| PERLFUNC(1) | Perl Programmers Reference Guide | PERLFUNC(1) |
In the syntax descriptions that follow, list operators that expect a list (and provide list context for 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. Commas should separate literal elements of the LIST.
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 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. Whitespace between the function and left parenthesis doesn't count, so sometimes you need to be careful:
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 "use warnings" pragma, 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 scalar context by returning the undefined value, and in list context by returning the empty 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 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 concatenation 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 ("syscalls") of the same name (like chown(2), fork(2), closedir(2), etc.) 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 include "wait", "waitpid", and "syscall". System calls also set the special $! variable on failure. Other functions do not, except accidentally.
Extension modules can also hook into the Perl parser to define new kinds of keyword-headed expression. These may look like functions, but may also look completely different. The syntax following the keyword is defined entirely by the extension. If you are an implementor, see "PL_keyword_plugin" in perlapi for the mechanism. If you are using such a module, see the module's documentation for details of the syntax that it defines.
"fc" is available only if the "fc" feature is enabled or if it is prefixed with "CORE::". The "fc" feature is enabled automatically with a "use v5.16" (or higher) declaration in the current scope.
"say" is available only if the "say" feature is enabled or if it is prefixed with "CORE::". The "say" feature is enabled automatically with a "use v5.10" (or higher) declaration in the current scope.
"break" is available only if you enable the experimental "switch" feature or use the "CORE::" prefix. The "switch" feature also enables the "default", "given" and "when" statements, which are documented in "Switch Statements" in perlsyn. The "switch" feature is enabled automatically with a "use v5.10" (or higher) declaration in the current scope. In Perl v5.14 and earlier, "continue" required the "switch" feature, like the other keywords.
"evalbytes" is only available with the "evalbytes" feature (see feature) or if prefixed with "CORE::". "__SUB__" is only available with the "current_sub" feature or if prefixed with "CORE::". Both the "evalbytes" and "current_sub" features are enabled automatically with a "use v5.16" (or higher) declaration in the current scope.
"state" is available only if the "state" feature is enabled or if it is prefixed with "CORE::". The "state" feature is enabled automatically with a "use v5.10" (or higher) declaration in the current scope.
"-X", "binmode", "chmod", "chown", "chroot", "crypt", "dbmclose", "dbmopen", "dump", "endgrent", "endhostent", "endnetent", "endprotoent", "endpwent", "endservent", "exec", "fcntl", "flock", "fork", "getgrent", "getgrgid", "gethostbyname", "gethostent", "getlogin", "getnetbyaddr", "getnetbyname", "getnetent", "getppid", "getpgrp", "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.
Despite the funny names, precedence is the same as any other named unary operator. The operator may be any of:
-r File is readable by effective uid/gid. -w File is writable by effective uid/gid. -x File is executable by effective uid/gid. -o File is owned by effective uid. -R File is readable by real uid/gid. -W File is writable by real uid/gid. -X File is executable by real uid/gid. -O File is owned by real uid. -e File exists. -z File has zero size (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 (false if symlinks aren't supported by the file system). -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 or UTF-8 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 $_; # ignore specials #... }
Note that "-s/a/b/" does not do a negated substitution. Saying "-exp($foo)" still works as expected, however: only single letters following a minus are interpreted as file tests.
These operators are exempt from the "looks like a function rule" described above. That is, an opening parenthesis after the operator does not affect how much of the following code constitutes the argument. Put the opening parentheses before the operator to separate it from code that follows (this applies only to operators with higher precedence than unary operators, of course):
-s($file) + 1024 # probably wrong; same as -s($file + 1024) (-s $file) + 1024 # correct
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: for example network filesystem access controls, ACLs (access control lists), read-only filesystems, and unrecognized executable formats. Note that the use of these six specific operators to verify if some operation is possible is usually a mistake, because it may be open to race conditions.
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 "use filetest 'access'", the above-mentioned filetests test whether the permission can(not) be granted using the access(2) family of system calls. Also note that the "-x" and "-X" tests 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. Note also that, due to the implementation of "use filetest 'access'", the "_" special filehandle won't cache the results of the file tests when this pragma is in effect. Read the documentation for the "filetest" pragma for more information.
The "-T" and "-B" tests work as follows. The first block or so of the file is examined to see if it is valid UTF-8 that includes non-ASCII characters. If so, it's a "-T" file. Otherwise, that same portion of the file is examined for odd characters such as strange control codes or characters with the high bit set. If more than a third of the characters are strange, it's a "-B" file; otherwise it's a "-T" file. Also, any file containing a zero byte in the examined portion is considered a binary file. (If executed within the scope of a use locale which includes "LC_CTYPE", odd characters are anything that isn't a printable nor space in the current locale.) 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 an empty 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" operator) is 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" leave values in the stat structure for the symbolic link, not the real file.) (Also, if the stat buffer was filled by an "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 _;
As of Perl 5.10.0, as a form of purely syntactic sugar, you can stack file test operators, in a way that "-f -w -x $file" is equivalent to "-x $file && -w _ && -f _". (This is only fancy syntax: if you use the return value of "-f $file" as an argument to another filetest operator, no special magic will happen.)
Portability issues: "-X" in perlport.
To avoid confusing would-be users of your code with mysterious syntax errors, put something like this at the top of your script:
use 5.010; # so filetest ops can stack
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, the Time::HiRes module (from CPAN, and starting from Perl 5.8 part of the standard distribution) provides "ualarm". You may also 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 setitimer(2) if your system supports it. See perlfaq8 for details.
It is usually a mistake to intermix "alarm" and "sleep" calls, because "sleep" may be internally implemented on 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; my $nread = sysread $socket, $buffer, $size; alarm 0; }; if ($@) { die unless $@ eq "alarm\n"; # propagate unexpected errors # timed out } else { # didn't }
For more information see perlipc.
Portability issues: "alarm" in perlport.
For the tangent operation, you may use the "Math::Trig::tan" function, or use the familiar relation:
sub tan { sin($_[0]) / cos($_[0]) }
The return value for "atan2(0,0)" is implementation-defined; consult your atan2(3) manpage for more information.
Portability issues: "atan2" in perlport.
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 always to use it when appropriate, and never to use it when it isn't appropriate. Also, people can set their I/O to be by default UTF8-encoded Unicode, not bytes.
In other words: regardless of platform, use "binmode" on binary data, like images, for example.
If LAYER is present it is a single string, but may contain multiple directives. The directives alter the behaviour of the filehandle. When LAYER is present, using binmode on a text file makes sense.
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, despite what may be implied in "Programming Perl" (the Camel, 3rd edition) or elsewhere, ":raw" is not simply the inverse of ":crlf". Other layers that would affect the binary nature of the stream are also disabled. See PerlIO, perlrun, and the discussion about the PERLIO environment variable.
The ":bytes", ":crlf", ":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.
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...
To mark FILEHANDLE as UTF-8, use ":utf8" or ":encoding(UTF-8)". ":utf8" just marks the data as UTF-8 without further checking, while ":encoding(UTF-8)" checks the data for actually being valid UTF-8. More details can be found in PerlIO::encoding.
In general, "binmode" should be called after "open" but before any I/O is done on the filehandle. Calling "binmode" normally flushes 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. The ":encoding" layer sometimes needs to be called in mid-stream, and it doesn't flush the stream. ":encoding" also implicitly pushes on top of itself the ":utf8" layer because internally Perl operates on UTF8-encoded Unicode characters.
The operating system, device drivers, C libraries, and Perl run-time system all conspire to let the programmer treat a single character ("\n") as the line terminator, irrespective of 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.
All variants of Unix, Mac OS (old and new), 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 old, pre-Darwin flavors of Mac OS, and is 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 contain "\cZ", the I/O subsystem will regard it as the end of the file, unless you use "binmode".
"binmode" is important not only 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.
Portability issues: "binmode" in perlport.
Consider always blessing objects in CLASSNAMEs that are mixed case. Namespaces with all lowercase names are considered reserved for Perl pragmas. Builtin types have all uppercase names. To prevent confusion, you may wish to avoid such package names as well. It is advised to avoid the class name 0, because much code erroneously uses the result of "ref" as a truth value.
See "Perl Modules" in perlmod.
"break" is available only if the "switch" feature is enabled or if it is prefixed with "CORE::". The "switch" feature is enabled automatically with a "use v5.10" (or higher) declaration in the current scope.
# 0 1 2 my ($package, $filename, $line) = caller;
With EXPR, it returns some extra information that the debugger uses to print a stack trace. The value of EXPR indicates how many call frames to go back before the current one.
# 0 1 2 3 4 my ($package, $filename, $line, $subroutine, $hasargs, # 5 6 7 8 9 10 $wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash) = caller($i);
Here, $subroutine is the function that the caller called (rather than the function containing the caller). Note that $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, $subroutine 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. $hints corresponds to $^H, and $bitmask corresponds to "${^WARNING_BITS}". The $hints and $bitmask values are subject to change between versions of Perl, and are not meant for external use.
$hinthash is a reference to a hash containing the value of "%^H" when the caller was compiled, or "undef" if "%^H" was empty. Do not modify the values of this hash, as they are the actual values stored in the optree.
Furthermore, when called from within the DB package in list context, and with an argument, 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 to, for "N > 1". In particular, @DB::args might have information from the previous time "caller" was called.
