indices instead
# for speed
@nums = @caps = ();
for (@old) {
push @nums, /=(\d+)/;
push @caps, uc($_);
}
@new = @old[ sort {
$nums[$b] <=> $nums[$a]
||
$caps[$a] cmp $caps[$b]
} 0..$#old
];
# same thing using a Schwartzian Transform (no temps)
@new = map { $_->[0] }
sort { $b->[1] <=> $a->[1]
||
$a->[2] cmp $b->[2]
} map { [$_, /=(\d+)/, uc($_)] } @old;
If you're using strict, you MUST NOT declare $a
and $b as lexicals. They are package globals. That means
if you're in the main package, it's
@articles = sort {$main::b <=> $main::a} @files;
or just
@articles = sort {$::b <=> $::a} @files;
but if you're in the FooPack package, it's
@articles = sort {$FooPack::b <=> $FooPack::a} @files;
The comparison function is required to behave. If it returns
inconsistent results (sometimes saying $x[1] is less than $x[2] and
sometimes saying the opposite, for example) the results are not
well-defined.
push(@a,$x,$y) splice(@a,@a,0,$x,$y)
pop(@a) splice(@a,-1)
shift(@a) splice(@a,0,1)
unshift(@a,$x,$y) splice(@a,0,0,$x,$y)
$a[$x] = $y splice(@a,$x,1,$y)
Example, assuming array lengths are passed before arrays:
sub aeq { # compare two list values
my(@a) = splice(@_,0,shift);
my(@b) = splice(@_,0,shift);
return 0 unless @a == @b; # same len?
while (@a) {
return 0 if pop(@a) ne pop(@b);
}
return 1;
}
if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
If not in list context, returns the number of fields found and splits into the @_ array. (In list context, you can force the split into @_ by using ?? as the pattern delimiters, but it still returns the list value.) The use of implicit split to @_ is deprecated, however, because it clobbers your subroutine arguments.
If EXPR is omitted, splits the $_ string. If PATTERN is also omitted, splits on whitespace (after skipping any leading whitespace). Anything matching PATTERN is taken to be a delimiter separating the fields. (Note that the delimiter may be longer than one character.)
If LIMIT is specified and positive, splits into no more than that many fields (though it may split into fewer). If LIMIT is unspecified or zero, trailing null fields are stripped (which potential users of pop() would do well to remember). If LIMIT is negative, it is treated as if an arbitrarily large LIMIT had been specified.
A pattern matching the null string (not to be confused with a null pattern //, which is just one member of the set of patterns matching a null string) will split the value of EXPR into separate characters at each point it matches that way. For example:
print join(':', split(/ */, 'hi there'));
produces the output `h:i:t:h:e:r:e'.
The LIMIT parameter can be used to split a line partially
($login, $passwd, $remainder) = split(/:/, $_, 3);When assigning to a list, if LIMIT is omitted, Perl supplies a LIMIT one larger than the number of variables in the list, to avoid unnecessary work. For the list above LIMIT would have been 4 by default. In time critical applications it behooves you not to split into more fields than you really need.
If the PATTERN contains parentheses, additional array elements are created from each matching substring in the delimiter.
split(/([,-])/, "1-10,20", 3);produces the list value
(1, '-', 10, ',', 20)If you had the entire header of a normal Unix email message in $header, you could split it up into fields and their values this way:
$header =~ s/\n\s+/ /g; # fix continuation lines
%hdrs = (UNIX_FROM => split /^(\S*?):\s*/m, $header);
The pattern /PATTERN/ may be replaced with an expression to specify
patterns that vary at runtime. (To do runtime compilation only once,
use /$variable/o.)
As a special case, specifying a PATTERN of space (' ') will split on white space just as split() with no arguments does. Thus, split(' ') can be used to emulate awk's default behavior, whereas split(/ /) will give you as many null initial fields as there are leading spaces. A split() on /\s+/ is like a split(' ') except that any leading whitespace produces a null first field. A split() with no arguments really does a split(' ', $_) internally.
Example:
open(PASSWD, '/etc/passwd');
while (<PASSWD>) {
($login, $passwd, $uid, $gid,
$gcos, $home, $shell) = split(/:/);
#...
}
(Note that $shell above will still have a newline on it. See the chop, chomp, and join entries elsewhere in this document.)
