perl
SYNOPSIS
perl [options] filename args
DESCRIPTION
Perl is an interpreted language optimized for scanning arbi-
trary 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, effi-
cient, 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. Unlike
most Unix utilities, perl does not arbitrarily limit the
size of your data--if you've got the memory, perl can slurp
in your whole file as a single string. Recursion is of
unlimited depth. And the hash tables used by associative
arrays grow as necessary to prevent degraded performance.
Perl uses sophisticated pattern matching techniques to scan
large amounts of data very quickly. Although optimized for
scanning text, perl can also deal with binary data, and can
make dbm files look like associative arrays (where dbm is
available). Setuid perl scripts are safer than C programs
through a dataflow tracing mechanism which prevents many
stupid security holes. If you have a problem that would
ordinarily use sed or awk or sh, but it exceeds their capa-
bilities 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 fol-
lowing 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 implicitly via standard input. This only
works if there are no filename arguments--to pass argu-
ments to a stdin script you must explicitly specify a -
for the script name.
After locating your script, perl compiles it to an internal
form. If the script is syntactically correct, it is exe-
cuted.
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 argu-
ment. Example:
#!/usr/bin/perl -spi.bak # same as -s -p -i.bak
...
Options include:
-0digits
specifies the record separator ($/) as an octal number.
If there are no digits, the null character is the
separator. Other switches may precede or follow the
digits. For example, if you have a version of find
which can print filenames terminated by the null char-
acter, you can say this:
find . -name '*.bak' -print0 perl -n0e unlink
The special value 00 will cause Perl to slurp files in
paragraph mode. The value 0777 will cause Perl to
slurp files whole since there is no legal character
with that value.
-a turns on autosplit mode when used with a -n or -p. An
implicit split command to the @F array is done as the
first thing inside the implicit while loop produced by
the -n or -p.
perl -ane 'print pop(@F), "\n";'
is equivalent to
while (<>) {
@F = split(' ');
print pop(@F), "\n";
}
-c causes perl to check the syntax of the script and then
exit without executing it.
-d runs the script under the perl debugger. See the sec-
tion on Debugging.
-Dnumber
sets debugging flags. To watch how it executes your
script, use -D14. (This only works if debugging is
compiled into your perl.) Another nice value is -D1024,
-e commandline
may be used to enter one line of script. Multiple -e
commands may be given to build up a multi-line script.
If -e is given, perl will not look for a script
filename in the argument list.
-iextension
specifies that files processed by the <> construct are
to be edited in-place. It does this by renaming the
input file, opening the output file by the same name,
and selecting that output file as the default for print
statements. The extension, if supplied, is added to
the name of the old file to make a backup copy. If no
extension is supplied, no backup is made. Saying "perl
-p -i.bak -e "s/foo/bar/;" ... " is the same as using
the script:
#!/usr/bin/perl -pi.bak
s/foo/bar/;
which is equivalent to
#!/usr/bin/perl
while (<>) {
if ($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 $oldargv to know when the filename has changed. It
does, however, use ARGVOUT for the selected filehandle.
Note that STDOUT is restored as the default output
filehandle after the loop.
You can use eof to locate the end of each input file,
in case you want to append to each file, or reset line
numbering (see example under eof).
-Idirectory
may be used in conjunction with -P to tell the C
preprocessor where to look for include files. By
default /usr/include and /usr/lib/perl are searched.
while (<>) {
... # your script goes here
}
Note that the lines are not printed by default. See -p
to have lines printed. Here is an efficient way to
delete all files older than a week:
find . -mtime +7 -print perl -ne 'chop;unlink;'
This is faster than using the -exec switch of find
because you don't have to start a process on every
filename found.
-p causes perl to assume the following loop around your
script, which makes it iterate over filename arguments
somewhat like sed:
while (<>) {
... # your script goes here
} continue {
print;
}
Note that the lines are printed automatically. To
suppress printing use the -n switch. A -p overrides a
-n switch.
-P causes your script to be run through the C preprocessor
before compilation by perl. (Since both comments and
cpp directives begin with the # character, you should
avoid starting comments with any words recognized by
the C preprocessor such as "if", "else" or "define".)
-s enables some rudimentary switch parsing for switches on
the command line after the script name but before any
filename arguments (or before a --). Any switch found
there is removed from @ARGV and sets the corresponding
variable in the perl script. The following script
prints "true" if and only if the script is invoked with
a -xyz switch.
#!/usr/bin/perl -s
if ($xyz) { print "true\n"; }
-S makes perl use the PATH environment variable to search
for the script (unless the name of the script starts
with a slash). Typically this is used to emulate #!
startup on machines that don't support #!, in the fol-
lowing manner:
The system ignores the first line and feeds the script
to /bin/sh, which proceeds to try to execute the perl
script as a shell script. The shell executes the
second line as a normal shell command, and thus starts
up the perl interpreter. On some systems $0 doesn't
always contain the full pathname, so the -S tells perl
to search for the script if necessary. After perl
locates the script, it parses the lines and ignores
them because the variable $running_under_some_shell is
never true. A better construct than $* would be
${1+"$@"}, which handles embedded spaces and such in
the filenames, but doesn't work if the script is being
interpreted by csh. In order to start up sh rather
than csh, some systems may have to replace the #! line
with a line containing just a colon, which will be pol-
itely ignored by perl. Other systems can't control
that, and need a totally devious construct that will
work under any of csh, sh or perl, such as the follow-
ing:
eval '(exit $?0)' && eval 'exec /usr/bin/perl -S $0 ${1+"$@"}'
& eval 'exec /usr/bin/perl -S $0 $argv:q'
if 0;
-u causes perl to dump core after compiling your script.
You can then take this core dump and turn it into an
executable file by using the undump program (not sup-
plied). This speeds startup at the expense of some
disk space (which you can minimize by stripping the
executable). (Still, a "hello world" executable comes
out to about 200K on my machine.) If you are going to
run your executable as a set-id program then you should
probably compile it using taintperl rather than normal
perl. If you want to execute a portion of your script
before dumping, use the dump operator instead. Note:
availability of undump is platform specific and may not
be available for a specific port of perl.
-U allows perl to do unsafe operations. Currently the
only "unsafe" operation is the unlinking of directories
while running as superuser.
-v prints the version and patchlevel of your perl execut-
able.
-w prints warnings about identifiers that are mentioned
only once, and scalar variables that are used before
being set. Also warns about redefined subroutines, and
references to undefined filehandles or filehandles
opened readonly that you are attempting to write on.
-xdirectory
tells perl that the script is embedded in a message.
Leading garbage will be discarded until the first line
that starts with #! and contains the string "perl".
Any meaningful switches on that line will be applied
(but only one group of switches, as with normal #! pro-
cessing). If a directory name is specified, Perl will
switch to that directory before running the script.
The -x switch only controls the the disposal of leading
garbage. The script must be terminated with __END__ if
there is trailing garbage to be ignored (the script can
process any or all of the trailing garbage via the DATA
filehandle if desired).
Data Types and Objects
Perl has three data types: scalars, arrays of scalars, and
associative arrays of scalars. Normal arrays are indexed by
number, and associative arrays by string.
The interpretation of operations and values in perl some-
times depends on the requirements of the context around the
operation or value. There are three major contexts: string,
numeric and array. Certain operations return array values
in contexts wanting an array, and scalar values otherwise.
(If this is true of an operation it will be mentioned in the
documentation for that operation.) Operations which return
scalars don't care whether the context is looking for a
string or a number, but scalar variables and values are
interpreted as strings or numbers as appropriate to the con-
text. A scalar 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.
There are actually two varieties of null string: defined and
undefined. Undefined null strings are returned when there
is no real value for something, such as when there was an
error, or at end of file, or when you refer to an uninitial-
ized variable or element of an array. An undefined null
string may become defined the first time you access it, but
prior to that you can use the defined() operator to deter-
mine whether the value is defined or not.
References to scalar variables always begin with '$', even
when referring to a scalar that is part of an array. Thus:
$days # a simple scalar variable
$days[28] # 29th element of array @days
$days{'Feb'} # one value from an associative array
$#days # last index of array @days
@days[3,4,5] # same as @days[3..5]
@days{'a','c'} # same as ($days{'a'},$days{'c'})
and entire associative arrays are denoted by '%':
%days # (key1, val1, key2, val2 ...)
Any of these eight constructs may serve as an lvalue, that
is, may be assigned to. (It also turns out that an assign-
ment is itself an lvalue in certain contexts--see examples
under s, tr and chop.) Assignment to a scalar evaluates the
righthand side in a scalar context, while assignment to an
array or array slice evaluates the righthand side in an
array context.
You may find the length of array @days by evaluating
"$#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.) Assigning to $#days changes
the length of the array. Shortening an array by this method
does not actually destroy any values. Lengthening an array
that was previously shortened recovers the values that were
in those elements. You can also gain some measure of effi-
ciency by preextending an array that is going to get big.
(You can also extend an array by assigning to an element
that is off the end of the array. This differs from assign-
ing to $#whatever in that intervening values are set to null
rather than recovered.) You can truncate an array down to
nothing by assigning the null list () to it. The following
are exactly equivalent
@whatever = ();
$#whatever = $[ - 1;
If you evaluate an array in a scalar context, it returns the
length of the array. The following is always true:
@whatever == $#whatever - $[ + 1;
Multi-dimensional arrays are not directly supported, but see
the discussion of the $; variable later for a means of emu-
lating multiple subscripts with an associative array. You
could also write a subroutine to turn multiple subscripts
into a single subscript.
Every data type has its own namespace. You can, without
fear of conflict, use the same name for a scalar variable,
an array, an associative array, a filehandle, a subroutine
name, and/or a label. Since variable and array references
always start with '$', '@', or '%', the "reserved" words
Hint: you could say open(LOG,'logfile') rather than
open(log,'logfile'). Using uppercase filehandles also
improves readability and protects you from conflict with
future reserved words.) 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 "$$". (Most of the one character names have a
predefined significance to perl. More later.)
Numeric literals are specified in any of the usual floating
point or integer formats:
12345
12345.67
.23E-10
0xffff # hex
0377 # octal
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 (except for \' and
\\). 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 to scalar variables, normal array
values, and array slices. (In other words, identifiers
beginning with $ or @, followed by an optional bracketed
expression as a subscript.) 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. Also note that
a single quoted string must be separated from a preceding
word by a space, since single quote is a valid character in
an identifier (see Packages).
Two special literals are __LINE__ and __FILE__, which
represent the current line number and filename at that point
in your program. They may only be used as separate tokens;
they will not be interpolated into strings. In addition,
the token __END__ may be used to indicate the logical end of
the script before the actual end of file. Any following
text is ignored (but may be read via the DATA filehandle).
A word that doesn't have any other interpretation in the
grammar will be treated as if it had single quotes around
it. For this purpose, a word consists only of alphanumeric
characters and underline, and must start with an alphabetic
character. As with filehandles and labels, a bare word that
consists entirely of lowercase letters risks conflict with
future reserved words, and if you use the -w switch, Perl
will warn you about any such words.
Array values are interpolated into double-quoted strings by
joining all the elements of the array with the delimiter
specified in the $" variable, space by default. (Since in
versions of perl prior to 3.0 the @ character was not a
metacharacter in double-quoted strings, the interpolation of
@array, $array[EXPR], @array[LIST], $array{EXPR}, or
@array{LIST} only happens if array is referenced elsewhere
in the program or is predefined.) The following are
equivalent:
$temp = join($",@ARGV);
system "echo $temp";
system "echo @ARGV";
Within search patterns (which also undergo double-quotish
substitution) there is a bad ambiguity: Is /$foo[bar]/ to
be interpreted as /${foo}[bar]/ (where [bar] is a character
class for the regular expression) or as /${foo[bar]}/ (where
[bar] is the subscript to array @foo)? If @foo doesn't oth-
erwise exist, then it's obviously a character class. If
@foo exists, perl takes a good guess about [bar], and is
almost always right. If it does guess wrong, or if you're
just plain paranoid, you can force the correct interpreta-
tion with curly brackets as above.
A line-oriented form of quoting is based on the shell here-
is syntax. Following a << you specify a string to terminate
the quoted material, and all lines following the current
line down to the terminating string are the value of the
item. The terminating string may be either an identifier (a
word), or some quoted text. If quoted, the type of quotes
you use determines the treatment of the text, just as in
regular quoting. An unquoted identifier works like double
quotes. There must be no space between the << and the iden-
tifier. (If you put a space it will be treated as a null
identifier, which is valid, and matches the first blank
line--see Merry Christmas example below.) The terminating
string must appear by itself (unquoted and with no surround-
ing whitespace) on the terminating line.
print <<EOF; # same as above
The price is $Price.
The price is $Price.