Be aware that setting @DB::args is best effort, intended for debugging or generating backtraces, and should not be relied upon. In particular, as @_ contains aliases to the caller's arguments, Perl does not take a copy of @_, so @DB::args will contain modifications the subroutine makes to @_ or its contents, not the original values at call time. @DB::args, like @_, does not hold explicit references to its elements, so under certain cases its elements may have become freed and reallocated for other variables or temporary values. Finally, a side effect of the current implementation is that the effects of "shift @_" can normally be undone (but not "pop @_" or other splicing, and not if a reference to @_ has been taken, and subject to the caveat about reallocated elements), so @DB::args is actually a hybrid of the current state and initial state of @_. Buyer beware.
On systems that support fchdir(2), you may pass a filehandle or directory handle as the argument. On systems that don't support fchdir(2), passing handles raises an exception.
my $cnt = chmod 0755, "foo", "bar"; chmod 0755, @executables; my $mode = "0644"; chmod $mode, "foo"; # !!! sets mode to # --w----r-T my $mode = "0644"; chmod oct($mode), "foo"; # this is better my $mode = 0644; chmod $mode, "foo"; # this is best
On systems that support fchmod(2), you may pass filehandles among the files. On systems that don't support fchmod(2), passing filehandles raises an exception. Filehandles must be passed as globs or glob references to be recognized; barewords are considered filenames.
open(my $fh, "<", "foo"); my $perm = (stat $fh)[2] & 07777; chmod($perm | 0600, $fh);
You can also import the symbolic "S_I*" constants from the "Fcntl" module:
use Fcntl qw( :mode ); chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables; # Identical to the chmod 0755 of the example above.
Portability issues: "chmod" in perlport.
while (<>) { chomp; # avoid \n on last field my @array = split(/:/); # ... }
If VARIABLE is a hash, it chomps the hash's values, but not its keys, resetting the "each" iterator in the process.
You can actually chomp anything that's an lvalue, including an assignment:
chomp(my $cwd = `pwd`); chomp(my $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".
my $cnt = chown $uid, $gid, 'foo', 'bar'; chown $uid, $gid, @filenames;
On systems that support fchown(2), you may pass filehandles among the files. On systems that don't support fchown(2), passing filehandles raises an exception. Filehandles must be passed as globs or glob references to be recognized; barewords are considered filenames.
Here's an example that looks up nonnumeric uids in the passwd file:
print "User: "; chomp(my $user = <STDIN>); print "Files: "; chomp(my $pattern = <STDIN>); my ($login,$pass,$uid,$gid) = getpwnam($user) or die "$user not in passwd file"; my @ary = glob($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); my $can_chown_giveaway = ! sysconf(_PC_CHOWN_RESTRICTED);
Portability issues: "chown" in perlport.
Negative values give the Unicode replacement character (chr(0xfffd)), except under the bytes pragma, where the low eight bits of the value (truncated to an integer) are used.
If NUMBER is omitted, uses $_.
For the reverse, use "ord".
Note that characters from 128 to 255 (inclusive) are by default internally not encoded as UTF-8 for backward compatibility reasons.
See perlunicode for more about Unicode.
NOTE: It is good security practice to do "chdir("/")" ("chdir" to the root directory) immediately after a "chroot".
Portability issues: "chroot" in perlport.
You don't have to close FILEHANDLE if you are immediately going to do another "open" on it, because "open" closes it for you. (See "open".) However, an explicit "close" on an input file resets the line counter ($.), while the implicit close done by "open" does not.
If the filehandle came from a piped open, "close" returns false if one of the other syscalls involved fails or if its 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 exit--in case you wish to look at the output of the pipe afterwards--and implicitly puts the exit status value of that command into $? and "${^CHILD_ERROR_NATIVE}".
If there are multiple threads running, "close" on a filehandle from a piped open returns true without waiting for the child process to terminate, if the filehandle is still open in another thread.
Closing the read end of a pipe before the process writing to it at the other end is done writing results in the writer receiving a SIGPIPE. 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 or an autovivified handle.
"last", "next", or "redo" may appear within a "continue" block; "last" and "redo" 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 equivalent to using an empty one, logically enough, so "next" goes directly back to check the condition at the top of the loop.
When there is no BLOCK, "continue" is a function that falls through the current "when" or "default" block instead of iterating a dynamically enclosing "foreach" or exiting a lexically enclosing "given". In Perl 5.14 and earlier, this form of "continue" was only available when the "switch" feature was enabled. See feature and "Switch Statements" in perlsyn for more information.
For the inverse cosine operation, you may use the "Math::Trig::acos" function, or use this relation:
sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
"crypt" is a one-way hash function. The PLAINTEXT and SALT are turned into a short string, called a digest, which is returned. The same PLAINTEXT and SALT will always return the same string, but there is no (known) way to get the original PLAINTEXT from the hash. Small changes in the PLAINTEXT or SALT will result in large changes in the digest.
There is no decrypt function. This function isn't all that useful for cryptography (for that, look for Crypt modules on your nearby CPAN mirror) and the name "crypt" is a bit of a misnomer. Instead it is primarily used to check if two pieces of text are the same without having to transmit or store the text itself. An example is checking if a correct password is given. The digest of the password is stored, not the password itself. The user types in a password that is "crypt"'d with the same salt as the stored digest. If the two digests match, the password is correct.
When verifying an existing digest string you should use the digest as the salt (like "crypt($plain, $digest) eq $digest"). The SALT used to create the digest is visible as part of the digest. This ensures "crypt" will hash the new string with the same salt as the digest. This allows your code to work with the standard "crypt" and with more exotic implementations. In other words, assume nothing about the returned string itself nor about how many bytes of SALT may 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 PLAINTEXT 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]"). This set of characters is just a recommendation; the characters allowed in the salt depend solely on your system's crypt library, and Perl can't restrict what salts "crypt" accepts.
Here's an example that makes sure that whoever runs this program knows their password:
my $pwd = (getpwuid($<))[1]; system "stty -echo"; print "Password: "; chomp(my $word = <STDIN>); print "\n"; system "stty echo"; if (crypt($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 hashing large quantities of data, not least of all because you can't get the information back. Look at the Digest module for more robust algorithms.
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 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".
Portability issues: "crypt" in perlport.
Breaks the binding between a DBM file and a hash.
Portability issues: "dbmclose" in perlport.
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"). To prevent creation of the database if it doesn't exist, you may specify a MODE of 0, and the function will return a false value if it can't find an existing database. If your system supports only the older DBM functions, you may make only one "dbmopen" call 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" to 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: $!";
Portability issues: "dbmopen" in perlport.
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 "func". A subroutine that 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 no longer supported. It used to report whether memory for that aggregate had ever been allocated. 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 are then 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, although it matched "nothing". It didn't really fail to match anything. 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 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".
In list context, usually returns the value or values deleted, or the last such element in scalar context. The return list's length corresponds to that of the argument list: deleting non-existent elements returns the undefined value in their corresponding positions. When a key/value hash slice is passed to "delete", the return value is a list of key/value pairs (two elements for each item deleted from the hash).
"delete" may also be used on arrays and array slices, but its behavior is less straightforward. Although "exists" will return false for deleted entries, deleting array elements never changes indices of existing values; use "shift" or "splice" for that. However, if any deleted elements fall at the end of an array, the array's size shrinks to the position of the highest element that still tests true for "exists", or to 0 if none do. In other words, an array won't have trailing nonexistent elements after a delete.
WARNING: Calling "delete" on array values is strongly discouraged. The notion of deleting or checking the existence of Perl array elements is not conceptually coherent, and can lead to surprising behavior.
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 "tied" hash or array may not necessarily return anything; it depends on the implementation of the "tied" package's DELETE method, which may do whatever it pleases.
The "delete local EXPR" construct localizes the deletion to the current block at run time. Until the block exits, elements locally deleted temporarily no longer exist. See "Localized deletion of elements of composite types" in perlsub.
my %hash = (foo => 11, bar => 22, baz => 33); my $scalar = delete $hash{foo}; # $scalar is 11 $scalar = delete @hash{qw(foo bar)}; # $scalar is 22 my @array = delete @hash{qw(foo baz)}; # @array is (undef,33)
The following (inefficiently) deletes all the values of %HASH and @ARRAY:
foreach my $key (keys %HASH) { delete $HASH{$key}; } foreach my $index (0 .. $#ARRAY) { delete $ARRAY[$index]; }
And so do these:
delete @HASH{keys %HASH}; delete @ARRAY[0 .. $#ARRAY];
But both are slower than assigning the empty list or undefining %HASH or @ARRAY, which is the customary way to empty out an aggregate:
%HASH = (); # completely empty %HASH undef %HASH; # forget %HASH ever existed @ARRAY = (); # completely empty @ARRAY undef @ARRAY; # forget @ARRAY ever existed
The EXPR can be arbitrarily complicated provided its final operation is an element or slice of an aggregate:
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"
Most of the time, "die" is called with a string to use as the exception. You may either give a single non-reference operand to serve as the exception, or a list of two or more items, which will be stringified and concatenated to make the exception.
If the string exception does not end in a newline, the current script line number and input line number (if any) and a newline are appended to it. 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.
If LIST was empty or made an empty string, and $@ already contains an exception value (typically from a previous "eval"), then that value is reused after appending "\t...propagated". This is useful for propagating exceptions:
eval { ... }; die unless $@ =~ /Expected exception/;
If LIST was empty or made an empty string, 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 $@; i.e., as if "$@ = eval { $@->PROPAGATE(__FILE__, __LINE__) };" were called.
If LIST was empty or made an empty string, and $@ is also empty, then the string "Died" is used.
