In the syntax descriptions that follow, list operators that expect a list (and provide list context for the elements of the list) are shown with LIST as an argument. Such a list may consist of any combination of scalar arguments or list values; the list values will be included in the list as if each individual element were interpolated at that point in the list, forming a longer single-dimensional list value. Elements of the LIST should be separated by commas.
Any function in the list below may be used either with or without parentheses around its arguments. (The syntax descriptions omit the parentheses.) If you use the parentheses, the simple (but occasionally surprising) rule is this: It looks like a function, therefore it is a function, and precedence doesn't matter. Otherwise it's a list operator or unary operator, and precedence does matter. And whitespace between the function and left parenthesis doesn't count---so you need to be careful sometimes:
print 1+2+4; # Prints 7.
print(1+2) + 4; # Prints 3.
print (1+2)+4; # Also prints 3!
print +(1+2)+4; # Prints 7.
print ((1+2)+4); # Prints 7.
If you run Perl with the -w switch it can warn you about this. For example, the third line above produces:
print (...) interpreted as function at - line 1.
Useless use of integer addition in void context at - line 1.
A few functions take no arguments at all, and therefore work as neither unary nor list operators. These include such functions as "time" and "endpwent". For example, "time+86_400" always means "time() + 86_400".
For functions that can be used in either a scalar or list context, nonabortive failure is generally indicated in a scalar context by returning the undefined value, and in a list context by returning the null list.
Remember the following important rule: There is no rule that relates the behavior of an expression in list context to its behavior in scalar context, or vice versa. It might do two totally different things. Each operator and function decides which sort of value it would be most appropriate to return in scalar context. Some operators return the length of the list that would have been returned in list context. Some operators return the first value in the list. Some operators return the last value in the list. Some operators return a count of successful operations. In general, they do what you want, unless you want consistency.
A 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), (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.
"-X", "binmode", "chmod", "chown", "chroot", "crypt", "dbmclose", "dbmopen", "dump", "endgrent", "endhostent", "endnetent", "endprotoent", "endpwent", "endservent", "exec", "fcntl", "flock", "fork", "getgrent", "getgrgid", "gethostent", "getlogin", "getnetbyaddr", "getnetbyname", "getnetent", "getppid", "getprgp", "getpriority", "getprotobynumber", "getprotoent", "getpwent", "getpwnam", "getpwuid", "getservbyport", "getservent", "getsockopt", "glob", "ioctl", "kill", "link", "lstat", "msgctl", "msgget", "msgrcv", "msgsnd", "open", "pipe", "readlink", "rename", "select", "semctl", "semget", "semop", "setgrent", "sethostent", "setnetent", "setpgrp", "setpriority", "setprotoent", "setpwent", "setservent", "setsockopt", "shmctl", "shmget", "shmread", "shmwrite", "socket", "socketpair", "stat", "symlink", "syscall", "sysopen", "system", "times", "truncate", "umask", "unlink", "utime", "wait", "waitpid"
For more information about the portability of these functions, see perlport and other available platform-specific documentation.
-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 (is empty).
-s File has nonzero size (returns size in bytes).
-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 an ASCII text file (heuristic guess).
-B File is a "binary" file (opposite of -T).
-M Script start time minus file modification time, in days.
-A Same for access time.
-C Same for inode change time (Unix, may differ for other platforms)
Example:
while (<>) {
chomp;
next unless -f $_; # ignore specials
#...
}
The interpretation of the file permission operators "-r", "-R", "-w", "-W", "-x", and "-X" is by default based solely on the mode of the file and the uids and gids of the user. There may be other reasons you can't actually read, write, or execute the file. Such reasons may be for example network filesystem access controls, ACLs (access control lists), read-only filesystems, and unrecognized executable formats.
Also note that, for the superuser on the local filesystems, the "-r", "-R", "-w", and "-W" tests always return 1, and "-x" and "-X" return 1 if any execute bit is set in the mode. Scripts run by the superuser may thus need to do a stat() to determine the actual mode of the file, or temporarily set their effective uid to something else.
If you are using ACLs, there is a pragma called "filetest" that may produce more accurate results than the bare stat() mode bits. When under the "use filetest 'access'" the above-mentioned filetests will test whether the permission can (not) be granted using the access() family of system calls. Also note that the "-x" and "-X" may under this pragma return true even if there are no execute permission bits set (nor any extra execute permission ACLs). This strangeness is due to the underlying system calls' definitions. Read the documentation for the "filetest" pragma for more information.
Note that "-s/a/b/" does not do a negated substitution. Saying "-exp($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 IO 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.) (Also, if the stat buffer was filled by a "lstat" call, "-T" and "-B" will reset it with the results of "stat _"). 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 _;
On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor, as determined by the value of $^F. See ``$^F'' in perlvar.
Only one timer may be counting at once. Each call disables the previous timer, and an argument of 0 may be supplied to cancel the previous timer without starting a new one. The returned value is the amount of time remaining on the previous timer.
For delays of finer granularity than one second, you may use Perl's four-argument version of select() leaving the first three arguments undefined, or you might be able to use the "syscall" interface to access (2) if your system supports it. The Time::HiRes module (from CPAN, and starting from Perl 5.8 part of the standard distribution) may also prove useful.