EOF
print << x 10; # null identifier is delimiter
Merry Christmas!
print <<`EOC`; # execute commands
echo hi there
echo lo there
EOC
print <<foo, <<bar; # you can stack them
I said foo.
foo
I said bar.
bar
Array literals are denoted by separating individual values
by commas, and enclosing the list in parentheses:
(LIST)
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. Note
that the value of an actual array in a scalar context is the
length of the array; the following assigns to $foo the value
3:
@foo = ('cc', '-E', $bar);
$foo = @foo; # $foo gets 3
You may have an optional comma before the closing
parenthesis of an array literal, so that you can say:
@foo = (
1,
2,
3,
);
When a LIST is evaluated, each element of the list is
evaluated in an array context, and the resulting array value
is interpolated into LIST just as if each individual element
(@foo,@bar,&SomeSub)
contains all the elements of @foo followed by all the ele-
ments of @bar, followed by all the elements returned by the
subroutine named SomeSub.
A list value may also be subscripted like a normal array.
Examples:
$time = (stat($file))[8]; # stat returns array value
$digit = ('a','b','c','d','e','f')[$digit-10];
return (pop(@foo),pop(@foo))[0];
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);
The final element may be an array or an associative array:
($a, $b, @rest) = split;
local($a, $b, %rest) = @_;
You can actually put an array anywhere in the list, but the
first array in the list will soak up all the values, and
anything after it will get a null value. This may be useful
in a local().
An associative array literal contains pairs of values to be
interpreted as a key and a value:
# same as map assignment above
%map = ('red',0x00f,'blue',0x0f0,'green',0xf00);
Array assignment in a scalar context returns the number of
elements produced by the expression on the right side of the
assignment:
$x = (($foo,$bar) = (3,2,1)); # set $x to 3, not 2
There are several other pseudo-literals that you should know
about. If a string is enclosed by backticks (grave
accents), it first undergoes variable substitution just like
a double quoted string. It is then interpreted as a com-
mand, and the output of that command is the value of the
pseudo-literal, like in a shell. In a scalar context, a
single string consisting of all the output is returned. In
an array context, an array of values is returned, one for
mand is returned in $? (see Predefined Names for the
interpretation of $?). Unlike in csh, no translation is
done on the return data--newlines remain newlines. Unlike
in any of the shells, single quotes do not hide variable
names in the command from interpretation. To pass a $
through to the shell you need to hide it with a backslash.
Evaluating a filehandle in angle brackets yields the next
line from that file (newline included, so it's never false
until EOF, at which time an undefined value is returned).
Ordinarily you must assign that value to a variable, but
there is one situation where an automatic assignment hap-
pens. 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>) { print; }
while (<STDIN>) { print; }
for (;<STDIN>;) { print; }
print while $_ = <STDIN>;
print while <STDIN>;
The filehandles STDIN, STDOUT and STDERR are predefined.
(The filehandles stdin, stdout and stderr will also work
except in packages, where they would be interpreted as local
identifiers rather than global.) Additional filehandles may
be created with the open function.
If a <FILEHANDLE> is used in a context that is looking for
an array, an array consisting of all the input lines is
returned, one line per array element. It's easy to make a
LARGE data space this way, so use with care.
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
}
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. (But
see example under eof for how to reset line numbers on each
file.)
If you want to set @ARGV to your 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.
If the string inside the angle brackets is a reference to a
scalar variable (e.g. <$foo>), then that variable contains
the name of the filehandle to input from.
If the string inside angle brackets is not a filehandle, it
is interpreted as a filename pattern to be globbed, and
either an array of filenames or the next filename in the
list is returned, depending on context. One level of $
interpretation is done first, but you can't say <$foo>
because that's an indirect filehandle as explained in the
previous paragraph. You could insert curly brackets to
force interpretation as a filename glob: <${foo}>. Example:
while (<*.c>) {
chmod 0644, $_;
}
while (<foo>) {
chop;
chmod 0644, $_;
}
In fact, it's currently implemented that way. (Which means
it will not work on filenames with spaces in them unless you
have /bin/csh on your machine.) Of course, the shortest way
to do the above is:
chmod 0644, <*.c>;
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 uninitial-
ized user-created objects are assumed to start with a null
or 0 value until they are defined by some explicit operation
such as assignment. The sequence of commands is executed
just once, unlike in sed and awk 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 con-
trol over which files and which lines you look at. (Actu-
ally, 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
commands-declarations all take effect at compile time. Typ-
ically 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 # 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 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:
LABEL while (EXPR) BLOCK
LABEL while (EXPR) BLOCK continue BLOCK
LABEL for (EXPR; EXPR; EXPR) BLOCK
LABEL foreach VAR (ARRAY) 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 brack-
ets 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) ? 'hi mom' : die "Can't open $foo: $!";
# 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 statement
(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:
is the same as
$i = 1;
while ($i < 10) {
...
} continue {
$i++;
}
The foreach loop iterates over a normal array value and sets
the variable VAR to be each element of the array in turn.
The variable is implicitly local to the loop, and regains
its former value upon exiting the loop. The "foreach" key-
word is actually identical to the "for" keyword, so you can
use "foreach" for readability or "for" for brevity. If VAR
is omitted, $_ is set to each value. If ARRAY is an actual
array (as opposed to an expression returning an array
value), you can modify each element of the array by modify-
ing VAR inside the loop. Examples:
for (@ary) { s/foo/bar/; }
foreach $elem (@elements) {
$elem *= 2;
}
for ((10,9,8,7,6,5,4,3,2,1,'BOOM')) {
print $_, "\n"; sleep(1);
}
for (1..15) { print "Merry Christmas\n"; }
foreach $item (split(/:[\\\n:]*/, $ENV{'TERMCAP'})) {
print "Item: $item\n";
}
The BLOCK by itself (labeled or not) is equivalent to a loop
that executes once. Thus you can use any of the loop con-
trol statements in it to leave or restart the block. The
continue 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;
}
There is no official switch statement in perl, because there
foo: {
$abc = 1, last foo if /^abc/;
$def = 1, last foo if /^def/;
$xyz = 1, last foo if /^xyz/;
$nothing = 1;
}
or
foo: {
/^abc/ && do { $abc = 1; last foo; };
/^def/ && do { $def = 1; last foo; };
/^xyz/ && do { $xyz = 1; last foo; };
$nothing = 1;
}
or
foo: {
/^abc/ && ($abc = 1, last foo);
/^def/ && ($def = 1, last foo);
/^xyz/ && ($xyz = 1, last foo);
$nothing = 1;
}
or even
if (/^abc/)
{ $abc = 1; }
elsif (/^def/)
{ $def = 1; }
elsif (/^xyz/)
{ $xyz = 1; }
else
{$nothing = 1;}
As it happens, these are all optimized internally to a
switch structure, so perl jumps directly to the desired
statement, and you needn't worry about perl executing a lot
of unnecessary statements when you have a string of 50
elsifs, as long as you are testing the same simple scalar
variable using ==, eq, or pattern matching as above. (If
you're curious as to whether the optimizer has done this for
a particular case statement, you can use the -D1024 switch
to list the syntax tree before execution.)
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 awk).
if EXPR
unless EXPR
while EXPR
until EXPR
The if and unless modifiers have the expected semantics.
The while and until modifiers also have the expected seman-
tics (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 do 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 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 exponentiation operator.
**= The exponentiation assignment operator.
() The null list, used to initialize an array to null.
. Concatenation of two strings.
.= The concatenation 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.
cmp String comparison, returning -1, 0, or 1.
<=> Numeric comparison, returning -1, 0, or 1.
=~ Certain operations search or modify the string "$_"
by default. This operator makes that kind of opera-
tion 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 suc-
cess of the operation. (If the right argument is an
expression other than a search pattern, substitu-
tion, 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 pre-
cedence of this operator is lower than unary minus
and autoincrement/decrement, but higher than every-
thing else.
!~ Just like =~ except the return value is negated.
x The repetition operator. Returns a string consist-
ing 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
x= The repetition assignment operator.
.. The range operator, which is really two different
operators depending on the context. In an array
context, returns an array of values counting (by
ones) from the left value to the right value. This
is useful for writing "for (1..10)" loops and for
doing slice operations on arrays.
In a scalar context, .. returns a boolean value.
The operator is bistable, like a flip-flop.. Each
.. operator maintains its own boolean state. It is
false as long as its left operand is false. Once
the left operand is true, the range operator stays
true until the right operand is true, AFTER which
the range operator becomes false again. (It doesn't
become false till the next time the range operator
is evaluated. It can become false on the same
operand is not evaluated while the operator is in
the "true" state. The scalar .. operator is pri-
marily intended for doing line number ranges after
the fashion of sed or awk. The precedence is a lit-
tle 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 operand of scalar .. is static, that
operand is implicitly compared to the $. variable,
the current line number. Examples:
As a scalar operator:
if (101 .. 200) { print; } # print 2nd hundred lines
next line if (1 .. /^$/); # skip header lines
s/^/> / if (/^$/ .. eof()); # quote body
As an array operator:
for (101 .. 200) { print; } # print $_ 100 times
@foo = @foo[$[ .. $#foo]; # an expensive no-op
@foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items
-x A file test. This unary operator takes one argu-
ment, either a filename or a filehandle, and tests
the associated file to see if something is true
about it. If the argument is omitted, tests $_,
except for -t, which tests STDIN. It returns 1 for
true and '' for false, or the undefined value if the
file doesn't exist. Precedence is higher than logi-
cal and relational operators, but lower than arith-
metic operators. The operator may be any of:
-r File is readable by effective uid.
-w File is writable by effective uid.
-x File is executable by effective uid.
-o File is owned by effective uid.
-R File is readable by real uid.
-W File is writable by real uid.
-X File is executable by real uid.
-O File is owned by real uid.
-e File exists.
-z File has zero size.
-s File has non-zero size (returns size).
-p File is a named pipe (FIFO).
-S File is a socket.
-b File is a block special file.
-c File is a character special file.
-u File has setuid bit set.
-g File has setgid bit set.
-k File has sticky bit set.
-t Filehandle is opened to a tty.
-T File is a text file.
-B File is a binary file (opposite of -T).
-M Age of file in days when script started.
-A Same for access time.
-C Same for inode change time.
The interpretation of the file permission operators
-r, -R, -w, -W, -x and -X is based solely on the
mode of the file and the uids and gids of the user.
There may be other reasons you can't actually read,
write or execute the file. Also note that, for the
superuser, -r, -R, -w and -W always return 1, and -x
and -X return 1 if any execute bit is set in the
mode. Scripts run by the superuser may thus need to
do a stat() in order to determine the actual mode of
the file, or temporarily set the uid to something
else.
Example:
while (<>) {
chop;
next unless -f $_; # ignore specials
...
}
Note that -s/a/b/ does not do a negated substitu-
tion. Saying -exp($foo) still works as expected,
however--only single letters following a minus are
interpreted as file tests.
The -T and -B switches work as follows. The first
block or so of the file is examined for odd charac-
ters such as strange control codes or metacharac-
ters. If too many odd characters (>10%) are found,
it's a -B file, otherwise it's a -T file. Also, any
file containing null in the first block is con-
sidered a binary file. If -T or -B is used on a
filehandle, the current stdio buffer is examined
rather than the first block. Both -T and -B return
TRUE on a null file, or a file at EOF when testing a
filehandle.
operator) is used, saving a system call. (This doesn't work
with -t, and you need to remember that lstat and -l will
leave values in the stat structure for the symbolic link,
not the real file.) Example:
print "Can do.\n" if -r $a -w _ -x _;
stat($filename);
print "Readable\n" if -r _;
print "Writable\n" if -w _;
print "Executable\n" if -x _;
print "Setuid\n" if -u _;
print "Setgid\n" if -g _;
print "Sticky\n" if -k _;
print "Text\n" if -T _;
print "Binary\n" if -B _;
Here is what C has that perl doesn't:
unary & Address-of operator.
unary * Dereference-address operator.
(TYPE) Type casting operator.
Like C, perl 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 substi-
tution. 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.
The autoincrement operator has a little extra built-in magic
to it. If you increment a variable that is numeric, or that
has ever been used in a numeric context, you get a normal
increment. If, however, the variable has only been used in
string contexts since it was set, and has a value that is
not null and matches the pattern /^[a-zA-Z]*[0-9]*$/, the
increment is done as a string, preserving each character
within its range, with carry:
print ++($foo = '99'); # prints '100'
print ++($foo = 'a0'); # prints 'a1'
print ++($foo = 'Az'); # prints 'Ba'
print ++($foo = 'zz'); # prints 'aaa'
magical autoincrement algorithm if the minimum and maximum
are strings. You can say
@alphabet = ('A' .. 'Z');
to get all the letters of the alphabet, or
$hexdigit = (0 .. 9, 'a' .. 'f')[$num & 15];
to get a hexadecimal digit, or
@z2 = ('01' .. '31'); print @z2[$mday];
to get dates with leading zeros. (If the final value speci-
fied is not in the sequence that the magical increment would
produce, the sequence goes until the next value would be
longer than the final value specified.)