Perl does its own sprintf() formatting -- it emulates the C function sprintf(), but it doesn't use it (except for floating-point numbers, and even then only the standard modifiers are allowed). As a result, any non-standard extensions in your local sprintf() are not available from Perl.
Perl's sprintf() permits the following universally-known conversions:
%% a percent sign %c a character with the given number %s a string %d a signed integer, in decimal %u an unsigned integer, in decimal %o an unsigned integer, in octal %x an unsigned integer, in hexadecimal %e a floating-point number, in scientific notation %f a floating-point number, in fixed decimal notation %g a floating-point number, in %e or %f notationIn 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 %fPerl permits the following universally-known flags between the % and the conversion letter:
space prefix positive number with a space
+ prefix positive number with a plus sign
- left-justify within the field
0 use zeros, not spaces, to right-justify
# prefix non-zero octal with "0", non-zero hex with "0x"
number minimum field width
.number "precision": digits after decimal point for
floating-point, max length for string, minimum length
for integer
l interpret integer as C type "long" or "unsigned long"
h interpret integer as C type "short" or "unsigned short"
There is also one Perl-specific flag:
V interpret integer as Perl's standard integer typeWhere 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.
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.)
In scalar context, stat() returns a boolean value indicating success
or failure, and, if successful, sets the information associated with
the special filehandle _.
For example, here is a loop that inserts index producing entries before any line containing a certain pattern:
while (<>) {
study;
print ".IX foo\n" if /\bfoo\b/;
print ".IX bar\n" if /\bbar\b/;
print ".IX blurfl\n" if /\bblurfl\b/;
# ...
print;
}
In searching for /\bfoo\b/, only those locations in $_ that contain "f"
will be looked at, because "f" is rarer than "o". In general, this is
a big win except in pathological cases. The only question is whether
it saves you more time than it took to build the linked list in the
first place.
Note that if you have to look for strings that you don't know till runtime, you can build an entire loop as a string and eval() that to avoid recompiling all your patterns all the time. Together with undefining $/ to input entire files as one record, this can be very fast, often faster than specialized programs like fgrep(1). The following scans a list of files (@files) for a list of words (@words), and prints out the names of those files that contain a match:
$search = 'while (<>) { study;';
foreach $word (@words) {
$search .= "++\$seen{\$ARGV} if /\\b$word\\b/;\n";
}
$search .= "}";
@ARGV = @files;
undef $/;
eval $search; # this screams
$/ = "\n"; # put back to normal input delimiter
foreach $file (sort keys(%seen)) {
print $file, "\n";
}
If you specify a substring that is partly outside the string, the part within the string is returned. If the substring is totally outside the string a warning is produced.
You can use the substr() function as an lvalue, in which case EXPR must be an lvalue. If you assign something shorter than LEN, the string will shrink, and if you assign something longer than LEN, the string will grow to accommodate it. To keep the string the same length you may need to pad or chop your value using sprintf().
An alternative to using substr() as an lvalue is to specify the
replacement string as the 4th argument. This allows you to replace
parts of the EXPR and return what was there before in one operation.
$symlink_exists = eval { symlink("",""); 1 };
require 'syscall.ph'; # may need to run h2ph
$s = "hi there\n";
syscall(&SYS_write, fileno(STDOUT), $s, length $s);
Note that Perl supports passing of up to only 14 arguments to your system call,
which in practice should usually suffice.
Syscall returns whatever value returned by the system call it calls. If the system call fails, syscall() returns -1 and sets $! (errno). Note that some system calls can legitimately return -1. The proper way to handle such calls is to assign $!=0; before the call and check the value of $! if syscall returns -1.
There's a problem with syscall(&SYS_pipe): it returns the file
number of the read end of the pipe it creates. There is no way
to retrieve the file number of the other end. You can avoid this
problem by using pipe() instead.
The possible values and flag bits of the MODE parameter are system-dependent; they are available via the standard module Fcntl. For historical reasons, some values work on almost every system supported by perl: zero means read-only, one means write-only, and two means read/write. We know that these values do not work under OS/390 Unix and on the Macintosh; you probably don't want to use them in new code.