You can also call "die" with a reference argument, and if this is trapped within an "eval", $@ contains that reference. This permits more elaborate exception handling using objects that maintain arbitrary state about the exception. Such a scheme is sometimes preferable to matching particular string values of $@ with regular expressions.
Because Perl stringifies uncaught exception messages before display, you'll probably want to overload stringification operations on exception objects. See overload for details about that. The stringified message should be non-empty, and should end in a newline, in order to fit in with the treatment of string exceptions. Also, because an exception object reference cannot be stringified without destroying it, Perl doesn't attempt to append location or other information to a reference exception. If you want location information with a complex exception object, you'll have to arrange to put the location information into the object yourself.
Because $@ is a global variable, be careful that analyzing an exception caught by "eval" doesn't replace the reference in the global variable. It's easiest to make a local copy of the reference before any manipulations. Here's an example:
use Scalar::Util "blessed"; eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) }; if (my $ev_err = $@) { if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) { # handle Some::Module::Exception } else { # handle all other possible exceptions } }
If an uncaught exception results in interpreter exit, the exit code is determined from the values of $! and $? with this pseudocode:
exit $! if $!; # errno exit $? >> 8 if $? >> 8; # child exit status exit 255; # last resort
As with "exit", $? is set prior to unwinding the call stack; any "DESTROY" or "END" handlers can then alter this value, and thus Perl's exit code.
The intent is to squeeze as much possible information about the likely cause into the limited space of the system exit code. However, as $! is the value of C's "errno", which can be set by any system call, this means that the value of the exit code used by "die" can be non-predictable, so should not be relied upon, other than to be non-zero.
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 is called with the exception as an argument, and can change the exception, if it sees fit, by calling "die" again. See "%SIG" in perlvar for details on setting %SIG entries, and "eval" for some examples. Although this feature was to be run only right before your program was to exit, this is not currently so: 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.
See also "exit", "warn", and the Carp module.
"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.
# load the exact specified file (./ and ../ special-cased) do '/foo/stat.pl'; do './stat.pl'; do '../foo/stat.pl'; # search for the named file within @INC do 'stat.pl'; do 'foo/stat.pl';
"do './stat.pl'" is largely like
eval `cat stat.pl`;
except that it's more concise, runs no external processes, and keeps track of the current filename for error messages. It also differs in that code evaluated with "do FILE" 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.
Using "do" with a relative path (except for ./ and ../), like
do 'foo/stat.pl';
will search the @INC directories, and update %INC if the file is found. See "@INC" in perlvar and "%INC" in perlvar for these variables. In particular, note that whilst historically @INC contained '.' (the current directory) making these two cases equivalent, that is no longer necessarily the case, as '.' is not included in @INC by default in perl versions 5.26.0 onwards. Instead, perl will now warn:
do "stat.pl" failed, '.' is no longer in @INC; did you mean do "./stat.pl"?
If "do" can read the file but cannot compile it, it returns "undef" and sets an error message in<EMAIL_ADDRESS>If "do" cannot read the file, it returns undef and sets $! to the error. Always check $@ first, as compilation could fail in a way that also sets $!. If the file is successfully compiled, "do" returns the value of the last expression evaluated.
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. # Beware of using relative pathnames here. 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 by Perl.
This function is now largely obsolete, mostly because it's very hard to convert a core file into an executable. As of Perl 5.30, it must be invoked as "CORE::dump()".
Unlike most named operators, this has the same precedence as assignment. It is also exempt from the looks-like-a-function rule, so "dump ("foo")."bar"" will cause "bar" to be part of the argument to "dump".
Portability issues: "dump" in perlport.
Hash entries are returned in an apparently random order. The actual random order is specific to a given hash; the exact same series of operations on two hashes may result in a different order for each hash. Any insertion into the hash may change the order, as will any deletion, with the exception that the most recent key returned by "each" or "keys" may be deleted without changing the order. So long as a given hash is unmodified you may rely on "keys", "values" and "each" to repeatedly return the same order as each other. See "Algorithmic Complexity Attacks" in perlsec for details on why hash order is randomized. Aside from the guarantees provided here the exact details of Perl's hash algorithm and the hash traversal order are subject to change in any release of Perl.
After "each" has returned all entries from the hash or array, the next call to "each" returns the empty list in list context and "undef" in scalar context; the next call following that one restarts iteration. Each hash or array has its own internal iterator, accessed by "each", "keys", and "values". The iterator is implicitly reset when "each" has reached the end as just described; it can be explicitly reset by calling "keys" or "values" on the hash or array, or by referencing the hash (but not array) in list context. If you add or delete a hash's elements while iterating over it, the effect on the iterator is unspecified; for example, entries may be skipped or duplicated--so don't do that. Exception: It is always safe to delete the item most recently returned by "each", so the following code works properly:
while (my ($key, $value) = each %hash) { print $key, "\n"; delete $hash{$key}; # This is safe }
Tied hashes may have a different ordering behaviour to perl's hash implementation.
The iterator used by "each" is attached to the hash or array, and is shared between all iteration operations applied to the same hash or array. Thus all uses of "each" on a single hash or array advance the same iterator location. All uses of "each" are also subject to having the iterator reset by any use of "keys" or "values" on the same hash or array, or by the hash (but not array) being referenced in list context. This makes "each"-based loops quite fragile: it is easy to arrive at such a loop with the iterator already part way through the object, or to accidentally clobber the iterator state during execution of the loop body. It's easy enough to explicitly reset the iterator before starting a loop, but there is no way to insulate the iterator state used by a loop from the iterator state used by anything else that might execute during the loop body. To avoid these problems, use a "foreach" loop rather than "while"-"each".
This prints out your environment like the printenv(1) program, but in a different order:
while (my ($key,$value) = each %ENV) { print "$key=$value\n"; }
Starting with Perl 5.14, an experimental feature allowed "each" to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24.
As of Perl 5.18 you can use a bare "each" in a "while" loop, which will set $_ on every iteration. If either an "each" expression or an explicit assignment of an "each" expression to a scalar is used as a "while"/"for" condition, then the condition actually tests for definedness of the expression's value, not for its regular truth value.
while (each %ENV) { print "$_=$ENV{$_}\n"; }
To avoid confusing would-be users of your code who are running earlier versions of Perl with mysterious syntax errors, put this sort of thing at the top of your file to signal that your code will work only on Perls of a recent vintage:
use 5.012; # so keys/values/each work on arrays use 5.018; # so each assigns to $_ in a lone while test
See also "keys", "values", and "sort".
An "eof" without an argument uses the last file read. Using "eof()" with empty parentheses is different. It refers to the pseudo file formed from the files listed on the command line and accessed via the "<>" operator. Since "<>" isn't explicitly opened, as a normal filehandle is, an "eof()" before "<>" has been used will cause @ARGV to be examined to determine if input is available. Similarly, an "eof()" after "<>" has returned end-of-file will assume you are processing another @ARGV list, and if you haven't set @ARGV, will read input from "STDIN"; see "I/O Operators" in perlop.
In a "while (<>)" loop, "eof" or "eof(ARGV)" can be used to detect the end of each file, whereas "eof()" will detect the end of the very last file only. Examples:
# reset line numbering on each input file while (<>) { next if /^\s*#/; # skip comments print "$.\t$_"; } continue { close ARGV if eof; # Not eof()! } # insert dashes just before last line of last file while (<>) { if (eof()) { # check for end of last file print "--------------\n"; } print; last if eof(); # needed if we're reading from a terminal }
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 encounter an error.
Plain "eval" with no argument is just "eval EXPR", where the expression is understood to be contained in $_. Thus there are only two real "eval" forms; the one with an EXPR is often called "string eval". In a string eval, the value of the expression (which is itself determined within scalar context) is first parsed, and if there were no errors, executed as a block within the lexical context of the current Perl program. This form is typically used to delay parsing and subsequent execution of the text of EXPR until run time. Note that the value is parsed every time the "eval" executes.
The other form is called "block eval". It is less general than string eval, but 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, while also providing the benefit of checking the code within BLOCK at compile time. BLOCK is parsed and compiled just once. Since errors are trapped, it often is used to check if a given feature is available.
In both forms, the value returned is the value of the last expression evaluated inside the mini-program; a return statement may also be 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, "eval" returns "undef" in scalar context, or an empty list in list context, and $@ is set to the error message. (Prior to 5.16, a bug caused "undef" to be returned in list context for syntax errors, but not for runtime errors.) If there was no error, $@ is set to the empty string. A control flow operator like "last" or "goto" can bypass the setting of<EMAIL_ADDRESS>Beware that using "eval" neither silences Perl from printing warnings to STDERR, nor does it stuff the text of warning messages into<EMAIL_ADDRESS>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, and warnings.
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.
Before Perl 5.14, the assignment to $@ occurred before restoration of localized variables, which means that for your code to run on older versions, a temporary is required if you want to mask some, but not all errors:
# alter $@ on nefarious repugnancy only { my $e; { local $@; # protect existing $@ eval { test_repugnancy() }; # $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only $@ =~ /nefarious/ and $e = $@; } die $e if defined $e }
There are some different considerations for each form:
In a plain "eval" without an EXPR argument, being in "use utf8" or not is irrelevant; the UTF-8ness of $_ itself determines the behavior.
Any "use utf8" or "no utf8" declarations within the string have no effect, and source filters are forbidden. ("unicode_strings", however, can appear within the string.) See also the "evalbytes" operator, which works properly with source filters.