It is usually a mistake to intermix "alarm" and "sleep" calls. ("sleep" may be internally implemented in your system with "alarm")
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 ``Signals'' in perlipc.
eval {
local $SIG{ALRM} = sub { die "alarm\n" }; # NB: \n required
alarm $timeout;
$nread = sysread SOCKET, $buffer, $size;
alarm 0;
};
if ($@) {
die unless $@ eq "alarm\n"; # propagate unexpected errors
# timed out
}
else {
# didn't
}
For the tangent operation, you may use the "Math::Trig::tan" function, or use the familiar relation:
sub tan { sin($_[0]) / cos($_[0]) }
If LAYER is omitted or specified as ":raw" the filehandle is made suitable for passing binary data. This includes turning off possible CRLF translation and marking it as bytes (as opposed to Unicode characters). Note that as desipite what may be implied in ``Programming Perl'' (the Camel) or elsewhere ":raw" is not the simply inverse of ":crlf" --- other layers which would affect binary nature of the stream are also disabled. See PerlIO, perlrun and the discussion about the PERLIO environment variable.
The LAYER parameter of the binmode() function is described as ``DISCIPLINE'' in ``Programming Perl, 3rd Edition''. However, since the publishing of this book, by many known as ``Camel III'', the consensus of the naming of this functionality has moved from ``discipline'' to ``layer''. All documentation of this version of Perl therefore refers to ``layers'' rather than to ``disciplines''. Now back to the regularly scheduled documentation...
On some systems (in general, DOS and Windows-based systems) binmode() is necessary when you're not working with a text file. For the sake of portability it is a good idea to always use it when appropriate, and to never use it when it isn't appropriate.
In other words: regardless of platform, use binmode() on binary files (like for example images).
If LAYER is present it is a single string, but may contain multiple directives. The directives alter the behaviour of the file handle. When LAYER is present using binmode on text file makes sense.
To mark FILEHANDLE as UTF-8, use ":utf8".
The ":bytes", ":crlf", and ":utf8", and any other directives of the form ":...", are called I/O layers. The "open" pragma can be used to establish default I/O layers. See open.
In general, binmode() should be called after open() but before any I/O is done on the filehandle. Calling binmode() will normally flush any pending buffered output data (and perhaps pending input data) on the handle. An exception to this is the ":encoding" layer that changes the default character encoding of the handle, see open. The ":encoding" layer sometimes needs to be called in mid-stream, and it doesn't flush the stream.
The operating system, device drivers, C libraries, and Perl run-time system all work together to let the programmer treat a single character ("\n") as the line terminator, irrespective of the external representation. On many operating systems, the native text file representation matches the internal representation, but on some platforms the external representation of "\n" is made up of more than one character.
Mac OS, all variants of Unix, and Stream_LF files on VMS use a single character to end each line in the external representation of text (even though that single character is CARRIAGE RETURN on Mac OS and LINE FEED on Unix and most VMS files). In other systems like OS/2, DOS and the various flavors of MS-Windows your program sees a "\n" as a simple "\cJ", but what's stored in text files are the two characters "\cM\cJ". That means that, if you don't use binmode() on these systems, "\cM\cJ" sequences on disk will be converted to "\n" on input, and any "\n" in your program will be converted back to "\cM\cJ" on output. This is what you want for text files, but it can be disastrous for binary files.
Another consequence of using binmode() (on some systems) is that special end-of-file markers will be seen as part of the data stream. For systems from the Microsoft family this means that if your binary data contains "\cZ", the I/O subsystem will regard it as the end of the file, unless you use binmode().
binmode() is not only important for readline() and print() operations, but also when using read(), seek(), sysread(), syswrite() and tell() (see perlport for more details). See the $/ and "$\" variables in perlvar for how to manually set your input and output line-termination sequences.
Consider always blessing objects in CLASSNAMEs that are mixed case. Namespaces with all lowercase names are considered reserved for Perl pragmata. Builtin types have all uppercase names, so to prevent confusion, you may wish to avoid such package names as well. Make sure that CLASSNAME is a true value.
See ``Perl Modules'' in perlmod.
($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, $hints, $bitmask) = caller($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 an "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.) $subroutine may also be "(unknown)" if this particular subroutine happens to have been deleted from the symbol table. $hasargs is true if a new instance of @_ was set up for the frame. $hints and $bitmask contain pragmatic hints that the caller was compiled with. The $hints and $bitmask values are subject to change between versions of Perl, and are not meant for external use.
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 $mode, 'foo'; # this is best
You can also import the symbolic "S_I*" constants from the Fcntl module:
use Fcntl ':mode';
chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
# This is identical to the chmod 0755 of the above example.
while (<>) {
chomp; # avoid \n on last field
@array = split(/:/);
# ...
}
If VARIABLE is a hash, it chomps the hash's values, but not its keys.
You can actually chomp anything that's an lvalue, including an assignment:
chomp($cwd = `pwd`);
chomp($answer = <STDIN>);
If you chomp a list, each element is chomped, and the total number of characters removed is returned.
Note that parentheses are necessary when you're chomping anything that is not a simple variable. This is because "chomp $cwd = `pwd`;" is interpreted as "(chomp $cwd) = `pwd`;", rather than as "chomp( $cwd = `pwd` )" which you might expect. Similarly, "chomp $a, $b" is interpreted as "chomp($a), $b" rather than as "chomp($a, $b)".
You can actually chop anything that's an lvalue, including an assignment.
If you chop a list, each element is chopped. Only the value of the last "chop" is returned.
Note that "chop" returns the last character. To return all but the last character, use "substr($string, 0, -1)".