If the file named by FILENAME does not exist and the open() call creates it (typically because MODE includes the O_CREAT flag), then the value of PERMS specifies the permissions of the newly created file. If you omit the PERMS argument to sysopen(), Perl uses the octal value 0666. These permission values need to be in octal, and are modified by your process's current umask. The umask value is a number representing disabled permissions bits---if your umask were 027 (group can't write; others can't read, write, or execute), then passing sysopen() 0666 would create a file with mode 0640 (0666 &~ 027 is 0640).
If you find this umask() talk confusing, here's some advice: supply a creation mode of 0666 for regular files and one of 0777 for directories (in mkdir()) and executable files. This gives users the freedom of choice: if they want protected files, they might choose process umasks of 022, 027, or even the particularly antisocial mask of 077. Programs should rarely if ever make policy decisions better left to the user. The exception to this is when writing files that should be kept private: mail files, web browser cookies, .rhosts files, and so on. In short, seldom if ever use 0644 as argument to sysopen() because that takes away the user's option to have a more permissive umask. Better to omit it.
The IO::File module provides a more object-oriented approach, if you're
into that kind of thing.
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.
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.
The return value is the exit status of the program as returned by the wait() call. To get the actual exit value divide by 256. See also the exec entry elsewhere in this document. This is NOT what you want to use to capture the output from a command, for that you should use merely backticks or qx//, as described in the section on `STRING` in the perlop manpage.
Like exec(), system() allows you to lie to a program about its name if you use the ``system PROGRAM LIST'' syntax. Again, see the exec entry elsewhere in this document.
Because system() and backticks block SIGINT and SIGQUIT, killing the program they're running doesn't actually interrupt your program.
@args = ("command", "arg1", "arg2");
system(@args) == 0
or die "system @args failed: $?"
You can check all the failure possibilities by inspecting
$? like this:
$exit_value = $? >> 8;
$signal_num = $? & 127;
$dumped_core = $? & 128;
When the arguments get executed via the system shell, results
and return codes will be subject to its quirks and capabilities.
See the section on `STRING` in the perlop manpage and the exec entry elsewhere in this documentfor details.
An OFFSET may be specified to write the data from some part of the
string other than the beginning. A negative OFFSET specifies writing
that many bytes counting backwards from the end of the string. In the
case the SCALAR is empty you can use OFFSET but only zero offset.
Note that functions such as keys() and values() may return huge lists when used on large objects, like DBM files. You may prefer to use the each() function to iterate over such. Example:
# print out history file offsets
use NDBM_File;
tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
untie(%HIST);
A class implementing a hash should have the following methods:
TIEHASH classname, LIST
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.
For further details see the perltie manpage, the section on tied VARIABLE.
($user,$system,$cuser,$csystem) = times;
If EXPR is omitted, uses $_.
If EXPR is omitted, uses $_.
If umask(2) is not implemented on your system and you are trying to restrict access for yourself (i.e., (EXPR & 0700) > 0), produces a fatal error at run time. If umask(2) is not implemented and you are not trying to restrict access for yourself, returns undef.
Remember that a umask is a number, usually given in octal; it is not a
string of octal digits. See also the oct entry elsewhere in this documentif all you have is a string.
undef $foo;
undef $bar{'blurfl'}; # Compare to: delete $bar{'blurfl'};
undef @ary;
undef %hash;
undef &mysub;
undef *xyz; # destroys $xyz, @xyz, %xyz, &xyz, etc.
return (wantarray ? (undef, $errmsg) : undef) if $they_blew_it;
select undef, undef, undef, 0.25;
($a, $b, undef, $c) = &foo; # Ignore third value returned
Note that this is a unary operator, not a list operator.
$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 $_.
sub substr {
my($what,$where,$howmuch) = @_;
unpack("x$where a$howmuch", $what);
}
and then there's
sub ordinal { unpack("c",$_[0]); } # same as ord()
In addition, you may prefix a field with a %<number> to indicate that
you want a <number>-bit checksum of the items instead of the items
themselves. Default is a 16-bit checksum. For example, the following
computes the same number as the System V sum program:
while (<>) {
$checksum += unpack("%16C*", $_);
}
$checksum %= 65536;
The following efficiently counts the number of set bits in a bit vector:
$setbits = unpack("%32b*", $selectmask);
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 that have changed in incompatible ways from older versions of Perl. (We try not to do this more than we have to.)