The and && operators differ
from C's in that, rather than returning 0 or 1, they return
the last value evaluated. Thus, a portable way to find out
the home directory might be:
$home = $ENV{'HOME'} $ENV{'LOGDIR'}
(getpwuid($<))[7] die "You're homeless!\n";
Along with the literals and variables mentioned earlier, the
operations in the following section can serve as terms in an
expression. Some of these operations take a LIST as an
argument. Such a list can consist of any combination of
scalar arguments or array values; the array 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 array value. Elements of the LIST should
be separated by commas. If an operation is listed both with
and without parentheses around its arguments, it means you
can either use it as a unary operator or as a function call.
To use it as a function call, the next token on the same
line must be a left parenthesis. (There may be intervening
white space.) Such a function then has highest precedence,
as you would expect from a function. If any token other
than a left parenthesis follows, then it is a unary opera-
tor, with a precedence depending only on whether it is a
LIST operator or not. LIST operators have lowest pre-
cedence. All other unary operators have a precedence
greater than relational operators but less than arithmetic
operators. See the section on Precedence.
/PATTERN/
See m/PATTERN/.
operator. This is a useful optimization when you
only want to see the first occurrence of something
in each file of a set of files, for instance. Only
?? patterns local to the current package are reset.
accept(NEWSOCKET,GENERICSOCKET)
Does the same thing that the accept system call
does. Returns true if it succeeded, false other-
wise. See example in section on Interprocess Com-
munication.
alarm(SECONDS)
alarm SECONDS
Arranges to have a SIGALRM delivered to this process
after the specified number of seconds (minus 1,
actually) have elapsed. Thus, alarm(15) will cause
a SIGALRM at some point more than 14 seconds in the
future. Only one timer may be counting at once.
Each call disables the previous timer, and an argu-
ment 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 previ-
ous timer.
atan2(Y,X)
Returns the arctangent of Y/X in the range -PI to
PI.
bind(SOCKET,NAME)
Does the same thing that the bind system call does.
Returns true if it succeeded, false otherwise. NAME
should be a packed address of the proper type for
the socket. See example in section on Interprocess
Communication.
binmode(FILEHANDLE)
binmode FILEHANDLE
Arranges for the file to be read in "binary" mode in
operating systems that distinguish between binary
and text files. Files that are not read in binary
mode have CR LF sequences translated to LF on input
and LF translated to CR LF on output. Binmode has
no effect under Unix. If FILEHANDLE is an expres-
sion, the value is taken as the name of the filehan-
dle.
caller(EXPR)
caller Returns the context of the current subroutine call:
debugger uses to print a stack trace. The value of
EXPR indicates how many call frames to go back
before the current one.
chdir(EXPR)
chdir EXPR
Changes the working directory to EXPR, if possible.
If EXPR is omitted, changes to home directory.
Returns 1 upon success, 0 otherwise. See example
under die.
chmod(LIST)
chmod LIST
Changes the permissions of a list of files. The
first element of the list must be the numerical
mode. Returns the number of files successfully
changed.
$cnt = chmod 0755, 'foo', 'bar';
chmod 0755, @executables;
chop(LIST)
chop(VARIABLE)
chop VARIABLE
chop Chops off the last character of a string and returns
the character chopped. 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(/:/);
...
}
You can actually chop anything that's an lvalue,
including an assignment:
chop($cwd = `pwd`);
chop($answer = <STDIN>);
If you chop a list, each element is chopped. Only
the value of the last chop is returned.
Changes the owner (and group) of a list of files.
The first two elements of the list must be the
NUMERICAL uid and gid, in that order. Returns the
number of files successfully changed.
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
Here's an example of looking up non-numeric uids:
print "User: ";
$user = <STDIN>;
chop($user);
print "Files: "
$pattern = <STDIN>;
chop($pattern);
open(pass, '/etc/passwd')
die "Can't open passwd: $!\n";
while (<pass>) {
($login,$pass,$uid,$gid) = split(/:/);
$uid{$login} = $uid;
$gid{$login} = $gid;
}
@ary = <${pattern}>; # get filenames
if ($uid{$user} eq '') {
die "$user not in passwd file";
}
else {
chown $uid{$user}, $gid{$user}, @ary;
}
chroot(FILENAME)
chroot FILENAME
Does the same as the system call of that name. If
you don't know what it does, don't worry about it.
If FILENAME is omitted, does chroot to $_.
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.) How-
ever, 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 complete, in
case you want to look at the output of the pipe
afterwards. Closing a pipe explicitly also puts the
... # print stuff to output
close OUTPUT; # wait for sort to finish
open(INPUT, 'foo'); # get sort's results
FILEHANDLE may be an expression whose value gives
the real filehandle name.
closedir(DIRHANDLE)
closedir DIRHANDLE
Closes a directory opened by opendir().
connect(SOCKET,NAME)
Does the same thing that the connect system call
does. Returns true if it succeeded, false other-
wise. NAME should be a package address of the
proper type for the socket. See example in section
on Interprocess Communication.
cos(EXPR)
cos EXPR
Returns the cosine of EXPR (expressed in radians).
If EXPR is omitted takes cosine of $_.
crypt(PLAINTEXT,SALT)
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.
dbmclose(ASSOC_ARRAY)
dbmclose ASSOC_ARRAY
Breaks the binding between a dbm file and an associ-
ative array. The values remaining in the associa-
tive array are meaningless unless you happen to want
to know what was in the cache for the dbm file.
This function is only useful if you have ndbm.
dbmopen(ASSOC,DBNAME,MODE)
This binds a dbm or ndbm file to an associative
array. ASSOC is the name of the associative array.
(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 the database does not exist, it
is created with protection specified by MODE (as
modified by the umask). If your system only sup-
ports the older dbm functions, you may only have one
dbmopen in your program. If your system has neither
dbm nor ndbm, calling dbmopen produces a fatal
the dbmopen are lost. A certain number of values
from the dbm file are cached in memory. By default
this number is 64, but you can increase it by preal-
locating that number of garbage entries in the asso-
ciative array before the dbmopen. You can flush the
cache if necessary with the reset command.
If you don't have write access to the dbm file, you
can only read associative array variables, not set
them. If you want to test whether you can write,
either use file tests or try setting a dummy array
entry inside an eval, which will trap the error.
Note that functions such as keys() and values() may
return huge array values 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);
defined(EXPR)
defined EXPR
Returns a boolean value saying whether the lvalue
EXPR has a real value or not. Many operations
return the undefined value under exceptional condi-
tions, such as end of file, uninitialized variable,
system error and such. This function allows you to
distinguish between an undefined null string and a
defined null string with operations that might
return a real null string, in particular referencing
elements of an array. You may also check to see if
arrays or subroutines exist. Use on predefined
variables is not guaranteed to produce intuitive
results. Examples:
print if defined $switch{'D'};
print "$val\n" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
eval '@foo = ()' if defined(@foo);
die "No XYZ package defined" unless defined %_XYZ;
sub foo { defined &bar ? &bar(@_) : die "No bar"; }
See also undef.
undefined value if nothing was deleted. Deleting
from $ENV{} modifies the environment. Deleting from
an array bound to a dbm file deletes the entry from
the dbm file.
The following deletes all the values of an associa-
tive array:
foreach $key (keys %ARRAY) {
delete $ARRAY{$key};
}
(But it would be faster to use the reset command.
Saying undef %ARRAY is faster yet.)
die(LIST)
die LIST
Outside of an eval, prints the value of LIST to
STDERR and exits with the current value of $!
(errno). If $! is 0, exits with the value of ($? >>
8) (`command` status). If ($? >> 8) is 0, exits
with 255. Inside an eval, the error message is
stuffed into $@ and the eval is terminated with the
undefined value.
Equivalent examples:
die "Can't cd to spool: $!\n"
unless chdir '/usr/spool/news';
chdir '/usr/spool/news' die "Can't cd to spool: $!\n"
If the value of EXPR does not end in a newline, the
current script line number and input line number (if
any) are also printed, and a newline is supplied.
Hint: sometimes appending ", stopped" to your mes-
sage will cause it to make better sense when the
string "at foo line 123" is appended. Suppose you
are running script "canasta".
die "/etc/games is no good";
die "/etc/games is no good, stopped";
produce, respectively
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
See also exit.
fied by a loop modifier, executes the BLOCK once
before testing the loop condition. (On other state-
ments the loop modifiers test the conditional
first.)
do SUBROUTINE (LIST)
Executes a SUBROUTINE declared by a sub declaration,
and returns the value of the last expression
evaluated in SUBROUTINE. If there is no subroutine
by that name, produces a fatal error. (You may use
the "defined" operator to determine if a subroutine
exists.) If you pass arrays as part of LIST you may
wish to pass the length of the array in front of
each array. (See the section on subroutines later
on.) SUBROUTINE may be a scalar variable, in which
case the variable contains the name of the subrou-
tine to execute. The parentheses are required to
avoid confusion with the "do EXPR" form.
As an alternate form, you may call a subroutine by
prefixing the name with an ampersand: &foo(@args).
If you aren't passing any arguments, you don't have
to use parentheses. If you omit the parentheses, no
@_ array is passed to the subroutine. The & form is
also used to specify subroutines to the defined and
undef operators.
do EXPR Uses the value of EXPR as a filename and executes
the contents of the file as a perl script. Its pri-
mary use is to include subroutines from a perl sub-
routine library.
do 'stat.pl';
is just like
eval `cat stat.pl`;
except that it's more efficient, more concise, keeps
track of the current filename for error messages,
and searches all the -I libraries if the file isn't
in the current directory (see also the @INC array in
Predefined Names). It's the same, however, in that
it does reparse the file every time you call it, so
if you are going to use the file inside a loop you
might prefer to use -P and #include, at the expense
of a little more startup time. (The main problem
with #include is that cpp doesn't grok # comments--a
workaround is to use ";#" for standalone comments.)
Note that the following are NOT equivalent:
Note that inclusion of library routines is better
done with the "require" operator.
dump LABEL
This causes an immediate core dump. Primarily this
is so that you can use the undump program to turn
your core dump into an executable binary after hav-
ing initialized all your variables at the beginning
of the program. When the new binary is executed it
will begin by executing a "goto LABEL" (with all the
restrictions that goto suffers). Think of it as a
goto with an intervening core dump and reincarna-
tion. If LABEL is omitted, restarts the program
from the top. WARNING: any files opened at the time
of the dump will NOT be open any more when the pro-
gram is reincarnated, with possible resulting confu-
sion on the part of perl. See also -u.
Example:
#!/usr/bin/perl
require 'getopt.pl';
require 'stat.pl';
%days = (
'Sun',1,
'Mon',2,
'Tue',3,
'Wed',4,
'Thu',5,
'Fri',6,
'Sat',7);
dump QUICKSTART if $ARGV[0] eq '-d';
QUICKSTART:
do Getopt('f');
each(ASSOC_ARRAY)
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 must not modify the
array while iterating over it. There is a single
iterator for each associative array, shared by all
order:
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}
See also keys() and values().
eof(FILEHANDLE)
eof()
eof Returns 1 if the next read on FILEHANDLE will return
end of file, or if FILEHANDLE is not open. FILEHAN-
DLE may be an expression whose value gives the real
filehandle name. (Note that this function actually
reads a character and then ungetc's it, so it is not
very useful in an interactive context.) An eof
without an argument returns the eof status for the
last file read. Empty parentheses () 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 to detect the end of only
the last file. Use eof(ARGV) or eof without the
parentheses to test EACH file in a while (<>) loop.
Examples:
# insert dashes just before last line of last file
while (<>) {
if (eof()) {
print "--------------\n";
}
print;
}
# reset line numbering on each input file
while (<>) {
print "$.\t$_";
if (eof) { # Not eof().
close(ARGV);
}
}
eval(EXPR)
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
returned by eval, and $@ is set to the error mes-
sage. If there was no error, $@ is guaranteed to be
a null string. If EXPR is omitted, evaluates $_.
The final semicolon, if any, may be omitted from the
expression.
Note that, since eval traps otherwise-fatal errors,
it is useful for determining whether a particular
feature (such as dbmopen or symlink) is implemented.
