perl(1)

NAME

perl - Practical Extraction and Report Language

SYNOPSIS

perl [options] filename args

DESCRIPTION

Perl is a interpreted language optimized for scanning
arbitrary text files, extracting information from those
text files, and printing reports based on that informa
tion. It's also a good language for many system manage
ment 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 BASICPLUS.) Expression syntax corresponds quite closely to C
expression syntax. If you have a problem that would ordi
narily use sed or awk or sh, but it exceeds their capabil ities or must run a little faster, and you don't want to
write the silly thing in C, then perl may be for you.
There are also translators to turn your sed and awk
scripts into perl scripts. OK, enough hype.

Upon startup, perl looks for your script in one of the
following places:

1. Specified line by line via -e switches on the command

line.

2. Contained in the file specified by the first filename
on the command line. (Note that systems supporting
the #! notation invoke interpreters this way.)

3. Passed in via standard input.

After locating your script, perl compiles it to an inter
nal 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 follow
ing 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:

-D<number>
sets debugging flags. To watch how it executes your
script, use -D14. (This only works if debugging is
compiled into your perl.)

-e commandline
may be used to enter one line of script. Multiple -e
commands may be given to build up a multi-line
script. If -e is given, perl will not look for a script filename in the argument list.

-i<extension>
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 file

name

}
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 file
handle. Note that stdout is restored as the default
output filehandle after the loop.

-I<directory>
may be used in conjunction with -P to tell the C pre
processor where to look for include files. By
default /usr/include and /usr/lib/perl are searched.

-n causes perl to assume the following loop around your script, which makes it iterate over filename argu
ments somewhat like "sed -n" or awk:

while (<>) {
... # your script goes here

}

Note that the lines are not printed by default. See
-p to have lines printed.

-p causes perl to assume the following loop around your script, which makes it iterate over filename argu
ments somewhat like sed:

while (<>) {
... # your script goes here

} continue {
print;

}

Note that the lines are printed automatically. To
suppress printing use the -n switch. A -p overrides
a -n switch.

-P causes your script to be run through the C preproces
sor before compilation by perl. (Since both comments
and cpp directives begin with the # character, you
should avoid starting comments with any words recog
nized by the C preprocessor such as "if", "else" or
"define".)

-s 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 corre
sponding variable in the perl script. The following
script prints "true" if and only if the script is
invoked with a -xyz switch.

#!/bin/perl -s
if ($xyz) { print "true0; }

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 opera
tors 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 com
piler lingo, may serve as an lvalue). (Additionally, you
may find the length of array @days by evaluating "$#days",
as in csh. [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 '$' or '@', the "reserved" words aren't
in fact reserved with respect to variable names. (They
ARE reserved with respect to labels and filehandles, how
ever, which don't have an initial special character.)
Case IS significant--"FOO", "Foo" and "foo" are all dif
ferent names. Names which start with a letter may also
contain digits and underscores. Names which do not start
with a letter are limited to one character, e.g. "$%" or
"$$". (Many one character names have a predefined signif
icance to perl. 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 sub
stitution; single-quoted strings are not. The usual back
slash 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.0;# 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 float
ing 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 exe
cuted 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 while
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 perl script you write.)
Anyway, the following lines are equivalent to each other:

while ($_ = <stdin>) {
while (<stdin>) {
for (;<stdin>;) {

The filehandles stdin, stdout and stderr are predefined. Additional filehandles may be created with the open func
tion.

The null filehandle <> is special and can be used to emu
late 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 vari
able 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 perl script consists of a sequence of declarations and
commands. The only things that need to be declared in
perl 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 sed
and awk scripts, where the sequence of commands is exe
cuted 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 -n or -p switch.)

A declaration can be put anywhere a command can, but has
no effect on the execution of the primary sequence of com
mands. Typically all the declarations are put at the
beginning or the end of the script.

Perl 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 # charac
ter, 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 perl, a sequence of commands may be treated as one com
mand 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 if statement is straightforward. Since BLOCKs are
always bounded by curly brackets, there is never any ambi
guity about which if an else goes with. If you use unless in place of if, the sense of the test is reversed.

The while 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 next, last and redo (see below). If there is a continue BLOCK, it is always executed just before the conditional is about to be
evaluated again, similarly to the third part of a for loop
in C. Thus it can be used to increment a loop variable,
even when the loop has been continued via the next state
ment (similar to the C "continue" statement).

If the word while is replaced by the word until, the sense of the test is reversed, but the conditional is still
tested before the first iteration.