Variables defined outside the "eval" and used inside it retain their original UTF-8ness. Everything inside the string follows the normal rules for a Perl program with the given state of "use utf8".
BEGIN { eval "use Semi::Semicolons; # not filtered" } # filtered here!
"evalbytes" fixes that to work the way one would expect:
use feature "evalbytes"; BEGIN { evalbytes "use Semi::Semicolons; # filtered" } # not filtered
Problems can arise if the string expands a scalar containing a floating point number. That scalar can expand to letters, such as "NaN" or "Infinity"; or, within the scope of a "use locale", the decimal point character may be something other than a dot (such as a comma). None of these are likely to parse as you are likely expecting.
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.
An "eval ''" executed within a subroutine defined in the "DB" package doesn't see the usual surrounding lexical scope, but rather the scope of the first non-DB piece of code that called it. You don't normally need to worry about this unless you are writing a Perl debugger.
The final semicolon, if any, may be omitted from the value of EXPR.
# 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 $@
If you want to trap errors when loading an XS module, some problems with the binary interface (such as Perl version skew) may be fatal even with "eval" unless $ENV{PERL_DL_NONLAZY} is set. See perlrun.
Using the "eval {}" form as an exception trap in libraries does have some issues. Due to the current arguably broken state of "__DIE__" hooks, 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 this example shows:
# a 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.
"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.
The final semicolon, if any, may be omitted from within the BLOCK.
If called when "use utf8" is in effect, the string will be assumed to be encoded in UTF-8, and "evalbytes" will make a temporary copy to work from, downgraded to non-UTF-8. If this is not possible (because one or more characters in it require UTF-8), the "evalbytes" will fail with the error stored in<EMAIL_ADDRESS>Bytes that correspond to ASCII-range code points will have their normal meanings for operators in the string. The treatment of the other bytes depends on if the "'unicode_strings"" feature is in effect.
Of course, variables that are UTF-8 and are referred to in the string retain that:
my $a = "\x{100}"; evalbytes 'print ord $a, "\n"';
prints
256
and $@ is empty.
Source filters activated within the evaluated code apply to the code itself.
"evalbytes" is available starting in Perl v5.16. To access it, you must say "CORE::evalbytes", but you can omit the "CORE::" if the "evalbytes" feature is enabled. This is enabled automatically with a "use v5.16" (or higher) declaration in the current scope.
Since it's a common mistake to use "exec" instead of "system", Perl warns you if "exec" is called in void context and if there is a following statement that isn't "die", "warn", or "exit" (if warnings are enabled--but you always do that, right?). 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, this calls execvp(3) with the arguments in LIST. If there is only one element in LIST, 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, as in "exec PROGRAM LIST". (This always forces interpretation of the LIST as a multivalued list, even if there is only a single scalar in the list.) Example:
my $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 are 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.
my @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 named "echo surprise", didn't find it, and set $? to a non-zero value indicating failure.
On Windows, only the "exec PROGRAM LIST" indirect object syntax will reliably avoid using the shell; "exec LIST", even with more than one element, will fall back to the shell if the first spawn fails.
Perl attempts 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 to avoid lost output.
Note that "exec" will not call your "END" blocks, nor will it invoke "DESTROY" methods on your objects.
Portability issues: "exec" in perlport.
print "Exists\n" if exists $hash{$key}; print "Defined\n" if defined $hash{$key}; print "True\n" if $hash{$key};
exists may also be called on array elements, but its behavior is much less obvious and is strongly tied to the use of "delete" on arrays.
WARNING: Calling "exists" on array values is strongly discouraged. The notion of deleting or checking the existence of Perl array elements is not conceptually coherent, and can lead to surprising behavior.
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 only 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 that 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 most deeply nested array or hash element 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 here:
undef $ref; if (exists $ref->{"Some key"}) { } print $ref; # prints HASH(0x80d3d5c)
Use of a subroutine call, rather than a subroutine name, as an argument to "exists" is an error.
exists ⊂ # OK exists &sub(); # Error
my $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. "END" routines and destructors can change the exit status by modifying $?. If this is a problem, you can call "POSIX::_exit($status)" to avoid "END" and destructor processing. See perlmod for details.
Portability issues: "exit" in perlport.
Casefolding is the process of mapping strings to a form where case differences are erased; comparing two strings in their casefolded form is effectively a way of asking if two strings are equal, regardless of case.
Roughly, if you ever found yourself writing this
lc($this) eq lc($that) # Wrong! # or uc($this) eq uc($that) # Also wrong! # or $this =~ /^\Q$that\E\z/i # Right!
Now you can write
fc($this) eq fc($that)
And get the correct results.
Perl only implements the full form of casefolding, but you can access the simple folds using "casefold()" in Unicode::UCD and "prop_invmap()" in Unicode::UCD. For further information on casefolding, refer to the Unicode Standard, specifically sections 3.13 "Default Case Operations", 4.2 "Case-Normative", and 5.18 "Case Mappings", available at <http://www.unicode.org/versions/latest/>, as well as the Case Charts available at <http://www.unicode.org/charts/case/>.
If EXPR is omitted, uses $_.
This function behaves the same way under various pragmas, such as within "use feature 'unicode_strings", as "lc" does, with the single exception of "fc" of LATIN CAPITAL LETTER SHARP S (U+1E9E) within the scope of "use locale". The foldcase of this character would normally be "ss", but as explained in the "lc" section, case changes that cross the 255/256 boundary are problematic under locales, and are hence prohibited. Therefore, this function under locale returns instead the string "\x{17F}\x{17F}", which is the LATIN SMALL LETTER LONG S. Since that character itself folds to "s", the string of two of them together should be equivalent to a single U+1E9E when foldcased.
While the Unicode Standard defines two additional forms of casefolding, one for Turkic languages and one that never maps one character into multiple characters, these are not provided by the Perl core. However, the CPAN module "Unicode::Casing" may be used to provide an implementation.
"fc" is available only if the "fc" feature is enabled or if it is prefixed with "CORE::". The "fc" feature is enabled automatically with a "use v5.16" (or higher) declaration in the current scope.
use Fcntl;
first to get the correct constant definitions. Argument processing and value returned work just like "ioctl" below. For example:
use Fcntl; my $flags = fcntl($filehandle, F_GETFL, 0) or die "Can't fcntl F_GETFL: $!";
You don't have to check for "defined" on the return from "fcntl". 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 "Argument "..." isn't numeric" warnings on improper numeric conversions.
Note that "fcntl" raises an exception 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.
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); my $flags = fcntl($REMOTE, F_GETFL, 0) or die "Can't get flags for the socket: $!\n"; fcntl($REMOTE, F_SETFL, $flags | O_NONBLOCK) or die "Can't set flags for the socket: $!\n";
Portability issues: "fcntl" in perlport.
This is mainly useful for constructing bitmaps for "select" and low-level POSIX tty-handling operations. If FILEHANDLE is an expression, the value is taken as an indirect filehandle, generally its name.
You can use this to find out whether two handles refer to the same underlying descriptor:
if (fileno($this) != -1 && fileno($this) == fileno($that)) { print "\$this and \$that are dups\n"; } elsif (fileno($this) != -1 && fileno($that) != -1) { print "\$this and \$that have different " . "underlying file descriptors\n"; } else { print "At least one of \$this and \$that does " . "not have a real file descriptor\n"; }
The behavior of "fileno" on a directory handle depends on the operating system. On a system with dirfd(3) or similar, "fileno" on a directory handle returns the underlying file descriptor associated with the handle; on systems with no such support, it returns the undefined value, and sets $! (errno).
Two potentially non-obvious but traditional "flock" semantics are that it waits indefinitely until the lock is granted, and that its locks are merely advisory. Such discretionary locks are more flexible, but offer fewer guarantees. This means that programs that do not also use "flock" may modify files locked with "flock". See perlport, your port's specific documentation, and 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" returns 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 and build a new Perl.

See also DB_File for other "flock" examples.
Portability issues: "flock" in perlport.
Perl attempts 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 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.
On some platforms such as Windows, where the fork(2) system call is not available, Perl can be built to emulate "fork" in the Perl interpreter. The emulation is designed, at the level of the Perl program, to be as compatible as possible with the "Unix" fork(2). However it has limitations that have to be considered in code intended to be portable. See perlfork for more details.
Portability issues: "fork" in perlport.
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 you are trying to use this instead of "write" to capture the output, you may find it easier to open a filehandle to a scalar ("open my $fh, ">", \$output") and write to that instead.
if ($BSD_STYLE) { system "stty cbreak </dev/tty >/dev/tty 2>&1"; } else { system "stty", '-icanon', 'eol', "\001"; } my $key = getc(STDIN); if ($BSD_STYLE) { system "stty -cbreak </dev/tty >/dev/tty 2>&1"; } else { system 'stty', 'icanon', 'eol', '^@'; # ASCII NUL } 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 on CPAN.
my $login = getlogin || getpwuid($<) || "Kilroy";
Do not consider "getlogin" for authentication: it is not as secure as "getpwuid".
Portability issues: "getlogin" in perlport.
use Socket; my $hersockaddr = getpeername($sock); my ($port, $iaddr) = sockaddr_in($hersockaddr); my $herhostname = gethostbyaddr($iaddr, AF_INET); my $herstraddr = inet_ntoa($iaddr);
Portability issues: "getpgrp" in perlport.
Note for Linux users: Between v5.8.1 and v5.16.0 Perl would work around non-POSIX thread semantics the minority of Linux systems (and Debian GNU/kFreeBSD systems) that used LinuxThreads, this emulation has since been removed. See the documentation for $$ for details.