It is also Perl's exception trapping mechanism,
where the die operator is used to raise exceptions.
exec(LIST)
exec LIST
If there is more than one argument in LIST, or if
LIST is an array with more than one value, calls
execvp() with the arguments in LIST. If there is
only one scalar 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. Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile uniq";
If you don't really want to execute the first argu-
ment, but want to lie to the program you are execut-
ing about its own name, you can specify the program
you actually want to run by assigning that to a
variable and putting the name of the variable in
front of the LIST without a comma. (This always
forces interpretation of the LIST as a multi-valued
list, even if there is only a single scalar in the
list.) Example:
$shell = '/bin/csh';
exec $shell '-sh'; # pretend it's a login shell
exit(EXPR)
exit EXPR
Evaluates EXPR and exits immediately with that
value. Example:
See also die. If EXPR is omitted, exits with 0
status.
exp(EXPR)
exp EXPR
Returns e to the power of EXPR. If EXPR is omitted,
gives exp($_).
fcntl(FILEHANDLE,FUNCTION,SCALAR)
Implements the fcntl(2) function. You'll probably
have to say
require "fcntl.ph"; # probably /usr/local/lib/perl/fcntl.ph
first to get the correct function definitions. If
fcntl.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/fcntl.h>. (There
is a perl script called h2ph that comes with the
perl kit which may help you in this.) Argument pro-
cessing and value return works just like ioctl
below. Note that fcntl will produce a fatal error
if used on a machine that doesn't implement
fcntl(2).
fileno(FILEHANDLE)
fileno FILEHANDLE
Returns the file descriptor for a filehandle. Use-
ful for constructing bitmaps for select(). If
FILEHANDLE is an expression, the value is taken as
the name of the filehandle.
flock(FILEHANDLE,OPERATION)
Calls flock(2) on FILEHANDLE. See manual page for
flock(2) for definition of OPERATION. Returns true
for success, false on failure. Will produce a fatal
error if used on a machine that doesn't implement
flock(2). Here's a mailbox appender for BSD sys-
tems.
$LOCK_UN = 8;
sub lock {
flock(MBOX,$LOCK_EX);
# and, in case someone appended
# while we were waiting...
seek(MBOX, 0, 2);
}
sub unlock {
flock(MBOX,$LOCK_UN);
}
open(MBOX, ">>/usr/spool/mail/$ENV{'USER'}")
die "Can't open mailbox: $!";
do lock();
print MBOX $msg,"\n\n";
do unlock();
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
processes, which means you may need to set
$ to avoid duplicate output.
getc(FILEHANDLE)
getc FILEHANDLE
getc Returns the next character from the input file
attached to FILEHANDLE, or a null string at EOF. If
FILEHANDLE is omitted, reads from STDIN.
getlogin
Returns the current login from /etc/utmp, if any.
If null, use getpwuid.
$login = getlogin
(getpwuid($<))[0]
"Somebody";
getpeername(SOCKET)
Returns the packed sockaddr address of other end of
the SOCKET connection.
# An internet sockaddr
$sockaddr = 'S n a4 x8';
$hersockaddr = getpeername(S);
($family, $port, $heraddr) =
getpgrp(PID)
getpgrp PID
Returns the current process group for the specified
PID, 0 for the current process. Will produce a
fatal error if used on a machine that doesn't imple-
ment getpgrp(2). If EXPR is omitted, returns pro-
cess group of current process.
getppid Returns the process id of the parent process.
getpriority(WHICH,WHO)
Returns the current priority for a process, a pro-
cess group, or a user. (See getpriority(2).) Will
produce a fatal error if used on a machine that
doesn't implement getpriority(2).
getpwnam(NAME)
getgrnam(NAME)
gethostbyname(NAME)
getnetbyname(NAME)
getprotobyname(NAME)
getpwuid(UID)
getgrgid(GID)
getservbyname(NAME,PROTO)
gethostbyaddr(ADDR,ADDRTYPE)
getnetbyaddr(ADDR,ADDRTYPE)
getprotobynumber(NUMBER)
getservbyport(PORT,PROTO)
getpwent
getgrent
gethostent
getnetent
getprotoent
setgrent
sethostent(STAYOPEN)
setnetent(STAYOPEN)
setprotoent(STAYOPEN)
setservent(STAYOPEN)
endpwent
endgrent
endhostent
endnetent
endprotoent
endservent
These routines perform the same functions as their
counterparts in the system library. The return
values from the various get routines are as follows:
($name,$passwd,$uid,$gid,
$quota,$comment,$gcos,$dir,$shell) = getpw...
($name,$passwd,$gid,$members) = getgr...
($name,$aliases,$addrtype,$length,@addrs) = gethost...
($name,$aliases,$addrtype,$net) = getnet...
($name,$aliases,$proto) = getproto...
($name,$aliases,$port,$proto) = getserv...
The $members value returned by getgr... is a space
separated list of the login names of the members of
the group.
The @addrs value returned by the gethost... func-
tions is a list of the raw addresses returned by the
corresponding system library call. In the Internet
domain, each address is four bytes long and you can
unpack it by saying something like:
($a,$b,$c,$d) = unpack('C4',$addr[0]);
getsockname(SOCKET)
Returns the packed sockaddr address of this end of
the SOCKET connection.
($family, $port, $myaddr) =
unpack($sockaddr,$mysockaddr);
getsockopt(SOCKET,LEVEL,OPTNAME)
Returns the socket option requested, or undefined if
there is an error.
gmtime(EXPR)
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 follows:
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
gmtime(time);
All array elements are numeric, and come straight
out of a struct tm. In particular this means that
$mon has the range 0..11 and $wday has the range
0..6. If EXPR is omitted, does gmtime(time).
goto LABEL
Finds the statement labeled with LABEL and resumes
execution there. Currently you may only go to
statements in the main body of the program that are
not nested inside a do {} construct. This statement
is not implemented very efficiently, and is here
only to make the sed-to-perl translator easier. I
may change its semantics at any time, consistent
with support for translated sed scripts. Use it at
your own risk. Better yet, don't use it at all.
grep(EXPR,LIST)
Evaluates EXPR for each element of LIST (locally
setting $_ to each element) and returns the array
value consisting of those elements for which the
expression evaluated to true. In a scalar context,
returns the number of times the expression was true.
@foo = grep(!/^#/, @bar); # weed out comments
Note that, since $_ is a reference into the array
value, it can be used to modify the elements of the
array. While this is useful and supported, it can
cause bizarre results if the LIST is not a named
array.
hex(EXPR)
with 0 or 0x see oct().) If EXPR is omitted, uses
$_.
index(STR,SUBSTR,POSITION)
index(STR,SUBSTR)
Returns the position of the first occurrence of
SUBSTR in STR at or after POSITION. If POSITION is
omitted, starts searching from the beginning of the
string. The return value is based at 0, or whatever
you've set the $[ variable to. If the substring is
not found, returns one less than the base, ordi-
narily -1.
int(EXPR)
int EXPR
Returns the integer portion of EXPR. If EXPR is
omitted, uses $_.
ioctl(FILEHANDLE,FUNCTION,SCALAR)
Implements the ioctl(2) function. You'll probably
have to say
require "ioctl.ph"; # probably /usr/local/lib/perl/ioctl.ph
first to get the correct function definitions. If
ioctl.ph doesn't exist or doesn't have the correct
definitions you'll have to roll your own, based on
your C header files such as <sys/ioctl.h>. (There
is a perl script called h2ph that comes with the
perl kit which may help you in this.) SCALAR will be
read and/or written depending on the FUNCTION--a
pointer to the string value of SCALAR will be passed
as the third argument of the actual ioctl call. (If
SCALAR has no string value but does have a numeric
value, that value will be passed rather than a
pointer to the string value. To guarantee this to
be true, add a 0 to the scalar before using it.) The
pack() and unpack() functions are useful for manipu-
lating the values of structures used by ioctl().
The following example sets the erase character to
DEL.
@ary = unpack($sgttyb_t,$sgttyb);
$ary[2] = 127;
$sgttyb = pack($sgttyb_t,@ary);
ioctl(STDIN,$TIOCSETP,$sgttyb)
ll die "Can't ioctl: $!";
}
The return value of ioctl (and fcntl) is as follows:
if OS returns: perl returns:
-1 undefined value
0 string "0 but true"
anything else that number
Thus perl returns true on success and false on
failure, yet you can still easily determine the
actual value returned by the operating system:
($retval = ioctl(...)) ll ($retval = -1);
printf "System returned %d\n", $retval;
join(EXPR,LIST)
join(EXPR,ARRAY)
Joins the separate strings of LIST or ARRAY into a
single string with fields separated by the value of
EXPR, and returns the string. Example:
$_ = join(':',
$login,$passwd,$uid,$gid,$gcos,$home,$shell);
See split.
keys(ASSOC_ARRAY)
keys ASSOC_ARRAY
Returns a normal array consisting of all the keys of
the named associative array. The keys are returned
in an apparently random order, but it is the same
order as either the values() or each() function pro-
duces (given that the associative array has not been
modified). Here is yet another way to print your
environment:
@keys = keys %ENV;
@values = values %ENV;
while ($#keys >= 0) {
print pop(@keys), '=', pop(@values), "\n";
}
or how about sorted by key:
kill(LIST)
kill LIST
Sends a signal to a list of processes. The first
element of the list must be the signal to send.
Returns the number of processes successfully sig-
naled.
$cnt = kill 1, $child1, $child2;
kill 9, @goners;
If the signal is negative, kills process groups
instead of processes. (On System V, a negative pro-
cess number will also kill process groups, but
that's not portable.) You may use a signal name in
quotes.
last LABEL
last The last command is like the break statement in C
(as used in loops); it immediately exits the loop in
question. If the LABEL is omitted, the command
refers to the innermost enclosing loop. The con-
tinue block, if any, is not executed:
line: while (<STDIN>) {
last line if /^$/; # exit when done with header
...
}
length(EXPR)
length EXPR
Returns the length in characters of the value of
EXPR. If EXPR is omitted, returns length of $_.
link(OLDFILE,NEWFILE)
Creates a new filename linked to the old filename.
Returns 1 for success, 0 otherwise.
listen(SOCKET,QUEUESIZE)
Does the same thing that the listen system call
does. Returns true if it succeeded, false other-
wise. See example in section on Interprocess Com-
munication.
local(LIST)
Declares the listed variables to be local to the
enclosing block, subroutine, eval or "do". All the
them upon exiting the block, subroutine or eval.
This means that called subroutines can also refer-
ence the local variable, but not the global one.
The LIST may be assigned to if desired, which allows
you to initialize your local variables. (If no ini-
tializer is given for a particular variable, it is
created with an undefined value.) Commonly this is
used to name the parameters to a subroutine. Exam-
ples:
sub RANGEVAL {
local($min, $max, $thunk) = @_;
local($result) = '';
local($i);
# Presumably $thunk makes reference to $i
for ($i = $min; $i < $max; $i++) {
$result .= eval $thunk;
}
$result;
}
if ($sw eq '-v') {
# init local array with global array
local(@ARGV) = @ARGV;
unshift(@ARGV,'echo');
system @ARGV;
}
# @ARGV restored
# temporarily add to digits associative array
if ($base12) {
# (NOTE: not claiming this is efficient!)
local(%digits) = (%digits,'t',10,'e',11);
do parse_num();
}
Note that local() is a run-time command, and so gets
executed every time through a loop, using up more
stack storage each time until it's all released at
once when the loop is exited.
localtime(EXPR)
localtime EXPR
Converts a time as returned by the time function to
a 9-element array with the time analyzed for the
local timezone. Typically used as follows:
All array elements are numeric, and come straight
out of a struct tm. In particular this means that
$mon has the range 0..11 and $wday has the range
0..6. If EXPR is omitted, does localtime(time).
log(EXPR)
log EXPR
Returns logarithm (base e) of EXPR. If EXPR is
omitted, returns log of $_.
lstat(FILEHANDLE)
lstat FILEHANDLE
lstat(EXPR)
lstat SCALARVARIABLE
Does the same thing as the stat() function, but
stats a symbolic link instead of the file the sym-
bolic link points to. If symbolic links are unim-
plemented on your system, a normal stat is done.
m/PATTERN/io
/PATTERN/io
Searches a string for a pattern match, 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 / is the delimiter then the initial 'm' is
optional. With the 'm' you can use any pair of
non-alphanumeric characters as delimiters. This is
particularly useful for matching Unix path names
that contain '/'. If the final delimiter is fol-
lowed by the optional letter 'i', the matching is
done in a case-insensitive manner. PATTERN may con-
tain references to scalar variables, which will be
interpolated (and the pattern recompiled) every time
the pattern search is evaluated. (Note that $) and
$l may not be interpolated because they look like
end-of-string tests.) If you want such a pattern to
be compiled only once, add an "o" after the trailing
delimiter. This avoids expensive run-time recompi-
lations, and is useful when the value you are inter-
polating won't change over the life of the script.
pattern, i.e. ($1, $2, $3...). It does NOT actually
set $1, $2, etc. in this case, nor does it set $+,
$`, $& or $'. If the match fails, a null array is
returned. If the match succeeds, but there were no
parentheses, an array value of (1) is returned.