The BEGIN forces the require and import() to happen at compile time. The require makes sure the module is loaded into memory if it hasn't been yet. The import() is not a builtin---it's just an ordinary static method call into the ``Module'' package to tell the module to import the list of features back into the current package. The module can implement its import() method any way it likes, though most modules just choose to derive their import() method via inheritance from the Exporter class that is defined in the Exporter module. See the Exporter manpage. If no import() method can be found then the error is currently silently ignored. This may change to a fatal error in a future version.
If you don't want your namespace altered, explicitly supply an empty list:
use Module ();That is exactly equivalent to
BEGIN { require Module }
If the VERSION argument is present between Module and LIST, then the
use will call the VERSION method in class Module with the given
version as an argument. The default VERSION method, inherited from
the Universal class, croaks if the given version is larger than the
value of the variable $Module::VERSION. (Note that there is not a
comma after VERSION!)
Because this is a wide-open interface, pragmas (compiler directives) are also implemented this way. Currently implemented pragmas are:
use integer;
use diagnostics;
use sigtrap qw(SEGV BUS);
use strict qw(subs vars refs);
use subs qw(afunc blurfl);
Some of these these pseudo-modules import semantics into the current
block scope (like strict or integer, unlike ordinary modules,
which import symbols into the current package (which are effective
through the end of the file).
There's a corresponding ``no'' command that unimports meanings imported by use, i.e., it calls unimport Module LIST instead of import().
no integer;
no strict 'refs';
If no unimport() method can be found the call fails with a fatal error.
See the perlmod manpage for a list of standard modules and pragmas.
#!/usr/bin/perl
$now = time;
utime $now, $now, @ARGV;
vec($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.
The following code will build up an ASCII string saying 'PerlPerlPerl'. The comments show the string after each step. Note that this code works in the same way on big-endian or little-endian machines.
my $foo = '';
vec($foo, 0, 32) = 0x5065726C; # 'Perl'
vec($foo, 2, 16) = 0x5065; # 'PerlPe'
vec($foo, 3, 16) = 0x726C; # 'PerlPerl'
vec($foo, 8, 8) = 0x50; # 'PerlPerlP'
vec($foo, 9, 8) = 0x65; # 'PerlPerlPe'
vec($foo, 20, 4) = 2; # 'PerlPerlPe' . "\x02"
vec($foo, 21, 4) = 7; # 'PerlPerlPer'
# 'r' is "\x72"
vec($foo, 45, 2) = 3; # 'PerlPerlPer' . "\x0c"
vec($foo, 93, 1) = 1; # 'PerlPerlPer' . "\x2c"
vec($foo, 94, 1) = 1; # 'PerlPerlPerl'
# 'l' is "\x6c"
To transform a bit vector into a string or array of 0's and 1's, use these:
$bits = unpack("b*", $vector);
@bits = split(//, unpack("b*", $vector));
If you know the exact length in bits, it can be used in place of the *.
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.)
See the perlipc manpage for other examples.
return unless defined wantarray; # don't bother doing more
my @a = complex_calculation();
return wantarray ? @a : "@a";
If LIST is empty and $@ already contains a value (typically from a previous eval) that value is used after appending "\t...caught" to $@. This is useful for staying almost, but not entirely similar to die().
If $@ is empty then the string "Warning: Something's wrong" is used.
No message is printed if there is a $SIG{__WARN__} handler installed. It is the handler's responsibility to deal with the message as it sees fit (like, for instance, converting it into a die()). Most handlers must therefore make arrangements to actually display the warnings that they are not prepared to deal with, by calling warn() again in the handler. Note that this is quite safe and will not produce an endless loop, since __WARN__ hooks are not called from inside one.
You will find this behavior is slightly different from that of $SIG{__DIE__} handlers (which don't suppress the error text, but can instead call die() again to change it).
Using a __WARN__ handler provides a powerful way to silence all warnings (even the so-called mandatory ones). An example:
# wipe out *all* compile-time warnings
BEGIN { $SIG{'__WARN__'} = sub { warn $_[0] if $DOWARN } }
my $foo = 10;
my $foo = 20; # no warning about duplicate my $foo,
# but hey, you asked for it!
# no compile-time or run-time warnings before here
$DOWARN = 1;
# run-time warnings enabled after here
warn "\$foo is alive and $foo!"; # does show up
See the perlvar manpage for details on setting %SIG entries, and for more
examples.
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.