See also ``chomp''.
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
Here's an example that looks up nonnumeric uids in the passwd file:
print "User: ";
chomp($user = <STDIN>);
print "Files: ";
chomp($pattern = <STDIN>);
($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. On POSIX systems, you can detect this condition this way:
use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
$can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
For the reverse, use ``ord''. See perlunicode and encoding for more about Unicode.
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.) Closing a pipe also waits for the process executing on the pipe to complete, in case you want to look at the output of the pipe afterwards, and implicitly puts the exit status value of that command into $?.
Prematurely closing the read end of a pipe (i.e. before the process writing to it at the other end has closed it) will result in a SIGPIPE being delivered to the writer. If the other end can't handle that, be sure to read all the data before closing the pipe.
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 "Math::Trig::acos()" function, or use this relation:
sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
Note that "crypt" is intended to be a one-way function, much like breaking eggs to make an omelette. There is no (known) corresponding decrypt function (in other words, the crypt() is a one-way hash function). As a result, this function isn't all that useful for cryptography. (For that, see your nearby CPAN mirror.)
When verifying an existing encrypted string you should use the encrypted text as the salt (like "crypt($plain, $crypted) eq $crypted"). This allows your code to work with the standard "crypt" and with more exotic implementations. In other words, do not assume anything about the returned string itself, or how many bytes in the encrypted string matter.
Traditionally the result is a string of 13 bytes: two first bytes of the salt, followed by 11 bytes from the set "[./0-9A-Za-z]", and only the first eight bytes of the encrypted string mattered, but alternative hashing schemes (like MD5), higher level security schemes (like C2), and implementations on non-UNIX platforms may produce different strings.
When choosing a new salt create a random two character string whose characters come from the set "[./0-9A-Za-z]" (like "join '', ('.', '/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]").
Here's an example that makes sure that whoever runs this program knows their own password:
$pwd = (getpwuid($<))[1];
system "stty -echo";
print "Password: ";
chomp($word = <STDIN>);
print "\n";
system "stty echo";
if (crypt($word, $pwd) ne $pwd) {
die "Sorry...\n";
} else {
print "ok\n";
}
Of course, typing in your own password to whoever asks you for it is unwise.
The crypt function is unsuitable for encrypting large quantities of data, not least of all because you can't get the information back. Look at the by-module/Crypt and by-module/PGP directories on your favorite CPAN mirror for a slew of potentially useful modules.
If using crypt() on a Unicode string (which potentially has characters with codepoints above 255), Perl tries to make sense of the situation by trying to downgrade (a copy of the string) the string back to an eight-bit byte string before calling crypt() (on that copy). If that works, good. If not, crypt() dies with "Wide character in crypt".
Breaks the binding between a DBM file and a hash.
This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a hash. HASH is the name of the hash. (Unlike normal "open", the first argument is not a filehandle, even though it looks like one). DBNAME is the name of the database (without the .dir or .pag extension if any). If the database does not exist, it is created with protection specified by MASK (as modified by the "umask"). If your system supports only the older DBM functions, you may perform only one "dbmopen" in your program. In older versions of Perl, if your system had neither DBM nor ndbm, calling "dbmopen" produced a fatal error; it now falls back to sdbm(3).
If you don't have write access to the DBM file, you can only read hash variables, not set them. If you want to test whether you can write, either use file tests or try setting a dummy hash entry inside an "eval", which will trap the error.
Note that functions such as "keys" and "values" may return huge lists when used on large DBM files. You may prefer to use the "each" function to iterate over large DBM files. Example:
# print out history file offsets
dbmopen(%HIST,'/usr/lib/news/history',0666);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "\n";
}
dbmclose(%HIST);
See also AnyDBM_File for a more general description of the pros and cons of the various dbm approaches, as well as DB_File for a particularly rich implementation.
You can control which DBM library you use by loading that library before you call dbmopen():
use DB_File;
dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
or die "Can't open netscape history file: $!";
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(&func)" to check whether subroutine &func has ever been defined. The return value is unaffected by any forward declarations of &foo. Note that a subroutine which is not defined may still be callable: its package may have an "AUTOLOAD" method that makes it spring into existence the first time that it is called --- see perlsub.
Use of "defined" on aggregates (hashes and arrays) is deprecated. It used to report whether memory for that aggregate has ever been allocated. This behavior may disappear in future versions of Perl. 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" }
When used on a hash element, it tells you whether the value is defined, not whether the key exists in the hash. Use ``exists'' for the latter purpose.
Examples:
print if defined $switch{'D'};
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
$debugging = 0 unless defined $debugging;
Note: Many folks tend to overuse "defined", and then are surprised to discover that the number 0 and "" (the zero-length string) are, in fact, defined values. For example, if you say
"ab" =~ /a(.*)b/;
The pattern match succeeds, and $1 is defined, despite the fact that it matched ``nothing''. But it didn't really match nothing---rather, it matched something that happened to be zero characters long. This is all very above-board and honest. When a function returns an undefined value, it's an admission that it couldn't give you an honest answer. So you should use "defined" only when you're questioning the integrity of what you're trying to do. At other times, a simple comparison to 0 or "" is what you want.
See also ``undef'', ``exists'', ``ref''.
Returns each element so deleted or the undefined value if there was no such element. Deleting from $ENV{} modifies the environment. Deleting from a hash tied to a DBM file deletes the entry from the DBM file. Deleting from a "tie"d hash or array may not necessarily return anything.
