perl | Practical Extraction and Report Language is a interpreted language optimized for scanning arbitrary text files, extracting information from those text files, and printing reports based on that information. It's also a good language for many system management tasks. The language is intended to be practical (easy to use, efficient, complete) rather than beautiful (tiny, elegant, minimal). It combines (in the author's opinion, anyway) some of the best features of C, sed, awk, and sh, so people familiar with those languages should have little difficulty with it. (Language historians will also note some vestiges of csh, Pascal, and even BASIC|PLUS.) Expression syntax corresponds quite closely to C expression syntax. If you have a problem that would ordinarily use sed or awk or sh, but it exceeds their capabilities or must run a little faster, and you don't want to write the silly thing in C, then may be for you. There are also translators to turn your sed and awk scripts into perl scripts. OK, enough hype. Upon startup, looks for your script in one of the following places: Specified line by line via switches on the command line. Contained in the file specified by the first filename on the command line. (Note that systems supporting the #! notation invoke interpreters this way.) Passed in via standard input. After locating your script, compiles it to an internal form. If the script is syntactically correct, it is executed.

Options
Note: on first reading this section may not make much sense to you. It's here at the front for easy reference. A single|character option may be combined with the following option, if any. This is particularly useful when invoking a script using the #! construct which only allows one argument. Example:
	#!/bin/perl |spi.bak	# same as |s |p |i.bak
	...
Options include: sets debugging flags. To watch how it executes your script, use (This only works if debugging is compiled into your may be used to enter one line of script. Multiple commands may be given to build up a multi|line script. If is given, will not look for a script filename in the argument list. specifies that files processed by the <> construct are to be edited in|place. It does this by renaming the input file, opening the output file by the same name, and selecting that output file as the default for print statements. The extension, if supplied, is added to the name of the old file to make a backup copy. If no extension is supplied, no backup is made. Saying "perl |p |i.bak |e "s/foo/bar/;" ... " is the same as using the script:
	#!/bin/perl |pi.bak
	s/foo/bar/;
which is equivalent to
	#!/bin/perl
	while (<>) {
		if ($ARGV ne $oldargv) {
			rename($ARGV,$ARGV . '.bak');
			open(ARGVOUT,">$ARGV");
			select(ARGVOUT);
			$oldargv = $ARGV;
		}
		s/foo/bar/;
	}
	continue {
	    print;	# this prints to original filename
	}
	select(stdout);
except that the |i form doesn't need to compare $ARGV to $oldargv to know when the filename has changed. It does, however, use ARGVOUT for the selected filehandle. Note that stdout is restored as the default output filehandle after the loop. may be used in conjunction with to tell the C preprocessor where to look for include files. By default /usr/include and /usr/lib/perl are searched. causes to assume the following loop around your script, which makes it iterate over filename arguments somewhat like "sed |n" or awk:
	while (<>) {
		...		# your script goes here
	}
Note that the lines are not printed by default. See to have lines printed. causes to assume the following loop around your script, which makes it iterate over filename arguments somewhat like sed:
	while (<>) {
		...		# your script goes here
	} continue {
		print;
	}
Note that the lines are printed automatically. To suppress printing use the switch. A overrides a switch. causes your script to be run through the C preprocessor before compilation by (Since both comments and cpp directives begin with the # character, you should avoid starting comments with any words recognized by the C preprocessor such as "if", "else" or "define".) enables some rudimentary switch parsing for switches on the command line after the script name but before any filename arguments (or before a ||). Any switch found there is removed from @ARGV and sets the corresponding variable in the script. The following script prints "true" if and only if the script is invoked with a |xyz switch.