Examples:
open(tty, '/dev/tty');
<tty> =~ /^y/i && do foo(); # do foo if desired
if (/Version: *([0-9.]*)/) { $version = $1; }
next if m#^/usr/spool/uucp#;
# poor man's grep
$arg = shift;
while (<>) {
print if /$arg/o; # compile only once
}
if (($F1, $F2, $Etc) = ($foo =~ /^(\S+)\s+(\S+)\s*(.*)/))
This last example splits $foo into the first two
words and the remainder of the line, and assigns
those three fields to $F1, $F2 and $Etc. The condi-
tional is true if any variables were assigned, i.e.
if the pattern matched.
mkdir(FILENAME,MODE)
Creates the directory specified by FILENAME, with
permissions specified by MODE (as modified by
umask). If it succeeds it returns 1, otherwise it
returns 0 and sets $! (errno).
msgctl(ID,CMD,ARG)
Calls the System V IPC function msgctl. If CMD is
&IPC_STAT, then ARG must be a variable which will
hold the returned msqid_ds structure. Returns like
ioctl: the undefined value for error, "0 but true"
for zero, or the actual return value otherwise.
msgget(KEY,FLAGS)
Calls the System V IPC function msgget. Returns the
message queue id, or the undefined value if there is
an error.
msgsnd(ID,MSG,FLAGS)
Calls the System V IPC function msgsnd to send the
message MSG to the message queue ID. MSG must begin
with the long integer message type, which may be
created with pack("L", $type). Returns true if
Calls the System V IPC function msgrcv to receive a
message from message queue ID into variable VAR with
a maximum message size of SIZE. Note that if a mes-
sage is received, the message type will be the first
thing in VAR, and the maximum length of VAR is SIZE
plus the size of the message type. Returns true if
successful, or false if there is an error.
next LABEL
next The next command is like the continue statement in
C; it starts the next iteration of the loop:
line: while (<STDIN>) {
next line if /^#/; # discard comments
...
}
Note that if there were a continue block on the
above, it would get executed even on discarded
lines. If the LABEL is omitted, the command refers
to the innermost enclosing loop.
oct(EXPR)
oct EXPR
Returns the decimal value of EXPR interpreted as an
octal string. (If EXPR happens to start off with
0x, interprets it as a hex string instead.) The fol-
lowing will handle decimal, octal and hex in the
standard notation:
$val = oct($val) if $val =~ /^0/;
If EXPR is omitted, uses $_.
open(FILEHANDLE,EXPR)
open(FILEHANDLE)
open FILEHANDLE
Opens the file whose filename is given by EXPR, and
associates it with FILEHANDLE. If FILEHANDLE is an
expression, its value is used as the name of the
real filehandle wanted. If EXPR is omitted, the
scalar variable of the same name as the FILEHANDLE
contains the filename. If the filename begins with
"<" or nothing, the file is opened for input. If
the filename begins with ">", the file is opened for
output. If the filename begins with ">>", the file
is opened for appending. (You can put a '+' in
front of the '>' or '<' to indicate that you want
the filename ends with a "l", the filename is inter-
preted as command which pipes input to us. (You may
not have a command that pipes both in and out.)
Opening '-' opens STDIN and opening '>-' opens
STDOUT. Open returns non-zero upon success, the
undefined value otherwise. If the open involved a
pipe, the return value happens to be the pid of the
subprocess. Examples:
$article = 100;
open article ll die "Can't find article $article: $!\n";
while (<article>) {...
open(LOG, '>>/usr/spool/news/twitlog');
# (log is reserved)
open(article, "caesar <$article l");
# decrypt article
open(extract, "lsort >/tmp/Tmp$$");
# $$ is our process#
# process argument list of files along with any includes
foreach $file (@ARGV) {
do process($file, 'fh00'); # no pun intended
}
sub process {
local($filename, $input) = @_;
$input++; # this is a string increment
unless (open($input, $filename)) {
print STDERR "Can't open $filename: $!\n";
return;
}
while (<$input>) { # note use of indirection
if (/^#include "(.*)"/) {
do process($1, $input);
next;
}
... # whatever
}
}
You may also, in the Bourne shell tradition, specify
an EXPR beginning with ">&", in which case the rest
of the string is interpreted as the name of a
filehandle (or file descriptor, if numeric) which is
to be duped and opened. You may use & after >, >>,
<, +>, +>> and +<. The mode you specify should
match the mode of the original filehandle. Here is
a script that saves, redirects, and restores STDOUT
open(SAVEOUT, ">&STDOUT");
open(SAVEERR, ">&STDERR");
open(STDOUT, ">foo.out") ll die "Can't redirect stdout";
open(STDERR, ">&STDOUT") ll die "Can't dup stdout";
select(STDERR); $l = 1; # make unbuffered
select(STDOUT); $l = 1; # make unbuffered
print STDOUT "stdout 1\n"; # this works for
print STDERR "stderr 1\n"; # subprocesses too
close(STDOUT);
close(STDERR);
open(STDOUT, ">&SAVEOUT");
open(STDERR, ">&SAVEERR");
print STDOUT "stdout 2\n";
print STDERR "stderr 2\n";
If you open a pipe on the command "-", i.e. either
"l-" or "-l", then there is an implicit fork done, and
the return value of open is the pid of the child
within the parent process, and 0 within the child
process. (Use defined($pid) to determine if the
open was successful.) The filehandle behaves nor-
mally for the parent, but i/o to that filehandle is
piped from/to the STDOUT/STDIN of the child process.
In the child process the filehandle isn't opened--
i/o happens from/to the new STDOUT or STDIN. Typi-
cally this is used like the normal piped open when
you want to exercise more control over just how the
pipe command gets executed, such as when you are
running setuid, and don't want to have to scan shell
commands for metacharacters. The following pairs
are more or less equivalent:
open(FOO, "2tr '[a-z]' '[A-Z]'");
open(FOO, "2-") 22 exec 'tr', '[a-z]', '[A-Z]';
open(FOO, "cat -n '$file'2");
open(FOO, "-2") 22 exec 'cat', '-n', $file;
Explicitly closing any piped filehandle causes the
parent process to wait for the child to finish, and
returns the status value in $?. Note: on any opera-
tion which may do a fork, unflushed buffers remain
unflushed in both processes, which means you may
need to set $2 to avoid duplicate output.
The filename that is passed to open will have
ing whitespace thusly:
$file =~ s#^(\s)#./$1#;
open(FOO, "< $file\0");
opendir(DIRHANDLE,EXPR)
Opens a directory named EXPR for processing by read-
dir(), telldir(), seekdir(), rewinddir() and
closedir(). Returns true if successful. DIRHANDLEs
have their own namespace separate from FILEHANDLEs.
ord(EXPR)
ord EXPR
Returns the numeric ascii value of the first charac-
ter of EXPR. If EXPR is omitted, uses $_.
pack(TEMPLATE,LIST)
Takes an array or list of values and packs it into a
binary structure, returning the string containing
the structure. The TEMPLATE is a sequence of char-
acters that give the order and type of values, as
follows:
A An ascii string, will be space padded.
a An ascii string, will be null padded.
c A signed char value.
C An unsigned char value.
s A signed short value.
S An unsigned short value.
i A signed integer value.
I An unsigned integer value.
l A signed long value.
L An unsigned long value.
n A short in "network" order.
N A long in "network" order.
f A single-precision float in the native format.
d A double-precision float in the native format.
p A pointer to a string.
x A null byte.
X Back up a byte.
@ Null fill to absolute position.
u A uuencoded string.
b A bit string (ascending bit order, like vec()).
B A bit string (descending bit order).
h A hex string (low nybble first).
H A hex string (high nybble first).
Each letter may optionally be followed by a number
which gives a repeat count. With all types except
"a", "A", "b", "B", "h" and "H", the pack function
value, but pack it as a string of length count, pad-
ding with nulls or spaces as necessary. (When
unpacking, "A" strips trailing spaces and nulls, but
"a" does not.) Likewise, the "b" and "B" fields pack
a string that many bits long. The "h" and "H"
fields pack a string that many nybbles long. Real
numbers (floats and doubles) are in the native
machine format only; due to the multiplicity of
floating formats around, and the lack of a standard
"network" representation, no facility for inter-
change has been made. This means that packed float-
ing point data written on one machine may not be
readable on another - even if both use IEEE floating
point arithmetic (as the endian-ness of the memory
representation is not part of the IEEE spec). Note
that perl uses doubles internally for all numeric
calculation, and converting from double -> float ->
double will lose precision (i.e. unpack("f",
pack("f", $foo)) will not in general equal $foo).
Examples:
$foo = pack("cccc",65,66,67,68);
# foo eq "ABCD"
$foo = pack("c4",65,66,67,68);
# same thing
$foo = pack("ccxxcc",65,66,67,68);
# foo eq "AB\0\0CD"
$foo = pack("s2",1,2);
# "\1\0\2\0" on little-endian
# "\0\1\0\2" on big-endian
$foo = pack("a4","abcd","x","y","z");
# "abcd"
$foo = pack("aaaa","abcd","x","y","z");
# "axyz"
$foo = pack("a14","abcdefg");
# "abcdefg\0\0\0\0\0\0\0"
$foo = pack("i9pl", gmtime);
# a real struct tm (on my system anyway)
sub bintodec {
unpack("N", pack("B32", substr("0" x 32 . shift, -32)));
}
The same template may generally also be used in the
unpack function.
piped processes, deadlock can occur unless you are
very careful. In addition, note that perl's pipes
use stdio buffering, so you may need to set $2 to
flush your WRITEHANDLE after each command, depending
on the application. [Requires version 3.0
patchlevel 9.]
pop(ARRAY)
pop ARRAY
Pops and returns the last value of the array, shor-
tening the array by 1. Has the same effect as
$tmp = $ARRAY[$#ARRAY--];
If there are no elements in the array, returns the
undefined value.
print(FILEHANDLE LIST)
print(LIST)
print FILEHANDLE LIST
print LIST
print Prints a string or a comma-separated list of
strings. Returns non-zero if successful. FILEHAN-
DLE may be a scalar variable name, in which case the
variable contains the name of the filehandle, thus
introducing one level of indirection. (NOTE: If
FILEHANDLE is a variable and the next token is a
term, it may be misinterpreted as an operator unless
you interpose a + or put parens around the argu-
ments.) If FILEHANDLE is omitted, prints by default
to standard output (or to the last selected output
channel--see select()). If LIST is also omitted,
prints $_ to STDOUT. To set the default output
channel to something other than STDOUT use the
select operation. Note that, because print takes a
LIST, anything in the LIST is evaluated in an array
context, and any subroutine that you call will have
one or more of its expressions evaluated in an array
context. Also be careful not to follow the print
keyword with a left parenthesis unless you want the
corresponding right parenthesis to terminate the
arguments to the print--interpose a + or put parens
around all the arguments.
printf FILEHANDLE LIST
printf LIST
Equivalent to a "print FILEHANDLE sprintf(LIST)".
push(ARRAY,LIST)
Treats ARRAY (@ is optional) as a stack, and pushes
the values of LIST onto the end of ARRAY. The
length of ARRAY increases by the length of LIST.
Has the same effect as
for $value (LIST) {
$ARRAY[++$#ARRAY] = $value;
}
but is more efficient.
q/STRING/
qq/STRING/
These are not really functions, but simply syntactic
sugar to let you avoid putting too many backslashes
into quoted strings. The q operator is a general-
ized single quote, and the qq operator a generalized
double quote. Any non-alphanumeric delimiter can be
used in place of /, including newline. If the del-
imiter is an opening bracket or parenthesis, the
final delimiter will be the corresponding closing
bracket or parenthesis. (Embedded occurrences of
the closing bracket need to be backslashed as
usual.) Examples:
$foo = q!I said, "You said, 'She said it.'"!;
$bar = q('This is it.');
$_ .= qq
*** The previous line contains the naughty word "$&".\n
if /(ibm2apple2awk)/; # :-)
rand(EXPR)
rand EXPR
rand Returns a random fractional number between 0 and the
value of EXPR. (EXPR should be positive.) If EXPR
is omitted, returns a value between 0 and 1. See
also srand().
Attempts to read LENGTH bytes of data into variable
SCALAR from the specified FILEHANDLE. Returns the
number of bytes actually read, or undef if there was
an error. SCALAR will be grown or shrunk to the
length actually read. An OFFSET may be specified to
place the read data at some other place than the
beginning of the string. This call is actually
implemented in terms of stdio's fread call. To get
a true read system call, see sysread.
readdir(DIRHANDLE)
readdir DIRHANDLE
Returns the next directory entry for a directory
opened by opendir(). If used in an array context,
returns all the rest of the entries in the direc-
tory. If there are no more entries, returns an
undefined value in a scalar context or a null list
in an array context.
readlink(EXPR)
readlink EXPR
Returns the value of a symbolic link, if symbolic
links are implemented. If not, gives a fatal error.