Deleting an array element effectively returns that position of the array to its initial, uninitialized state. Subsequently testing for the same element with exists() will return false. Note that deleting array elements in the middle of an array will not shift the index of the ones after them down---use splice() for that. See ``exists''.
The following (inefficiently) deletes all the values of %HASH and @ARRAY:
foreach $key (keys %HASH) {
delete $HASH{$key};
}
foreach $index (0 .. $#ARRAY) {
delete $ARRAY[$index];
}
And so do these:
delete @HASH{keys %HASH};
delete @ARRAY[0 .. $#ARRAY];
But both of these are slower than just assigning the empty list or undefining %HASH or @ARRAY:
%HASH = (); # completely empty %HASH
undef %HASH; # forget %HASH ever existed
@ARRAY = (); # completely empty @ARRAY
undef @ARRAY; # forget @ARRAY ever existed
Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash element, array element, hash slice, or array slice lookup:
delete $ref->[$x][$y]{$key};
delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
delete $ref->[$x][$y][$index];
delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
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 last element of LIST does not end in a newline, the current script line number and input line number (if any) are also printed, and a newline is supplied. Note that the ``input line number'' (also known as ``chunk'') is subject to whatever notion of ``line'' happens to be currently in effect, and is also available as the special variable $.. See ``$/'' in perlvar and ``$.'' in perlvar.
Hint: sometimes appending ", stopped" to your message will cause it to make better sense when the string "at foo line 123" is appended. Suppose you are running script ``canasta''.
die "/etc/games is no good";
die "/etc/games is no good, stopped";
produce, respectively
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
See also exit(), warn(), and the Carp module.
If LIST is empty and $@ already contains a value (typically from a previous eval) that value is reused after appending "\t...propagated". This is useful for propagating exceptions:
eval { ... };
die unless $@ =~ /Expected exception/;
If LIST is empty and $@ contains an object reference that has a "PROPAGATE" method, that method will be called with additional file and line number parameters. The return value replaces the value in $@. ie. as if "<$@ = eval { $@-"PROPAGATE(__FILE__, __LINE__) };>> were called.
If $@ is empty then the string "Died" is used.
die() can also be called with a reference argument. If this happens to be trapped within an eval(), $@ contains the reference. This behavior permits a more elaborate exception handling implementation using objects that maintain arbitrary state about the nature of the exception. Such a scheme is sometimes preferable to matching particular string values of $@ using regular expressions. Here's an example:
eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
if ($@) {
if (ref($@) && UNIVERSAL::isa($@,"Some::Module::Exception")) {
# handle Some::Module::Exception
}
else {
# handle all other possible exceptions
}
}
Because perl will stringify uncaught exception messages before displaying them, you may want to overload stringification operations on such custom exception objects. See overload for details about that.
You can arrange for a callback to be run just before the "die" does its deed, by setting the $SIG{__DIE__} hook. The associated handler will be called with the error text and can change the error message, if it sees fit, by calling "die" again. See ``$SIG{expr}'' in perlvar for details on setting %SIG entries, and ``eval BLOCK'' for some examples. Although this feature was meant to be run only right before your program was to exit, this is not currently the case---the $SIG{__DIE__} hook is currently called even inside eval()ed blocks/strings! If one wants the hook to do nothing in such situations, put
die @_ if $^S;
as the first line of the handler (see ``$^S'' in perlvar). Because this promotes strange action at a distance, this counterintuitive behavior may be fixed in a future release.
"do BLOCK" does not count as a loop, so the loop control statements "next", "last", or "redo" cannot be used to leave or restart the block. See perlsyn for alternative strategies.
do 'stat.pl';
is just like
eval `cat stat.pl`;
except that it's more efficient and concise, keeps track of the current filename for error messages, searches the @INC libraries, and updates %INC if the file is found. See ``Predefined Names'' in perlvar for these variables. It also differs in that code evaluated with "do FILENAME" cannot see lexicals in the enclosing scope; "eval STRING" does. It's the same, however, in that it does reparse the file every time you call it, so you probably don't want to do this inside a loop.
If "do" cannot read the file, it returns undef and sets $! to the error. If "do" can read the file but cannot compile it, it returns undef and sets an error message in $@. If the file is successfully compiled, "do" returns the value of the last expression evaluated.
Note that inclusion of library modules is better done with the "use" and "require" operators, which also do automatic error checking and raise an exception if there's a problem.
You might like to use "do" to read in a program configuration file. Manual error checking can be done this way:
# read in config files: system first, then user
for $file ("/share/prog/defaults.rc",
"$ENV{HOME}/.someprogrc")
{
unless ($return = do $file) {
warn "couldn't parse $file: $@" if $@;
warn "couldn't do $file: $!" unless defined $return;
warn "couldn't run $file" unless $return;
}
}
WARNING: Any files opened at the time of the dump will not be open any more when the program is reincarnated, with possible resulting confusion on the part of Perl.
This function is now largely obsolete, partly because it's very hard to convert a core file into an executable, and because the real compiler backends for generating portable bytecode and compilable C code have superseded it. That's why you should now invoke it as "CORE::dump()", if you don't want to be warned against a possible typo.
If you're looking to use dump to speed up your program, consider generating bytecode or native C code as described in perlcc. If you're just trying to accelerate a CGI script, consider using the "mod_perl" extension to Apache, or the CPAN module, CGI::Fast. You might also consider autoloading or selfloading, which at least make your program appear to run faster.