	#!/bin/perl |s
	if ($xyz) { print "true\n"; }

Data Types and Objects
Perl has about two and a half data types: strings, arrays of strings, and associative arrays. Strings and arrays of strings are first class objects, for the most part, in the sense that they can be used as a whole as values in an expression. Associative arrays can only be accessed on an association by association basis; they don't have a value as a whole (at least not yet). Strings are interpreted numerically as appropriate. A string is interpreted as TRUE in the boolean sense if it is not the null string or 0. Booleans returned by operators are 1 for true and '0' or '' (the null string) for false. References to string variables always begin with '$', even when referring to a string that is part of an array. Thus:
    $days	# a simple string variable
    $days[28]	# 29th element of array @days
    $days{'Feb'}# one value from an associative array
but entire arrays are denoted by '@':
    @days	# ($days[0], $days[1],... $days[n])
Any of these four constructs may be assigned to (in compiler lingo, may serve as an lvalue). (Additionally, you may find the length of array @days by evaluating "$#days", as in [Actually, it's not the length of the array, it's the subscript of the last element, since there is (ordinarily) a 0th element.]) Every data type has its own namespace. You can, without fear of conflict, use the same name for a string variable, an array, an associative array, a filehandle, a subroutine name, and/or a label. Since variable and array references always start with '$' or '@', the "reserved" words aren't in fact reserved with respect to variable names. (They ARE reserved with respect to labels and filehandles, however, which don't have an initial special character.) Case IS significant|"FOO", "Foo" and "foo" are all different names. Names which start with a letter may also contain digits and underscores. Names which do not start with a letter are limited to one character, e.g. "$%" or "$$". (Many one character names have a predefined significance to More later.) String literals are delimited by either single or double quotes. They work much like shell quotes: double|quoted string literals are subject to backslash and variable substitution; single|quoted strings are not. The usual backslash rules apply for making characters such as newline, tab, etc. You can also embed newlines directly in your strings, i.e. they can end on a different line than they begin. This is nice, but if you forget your trailing quote, the error will not be reported until perl finds another line containing the quote character, which may be much further on in the script. Variable substitution inside strings is limited (currently) to simple string variables. The following code segment prints out "The price is $100."
    $Price = '$100';# not interpreted
    print "The price is $Price.\n";# interpreted
Note that you can put curly brackets around the identifier to delimit it from following alphanumerics. Array literals are denoted by separating individual values by commas, and enclosing the list in parentheses. In a context not requiring an array value, the value of the array literal is the value of the final element, as in the C comma operator. For example,
    @foo = ('cc', '|E', $bar);
assigns the entire array value to array foo, but
    $foo = ('cc', '|E', $bar);
assigns the value of variable bar to variable foo. Array lists may be assigned to if and only if each element of the list is an lvalue:
    ($a, $b, $c) = (1, 2, 3);
    ($map{'red'}, $map{'blue'}, $map{'green'}) = (0x00f, 0x0f0, 0xf00);
Numeric literals are specified in any of the usual floating point or integer formats. There are several other pseudo|literals that you should know about. If a string is enclosed by backticks (grave accents), it is interpreted as a command, and the output of that command is the value of the pseudo|literal, just like in any of the standard shells. The command is executed each time the pseudo|literal is evaluated. Unlike in csh, no interpretation is done on the data|newlines remain newlines. The status value of the command is returned in $?. Evaluating a filehandle in angle brackets yields the next line from that file (newline included, so it's never false until EOF). Ordinarily you must assign that value to a variable, but there is one situation where in which an automatic assignment happens. If (and only if) the input symbol is the only thing inside the conditional of a loop, the value is automatically assigned to the variable "$_". (This may seem like an odd thing to you, but you'll use the construct in almost every script you write.) Anyway, the following lines are equivalent to each other:
    while ($_ = <stdin>) {
    while (<stdin>) {
    for (;<stdin>;) {
The filehandles and are predefined. Additional filehandles may be created with the function. The null filehandle <> is special and can be used to emulate the behavior of sed and awk. Input from <> comes either from standard input, or from each file listed on the command line. Here's how it works: the first time <> is evaluated, the ARGV array is checked, and if it is null, $ARGV[0] is set to '|', which when opened gives you standard input. The ARGV array is then processed as a list of filenames. The loop
	while (<>) {
		...			# code for each line
	}
is equivalent to
	unshift(@ARGV, '|') if $#ARGV < $[;
	while ($ARGV = shift) {
		open(ARGV, $ARGV);
		while (<ARGV>) {
			...		# code for each line
		}
	}
except that it isn't as cumbersome to say. It really does shift array ARGV and put the current filename into variable ARGV. It also uses filehandle ARGV internally. You can modify @ARGV before the first <> as long as you leave the first filename at the beginning of the array. Line numbers ($.) continue as if the input was one big happy file. If you want to set @ARGV to you own list of files, go right ahead. If you want to pass switches into your script, you can put a loop on the front like this:
	while ($_ = $ARGV[0], /^|/) {
		shift;
	    last if /^||$/;
		/^|D(.*)/ && ($debug = $1);
		/^|v/ && $verbose++;
		...		# other switches
	}
	while (<>) {
		...		# code for each line
	}
The <> symbol will return FALSE only once. If you call it again after this it will assume you are processing another @ARGV list, and if you haven't set @ARGV, will input from stdin.