Portability issues: "getppid" in perlport.
"WHICH" can be any of "PRIO_PROCESS", "PRIO_PGRP" or "PRIO_USER" imported from "RESOURCE CONSTANTS" in POSIX.
Portability issues: "getpriority" in perlport.
# 0 1 2 3 4 my ( $name, $passwd, $gid, $members ) = getgr* my ( $name, $aliases, $addrtype, $net ) = getnet* my ( $name, $aliases, $port, $proto ) = getserv* my ( $name, $aliases, $proto ) = getproto* my ( $name, $aliases, $addrtype, $length, @addrs ) = gethost* my ( $name, $passwd, $uid, $gid, $quota, $comment, $gcos, $dir, $shell, $expire ) = getpw* # 5 6 7 8 9
(If the entry doesn't exist, the return value is a single meaningless true value.)
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, for 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:
my $uid = getpwnam($name); my $name = getpwuid($num); my $name = getpwent(); my $gid = getgrnam($name); my $name = getgrgid($num); my $name = getgrent(); # etc.
In getpw*() the fields $quota, $comment, and $expire are special in that they are unsupported on many systems. 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 getpwnam(3) and your system's 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 supported only 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 that 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 raw addresses returned by the corresponding library call. In the Internet domain, each address is four bytes long; you can unpack it by saying something like:
my ($w,$x,$y,$z) = unpack('W4',$addr[0]);
The Socket library makes this slightly easier:
use Socket; my $iaddr = inet_aton("127.1"); # or whatever address my $name = gethostbyaddr($iaddr, AF_INET); # or going the other way my $straddr = inet_ntoa($iaddr);
In the opposite way, to resolve a hostname to the IP address you can write this:
use Socket; my $packed_ip = gethostbyname("www.perl.org"); my $ip_address; if (defined $packed_ip) { $ip_address = inet_ntoa($packed_ip); }
Make sure "gethostbyname" is called in SCALAR context and that its return value is checked for definedness.
The "getprotobynumber" function, even though it only takes one argument, has the precedence of a list operator, so beware:
getprotobynumber $number eq 'icmp' # WRONG getprotobynumber($number eq 'icmp') # actually means this getprotobynumber($number) eq 'icmp' # better this way
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; my $is_his = (stat($filename)->uid == pwent($whoever)->uid);
Even though it looks as though they're the same method calls (uid), they aren't, because a "File::stat" object is different from a "User::pwent" object.
Many of these functions are not safe in a multi-threaded environment where more than one thread can be using them. In particular, functions like "getpwent()" iterate per-process and not per-thread, so if two threads are simultaneously iterating, neither will get all the records.
Some systems have thread-safe versions of some of the functions, such as "getpwnam_r()" instead of "getpwnam()". There, Perl automatically and invisibly substitutes the thread-safe version, without notice. This means that code that safely runs on some systems can fail on others that lack the thread-safe versions.
Portability issues: "getpwnam" in perlport to "endservent" in perlport.
use Socket; my $mysockaddr = getsockname($sock); my ($port, $myaddr) = sockaddr_in($mysockaddr); printf "Connect to %s [%s]\n", scalar gethostbyaddr($myaddr, AF_INET), inet_ntoa($myaddr);
The function returns a packed string representing the requested socket option, or "undef" on error, with the reason for the error placed in $!. Just what is in the packed string depends on LEVEL and OPTNAME; consult getsockopt(2) for details. A common case is that the option is an integer, in which case the result is a packed integer, which you can decode using "unpack" with the "i" (or "I") format.
Here's an example to test whether Nagle's algorithm is enabled on a socket:
use Socket qw(:all); defined(my $tcp = getprotobyname("tcp")) or die "Could not determine the protocol number for tcp"; # my $tcp = IPPROTO_TCP; # Alternative my $packed = getsockopt($socket, $tcp, TCP_NODELAY) or die "getsockopt TCP_NODELAY: $!"; my $nodelay = unpack("I", $packed); print "Nagle's algorithm is turned ", $nodelay ? "off\n" : "on\n";
Portability issues: "getsockopt" in perlport.
Note that "glob" splits its arguments on whitespace and treats each segment as separate pattern. As such, "glob("*.c *.h")" matches all files with a .c or .h extension. The expression "glob(".* *")" matches all files in the current working directory. If you want to glob filenames that might contain whitespace, you'll have to use extra quotes around the spacey filename to protect it. For example, to glob filenames that have an "e" followed by a space followed by an "f", use one of:
my @spacies = <"*e f*">; my @spacies = glob '"*e f*"'; my @spacies = glob q("*e f*");
If you had to get a variable through, you could do this:
my @spacies = glob "'*${var}e f*'"; my @spacies = glob qq("*${var}e f*");
If non-empty braces are the only wildcard characters used in the "glob", no filenames are matched, but potentially many strings are returned. For example, this produces nine strings, one for each pairing of fruits and colors:
my @many = glob "{apple,tomato,cherry}={green,yellow,red}";
This operator is implemented using the standard "File::Glob" extension. See File::Glob for details, including "bsd_glob", which does not treat whitespace as a pattern separator.
If a "glob" expression is used as the condition of a "while" or "for" loop, then it will be implicitly assigned to $_. If either a "glob" expression or an explicit assignment of a "glob" expression to a scalar is used as a "while"/"for" condition, then the condition actually tests for definedness of the expression's value, not for its regular truth value.
Portability issues: "glob" in perlport.
Note: When called in list context, $isdst, the last value returned by gmtime, is always 0. There is no Daylight Saving Time in GMT.
Portability issues: "gmtime" in perlport.
The "goto EXPR" form expects to evaluate "EXPR" to a code reference or a label name. If it evaluates to a code reference, it will be handled like "goto &NAME", below. This is especially useful for implementing tail recursion via "goto __SUB__".
If the expression evaluates to a label name, its 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];
As shown in this example, "goto EXPR" is exempt from the "looks like a function" rule. A pair of parentheses following it does not (necessarily) delimit its argument. "goto("NE")."XT"" is equivalent to "goto NEXT". Also, unlike most named operators, this has the same precedence as assignment.
Use of "goto LABEL" or "goto EXPR" to jump into a construct is deprecated and will issue a warning. Even then, it may not be used to go into any construct that requires initialization, such as a subroutine, a "foreach" loop, or a "given" block. In general, it may not be used to jump into the parameter of a binary or list operator, but it may be used to jump into the first parameter of a binary operator. (The "=" assignment operator's "first" operand is its right-hand operand.) It also can't be used to go into a construct that is optimized away.
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 that 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.
my @foo = grep(!/^#/, @bar); # weed out comments
or equivalently,
my @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 $valid_input =~ /\A(?:0?[xX])?(?:_?[0-9a-fA-F])*\z/
A hex string consists of hex digits and an optional "0x" or "x" prefix. Each hex digit may be preceded by a single underscore, which will be ignored. Any other character triggers a warning and causes the rest of the string to be ignored (even leading whitespace, unlike "oct"). Only integers can be represented, and integer overflow triggers a warning.
To convert strings that might start with any of 0, "0x", or "0b", see "oct". To present something as hex, look into "printf", "sprintf", and "unpack".
require "sys/ioctl.ph"; # probably in # $Config{archlib}/sys/ioctl.ph
to get the correct function definitions. If sys/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 C 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:
my $retval = ioctl(...) || -1; printf "System returned %d\n", $retval;
The special string "0 but true" is exempt from "Argument "..." isn't numeric" warnings on improper numeric conversions.
Portability issues: "ioctl" in perlport.
my $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".
Hash entries are returned in an apparently random order. The actual random order is specific to a given hash; the exact same series of operations on two hashes may result in a different order for each hash. Any insertion into the hash may change the order, as will any deletion, with the exception that the most recent key returned by "each" or "keys" may be deleted without changing the order. So long as a given hash is unmodified you may rely on "keys", "values" and "each" to repeatedly return the same order as each other. See "Algorithmic Complexity Attacks" in perlsec for details on why hash order is randomized. Aside from the guarantees provided here the exact details of Perl's hash algorithm and the hash traversal order are subject to change in any release of Perl. Tied hashes may behave differently to Perl's hashes with respect to changes in order on insertion and deletion of items.
As a side effect, calling "keys" resets the internal iterator of the HASH or ARRAY (see "each") before yielding the keys. In particular, calling "keys" in void context resets the iterator with no other overhead.
Here is yet another way to print your environment:
my @keys = keys %ENV; my @values = values %ENV; while (@keys) { print pop(@keys), '=', pop(@values), "\n"; }
or how about sorted by key:
foreach my $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 my $key (sort { $hash{$b} <=> $hash{$a} } keys %hash) { printf "%4d %s\n", $hash{$key}, $key; }
Used 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). "keys @array" in an lvalue context is a syntax error.
Starting with Perl 5.14, an experimental feature allowed "keys" to take a scalar expression. This experiment has been deemed unsuccessful, and was removed as of Perl 5.24.
To avoid confusing would-be users of your code who are running earlier versions of Perl with mysterious syntax errors, put this sort of thing at the top of your file to signal that your code will work only on Perls of a recent vintage:
use 5.012; # so keys/values/each work on arrays
See also "each", "values", and "sort".
my $cnt = kill 'HUP', $child1, $child2; kill 'KILL', @goners;
SIGNAL may be either a signal name (a string) or a signal number. A signal name may start with a "SIG" prefix, thus "FOO" and "SIGFOO" refer to the same signal. The string form of SIGNAL is recommended for portability because the same signal may have different numbers in different operating systems.