Entries are returned in an apparently random order. The actual random order is subject to change in future versions of perl, but it is guaranteed to be in the same order as either the "keys" or "values" function would produce on the same (unmodified) hash.
When the hash is entirely read, a null array is returned in list context (which when assigned produces a false (0) value), and "undef" in scalar context. The next call to "each" after that will start iterating again. There is a single iterator for each hash, shared by all "each", "keys", and "values" function calls in the program; it can be reset by reading all the elements from the hash, or by evaluating "keys HASH" or "values HASH". If you add or delete elements of a hash while you're iterating over it, you may get entries skipped or duplicated, so don't. Exception: It is always safe to delete the item most recently returned by "each()", which means that the following code will work:
while (($key, $value) = each %hash) {
print $key, "\n";
delete $hash{$key}; # This is safe
}
The following prints out your environment like the printenv(1) program, only in a different order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}
See also "keys", "values" and "sort".
An "eof" without an argument uses the last file read. Using "eof()" with empty parentheses is very different. It refers to the pseudo file formed from the files listed on the command line and accessed via the "<>" operator. Since "<>" isn't explicitly opened, as a normal filehandle is, an "eof()" before "<>" has been used will cause @ARGV to be examined to determine if input is available. Similarly, an "eof()" after "<>" has returned end-of-file will assume you are processing another @ARGV list, and if you haven't set @ARGV, will read input from "STDIN"; see ``I/O Operators'' in perlop.
In a "while (<>)" loop, "eof" or "eof(ARGV)" can be used to detect the end of each file, "eof()" will only detect the end of the last file. Examples:
# reset line numbering on each input file
while (<>) {
next if /^\s*#/; # skip comments
print "$.\t$_";
} continue {
close ARGV if eof; # Not eof()!
}
# insert dashes just before last line of last file
while (<>) {
if (eof()) { # check for end of current file
print "--------------\n";
close(ARGV); # close or last; is needed if we
# are reading from the terminal
}
print;
}
Practical hint: you almost never need to use "eof" in Perl, because the input operators typically return "undef" 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 ``wantarray'' for more on how the evaluation context can be determined.
If there is a syntax error or runtime error, or a "die" statement is executed, an undefined value is returned by "eval", and $@ is set to the error message. If there was no error, $@ is guaranteed to be a null string. Beware that using "eval" neither silences perl from printing warnings to STDERR, nor does it stuff the text of warning messages into $@. To do either of those, you have to use the $SIG{__WARN__} facility, or turn off warnings inside the BLOCK or EXPR using "no warnings 'all'". See ``warn'', perlvar, warnings and perllexwarn.
Note that, because "eval" traps otherwise-fatal errors, it is useful for determining whether a particular feature (such as "socket" or "symlink") is implemented. It is also Perl's exception trapping mechanism, where the die operator is used to raise exceptions.
If the code to be executed doesn't vary, you may use the eval-BLOCK form to trap run-time errors without incurring the penalty of recompiling each time. The error, if any, is still returned in $@. Examples:
# make divide-by-zero nonfatal
eval { $answer = $a / $b; }; warn $@ if $@;
# same thing, but less efficient
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
eval { $answer = }; # WRONG
# a run-time error
eval '$answer ='; # sets $@
Due to the current arguably broken state of "__DIE__" hooks, when using the "eval{}" form as an exception trap in libraries, you may wish not to trigger any "__DIE__" hooks that user code may have installed. You can use the "local $SIG{__DIE__}" construct for this purpose, as shown in this example:
# a very private exception trap for divide-by-zero
eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
warn $@ if $@;
This is especially significant, given that "__DIE__" hooks can call "die" again, which has the effect of changing their error messages:
# __DIE__ hooks may modify error messages
{
local $SIG{'__DIE__'} =
sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
eval { die "foo lives here" };
print $@ if $@; # prints "bar lives here"
}
Because this promotes action at a distance, this counterintuitive behavior may be fixed in a future release.
With an "eval", you should be especially careful to remember what's being looked at when:
eval $x; # CASE 1
eval "$x"; # CASE 2
eval '$x'; # CASE 3
eval { $x }; # CASE 4
eval "\$$x++"; # CASE 5
$$x++; # CASE 6
Cases 1 and 2 above behave identically: they run the code contained in the variable $x. (Although case 2 has misleading double quotes making the reader wonder what else might be happening (nothing is).) Cases 3 and 4 likewise behave in the same way: they run the code '$x', which does nothing but return the value of $x. (Case 4 is preferred for purely visual reasons, but it also has the advantage of compiling at compile-time instead of at run-time.) Case 5 is a place where normally you would like to use double quotes, except that in this particular situation, you can just use symbolic references instead, as in case 6.
"eval BLOCK" does not count as a loop, so the loop control statements "next", "last", or "redo" cannot be used to leave or restart the block.
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. 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 ```STRING`'' in perlop for details.
Using an indirect object with "exec" or "system" is also more secure. This usage (which also works fine with system()) forces interpretation of the arguments as a multivalued list, even if the list had just one argument. That way you're safe from the shell expanding wildcards or splitting up words with whitespace in them.
@args = ( "echo surprise" );
exec @args; # subject to shell escapes
# if @args == 1
exec { $args[0] } @args; # safe even with one-arg list
The first version, the one without the indirect object, ran the echo program, passing it "surprise" an argument. The second version didn't---it tried to run a program literally called ``echo surprise'', didn't find it, and set $? to a non-zero value indicating failure.