Syntax
A script consists of a sequence of declarations and commands. The only things that need to be declared in are report formats and subroutines. See the sections below for more information on those declarations. All objects are assumed to start with a null or 0 value. The sequence of commands is executed just once, unlike in and scripts, where the sequence of commands is executed for each input line. While this means that you must explicitly loop over the lines of your input file (or files), it also means you have much more control over which files and which lines you look at. (Actually, I'm lying|it is possible to do an implicit loop with either the or switch.) A declaration can be put anywhere a command can, but has no effect on the execution of the primary sequence of commands. Typically all the declarations are put at the beginning or the end of the script. is, for the most part, a free|form language. (The only exception to this is format declarations, for fairly obvious reasons.) Comments are indicated by the # character, and extend to the end of the line. If you attempt to use /* */ C comments, it will be interpreted either as division or pattern matching, depending on the context. So don't do that.

Compound statements
In a sequence of commands may be treated as one command by enclosing it in curly brackets. We will call this a BLOCK. The following compound commands may be used to control flow:
	if (EXPR) BLOCK
	if (EXPR) BLOCK else BLOCK
	if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK
	LABEL while (EXPR) BLOCK
	LABEL while (EXPR) BLOCK continue BLOCK
	LABEL for (EXPR; EXPR; EXPR) BLOCK
	LABEL BLOCK continue BLOCK
Note that, unlike C and Pascal, these are defined in terms of BLOCKs, not statements. This means that the curly brackets are required|no dangling statements allowed. If you want to write conditionals without curly brackets there are several other ways to do it. The following all do the same thing:
    if (!open(foo)) { die "Can't open $foo"; }
    die "Can't open $foo" unless open(foo);
    open(foo) || die "Can't open $foo";	# foo or bust!
    open(foo) ? die "Can't open $foo" : 'hi mom';
			    # a bit exotic, that last one
The statement is straightforward. Since BLOCKs are always bounded by curly brackets, there is never any ambiguity about which an goes with. If you use in place of the sense of the test is reversed. The statement executes the block as long as the expression is true (does not evaluate to the null string or 0). The LABEL is optional, and if present, consists of an identifier followed by a colon. The LABEL identifies the loop for the loop control statements and (see below). If there is a BLOCK, it is always executed just before the conditional is about to be evaluated again, similarly to the third part of a loop in C. Thus it can be used to increment a loop variable, even when the loop has been continued via the statement (similar to the C "continue" statement). If the word is replaced by the word the sense of the test is reversed, but the conditional is still tested before the first iteration. In either the or the statement, you may replace "(EXPR)" with a BLOCK, and the conditional is true if the value of the last command in that block is true. The loop works exactly like the corresponding loop:
	for ($i = 1; $i < 10; $i++) {
		...
	}
is the same as
	$i = 1;
	while ($i < 10) {
		...
	} continue {
		$i++;
	}
The BLOCK by itself (labeled or not) is equivalent to a loop that executes once. Thus you can use any of the loop control statements in it to leave or restart the block. The block is optional. This construct is particularly nice for doing case structures.
	foo: {
		if (/abc/) { $abc = 1; last foo; }
		if (/def/) { $def = 1; last foo; }
		if (/xyz/) { $xyz = 1; last foo; }
		$nothing = 1;
	}

Simple statements
The only kind of simple statement is an expression evaluated for its side effects. Every expression (simple statement) must be terminated with a semicolon. Note that this is like C, but unlike Pascal (and Any simple statement may optionally be followed by a single modifier, just before the terminating semicolon. The possible modifiers are:
	if EXPR
	unless EXPR
	while EXPR
	until EXPR
The and modifiers have the expected semantics. The and modifiers also have the expected semantics (conditional evaluated first), except when applied to a do|BLOCK command, in which case the block executes once before the conditional is evaluated. This is so that you can write loops like:
	do {
		$_ = <stdin>;
		...
	} until $_ eq ".\n";
(See the operator below. Note also that the loop control commands described later will NOT work in this construct, since modifiers don't take loop labels. Sorry.)