A list of signal names supported by the current platform can be found in $Config{sig_name}, which is provided by the "Config" module. See Config for more details.
A negative signal name is the same as a negative signal number, killing process groups instead of processes. For example, "kill '-KILL', $pgrp" and "kill -9, $pgrp" will send "SIGKILL" to the entire process group specified. That means you usually want to use positive not negative signals.
If SIGNAL is either the number 0 or the string "ZERO" (or "SIGZERO"), no signal is sent to the process, but "kill" checks whether it's possible to send a signal to it (that means, to be brief, that the process is owned by the same user, or we are the super-user). This is useful to check that a child process is still alive (even if only as a zombie) and hasn't changed its UID. See perlport for notes on the portability of this construct.
The behavior of kill when a PROCESS number is zero or negative depends on the operating system. For example, on POSIX-conforming systems, zero will signal the current process group, -1 will signal all processes, and any other negative PROCESS number will act as a negative signal number and kill the entire process group specified.
If both the SIGNAL and the PROCESS are negative, the results are undefined. A warning may be produced in a future version.
See "Signals" in perlipc for more details.
On some platforms such as Windows where the fork(2) system call is not available, Perl can be built to emulate "fork" at the interpreter level. This emulation has limitations related to kill that have to be considered, for code running on Windows and in code intended to be portable.
See perlfork for more details.
If there is no LIST of processes, no signal is sent, and the return value is 0. This form is sometimes used, however, because it causes tainting checks to be run. But see "Laundering and Detecting Tainted Data" in perlsec.
Portability issues: "kill" in perlport.
LINE: while (<STDIN>) { last LINE if /^$/; # exit when done with header #... }
"last" cannot return a value from a block that typically returns a value, such as "eval {}", "sub {}", or "do {}". It will perform its flow control behavior, which precludes any return value. It 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.
Unlike most named operators, this has the same precedence as assignment. It is also exempt from the looks-like-a-function rule, so "last ("foo")."bar"" will cause "bar" to be part of the argument to "last".
If EXPR is omitted, uses $_.
What gets returned depends on several factors:
Starting in v5.20, Perl uses full Unicode rules if the locale is UTF-8. Otherwise, there is a deficiency in this scheme, which is that case changes that cross the 255/256 boundary are not well-defined. For example, the lower case of LATIN CAPITAL LETTER SHARP S (U+1E9E) in Unicode rules is U+00DF (on ASCII platforms). But under "use locale" (prior to v5.20 or not a UTF-8 locale), the lower case of U+1E9E is itself, because 0xDF may not be LATIN SMALL LETTER SHARP S in the current locale, and Perl has no way of knowing if that character even exists in the locale, much less what code point it is. Perl returns a result that is above 255 (almost always the input character unchanged), for all instances (and there aren't many) where the 255/256 boundary would otherwise be crossed; and starting in v5.22, it raises a locale warning.
If EXPR is omitted, uses $_.
This function behaves the same way under various pragmas, such as in a locale, as "lc" does.
This function cannot be used on an entire array or hash to find out how many elements these have. For that, use "scalar @array" and "scalar keys %hash", respectively.
Like all Perl character operations, "length" normally deals in logical characters, not physical bytes. For how many bytes a string encoded as UTF-8 would take up, use "length(Encode::encode('UTF-8', EXPR))" (you'll have to "use Encode" first). See Encode and perlunicode.
Portability issues: "link" in perlport.
A local modifies the listed variables to be local to the enclosing block, file, or eval. If more than one value is listed, the list must be placed in parentheses. See "Temporary Values via local()" in perlsub for details, including issues with tied arrays and hashes.
The "delete local EXPR" construct can also be used to localize the deletion of array/hash elements to the current block. See "Localized deletion of elements of composite types" in perlsub.
# 0 1 2 3 4 5 6 7 8 my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime(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 the month in the range 0..11, with 0 indicating January and 11 indicating December. This makes it easy to get a month name from a list:
my @abbr = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec); print "$abbr[$mon] $mday"; # $mon=9, $mday=18 gives "Oct 18"
$year contains the number of years since 1900. To get a 4-digit year write:
$year += 1900;
To get the last two digits of the year (e.g., "01" in 2001) do:
$year = sprintf("%02d", $year % 100);
$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.)
$isdst is true if the specified time occurs during Daylight Saving Time, false otherwise.
If EXPR is omitted, "localtime" uses the current time (as returned by "time").
In scalar context, "localtime" returns the ctime(3) value:
my $now_string = localtime; # e.g., "Thu Oct 13 04:54:34 1994"
The format of this scalar value is not locale-dependent but built into Perl. For GMT instead of local time use the "gmtime" builtin. See also the "Time::Local" module (for converting seconds, minutes, hours, and such back to the integer value returned by "time"), and the POSIX module's "strftime" and "mktime" functions.
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); my $now_string = strftime "%a %b %e %H:%M:%S %Y", localtime; # or for GMT formatted appropriately for your locale: my $now_string = strftime "%a %b %e %H:%M:%S %Y", gmtime;
Note that %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 Time::gmtime and Time::localtime modules provide a convenient, by-name access mechanism to the "gmtime" and "localtime" functions, respectively.
For a comprehensive date and time representation look at the DateTime module on CPAN.
Portability issues: "localtime" in perlport.
The value returned is the scalar itself, if the argument is a scalar, or a reference, if the argument is a hash, array or subroutine.
"lock" is a "weak keyword"; this means that if you've defined a function by this name (before any calls to it), that function will be called instead. If you are not under "use threads::shared" this does nothing. See threads::shared.
sub log10 { my $n = shift; return log($n)/log(10); }
See also "exp" for the inverse operation.
If EXPR is omitted, stats $_.
Portability issues: "lstat" in perlport.
my @chars = map(chr, @numbers);
translates a list of numbers to the corresponding characters.
my @squares = map { $_ * $_ } @numbers;
translates a list of numbers to their squared values.
my @squares = map { $_ > 5 ? ($_ * $_) : () } @numbers;
shows that number of returned elements can differ from the number of input elements. To omit an element, return an empty list (). This could also be achieved by writing
my @squares = map { $_ * $_ } grep { $_ > 5 } @numbers;
which makes the intention more clear.
Map always returns a list, which can be assigned to a hash such that the elements become key/value pairs. See perldata for more details.
my %hash = map { get_a_key_for($_) => $_ } @array;
is just a funny way to write
my %hash; foreach (@array) { $hash{get_a_key_for($_)} = $_; }
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. Using a regular "foreach" loop for this purpose would be clearer in most cases. See also "grep" for a list composed of those items of the original list for which the BLOCK or EXPR evaluates to true.
"{" starts both hash references and blocks, so "map { ..." could be either the start of map BLOCK LIST or map EXPR, LIST. Because Perl doesn't look ahead for the closing "}" it has to take a guess at which it's dealing with based on what it finds just after the "{". Usually it gets it right, but if it doesn't it won't realize something is wrong until it gets to the "}" and encounters the missing (or unexpected) comma. The syntax error will be reported close to the "}", but you'll need to change something near the "{" such as using a unary "+" or semicolon to give Perl some help:
my %hash = map { "\L$_" => 1 } @array # perl guesses EXPR. wrong my %hash = map { +"\L$_" => 1 } @array # perl guesses BLOCK. right my %hash = map {; "\L$_" => 1 } @array # this also works my %hash = map { ("\L$_" => 1) } @array # as does this my %hash = map { lc($_) => 1 } @array # and this. my %hash = map +( lc($_) => 1 ), @array # this is EXPR and works! my %hash = map ( lc($_), 1 ), @array # evaluates to (1, @array)
or to force an anon hash constructor use "+{":
my @hashes = map +{ lc($_) => 1 }, @array # EXPR, so needs # comma at end
to get a list of anonymous hashes each with only one entry apiece.
In general, it is better to create directories with a permissive MODE and let the user modify that with their "umask" than it is to supply a restrictive MODE and give the user no way to be more permissive. The exceptions to this rule are when the file or directory should be kept private (mail files, for instance). The documentation for "umask" discusses the choice of MODE in more detail.
Note that according to the POSIX 1003.1-1996 the FILENAME may have any number of trailing slashes. Some operating and filesystems do not get this right, so Perl automatically removes all trailing slashes to keep everyone happy.
To recursively create a directory structure, look at the "make_path" function of the File::Path module.
use IPC::SysV;
first to get the correct constant definitions. If CMD is "IPC_STAT", then ARG must be a variable that 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 "SysV IPC" in perlipc and the documentation for "IPC::SysV" and "IPC::Semaphore".
Portability issues: "msgctl" in perlport.
Portability issues: "msgget" in perlport.
Portability issues: "msgrcv" in perlport.
Portability issues: "msgsnd" in perlport.
The exact semantics and interface of TYPE and ATTRS are still evolving. TYPE may be a bareword, a constant declared with "use constant", or "__PACKAGE__". It is currently bound to the use of the fields pragma, and attributes are handled using the attributes pragma, or starting from Perl 5.8.0 also via the Attribute::Handlers module. See "Private Variables via my()" in perlsub for details.
Note that with a parenthesised list, "undef" can be used as a dummy placeholder, for example to skip assignment of initial values:
my ( undef, $min, $hour ) = localtime;
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 LABEL is omitted, the command refers to the innermost enclosing loop. The "next EXPR" form, available as of Perl 5.18.0, allows a label name to be computed at run time, being otherwise identical to "next LABEL".