If there is some system error, returns the undefined
value and sets $! (errno). If EXPR is omitted, uses
$_.
recv(SOCKET,SCALAR,LEN,FLAGS)
Receives a message on a socket. Attempts to receive
LENGTH bytes of data into variable SCALAR from the
specified SOCKET filehandle. Returns the address of
the sender, or the undefined value if there's an
error. SCALAR will be grown or shrunk to the length
actually read. Takes the same flags as the system
call of the same name.
redo LABEL
redo The redo command restarts the loop block without
evaluating the conditional again. The continue
block, if any, is not executed. If the LABEL is
omitted, the command refers to the innermost enclos-
ing loop. This command is normally used by programs
that want to lie to themselves about what was just
input:
while (s2({.*}.*){.*}2$1 2) {}
s2{.*}2 2;
if (s2{.*2 2) {
$front = $_;
while (<STDIN>) {
if (/}/) { # end of comment?
s2^2$front{2;
redo line;
}
}
}
print;
}
rename(OLDNAME,NEWNAME)
Changes the name of a file. Returns 1 for success,
0 otherwise. Will not work across filesystem boun-
daries.
require(EXPR)
require EXPR
require Includes the library file specified by EXPR, or by
$_ if EXPR is not supplied. Has semantics similar
to the following subroutine:
sub require {
local($filename) = @_;
return 1 if $INC{$filename};
local($realfilename,$result);
ITER: {
foreach $prefix (@INC) {
$realfilename = "$prefix/$filename";
if (-f $realfilename) {
$result = do $realfilename;
last ITER;
}
}
die "Can't find $filename in \@INC";
}
die $@ if $@;
die "$filename did not return true value" unless $result;
$INC{$filename} = $realfilename;
$result;
}
Note that the file will not be included twice under
the same specified name.
reset Generally used in a continue block at the end of a
loop to clear variables and reset ?? searches so
that they work again. The expression is interpreted
as a list of single characters (hyphens allowed for
ranges). All variables and arrays beginning with
one of those letters are reset to their pristine
state. If the expression is omitted, one-match
searches (?pattern?) are reset to match again. Only
resets variables or searches in the current package.
Always returns 1. Examples:
reset 'X'; # reset all X variables
reset 'a-z'; # reset lower case variables
reset; # just reset ?? searches
Note: resetting "A-Z" is not recommended since
you'll wipe out your ARGV and ENV arrays.
The use of reset on dbm associative arrays does not
change the dbm file. (It does, however, flush any
entries cached by perl, which may be useful if you
are sharing the dbm file. Then again, maybe not.)
return LIST
Returns from a subroutine with the value specified.
(Note that a subroutine can automatically return the
value of the last expression evaluated. That's the
preferred method--use of an explicit return is a bit
slower.)
reverse(LIST)
reverse LIST
In an array context, returns an array value consist-
ing of the elements of LIST in the opposite order.
In a scalar context, returns a string value consist-
ing of the bytes of the first element of LIST in the
opposite order.
rewinddir(DIRHANDLE)
rewinddir DIRHANDLE
Sets the current position to the beginning of the
directory for the readdir() routine on DIRHANDLE.
rindex(STR,SUBSTR,POSITION)
rindex(STR,SUBSTR)
Works just like index except that it returns the
position of the LAST occurrence of SUBSTR in STR.
If POSITION is specified, returns the last
rmdir FILENAME
Deletes the directory specified by FILENAME if it is
empty. If it succeeds it returns 1, otherwise it
returns 0 and sets $! (errno). If FILENAME is omit-
ted, uses $_.
s/PATTERN/REPLACEMENT/gieo
Searches a string for a pattern, and if found,
replaces that pattern with the replacement text and
returns the number of substitutions made. Otherwise
it returns false (0). The "g" is optional, and if
present, indicates that all occurrences of the pat-
tern are to be replaced. The "i" is also optional,
and if present, indicates that matching is to be
done in a case-insensitive manner. The "e" is like-
wise optional, and if present, indicates that the
replacement string is to be evaluated as an expres-
sion rather than just as a double-quoted string.
Any non-alphanumeric delimiter may replace the
slashes; if single quotes are used, no interpreta-
tion is done on the replacement string (the e modif-
ier overrides this, however); if backquotes are
used, the replacement string is a command to execute
whose output will be used as the actual replacement
text. If no string is specified via the =~ or !~
operator, the $_ string is searched and modified.
(The string specified with =~ must be a scalar vari-
able, an array element, or an assignment to one of
those, i.e. an lvalue.) If the pattern contains a $
that looks like a variable rather than an end-of-
string test, the variable will be interpolated into
the pattern at run-time. If you only want the pat-
tern compiled once the first time the variable is
interpolated, add an "o" at the end. See also the
section on regular expressions. Examples:
s/\bgreen\b/mauve/g; # don't change wintergreen
$path =~ s2/usr/bin2/usr/local/bin2;
s/Login: $foo/Login: $bar/; # run-time pattern
($foo = $bar) =~ s/bar/foo/;
$_ = 'abc123xyz';
s/\d+/$&*2/e; # yields 'abc246xyz'
s/\d+/sprintf("%5d",$&)/e; # yields 'abc 246xyz'
s/\w/$& x 2/eg; # yields 'aabbcc 224466xxyyzz'
s/([^ ]*) *([^ ]*)/$2 $1/; # reverse 1st two fields
scalar(EXPR)
Forces EXPR to be interpreted in a scalar context
and returns the value of EXPR.
seek(FILEHANDLE,POSITION,WHENCE)
Randomly positions the file pointer for FILEHANDLE,
just like the fseek() call of stdio. FILEHANDLE may
be an expression whose value gives the name of the
filehandle. Returns 1 upon success, 0 otherwise.
seekdir(DIRHANDLE,POS)
Sets the current position for the readdir() routine
on DIRHANDLE. POS must be a value returned by
telldir(). Has the same caveats about possible
directory compaction as the corresponding system
library routine.
select(FILEHANDLE)
select Returns the currently selected filehandle. Sets the
current default filehandle for output, if FILEHANDLE
is supplied. This has two effects: first, a write
or a print without a filehandle will default to this
FILEHANDLE. Second, references to variables related
to output will refer to this output channel. For
example, if you have to set the top of form format
for more than one output channel, you might do the
following:
select(REPORT1);
$^ = 'report1_top';
select(REPORT2);
$^ = 'report2_top';
FILEHANDLE may be an expression whose value gives
the name of the actual filehandle. Thus:
$oldfh = select(STDERR); $2 = 1; select($oldfh);
select(RBITS,WBITS,EBITS,TIMEOUT)
This calls the select system call with the bitmasks
specified, which can be constructed using fileno()
and vec(), along these lines:
$rin = $win = $ein = '';
vec($rin,fileno(STDIN),1) = 1;
vec($win,fileno(STDOUT),1) = 1;
$ein = $rin 2 $win;
If you want to select on many filehandles you might
wish to write a subroutine:
for (@fhlist) {
vec($bits,fileno($_),1) = 1;
}
$bits;
}
$rin = &fhbits('STDIN TTY SOCK');
The usual idiom is:
($nfound,$timeleft) =
select($rout=$rin, $wout=$win, $eout=$ein, $timeout);
or to block until something becomes ready:
$nfound = select($rout=$rin, $wout=$win,
$eout=$ein, undef);
Any of the bitmasks can also be undef. The timeout,
if specified, is in seconds, which may be frac-
tional. NOTE: not all implementations are capable
of returning the $timeleft. If not, they always
return $timeleft equal to the supplied $timeout.
semctl(ID,SEMNUM,CMD,ARG)
Calls the System V IPC function semctl. If CMD is
&IPC_STAT or &GETALL, then ARG must be a variable
which will hold the returned semid_ds structure or
semaphore value array. Returns like ioctl: the
undefined value for error, "0 but true" for zero, or
the actual return value otherwise.
semget(KEY,NSEMS,SIZE,FLAGS)
Calls the System V IPC function semget. Returns the
semaphore id, or the undefined value if there is an
error.
semop(KEY,OPSTRING)
Calls the System V IPC function semop to perform
semaphore operations such as signaling and waiting.
OPSTRING must be a packed array of semop structures.
Each semop structure can be generated with
'pack("sss", $semnum, $semop, $semflag)'. The
number of semaphore operations is implied by the
length of OPSTRING. Returns true if successful, or
false if there is an error. As an example, the fol-
lowing code waits on semaphore $semnum of semaphore
id $semid:
$semop = pack("sss", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid, $semop);
To signal the semaphore, replace "-1" with "1".
Sends a message on a socket. Takes the same flags
as the system call of the same name. On unconnected
sockets you must specify a destination to send TO.
Returns the number of characters sent, or the unde-
fined value if there is an error.
setpgrp(PID,PGRP)
Sets the current process group for the specified
PID, 0 for the current process. Will produce a
fatal error if used on a machine that doesn't imple-
ment setpgrp(2).
setpriority(WHICH,WHO,PRIORITY)
Sets the current priority for a process, a process
group, or a user. (See setpriority(2).) Will pro-
duce a fatal error if used on a machine that doesn't
implement setpriority(2).
setsockopt(SOCKET,LEVEL,OPTNAME,OPTVAL)
Sets the socket option requested. Returns undefined
if there is an error. OPTVAL may be specified as
undef if you don't want to pass an argument.
shift(ARRAY)
shift ARRAY
shift Shifts the first value of the array off and returns
it, shortening the array by 1 and moving everything
down. If there are no elements in the array,
returns the undefined value. If ARRAY is omitted,
shifts the @ARGV array in the main program, and the
@_ array in subroutines. (This is determined lexi-
cally.) See also unshift(), push() and pop().
Shift() and unshift() do the same thing to the left
end of an array that push() and pop() do to the
right end.
shmctl(ID,CMD,ARG)
Calls the System V IPC function shmctl. If CMD is
&IPC_STAT, then ARG must be a variable which will
hold the returned shmid_ds structure. Returns like
ioctl: the undefined value for error, "0 but true"
for zero, or the actual return value otherwise.
shmget(KEY,SIZE,FLAGS)
Calls the System V IPC function shmget. Returns the
shared memory segment id, or the undefined value if
there is an error.
Reads or writes the System V shared memory segment
ID starting at position POS for size SIZE by attach-
ing to it, copying in/out, and detaching from it.
When reading, VAR must be a variable which will hold
the data read. When writing, if STRING is too long,
only SIZE bytes are used; if STRING is too short,
nulls are written to fill out SIZE bytes. Return
true if successful, or false if there is an error.
shutdown(SOCKET,HOW)
Shuts down a socket connection in the manner indi-
cated by HOW, which has the same interpretation as
in the system call of the same name.
sin(EXPR)
sin EXPR
Returns the sine of EXPR (expressed in radians). If
EXPR is omitted, returns sine of $_.
sleep(EXPR)
sleep EXPR
sleep Causes the script to sleep for EXPR seconds, or for-
ever if no EXPR. May be interrupted by sending the
process a SIGALARM. Returns the number of seconds
actually slept.
socket(SOCKET,DOMAIN,TYPE,PROTOCOL)
Opens a socket of the specified kind and attaches it
to filehandle SOCKET. DOMAIN, TYPE and PROTOCOL are
specified the same as for the system call of the
same name. You may need to run h2ph on sys/socket.h
to get the proper values handy in a perl library
file. Return true if successful. See the example
in the section on Interprocess Communication.
socketpair(SOCKET1,SOCKET2,DOMAIN,TYPE,PROTOCOL)
Creates an unnamed pair of sockets in the specified
domain, of the specified type. DOMAIN, TYPE and
PROTOCOL are specified the same as for the system
call of the same name. If unimplemented, yields a
fatal error. Return true if successful.
sort SUBROUTINE LIST
sort LIST
Sorts the LIST and returns the sorted array value.
Nonexistent values of arrays are stripped out. If
SUBROUTINE is omitted, sorts in standard string com-
parison order. If SUBROUTINE is specified, gives
the name of a subroutine that returns an integer
less than, equal to, or greater than 0, depending on
how the elements of the array are to be ordered. In
the interests of efficiency the normal calling code
for subroutines is bypassed, with the following
effects: the subroutine may not be a recursive sub-
routine, and the two elements to be compared are
passed into the subroutine not via @_ but as $a and
$b (see example below). They are passed by refer-
ence so don't modify $a and $b. SUBROUTINE may be a
scalar variable name, in which case the value pro-
vides the name of the subroutine to use. Examples:
sub byage {
$age{$a} - $age{$b}; # presuming integers
}
@sortedclass = sort byage @class;
sub reverse { $a lt $b ? 1 : $a gt $b ? -1 : 0; }
@harry = ('dog','cat','x','Cain','Abel');
@george = ('gone','chased','yz','Punished','Axed');
print sort @harry;
# prints AbelCaincatdogx
print sort reverse @harry;
# prints xdogcatCainAbel
print sort @george, 'to', @harry;
# prints AbelAxedCainPunishedcatchaseddoggonetoxyz
splice(ARRAY,OFFSET,LENGTH,LIST)
splice(ARRAY,OFFSET,LENGTH)
splice(ARRAY,OFFSET)
Removes the elements designated by OFFSET and LENGTH
from an array, and replaces them with the elements
of LIST, if any. Returns the elements removed from
the array. The array grows or shrinks as necessary.