Expressions
Since expressions work almost exactly like C expressions, only the differences will be mentioned here. Here's what has that C doesn't: The null list, used to initialize an array to null. Concatenation of two strings. The corresponding assignment operator. String equality (== is numeric equality). For a mnemonic just think of "eq" as a string. (If you are used to the behavior of using == for either string or numeric equality based on the current form of the comparands, beware! You must be explicit here.) String inequality (!= is numeric inequality). String less than. String greater than. String less than or equal. String greater than or equal. Certain operations search or modify the string "$_" by default. This operator makes that kind of operation work on some other string. The right argument is a search pattern, substitution, or translation. The left argument is what is supposed to be searched, substituted, or translated instead of the default "$_". The return value indicates the success of the operation. (If the right argument is an expression other than a search pattern, substitution, or translation, it is interpreted as a search pattern at run time. This is less efficient than an explicit search, since the pattern must be compiled every time the expression is evaluated.) The precedence of this operator is lower than unary minus and autoincrement/decrement, but higher than everything else. Just like =~ except the return value is negated. The repetition operator. Returns a string consisting of the left operand repeated the number of times specified by the right operand.
	print '|' x 80;		# print row of dashes
	print '|' x80;		# illegal, x80 is identifier
	print "\t" x ($tab/8), ' ' x ($tab%8);	# tab over
The corresponding assignment operator. The range operator, which is bistable. It is false as long as its left argument is false. Once the left argument is true, it stays true until the right argument is true, AFTER which it becomes false again. (It doesn't become false till the next time it's evaluated. It can become false on the same evaluation it became true, but it still returns true once.) The .. operator is primarily intended for doing line number ranges after the fashion of sed or awk. The precedence is a little lower than || and &&. The value returned is either the null string for false, or a sequence number (beginning with 1) for true. The sequence number is reset for each range encountered. The final sequence number in a range has the string 'E0' appended to it, which doesn't affect its numeric value, but gives you something to search for if you want to exclude the endpoint. You can exclude the beginning point by waiting for the sequence number to be greater than 1. If either argument to .. is static, that argument is implicitly compared to the $. variable, the current line number. Examples:
    if (101 .. 200) { print; }	# print 2nd hundred lines
    next line if (1 .. /^$/);	# skip header lines
    s/^/> / if (/^$/ .. eof());	# quote body
Here is what C has that doesn't: Address|of operator. Dereference|address operator. Like C, does a certain amount of expression evaluation at compile time, whenever it determines that all of the arguments to an operator are static and have no side effects. In particular, string concatenation happens at compile time between literals that don't do variable substitution. Backslash interpretation also happens at compile time. You can say
	'Now is the time for all' . "\n" .
	'good men to come to.'
and this all reduces to one string internally. Along with the literals and variables mentioned earlier, the following operations can serve as terms in an expression: Searches a string for a pattern, and returns true (1) or false (''). If no string is specified via the =~ or !~ operator, the $_ string is searched. (The string specified with =~ need not be an lvalue|it may be the result of an expression evaluation, but remember the =~ binds rather tightly.) See also the section on regular expressions. If you prepend an `m' you can use any pair of characters as delimiters. This is particularly useful for matching Unix path names that contain `/'. Examples:
    open(tty, '/dev/tty');
    <tty> =~ /^[Yy]/ && do foo();	# do foo if desired
    if (/Version: *([0|9.]*)/) { $version = $1; }
    next if m#^/usr/spool/uucp#;
This is just like the /pattern/ search, except that it matches only once between calls to the operator. This is a useful optimization when you only want to see the first occurence of something in each of a set of files, for instance. Changes the working director to EXPR, if possible. Returns 1 upon success, 0 otherwise. See example under die(). Changes the permissions of a list of files. The first element of the list must be the numerical mode. LIST may be an array, in which case you may wish to use the unshift() command to put the mode on the front of the array. Returns the number of files successfully changed. Note: in order to use the value you must put the whole thing in parentheses.
	$cnt = (chmod 0755,'foo','bar');
Chops off the last character of a string and returns it. It's used primarily to remove the newline from the end of an input record, but is much more efficient than s/\n// because it neither scans nor copies the string. If VARIABLE is omitted, chops $_. Example:
	while (<>) {
		chop;	# avoid \n on last field
		@array = split(/:/);
		...