"next" cannot return a value from a block that typically returns a value, such as "eval {}", "sub {}", or "do {}". It will perform its flow control behavior, which precludes any return value. It 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 "next" will exit such a block early.
See also "continue" for an illustration of how "last", "next", and "redo" work.
Unlike most named operators, this has the same precedence as assignment. It is also exempt from the looks-like-a-function rule, so "next ("foo")."bar"" will cause "bar" to be part of the argument to "next".
$val = oct($val) if $val =~ /^0/;
If EXPR is omitted, uses $_. To go the other way (produce a number in octal), use "sprintf" or "printf":
my $dec_perms = (stat("filename"))[2] & 07777; my $oct_perm_str = sprintf "%o", $perms;
The "oct" function is commonly used when a string such as 644 needs to be converted into a file mode, for example. Although Perl automatically converts strings into numbers as needed, this automatic conversion assumes base 10.
Leading white space is ignored without warning, as too are any trailing non-digits, such as a decimal point ("oct" only handles non-negative integers, not negative integers or floating point).
Simple examples to open a file for reading:
open(my $fh, "<", "input.txt") or die "Can't open < input.txt: $!";
and for writing:
open(my $fh, ">", "output.txt") or die "Can't open > output.txt: $!";
(The following is a comprehensive reference to "open": for a gentler introduction you may consider perlopentut.)
If FILEHANDLE is an undefined scalar variable (or array or hash element), a new filehandle is autovivified, meaning that the variable is assigned a reference to a newly allocated anonymous filehandle. Otherwise if FILEHANDLE is an expression, its value is the real filehandle. (This is considered a symbolic reference, so "use strict "refs"" should not be in effect.)
If three (or more) arguments are specified, the open mode (including optional encoding) in the second argument are distinct from the filename in the third. If MODE is "<" or nothing, the file is opened for input. If MODE is ">", the file is opened for output, with existing files first being truncated ("clobbered") and nonexisting files newly created. If MODE is ">>", 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 perlrun for a better approach. The file is created with permissions of 0666 modified by the process's "umask" value.
These various prefixes correspond to the fopen(3) modes of "r", "r+", "w", "w+", "a", and "a+".
In the one- and two-argument forms of the call, the mode and filename should be concatenated (in that order), preferably separated by white space. You can--but shouldn't--omit the mode in these forms when that mode is "<". It is safe to use the two-argument form of "open" if the filename argument is a known literal.
For three or more arguments if MODE is "|-", the filename is interpreted as a command to which output is to be piped, and if MODE is "-|", the filename is interpreted as a command that pipes output to us. In the two-argument (and one-argument) form, one should replace dash ("-") with the command. See "Using open() for IPC" in perlipc for more examples of this. (You are not allowed to "open" to a command that pipes both in and out, but see IPC::Open2, IPC::Open3, and "Bidirectional Communication with Another Process" in perlipc for alternatives.)
In the form of pipe opens taking three or more arguments, if LIST is specified (extra arguments after the command name) then LIST becomes arguments to the command invoked if the platform supports it. The meaning of "open" with more than three arguments for non-pipe modes is not yet defined, but experimental "layers" may give extra LIST arguments meaning.
In the two-argument (and one-argument) form, opening "<-" or "-" opens STDIN and opening ">-" opens STDOUT.
You may (and usually should) use the three-argument form of open to specify I/O layers (sometimes referred to as "disciplines") to apply to the handle that affect how the input and output are processed (see open and PerlIO for more details). For example:
open(my $fh, "<:encoding(UTF-8)", $filename) || die "Can't open UTF-8 encoded $filename: $!";
opens the UTF8-encoded file containing Unicode characters; see perluniintro. Note that if layers are specified in the three-argument form, then default layers stored in ${^OPEN} (see perlvar; usually set by the open pragma or the switch "-CioD") are ignored. Those layers will also be ignored if you specify a colon with no name following it. In that case the default layer for the operating system (:raw on Unix, :crlf on Windows) is used.
Open returns nonzero on success, the undefined value otherwise. If the "open" involved a pipe, the return value happens to be the pid of the subprocess.
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 always to use it when appropriate, and never to use it when it isn't appropriate. Also, people can set their I/O to be by default UTF8-encoded Unicode, not bytes.
When opening a file, it's seldom a good idea to continue if the request failed, so "open" is frequently used with "die". Even if "die" won't do what you want (say, in a CGI script, where you want to format a suitable error message (but there are modules that can help with that problem)) always check the return value from opening a file.
The filehandle will be closed when its reference count reaches zero. If it is a lexically scoped variable declared with "my", that usually means the end of the enclosing scope. However, this automatic close does not check for errors, so it is better to explicitly close filehandles, especially those used for writing:
close($handle) || warn "close failed: $!";
An older style is to use a bareword as the filehandle, as
open(FH, "<", "input.txt") or die "Can't open < input.txt: $!";
Then you can use "FH" as the filehandle, in "close FH" and "<FH>" and so on. Note that it's a global variable, so this form is not recommended in new code.
As a shortcut a one-argument call takes the filename from the global scalar variable of the same name as the filehandle:
$ARTICLE = 100; open(ARTICLE) or die "Can't find article $ARTICLE: $!\n";
Here $ARTICLE must be a global (package) scalar variable - not one declared with "my" or "state".
As a special case the three-argument form with a read/write mode and the third argument being "undef":
open(my $tmp, "+>", undef) or die ...
opens a filehandle to a newly created empty anonymous temporary file. (This happens under any mode, which makes "+>" the only useful and sensible mode to use.) You will need to "seek" to do the reading.
Perl is built using PerlIO by default. Unless you've changed this (such as building Perl with "Configure -Uuseperlio"), you can open filehandles directly to Perl scalars via:
open(my $fh, ">", \$variable) || ..
To (re)open "STDOUT" or "STDERR" as an in-memory file, close it first:
close STDOUT; open(STDOUT, ">", \$variable) or die "Can't open STDOUT: $!";
The scalars for in-memory files are treated as octet strings: unless the file is being opened with truncation the scalar may not contain any code points over 0xFF.
Opening in-memory files can fail for a variety of reasons. As with any other "open", check the return value for success.
See perliol for detailed info on PerlIO.
General examples:
open(my $log, ">>", "/usr/spool/news/twitlog"); # if the open fails, output is discarded open(my $dbase, "+<", "dbase.mine") # open for update or die "Can't open 'dbase.mine' for update: $!"; open(my $dbase, "+<dbase.mine") # ditto or die "Can't open 'dbase.mine' for update: $!"; open(my $article_fh, "-|", "caesar <$article") # decrypt # article or die "Can't start caesar: $!"; open(my $article_fh, "caesar <$article |") # ditto or die "Can't start caesar: $!"; open(my $out_fh, "|-", "sort >Tmp$$") # $$ is our process id or die "Can't start sort: $!"; # in-memory files open(my $memory, ">", \$var) or die "Can't open memory file: $!"; print $memory "foo!\n"; # output will appear in $var
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 (as in dup(2)) 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 IO buffers.) If you use the three-argument form, then you can pass either a number, the name of a filehandle, or the normal "reference to a glob".
 For example:
# open for input, reusing the fileno of $fd open(my $fh, "<&=", $fd)
or
open(my $fh, "<&=$fd")
or
# open for append, using the fileno of $oldfh open(my $fh, ">>&=", $oldfh)
Being parsimonious on filehandles is also useful (besides being parsimonious) for example when something is dependent on file descriptors, like for example locking using "flock". If you do just "open(my $A, ">>&", $B)", the filehandle $A will not have the same file descriptor as $B, and therefore "flock($A)" will not "flock($B)" nor vice versa. But with "open(my $A, ">>&=", $B)", the filehandles will share the same underlying system file descriptor.
Note that under Perls older than 5.8.0, Perl uses the standard C library's' fdopen(3) to implement the "=" functionality. On many Unix systems, fdopen(3) fails when file descriptors exceed a certain value, typically 255. For Perls 5.8.0 and later, PerlIO is (most often) the default.
You can see whether your Perl was built with PerlIO by running "perl -V:useperlio". If it says 'define', you have PerlIO; otherwise you don't.
If you open a pipe on the command "-" (that is, specify either "|-" or "-|" with the one- or two-argument forms of "open"), an implicit "fork" is done, so "open" returns twice: in the parent process it returns the pid of the child process, and in the child process it returns (a defined) 0. Use "defined($pid)" or "//" to determine whether the open was successful.
For example, use either
my $child_pid = open(my $from_kid, "-|") // die "Can't fork: $!";
or
my $child_pid = open(my $to_kid, "|-") // die "Can't fork: $!";
followed by
if ($child_pid) { # am the parent: # either write $to_kid or else read $from_kid ... waitpid $child_pid, 0; } else { # am the child; use STDIN/STDOUT normally ... exit; }
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/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 running setuid and you don't want to have to scan shell commands for metacharacters.
The following blocks are more or less equivalent:
open(my $fh, "|tr '[a-z]' '[A-Z]'"); open(my $fh, "|-", "tr '[a-z]' '[A-Z]'"); open(my $fh, "|-") || exec 'tr', '[a-z]', '[A-Z]'; open(my $fh, "|-", "tr", '[a-z]', '[A-Z]'); open(my $fh, "cat -n '$file'|"); open(my $fh, "-|", "cat -n '$file'"); open(my $fh, "-|") || exec "cat", "-n", $file; open(my $fh, "-|", "cat", "-n", $file);
The last two examples in each block show the pipe as "list form", which is not yet supported on all platforms. A good rule of thumb is that if your platform has a real "fork" (in other words, if your platform is Unix, including Linux and MacOS X), you can use the list form. You would want to use the list form of the pipe so you can pass literal arguments to the command without risk of the shell interpreting any shell metacharacters in them. However, this also bars you from opening pipes to commands that intentionally contain shell metacharacters, such as:
open(my $fh, "|cat -n | expand -4 | lpr") || die "Can't open pipeline to lpr: $!";
See "Safe Pipe Opens" in perlipc for more examples of this.