Beginning with v5.6.0, Perl will attempt to flush all files opened for output before the exec, but this may not be supported on some platforms (see perlport). To be safe, you may need to set $| ($AUTOFLUSH in English) or call the "autoflush()" method of "IO::Handle" on any open handles in order to avoid lost output.
Note that "exec" will not call your "END" blocks, nor will it call any "DESTROY" methods in your objects.
print "Exists\n" if exists $hash{$key};
print "Defined\n" if defined $hash{$key};
print "True\n" if $hash{$key};
print "Exists\n" if exists $array[$index];
print "Defined\n" if defined $array[$index];
print "True\n" if $array[$index];
A hash or array element can be true only if it's defined, and defined if it exists, but the reverse doesn't necessarily hold true.
Given an expression that specifies the name of a subroutine, returns true if the specified subroutine has ever been declared, even if it is undefined. Mentioning a subroutine name for exists or defined does not count as declaring it. Note that a subroutine which does not exist may still be callable: its package may have an "AUTOLOAD" method that makes it spring into existence the first time that it is called --- see perlsub.
print "Exists\n" if exists &subroutine;
print "Defined\n" if defined &subroutine;
Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash or array key lookup or subroutine name:
if (exists $ref->{A}->{B}->{$key}) { }
if (exists $hash{A}{B}{$key}) { }
if (exists $ref->{A}->{B}->[$ix]) { }
if (exists $hash{A}{B}[$ix]) { }
if (exists &{$ref->{A}{B}{$key}}) { }
Although the deepest nested array or hash will not spring into existence just because its existence was tested, any intervening ones will. Thus "$ref->{"A"}" and "$ref->{"A"}->{"B"}" will spring into existence due to the existence test for the $key element above. This happens anywhere the arrow operator is used, including even:
undef $ref;
if (exists $ref->{"Some key"}) { }
print $ref; # prints HASH(0x80d3d5c)
This surprising autovivification in what does not at first---or even second---glance appear to be an lvalue context may be fixed in a future release.
See ``Pseudo-hashes: Using an array as a hash'' in perlref for specifics on how exists() acts when used on a pseudo-hash.
Use of a subroutine call, rather than a subroutine name, as an argument to exists() is an error.
exists ⊂ # OK
exists &sub(); # Error
$ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
See also "die". If EXPR is omitted, exits with 0 status. The only universally recognized values for EXPR are 0 for success and 1 for error; other values are subject to interpretation depending on the environment in which the Perl program is running. For example, exiting 69 (EX_UNAVAILABLE) from a sendmail incoming-mail filter will cause the mailer to return the item undelivered, but that's not true everywhere.
Don't use "exit" to abort a subroutine if there's any chance that someone might want to trap whatever error happened. Use "die" instead, which can be trapped by an "eval".
The exit() function does not always exit immediately. It calls any defined "END" routines first, but these "END" routines may not themselves abort the exit. Likewise any object destructors that need to be called are called before the real exit. If this is a problem, you can call "POSIX:_exit($status)" to avoid END and destructor processing. See perlmod 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). See the Fcntl module or your fcntl(2) manpage to learn what functions are available on your system.
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";
}
(Filehandles connected to memory objects via new features of "open" may return undefined even though they are open.)
Two potentially non-obvious but traditional "flock" semantics are that it waits indefinitely until the lock is granted, and that its locks merely advisory. Such discretionary locks are more flexible, but offer fewer guarantees. This means that files locked with "flock" may be modified by programs that do not also use "flock". See perlport, your port's specific documentation, or your system-specific local manpages for details. It's best to assume traditional behavior if you're writing portable programs. (But if you're not, you should as always feel perfectly free to write for your own system's idiosyncrasies (sometimes called ``features''). Slavish adherence to portability concerns shouldn't get in the way of your getting your job done.)
OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but you can use the symbolic names if you 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 bitwise-or'ed with LOCK_SH or LOCK_EX then "flock" will return immediately rather than blocking waiting for the lock (check the return status to see if you got it).
To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE before locking or unlocking it.
Note that the emulation built with lockf(3) doesn't provide shared locks, and it requires that FILEHANDLE be open with write intent. These are the semantics that lockf(3) implements. Most if not all systems implement lockf(3) in terms of fcntl(2) locking, though, so the differing semantics shouldn't bite too many people.
Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE be open with read intent to use LOCK_SH and requires that it be open with write intent to use LOCK_EX.
Note also that some versions of "flock" cannot lock things over the network; you would need to use the more system-specific "fcntl" for that. If you like you can force Perl to ignore your system's flock(2) function, and so provide its own fcntl(2)-based emulation, by passing the switch "-Ud_flock" to the Configure program when you configure perl.
Here's a mailbox appender for BSD systems.
use Fcntl ':flock'; # import LOCK_* constants
sub lock {
flock(MBOX,LOCK_EX);
# and, in case someone appended
# while we were waiting...
seek(MBOX, 0, 2);
}
sub unlock {
flock(MBOX,LOCK_UN);
}
open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
or die "Can't open mailbox: $!";
lock();
print MBOX $msg,"\n\n";
unlock();
On systems that support a real flock(), locks are inherited across fork() calls, whereas those that must resort to the more capricious fcntl() function lose the locks, making it harder to write servers.
See also DB_File for other flock() examples.