	}
Changes the owner (and group) of a list of files. LIST may be an array. The first two elements of the list must be the NUMERICAL uid and gid, in that order. Returns the number of files successfully changed. Note: in order to use the value you must put the whole thing in parentheses.
	$cnt = (chown $uid,$gid,'foo');
Here's an example of looking up non|numeric uids:
	print "User: ";
	$user = <stdin>;
	open(pass,'/etc/passwd') || die "Can't open passwd";
	while (<pass>) {
		($login,$pass,$uid,$gid) = split(/:/);
		$uid{$login} = $uid;
		$gid{$login} = $gid;
	}
	@ary = ('foo','bar','bie','doll');
	if ($uid{$user} eq '') {
		die "$user not in passwd file";
	}
	else {
		unshift(@ary,$uid{$user},$gid{$user});
		chown @ary;
	}
Closes the file or pipe associated with the file handle. You don't have to close FILEHANDLE if you are immediately going to do another open on it, since open will close it for you. (See However, an explicit close on an input file resets the line counter ($.), while the implicit close done by does not. Also, closing a pipe will wait for the process executing on the pipe to complete, in case you want to look at the output of the pipe afterwards. Example:
	open(output,'|sort >foo');	# pipe to sort
	...	# print stuff to output
	close(output);		# wait for sort to finish
	open(input,'foo');	# get sort's results
Encrypts a string exactly like the crypt() function in the C library. Useful for checking the password file for lousy passwords. Only the guys wearing white hats should do this. Prints the value of EXPR to stderr and exits with a non|zero status. Equivalent examples:
	die "Can't cd to spool." unless chdir '/usr/spool/news';
	(chdir '/usr/spool/news') || die "Can't cd to spool."
Note that the parens are necessary above due to precedence. See also Returns the value of the last command in the sequence of commands indicated by BLOCK. When modified by a loop modifier, executes the BLOCK once before testing the loop condition. (On other statements the loop modifiers test the conditional first.) Executes a SUBROUTINE declared by a declaration, and returns the value of the last expression evaluated in SUBROUTINE. (See the section on subroutines later on.) Returns a 2 element array consisting of the key and value for the next value of an associative array, so that you can iterate over it. Entries are returned in an apparently random order. When the array is entirely read, a null array is returned (which when assigned produces a FALSE (0) value). The next call to each() after that will start iterating again. The iterator can be reset only by reading all the elements from the array. You should not modify the array while iterating over it. The following prints out your environment like the printenv program, only in a different order:
	while (($key,$value) = each(ENV)) {
		print "$key=$value\n";
	}
See also keys() and values(). Returns 1 if the next read on FILEHANDLE will return end of file, or if FILEHANDLE is not open. If (FILEHANDLE) is omitted, the eof status is returned for the last file read. The null filehandle may be used to indicate the pseudo file formed of the files listed on the command line, i.e. eof() is reasonable to use inside a while (<>) loop. Example:
	# insert dashes just before last line
	while (<>) {
		if (eof()) {
			print "||||||||||||||\n";
		}
		print;
	}
EXPR is parsed and executed as if it were a little perl program. It is executed in the context of the current perl program, so that any variable settings, subroutine or format definitions remain afterwards. The value returned is the value of the last expression evaluated, just as with subroutines. If there is a syntax error or runtime error, a null string is returned by eval, and $@ is set to the error message. If there was no error, $@ is null. If there is more than one argument in LIST, calls execvp() with the arguments in LIST. If there is only one argument, the argument is checked for shell metacharacters. If there are any, the entire argument is passed to /bin/sh |c for parsing. If there are none, the argument is split into words and passed directly to execvp(), which is more efficient. Note: exec (and system) do not flush your output buffer, so you may need to set $| to avoid lost output. Evaluates EXPR and exits immediately with that value. Example:
	$ans = <stdin>;
	exit 0 if $ans =~ /^[Xx]/;
See also Returns e to the power of EXPR. Does a fork() call. Returns the child pid to the parent process and 0 to the child process. Note: unflushed buffers remain unflushed in both processes, which means you may need to set $| to avoid duplicate output. Converts a time as returned by the time function to a 9|element array with the time analyzed for the Greenwich timezone. Typically used as follows:
    ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst)
       = gmtime(time);
All array elements are numeric.