Perl will attempt to flush all files opened for output before any operation that may do a fork, 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.
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.
Closing any piped filehandle causes the parent process to wait for the child to finish, then returns the status value in $? and "${^CHILD_ERROR_NATIVE}".
The filename passed to the one- and two-argument forms of "open" will have leading and trailing whitespace deleted and 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(my $fh, $filename) or die "Can't open $filename: $!";
Use the three-argument form to open a file with arbitrary weird characters in it,
open(my $fh, "<", $file) || die "Can't open $file: $!";
otherwise it's necessary to protect any leading and trailing whitespace:
$file =~ s#^(\s)#./$1#; open(my $fh, "< $file\0") || die "Can't open $file: $!";
(this may not work on some bizarre filesystems). One should conscientiously choose between the magic and three-argument form of "open":
open(my $in, $ARGV[0]) || die "Can't open $ARGV[0]: $!";
will allow the user to specify an argument of the form "rsh cat file |", but will not work on a filename that happens to have a trailing space, while
open(my $in, "<", $ARGV[0]) || die "Can't open $ARGV[0]: $!";
will have exactly the opposite restrictions. (However, some shells support the syntax "perl your_program.pl <( rsh cat file )", which produces a filename that can be opened normally.)
If you want a "real" C open(2), then you should use the "sysopen" function, which involves no such magic (but uses different filemodes than Perl "open", which corresponds to C fopen(3)). This is another way to protect your filenames from interpretation. For example:
use IO::Handle; sysopen(my $fh, $path, O_RDWR|O_CREAT|O_EXCL) or die "Can't open $path: $!"; $fh->autoflush(1); print $fh "stuff $$\n"; seek($fh, 0, 0); print "File contains: ", readline($fh);
See "seek" for some details about mixing reading and writing.
Portability issues: "open" in perlport.
See the example at "readdir".
For the reverse, see "chr". See perlunicode for more about Unicode.
"our" has the same scoping rules as "my" or "state", meaning that it is only valid within a lexical scope. Unlike "my" and "state", which both declare new (lexical) variables, "our" only creates an alias to an existing variable: a package variable of the same name.
This means that when "use strict 'vars'" is in effect, "our" lets you use a package variable without qualifying it with the package name, but only within the lexical scope of the "our" declaration. This applies immediately--even within the same statement.
package Foo; use strict; $Foo::foo = 23; { our $foo; # alias to $Foo::foo print $foo; # prints 23 } print $Foo::foo; # prints 23 print $foo; # ERROR: requires explicit package name
This works even if the package variable has not been used before, as package variables spring into existence when first used.
package Foo; use strict; our $foo = 23; # just like $Foo::foo = 23 print $Foo::foo; # prints 23
Because the variable becomes legal immediately under "use strict 'vars'", so long as there is no variable with that name is already in scope, you can then reference the package variable again even within the same statement.
package Foo; use strict; my $foo = $foo; # error, undeclared $foo on right-hand side our $foo = $foo; # no errors
If more than one variable is listed, the list must be placed in parentheses.
our($bar, $baz);
An "our" declaration declares an alias for a package variable that will be visible across its entire lexical scope, even across package boundaries. The package in which the variable is entered is determined at the point of the declaration, not at the point of use. This means the following behavior holds:
package Foo; our $bar; # declares $Foo::bar for rest of lexical scope $bar = 20; package Bar; print $bar; # prints 20, as it refers to $Foo::bar
Multiple "our" declarations with the same name in the same lexical scope are allowed if they are in different packages. If they happen to be in the same package, Perl will emit warnings if you have asked for them, just like multiple "my" declarations. Unlike a second "my" declaration, which will bind the name to a fresh variable, a second "our" declaration in the same package, in the same scope, is merely redundant.
use warnings; package Foo; our $bar; # declares $Foo::bar for rest of lexical scope $bar = 20; package Bar; our $bar = 30; # declares $Bar::bar for rest of lexical scope print $bar; # prints 30 our $bar; # emits warning but has no other effect print $bar; # still prints 30
An "our" declaration may also have a list of attributes associated with it.
The exact semantics and interface of TYPE and ATTRS are still evolving. TYPE is currently bound to the use of the fields pragma, and attributes are handled using the attributes pragma, or, starting from Perl 5.8.0, also via the Attribute::Handlers module. See "Private Variables via my()" in perlsub for details.
Note that with a parenthesised list, "undef" can be used as a dummy placeholder, for example to skip assignment of initial values:
our ( undef, $min, $hour ) = localtime;
"our" differs from "use vars", which allows use of an unqualified name only within the affected package, but across scopes.
See perlpacktut for an introduction to this function.
The TEMPLATE is a sequence of characters that give the order and type of values, as follows:
a A string with arbitrary binary data, will be null padded. A A text (ASCII) string, will be space padded. Z A null-terminated (ASCIZ) string, will be null padded. b A bit string (ascending bit order inside each byte, like vec()). B A bit string (descending bit order inside each byte). h A hex string (low nybble first). H A hex string (high nybble first). c A signed char (8-bit) value. C An unsigned char (octet) value. W An unsigned char value (can be greater than 255). s A signed short (16-bit) value. S An unsigned short value. l A signed long (32-bit) value. L An unsigned long value. q A signed quad (64-bit) value. Q An unsigned quad value. (Quads are available only if your system supports 64-bit integer values _and_ if Perl has been compiled to support those. Raises an exception otherwise.) 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'.) n An unsigned short (16-bit) in "network" (big-endian) order. N An unsigned long (32-bit) in "network" (big-endian) order. v An unsigned short (16-bit) in "VAX" (little-endian) order. V An unsigned long (32-bit) in "VAX" (little-endian) order. j A Perl internal signed integer value (IV).  F A Perl internal floating-point value (NV) in native format D A float of long-double precision in native format. (Long doubles are available only if your system supports long double values _and_ if Perl has been compiled to support those. Raises an exception otherwise. Note that there are different long double formats.) p A pointer to a null-terminated string.  U A Unicode character number. Encodes to a character in char- acter mode and UTF-8 (or UTF-EBCDIC in EBCDIC platforms) in byte mode. w A BER compressed integer (not an ASN.1 BER, see perlpacktut for details). 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 (a.k.a ASCII NUL, "\000", chr(0)) X Back up a byte. @ Null-fill or truncate to absolute position, counted from the start of the innermost ()-group. . Null-fill or truncate to absolute position specified by the value. ( Start of a ()-group.
One or more modifiers below may optionally follow certain letters in the TEMPLATE (the second column lists letters for which the modifier is valid):
! sSlLiI Forces native (short, long, int) sizes instead of fixed (16-/32-bit) sizes. ! xX Make x and X act as alignment commands. ! nNvV Treat integers as signed instead of unsigned. ! @. Specify position as byte offset in the internal representation of the packed string. Efficient but dangerous. > sSiIlLqQ Force big-endian byte-order on the type. jJfFdDpP (The "big end" touches the construct.) < sSiIlLqQ Force little-endian byte-order on the type. jJfFdDpP (The "little end" touches the construct.)
The ">" and "<" modifiers can also be used on "()" groups to force a particular byte-order on all components in that group, including all its subgroups.
The following rules apply:
One can replace a numeric repeat count with a template letter enclosed in brackets to use the packed byte length of the bracketed template for the repeat count.
For example, the template "x[L]" skips as many bytes as in a packed long, and the template "$t X[$t] $t" unpacks twice whatever $t (when variable-expanded) unpacks. If the template in brackets contains alignment commands (such as "x![d]"), its packed length is calculated as if the start of the template had the maximal possible alignment.
When used with "Z", a "*" as the repeat count is guaranteed to add a trailing null byte, so the resulting string is always one byte longer than the byte length of the item itself.
When used with "@", the repeat count represents an offset from the start of the innermost "()" group.
When used with ".", the repeat count determines the starting position to calculate the value offset as follows:
The repeat count for "u" is interpreted as the maximal number of bytes to encode per line of output, with 0, 1 and 2 replaced by 45. The repeat count should not be more than 65.
If the value to pack is too long, the result is truncated. If it's too long and an explicit count is provided, "Z" packs only "$count-1" bytes, followed by a null byte. Thus "Z" always packs a trailing null, except when the count is 0.
Each result bit is based on the least-significant bit of the corresponding input character, i.e., on "ord($char)%2". In particular, characters "0" and "1" generate bits 0 and 1, as do characters "\000" and "\001".
Starting from the beginning of the input string, each 8-tuple of characters is converted to 1 character of output. With format "b", the first character of the 8-tuple determines the least-significant bit of a character; with format "B", it determines the most-significant bit of a character.
If the length of the input string is not evenly divisible by 8, the remainder is packed as if the input string were padded by null characters at the end. Similarly during unpacking, "extra" bits are ignored.
If the input string is longer than needed, remaining characters are ignored.
A "*" for the repeat count uses all characters of the input field. On unpacking, bits are converted to a string of 0s and 1s.