If LENGTH is omitted, removes everything from OFFSET
onward. The following equivalencies hold (assuming
$[ == 0):
push(@a,$x,$y) splice(@a,$#a+1,0,$x,$y)
pop(@a) splice(@a,-1)
Example, assuming array lengths are passed before arrays:
sub aeq { # compare two array values
local(@a) = splice(@_,0,shift);
local(@b) = splice(@_,0,shift);
return 0 unless @a == @b; # same len?
while (@a) {
return 0 if pop(@a) ne pop(@b);
}
return 1;
}
if (&aeq($len,@foo[1..$len],0+@bar,@bar)) { ... }
split(/PATTERN/,EXPR,LIMIT)
split(/PATTERN/,EXPR)
split(/PATTERN/)
split Splits a string into an array of strings, and
returns it. (If not in an array context, returns
the number of fields found and splits into the @_
array. (In an array context, you can force the
split into @_ by using ?? as the pattern delimiters,
but it still returns the array value.)) If EXPR is
omitted, splits the $_ string. If PATTERN is also
omitted, splits on whitespace (/[ \t\n]+/). Any-
thing matching PATTERN is taken to be a delimiter
separating the fields. (Note that the delimiter may
be longer than one character.) If LIMIT is speci-
fied, splits into no more than that many fields
(though it may split into fewer). If LIMIT is
unspecified, trailing null fields are stripped
(which potential users of pop() would do well to
remember). A pattern matching the null string (not
to be confused with a null pattern //, which is just
one member of the set of patterns matching a null
string) will split the value of EXPR into separate
characters at each point it matches that way. For
example:
print join(':', split(/ */, 'hi there'));
produces the output 'h:i:t:h:e:r:e'.
The LIMIT parameter can be used to partially split a
line
($login, $passwd, $remainder) = split(/:/, $_, 3);
the list above LIMIT would have been 4 by default.
In time critical applications it behooves you not to
split into more fields than you really need.)
If the PATTERN contains parentheses, additional
array elements are created from each matching sub-
string in the delimiter.
split(/([,-])/,"1-10,20");
produces the array value
(1,'-',10,',',20)
The pattern /PATTERN/ may be replaced with an
expression to specify patterns that vary at runtime.
(To do runtime compilation only once, use
/$variable/o.) As a special case, specifying a space
(' ') will split on white space just as split with
no arguments does, but leading white space does NOT
produce a null first field. Thus, split(' ') can be
used to emulate awk's default behavior, whereas
split(/ /) will give you as many null initial fields
as there are leading spaces.
Example:
open(passwd, '/etc/passwd');
while (<passwd>) {
($login, $passwd, $uid, $gid, $gcos, $home, $shell)
= split(/:/);
...
}
(Note that $shell above will still have a newline on
it. See chop().) See also join.
sprintf(FORMAT,LIST)
Returns a string formatted by the usual printf con-
ventions. The * character is not supported.
sqrt(EXPR)
sqrt EXPR
Return the square root of EXPR. If EXPR is omitted,
returns square root of $_.
srand(EXPR)
srand EXPR
Sets the random number seed for the rand operator.
If EXPR is omitted, does srand(time).
stat(EXPR)
stat SCALARVARIABLE
Returns a 13-element array giving the statistics for
a file, either the file opened via FILEHANDLE, or
named by EXPR. Typically used as follows:
($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
$atime,$mtime,$ctime,$blksize,$blocks)
= stat($filename);
If stat is passed the special filehandle consisting
of an underline, no stat is done, but the current
contents of the stat structure from the last stat or
filetest are returned. Example:
if (-x $file && (($d) = stat(_)) && $d < 0) {
print "$file is executable NFS file\n";
}
study(SCALAR)
study SCALAR
study Takes extra time to study SCALAR ($_ if unspecified)
in anticipation of doing many pattern matches on the
string before it is next modified. This may or may
not save time, depending on the nature and number of
patterns you are searching on, and on the distribu-
tion of character frequencies in the string to be
searched--you probably want to compare runtimes with
and without it to see which runs faster. Those
loops which scan for many short constant strings
(including the constant parts of more complex pat-
terns) will benefit most. You may have only one
study active at a time--if you study a different
scalar the first is "unstudied". (The way study
works is this: a linked list of every character in
the string to be searched is made, so we know, for
example, where all the 'k' characters are. From
each search string, the rarest character is
selected, based on some static frequency tables con-
structed from some C programs and English text.
Only those places that contain this "rarest" charac-
ter are examined.)
For example, here is a loop which inserts index pro-
ducing entries before any line containing a certain
pattern:
print ".IX bar\n" if /\bbar\b/;
print ".IX blurfl\n" if /\bblurfl\b/;
...
print;
}
In searching for /\bfoo\b/, only those locations in
$_ that contain 'f' will be looked at, because 'f'
is rarer than 'o'. In general, this is a big win
except in pathological cases. The only question is
whether it saves you more time than it took to build
the linked list in the first place.
Note that if you have to look for strings that you
don't know till runtime, you can build an entire
loop as a string and eval that to avoid recompiling
all your patterns all the time. Together with set-
ting $/ to input entire files as one record, this
can be very fast, often faster than specialized pro-
grams like fgrep. The following scans a list of
files (@files) for a list of words (@words), and
prints out the names of those files that contain a
match:
$search = 'while (<>) { study;';
foreach $word (@words) {
$search .= "++\$seen{\$ARGV} if /\b$word\b/;\n";
}
$search .= "}";
@ARGV = @files;
$/ = "\177"; # something that doesn't occur
eval $search; # this screams
$/ = "\n"; # put back to normal input delim
foreach $file (sort keys(%seen)) {
print $file, "\n";
}
substr(EXPR,OFFSET,LEN)
substr(EXPR,OFFSET)
Extracts a substring out of EXPR and returns it.
First character is at offset 0, or whatever you've
set $[ to. If OFFSET is negative, starts that far
from the end of the string. If LEN is omitted,
returns everything to the end of the string. You
can use the substr() function as an lvalue, in which
case EXPR must be an lvalue. If you assign some-
thing shorter than LEN, the string will shrink, and
if you assign something longer than LEN, the string
will grow to accommodate it. To keep the string the
same length you may need to pad or chop your value
Creates a new filename symbolically linked to the
old filename. Returns 1 for success, 0 otherwise.
On systems that don't support symbolic links, pro-
duces a fatal error at run time. To check for that,
use eval:
$symlink_exists = (eval 'symlink("","");', $@ eq '');
syscall(LIST)
syscall LIST
Calls the system call specified as the first element
of the list, passing the remaining elements as argu-
ments to the system call. If unimplemented, pro-
duces a fatal error. The arguments are interpreted
as follows: if a given argument is numeric, the
argument is passed as an int. If not, the pointer
to the string value is passed. You are responsible
to make sure a string is pre-extended long enough to
receive any result that might be written into a
string. If your integer arguments are not literals
and have never been interpreted in a numeric con-
text, you may need to add 0 to them to force them to
look like numbers.
require 'syscall.ph'; # may need to run h2ph
syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
sysread(FILEHANDLE,SCALAR,LENGTH,OFFSET)
sysread(FILEHANDLE,SCALAR,LENGTH)
Attempts to read LENGTH bytes of data into variable
SCALAR from the specified FILEHANDLE, using the sys-
tem call read(2). It bypasses stdio, so mixing this
with other kinds of reads may cause confusion.
Returns the number of bytes actually read, or undef
if there was an error. SCALAR will be grown or
shrunk to the length actually read. An OFFSET may
be specified to place the read data at some other
place than the beginning of the string.
system(LIST)
system LIST
Does exactly the same thing as "exec LIST" except
that a fork is done first, and the parent process
waits for the child process to complete. Note that
argument processing varies depending on the number
of arguments. The return value is the exit status
of the program as returned by the wait() call. To
syswrite(FILEHANDLE,SCALAR,LENGTH,OFFSET)
syswrite(FILEHANDLE,SCALAR,LENGTH)
Attempts to write LENGTH bytes of data from variable
SCALAR to the specified FILEHANDLE, using the system
call write(2). It bypasses stdio, so mixing this
with prints may cause confusion. Returns the number
of bytes actually written, or undef if there was an
error. An OFFSET may be specified to place the read
data at some other place than the beginning of the
string.
tell(FILEHANDLE)
tell FILEHANDLE
tell Returns the current file position for FILEHANDLE.
FILEHANDLE may be an expression whose value gives
the name of the actual filehandle. If FILEHANDLE is
omitted, assumes the file last read.
telldir(DIRHANDLE)
telldir DIRHANDLE
Returns the current position of the readdir() rou-
tines on DIRHANDLE. Value may be given to seekdir()
to access a particular location in a directory. Has
the same caveats about possible directory compaction
as the corresponding system library routine.
time Returns the number of non-leap seconds since
00:00:00 UTC, January 1, 1970. Suitable for feeding
to gmtime() and localtime().
times Returns a four-element array giving the user and
system times, in seconds, for this process and the
children of this process.
($user,$system,$cuser,$csystem) = times;
tr/SEARCHLIST/REPLACEMENTLIST/cds
y/SEARCHLIST/REPLACEMENTLIST/cds
Translates all occurrences of the characters found
in the search list with the corresponding character
in the replacement list. It returns the number of
characters replaced or deleted. If no string is
specified via the =~ or !~ operator, the $_ string
is translated. (The string specified with =~ must
be a scalar variable, an array element, or an
assignment to one of those, i.e. an lvalue.) For sed
acter set is complemented. If the d modifier is
specified, any characters specified by SEARCHLIST
that are not found in REPLACEMENTLIST are deleted.
(Note that this is slightly more flexible than the
behavior of some tr programs, which delete anything
they find in the SEARCHLIST, period.) If the s
modifier is specified, sequences of characters that
were translated to the same character are squashed
down to 1 instance of the character.
If the d modifier was used, the REPLACEMENTLIST is
always interpreted exactly as specified. Otherwise,
if the REPLACEMENTLIST is shorter than the SEAR-
CHLIST, the final character is replicated till it is
long enough. If the REPLACEMENTLIST is null, the
SEARCHLIST is replicated. This latter is useful for
counting characters in a class, or for squashing
character sequences in a class.
Examples:
$ARGV[1] =~ y/A-Z/a-z/; # canonicalize to lower case
$cnt = tr/*/*/; # count the stars in $_
$cnt = tr/0-9//; # count the digits in $_
tr/a-zA-Z//s; # bookkeeper -> bokeper
($HOST = $host) =~ tr/a-z/A-Z/;
y/a-zA-Z/ /cs; # change non-alphas to single space
tr/\200-\377/\0-\177/; # delete 8th bit
truncate(FILEHANDLE,LENGTH)
truncate(EXPR,LENGTH)
Truncates the file opened on FILEHANDLE, or named by
EXPR, to the specified length. Produces a fatal
error if truncate isn't implemented on your system.
umask(EXPR)
umask EXPR
umask Sets the umask for the process and returns the old
one. If EXPR is omitted, merely returns current
umask.
undef Undefines the value of EXPR, which must be an
lvalue. Use only on a scalar value, an entire
array, or a subroutine name (using &). (Undef will
probably not do what you expect on most predefined
variables or dbm array values.) Always returns the
undefined value. You can omit the EXPR, in which
case nothing is undefined, but you still get an
undefined value that you could, for instance, return
from a subroutine. Examples:
undef $foo;
undef $bar{'blurfl'};
undef @ary;
undef %assoc;
undef &mysub;
return (wantarray ? () : undef) if $they_blew_it;
unlink(LIST)
unlink LIST
Deletes a list of files. Returns the number of
files successfully deleted.
$cnt = unlink 'a', 'b', 'c';
unlink @goners;
unlink <*.bak>;
Note: unlink will not delete directories unless you
are superuser and the -U flag is supplied to perl.
Even if these conditions are met, be warned that
unlinking a directory can inflict damage on your
filesystem. Use rmdir instead.
unpack(TEMPLATE,EXPR)
Unpack does the reverse of pack: it takes a string
representing a structure and expands it out into an
array value, returning the array value. (In a
scalar context, it merely returns the first value
produced.) The TEMPLATE has the same format as in
the pack function. Here's a subroutine that does
substring:
sub substr {
local($what,$where,$howmuch) = @_;
unpack("x$where a$howmuch", $what);
}
and then there's
sub ord { unpack("c",$_[0]); }
is a 16-bit checksum. For example, the following
computes the same number as the System V sum pro-
gram:
while (<>) {
$checksum += unpack("%16C*", $_);
}
$checksum %= 65536;
unshift(ARRAY,LIST)
Does the opposite of a shift. Or the opposite of a
push, depending on how you look at it. Prepends
list to the front of the array, and returns the
number of elements in the new array.
unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
utime(LIST)
utime LIST
Changes the access and modification times on each
file of a list of files. The first two elements of
the list must be the NUMERICAL access and modifica-
tion times, in that order. Returns the number of
files successfully changed. The inode modification
time of each file is set to the current time. Exam-
ple of a "touch" command:
#!/usr/bin/perl
$now = time;
utime $now, $now, @ARGV;
values(ASSOC_ARRAY)
values ASSOC_ARRAY
Returns a normal array consisting of all the values
of the named associative array. The values are
returned in an apparently random order, but it is
the same order as either the keys() or each() func-
tion would produce on the same array. See also
keys() and each().
vec(EXPR,OFFSET,BITS)
Treats a string as a vector of unsigned integers,
and returns the value of the bitfield specified.