In either the if or the while 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 for loop works exactly like the corresponding while 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 continue block is optional. This construct is partic ularly 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 evalu
ated for its side effects. Every expression (simple
statement) must be terminated with a semicolon. Note that
this is like C, but unlike Pascal (and awk).

Any simple statement may optionally be followed by a sin
gle modifier, just before the terminating semicolon. The
possible modifiers are:

if EXPR
unless EXPR
while EXPR
until EXPR

The if and unless modifiers have the expected semantics. The while and unless 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 ".0;

(See the do operator below. Note also that the loop con
trol commands described later will NOT work in this con
struct, since modifiers don't take loop labels. Sorry.)

Expressions

Since perl expressions work almost exactly like C expres
sions, only the differences will be mentioned here.

Here's what perl has that C doesn't:

() The null list, used to initialize an array to
null.

. Concatenation of two strings.

.= The corresponding assignment operator.

eq String equality (== is numeric equality). For a
mnemonic just think of "eq" as a string. (If you
are used to the awk behavior of using == for
either string or numeric equality based on the
current form of the comparands, beware! You must
be explicit here.)

ne String inequality (!= is numeric inequality).

lt String less than.

gt String greater than.

le String less than or equal.

ge String greater than or equal.

=~ Certain operations search or modify the string
"$_" by default. This operator makes that kind of
operation work on some other string. The right
argument is a search pattern, substitution, or
translation. The left argument is what is sup
posed to be searched, substituted, or translated
instead of the default "$_". 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 autoincre
ment/decrement, but higher than everything else.

!~ Just like =~ except the return value is negated.

x The repetition operator. Returns a string con
sisting of the left operand repeated the number of
times specified by the right operand.

print '-' x 80; # print row of dash

es
print '-' x80; # illegal, x80 is identi

fier

print "" x ($tab/8), ' ' x ($tab%8); # tab

over

x= 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 implic
itly 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 perl doesn't:

unary & Address-of operator.

unary * Dereference-address operator.

Like C, perl does a certain amount of expression evalua
tion 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 sub
stitution. Backslash interpretation also happens at com
pile time. You can say

'Now is the time for all' . "0 .
'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 expres
sion:

/PATTERN/
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 expres
sions.

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 de

sired

if (/Version: *([0-9.]*)/) { $version = $1; }

next if m#^/usr/spool/uucp#;

?PATTERN?
This is just like the /pattern/ search, except
that it matches only once between calls to the
reset 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.

chdir EXPR
Changes the working director to EXPR, if possible.
Returns 1 upon success, 0 otherwise. See example
under die().

chmod LIST
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 paren
theses.

$cnt = (chmod 0755,'foo','bar');

chop(VARIABLE)

chop 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/0/ because it neither
scans nor copies the string. If VARIABLE is omit
ted, chops $_. Example:

while (<>) {
chop; # avoid on last field
@array = split(/:/);
...

}

chown LIST
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 success
fully 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;

}

close(FILEHANDLE)

close FILEHANDLE
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 open.)
However, an explicit close on an input file resets
the line counter ($.), while the implicit close
done by open does not. Also, closing a pipe will
wait for the process executing on the pipe to com
plete, 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

crypt(PLAINTEXT,SALT)
Encrypts a string exactly like the crypt() func
tion in the C library. Useful for checking the
password file for lousy passwords. Only the guys
wearing white hats should do this.

die EXPR
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 exit.

do BLOCK
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.)

do SUBROUTINE (LIST)
Executes a SUBROUTINE declared by a sub declara
tion, and returns the value of the last expression
evaluated in SUBROUTINE. (See the section on sub
routines later on.)

each(ASSOC_ARRAY)
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 environ
ment like the printenv program, only in a differ
ent order:

while (($key,$value) = each(ENV)) {
print "$key=$value0;

}

See also keys() and values().

eof(FILEHANDLE)

eof 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 file
handle 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 "--------------0;

}
print;

}

eval EXPR
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 sub
routines. 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.

exec LIST
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 pars
ing. 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.

exit EXPR
Evaluates EXPR and exits immediately with that
value. Example:

$ans = <stdin>;
exit 0 if $ans =~ /^[Xx]/;

See also die.

exp(EXPR)
Returns e to the power of EXPR.

fork 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 pro
cesses, which means you may need to set $| to
avoid duplicate output.

gmtime(EXPR)
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 fol
lows:

($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$is

dst)
= gmtime(time);

All array elements are numeric.

LOCAL PERL(1)