Beginning with v5.6.0, Perl will attempt to flush all files opened for output before forking the child process, but this may not be supported on some platforms (see perlport). To be safe, you may need to set $| ($AUTOFLUSH in English) or call the "autoflush()" method of "IO::Handle" on any open handles in order to avoid duplicate output.
If you "fork" without ever waiting on your children, you will accumulate zombies. On some systems, you can avoid this by setting $SIG{CHLD} to "IGNORE". See also perlipc for more examples of forking and reaping moribund children.
Note that if your forked child inherits system file descriptors like STDIN and STDOUT that are actually connected by a pipe or socket, even if you exit, then the remote server (such as, say, a CGI script or a backgrounded job launched from a remote shell) 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;
$~ = 'Something';
write;
See perlform for many details and examples.
Be careful if you put double quotes around the picture, because an "@" character may be taken to mean the beginning of an array name. "formline" always returns true. See perlform for other examples.
if ($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 ``CPAN'' in perlmodlib.
$login = getlogin || getpwuid($<) || "Kilroy";
Do not consider "getlogin" for authentication: it is not as secure as "getpwuid".
use Socket;
$hersockaddr = getpeername(SOCK);
($port, $iaddr) = sockaddr_in($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.)
The exact meaning of the $gcos field varies but it usually contains the real name of the user (as opposed to the login name) and other information pertaining to the user. Beware, however, that in many system users are able to change this information and therefore it cannot be trusted and therefore the $gcos is tainted (see perlsec). The $passwd and $shell, user's encrypted password and login shell, are also tainted, because of the same reason.
In scalar context, you get the name, unless the function was a lookup by name, in which case you get the other thing, whatever it is. (If the entry doesn't exist you get the undefined value.) For example:
$uid = getpwnam($name);
$name = getpwuid($num);
$name = getpwent();
$gid = getgrnam($name);
$name = getgrgid($num;
$name = getgrent();
#etc.
In getpw*() the fields $quota, $comment, and $expire are special cases in the sense that in many systems they are unsupported. If the $quota is unsupported, it is an empty scalar. If it is supported, it usually encodes the disk quota. If the $comment field is unsupported, it is an empty scalar. If it is supported it usually encodes some administrative comment about the user. In some systems the $quota field may be $change or $age, fields that have to do with password aging. In some systems the $comment field may be $class. The $expire field, if present, encodes the expiration period of the account or the password. For the availability and the exact meaning of these fields in your system, please consult your getpwnam(3) documentation and your pwd.h file. You can also find out from within Perl what your $quota and $comment fields mean and whether you have the $expire field by using the "Config" module and the values "d_pwquota", "d_pwage", "d_pwchange", "d_pwcomment", and "d_pwexpire". Shadow password files are only supported if your vendor has implemented them in the intuitive fashion that calling the regular C library routines gets the shadow versions if you're running under privilege or if there exists the shadow(3) functions as found in System V ( this includes Solaris and Linux.) Those systems which implement a proprietary shadow password facility are unlikely to be supported.
The $members value returned by getgr*() is a space separated list of the login names of the members of the group.
For the gethost*() functions, if the "h_errno" variable is supported in C, it will be returned to you via $? if the function call fails. The @addrs value returned by a successful call is a list of the raw addresses returned by the corresponding system library call. In the Internet domain, each address is four bytes long and you can unpack it by saying something like:
($a,$b,$c,$d) = unpack('C4',$addr[0]);
The Socket library makes this slightly easier:
use Socket;
$iaddr = inet_aton("127.1"); # or whatever address
$name = gethostbyaddr($iaddr, AF_INET);
# or going the other way
$straddr = inet_ntoa($iaddr);
If you get tired of remembering which element of the return list contains which return value, by-name interfaces are provided in standard modules: "File::stat", "Net::hostent", "Net::netent", "Net::protoent", "Net::servent", "Time::gmtime", "Time::localtime", and "User::grent". These override the normal built-ins, supplying 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, $myaddr) = sockaddr_in($mysockaddr);
printf "Connect to %s [%s]\n",
scalar gethostbyaddr($myaddr, AF_INET),
inet_ntoa($myaddr);
Beginning with v5.6.0, this operator is implemented using the standard "File::Glob" extension. See File::Glob for details.
# 0 1 2 3 4 5 6 7
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday) =
gmtime(time);
All list elements are numeric, and come straight out of the C `struct tm'. $sec, $min, and $hour are the seconds, minutes, and hours of the specified time. $mday is the day of the month, and $mon is the month itself, in the range 0..11 with 0 indicating January and 11 indicating December. $year is the number of years since 1900. That is, $year is 123 in year 2023. $wday is the day of the week, with 0 indicating Sunday and 3 indicating Wednesday. $yday is the day of the year, in the range 0..364 (or 0..365 in leap years.)
Note that the $year element is not simply the last two digits of the year. If you assume it is, then you create non-Y2K-compliant programs---and you wouldn't want to do that, would you?
The proper way to get a complete 4-digit year is simply:
$year += 1900;
And to get the last two digits of the year (e.g., '01' in 2001) do:
$year = sprintf("%02d", $year % 100);
If EXPR is omitted, "gmtime()" uses the current time ("gmtime(time)").
In scalar context, "gmtime()" 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 perllocale), but is instead a Perl builtin. Also see the "Time::Local" module, and the strftime(3) and mktime(3) functions available via the POSIX module. To get somewhat similar but locale dependent date strings, set up your locale environment variables appropriately (please see perllocale) and try for example:
use POSIX qw(strftime);
$now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
Note that the %a and %b escapes, which represent the short forms of the day of the week and the month of the year, may not necessarily be three characters wide in all locales.
The "goto-EXPR" form expects a label name, whose scope will be resolved dynamically. This allows for computed "goto"s per FORTRAN, but isn't necessarily recommended if you're optimizing for maintainability:
goto ("FOO", "BAR", "GLARCH")[$i];
The "goto-&NAME" form is quite different from the other forms of "goto". In fact, it isn't a goto in the normal sense at all, and doesn't have the stigma associated with other gotos. Instead, it exits the current subroutine (losing any changes set by local()) and immediately calls in its place the named subroutine using the current value of @_. 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.
NAME needn't be the name of a subroutine; it can be a scalar variable containing a code reference, or a block which evaluates to a code reference.
Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each element) and returns the list value consisting of those elements for which the expression evaluated to true. In scalar context, returns the number of times the expression was true.
@foo = grep(!/^#/, @bar); # weed out comments
or equivalently,
@foo = grep {!/^#/} @bar; # weed out comments
Note that $_ is an alias to the list value, so it can be used to modify the elements of the LIST. While this is useful and supported, it can cause bizarre results if the elements of LIST are not variables. Similarly, grep returns aliases into the original list, much as a for loop's index variable aliases the list elements. That is, modifying an element of a list returned by grep (for example, in a "foreach", "map" or another "grep") actually modifies the element in the original list. This is usually something to be avoided when writing clear code.
See also ``map'' for a list composed of the results of the BLOCK or EXPR.
print hex '0xAf'; # prints '175'
print hex 'aF'; # same
Hex strings may only represent integers. Strings that would cause integer overflow trigger a warning. Leading whitespace is not stripped, unlike oct().
require "ioctl.ph"; # probably in /usr/local/lib/perl/ioctl.ph
to get the correct function definitions. If ioctl.ph doesn't exist or doesn't have the correct definitions you'll have to roll your own, based on your C header files such as <sys/ioctl.h>. (There is a Perl script called h2ph that comes with the Perl kit that may help you in this, but it's nontrivial.) SCALAR will be read and/or written depending on the FUNCTION---a pointer to the string value of SCALAR will be passed as the third argument of the actual "ioctl" call. (If SCALAR has no string value but does have a numeric value, that value will be passed rather than a pointer to the string value. To guarantee this to be true, add a 0 to the scalar before using it.) The "pack" and "unpack" functions may be needed to manipulate the values of structures used by "ioctl".
The return value of "ioctl" (and "fcntl") is as follows:
if OS returns: then Perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
Thus Perl returns true on success and false on failure, yet you can still easily determine the actual value returned by the operating system:
$retval = ioctl(...) || -1;
printf "System returned %d\n", $retval;
The special string "0 but true" is exempt from -w complaints about improper numeric conversions.
Here's an example of setting a filehandle named "REMOTE" to be non-blocking at the system level. You'll have to negotiate $| on your own, though.
use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
$flags = fcntl(REMOTE, F_GETFL, 0)
or die "Can't get flags for the socket: $!\n";
$flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
or die "Can't set flags for the socket: $!\n";
$rec = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
Beware that unlike "split", "join" doesn't take a pattern as its first argument. Compare ``split''.
Here is yet another way to print your environment:
@keys = keys %ENV;
@values = values %ENV;
while (@keys) {
print pop(@keys), '=', pop(@values), "\n";
}
or how about sorted by key:
foreach $key (sort(keys %ENV)) {
print $key, '=', $ENV{$key}, "\n";
}
The returned values are copies of the original keys in the hash, so modifying them will not affect the original hash. Compare ``values''.
To sort a hash by value, you'll need to use a "sort" function. Here's a descending numeric sort of a hash by its values:
foreach $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) {
printf "%4d %s\n", $hash{$key}, $key;
}
As an lvalue "keys" allows you to increase the number of hash buckets allocated for the given hash. This can gain you a measure of efficiency if you know the hash is going to get big. (This is similar to pre-extending an array by assigning a larger number to $#array.) If you say
keys %hash = 200;
then %hash will have at least 200 buckets allocated for it--256 of them, in fact, since it rounds up to the next power of two. These buckets will be retained even if you do "%hash = ()", use "undef %hash" if you want to free the storage while %hash is still in scope. You can't shrink the number of buckets allocated for the hash using "keys" in this way (but you needn't worry about doing this by accident, as trying has no effect).
See also "each", "values" and "sort".
$cnt = kill 1, $child1, $child2;
kill 9, @goners;
If SIGNAL is zero, no signal is sent to the process. This is a useful way to check that the process is alive and hasn't changed its UID. See perlport for notes on the portability of this construct.
Unlike in the shell, if SIGNAL is negative, it kills process groups instead of processes. (On System V, a negative PROCESS number will also kill process groups, but that's not portable.) That means you usually want to use positive not negative signals. You may also use a signal name in quotes. See ``Signals'' in perlipc for details.
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
#...
}
"last" cannot be used to exit a block which returns a value such as "eval {}", "sub {}" or "do {}", and should not be used to exit a grep() or map() operation.
Note that a block by itself is semantically identical to a loop that executes once. Thus "last" can be used to effect an early exit out of such a block.
See also ``continue'' for an illustration of how "last", "next", and "redo" work.