May also be assigned to. BITS must be a power of
two from 1 to 32.
Vectors created with vec() can also be manipulated
enabled unless there is at least one vec() in your
program, to protect older programs.
To transform a bit vector into a string or array of
0's and 1's, use these:
$bits = unpack("b*", $vector);
@bits = split(//, unpack("b*", $vector));
If you know the exact length in bits, it can be used
in place of the *.
wait Waits for a child process to terminate and returns
the pid of the deceased process, or -1 if there are
no child processes. The status is returned in $?.
waitpid(PID,FLAGS)
Waits for a particular child process to terminate
and returns the pid of the deceased process, or -1
if there is no such child process. The status is
returned in $?. If you say
require "sys/wait.h";
...
waitpid(-1,&WNOHANG);
then you can do a non-blocking wait for any process.
Non-blocking wait is only available on machines sup-
porting either the waitpid (2) or wait4 (2) system
calls. However, waiting for a particular pid with
FLAGS of 0 is implemented everywhere. (Perl emu-
lates the system call by remembering the status
values of processes that have exited but have not
been harvested by the Perl script yet.)
wantarray
Returns true if the context of the currently execut-
ing subroutine is looking for an array value.
Returns false if the context is looking for a
scalar.
return wantarray ? () : undef;
warn(LIST)
warn LIST
Produces a message on STDERR just like "die", but
doesn't exit.
write Writes a formatted record (possibly multi-line) to
the specified file, using the format associated with
that file. By default the format for a file is the
one having the same name is the filehandle, but the
format for the current output channel (see select)
may be set explicitly by assigning the name of the
format to the $~ variable.
Top of form processing is handled automatically: if
there is insufficient room on the current page for
the formatted record, the page is advanced by writ-
ing a form feed, a special top-of-page format is
used to format the new page header, and then the
record is written. By default the top-of-page for-
mat is "top", but it may be set to the format of
your choice by assigning the name to the $^ vari-
able. The number of lines remaining on the current
page is in variable $-, which can be set to 0 to
force a new page.
If FILEHANDLE is unspecified, output goes to the
current default output channel, which starts out as
STDOUT but may be changed by the select operator.
If the FILEHANDLE is an EXPR, then the expression is
evaluated and the resulting string is used to look
up the name of the FILEHANDLE at run time. For more
on formats, see the section on formats later on.
Note that write is NOT the opposite of read.
Precedence
Perl operators have the following associativity and pre-
cedence:
nonassoc print printf exec system sort reverse
chmod chown kill unlink utime die return
left ,
right = += -= *= etc.
right ?:
nonassoc ..
left 22
left &&
left 2 ^
left &
nonassoc == != <=> eq ne cmp
nonassoc < > <= >= lt gt le ge
nonassoc chdir exit eval reset sleep rand umask
nonassoc -r -w -x etc.
left << >>
left + - .
right **
nonassoc ++ --
left '('
As mentioned earlier, if any list operator (print, etc.) or
any unary operator (chdir, etc.) is followed by a left
parenthesis as the next token on the same line, the operator
and arguments within parentheses are taken to be of highest
precedence, just like a normal function call. Examples:
chdir $foo 22 die; # (chdir $foo) 22 die
chdir($foo) 22 die; # (chdir $foo) 22 die
chdir ($foo) 22 die; # (chdir $foo) 22 die
chdir +($foo) 22 die; # (chdir $foo) 22 die
but, because * is higher precedence than 22:
chdir $foo * 20; # chdir ($foo * 20)
chdir($foo) * 20; # (chdir $foo) * 20
chdir ($foo) * 20; # (chdir $foo) * 20
chdir +($foo) * 20; # chdir ($foo * 20)
rand 10 * 20; # rand (10 * 20)
rand(10) * 20; # (rand 10) * 20
rand (10) * 20; # (rand 10) * 20
rand +(10) * 20; # rand (10 * 20)
In the absence of parentheses, the precedence of list opera-
tors such as print, sort or chmod is either very high or
very low depending on whether you look at the left side of
operator or the right side of it. For example, in
@ary = (1, 3, sort 4, 2);
print @ary; # prints 1324
the commas on the right of the sort are evaluated before the
sort, but the commas on the left are evaluated after. In
other words, list operators tend to gobble up all the argu-
ments that follow them, and then act like a simple term with
regard to the preceding expression. Note that you have to
be careful with parens:
# These evaluate exit before doing the print:
print($foo, exit); # Obviously not what you want.
print $foo, exit; # Nor is this.
# These do the print before evaluating exit:
(print $foo), exit; # This is what you want.
print($foo), exit; # Or this.
print ($foo), exit; # Or even this.
Also note that
Subroutines
A subroutine may be declared as follows:
sub NAME BLOCK
Any arguments passed to the routine come in as array @_,
that is ($_[0], $_[1], ...). The array @_ is a local array,
but its values are references to the actual scalar parame-
ters. The return value of the subroutine is the value of
the last expression evaluated, and can be either an array
value or a scalar value. Alternately, a return statement
may be used to specify the returned value and exit the sub-
routine. To create local variables see the local operator.
A subroutine is called using the do operator or the & opera-
tor.
Example:
sub MAX {
local($max) = pop(@_);
foreach $foo (@_) {
$max = $foo if $max < $foo;
}
$max;
}
...
$bestday = &MAX($mon,$tue,$wed,$thu,$fri);
# that start with whitespace
sub get_line {
$thisline = $lookahead;
line: while ($lookahead = <STDIN>) {
if ($lookahead =~ /^[ \t]/) {
$thisline .= $lookahead;
}
else {
last line;
}
}
$thisline;
}
$lookahead = <STDIN>; # get first line
while ($_ = do get_line()) {
...
}
Use array assignment to a local list to name your formal arguments:
sub maybeset {
local($key, $value) = @_;
$foo{$key} = $value unless $foo{$key};
}
This also has the effect of turning call-by-reference into
call-by-value, since the assignment copies the values.
Subroutines may be called recursively. If a subroutine is
called using the & form, the argument list is optional. If
omitted, no @_ array is set up for the subroutine; the @_
array at the time of the call is visible to subroutine
instead.
do foo(1,2,3); # pass three arguments
&foo(1,2,3); # the same
do foo(); # pass a null list
&foo(); # the same
&foo; # pass no arguments--more efficient
Passing By Reference
Sometimes you don't want to pass the value of an array to a
subroutine but rather the name of it, so that the subroutine
can modify the global copy of it rather than working with a
local copy. In perl you can refer to all the objects of a
particular name by prefixing the name with a star: *foo.
When evaluated, it produces a scalar value that represents
all the objects of that name, including any filehandle,
sub doubleary {
local(*someary) = @_;
foreach $elem (@someary) {
$elem *= 2;
}
}
do doubleary(*foo);
do doubleary(*bar);
Assignment to *name is currently recommended only inside a
local(). You can actually assign to *name anywhere, but the
previous referent of *name may be stranded forever. This
may or may not bother you.
Note that scalars are already passed by reference, so you
can modify scalar arguments without using this mechanism by
referring explicitly to the $_[nnn] in question. You can
modify all the elements of an array by passing all the ele-
ments as scalars, but you have to use the * mechanism to
push, pop or change the size of an array. The * mechanism
will probably be more efficient in any case.
Since a *name value contains unprintable binary data, if it
is used as an argument in a print, or as a %s argument in a
printf or sprintf, it then has the value '*name', just so it
prints out pretty.
Even if you don't want to modify an array, this mechanism is
useful for passing multiple arrays in a single LIST, since
normally the LIST mechanism will merge all the array values
so that you can't extract out the individual arrays.
Regular Expressions
The patterns used in pattern matching are regular expres-
sions such as those supplied in the Version 8 regexp rou-
tines. (In fact, the routines are derived from Henry
Spencer's freely redistributable reimplementation of the V8
routines.) In addition, \w matches an alphanumeric character
(including "_") and \W a nonalphanumeric. Word boundaries
may be matched by \b, and non-boundaries by \B. A whi-
tespace character is matched by \s, non-whitespace by \S. A
numeric character is matched by \d, non-numeric by \D. You
may use \w, \s and \d within character classes. Also, \n,
\r, \f, \t and \NNN have their normal interpretations.
Within character classes \b represents backspace rather than
a word boundary. Alternatives may be separated by 2. The
bracketing construct ( ... ) may also be used, in which case
\<digit> matches the digit'th substring, where digit can
range from 1 to 9. (Outside of the pattern, always use $
instead of \ in front of the digit. The scope of $<digit>
side the current pattern, but should not be relied upon.) $+
returns whatever the last bracket match matched. $& returns
the entire matched string. ($0 used to return the same
thing, but not any more.) $` returns everything before the
matched string. $' returns everything after the matched
string. Examples:
s/^([^ ]*) *([^ ]*)/$2 $1/; # swap first two words
if (/Time: (..):(..):(..)/) {
$hours = $1;
$minutes = $2;
$seconds = $3;
}
By default, the ^ character is only guaranteed to match at
the beginning of the string, the $ character only at the end
(or before the newline at the end) and perl does certain
optimizations with the assumption that the string contains
only one line. The behavior of ^ and $ on embedded newlines
will be inconsistent. You may, however, wish to treat a
string as a multi-line buffer, such that the ^ will match
after any newline within the string, and $ will match before
any newline. At the cost of a little more overhead, you can
do this by setting the variable $* to 1. Setting it back to
0 makes perl revert to its old behavior.
To facilitate multi-line substitutions, the . character
never matches a newline (even when $* is 0). In particular,
the following leaves a newline on the $_ string:
$_ = <STDIN>;
s/.*(some_string).*/$1/;
If the newline is unwanted, try one of
s/.*(some_string).*\n/$1/;
s/.*(some_string)[^\000]*/$1/;
s/.*(some_string)(.2\n)*/$1/;
chop; s/.*(some_string).*/$1/;
/(some_string)/ && ($_ = $1);
Any item of a regular expression may be followed with digits
in curly brackets of the form {n,m}, where n gives the
minimum number of times to match the item and m gives the
maximum. The form {n} is equivalent to {n,n} and matches
exactly n times. The form {n,} matches n or more times.
(If a curly bracket occurs in any other context, it is
treated as a regular character.) The * modifier is
equivalent to {0,}, the + modifier to {1,} and the ? modif-
ier to {0,1}. There is no limit to the size of n or m, but
large numbers will chew up more memory.
bols that aren't alphanumeric. So anything that looks like
\\, \(, \), \<, \>, \{, or \} is always interpreted as a
literal character, not a metacharacter. This makes it sim-
ple to quote a string that you want to use for a pattern but
that you are afraid might contain metacharacters. Simply
quote all the non-alphanumeric characters:
$pattern =~ s/(\W)/\\$1/g;
Formats
Output record formats for use with the write operator may
declared as follows:
format NAME =
FORMLIST
.
If name is omitted, format "STDOUT" is defined. FORMLIST
consists of a sequence of lines, each of which may be of one
of three types:
1. A comment.
2. A "picture" line giving the format for one output line.
3. An argument line supplying values to plug into a picture
line.
Picture lines are printed exactly as they look, except for
certain fields that substitute values into the line. Each
picture field starts with either @ or ^. The @ field (not
to be confused with the array marker @) is the normal case;
^ fields are used to do rudimentary multi-line text block
filling. The length of the field is supplied by padding out
the field with multiple <, >, or 2 characters to specify,
respectively, left justification, right justification, or
centering. As an alternate form of right justification, you
may also use # characters (with an optional .) to specify a
numeric field. (Use of ^ instead of @ causes the field to
be blanked if undefined.) If any of the values supplied for
these fields contains a newline, only the text up to the
newline is printed. The special field @* can be used for
printing multi-line values. It should appear by itself on a
line.
The values are specified on the following line, in the same
order as the picture fields. The values should be separated
by commas.
as it can into the field, and then chops off the front of
the string so that the next time the variable is referenced,
more of the text can be printed. Normally you would use a
sequence of fields in a vertical stack to print out a block
of text. If you like, you can end the final field with ...,
which will appear in the output if the text was too long to
appear in its entirety. You can change which characters are
legal to break on by changing the variable $: to a list of
the desired characters.
Since use of ^ fields can produce variable length records if
the text to be formatted is short, you can suppress blank
lines by putting the tilde (~) character anywhere in the
line. (Normally you should put it in the front if possible,
for visibility.) The tilde will be translated to a space
upon output. If you put a second tilde contiguous to the
first, the line will be repeated until all the fields on the
line are exhausted. (If you use a field of the @ variety,
the expression you supply had better not give the same value
every time forever!)
Examples:
# a report on the /etc/passwd file
format top =
Passwd File
Name Login Office Uid Gid Home
------------------------------------------------------------------
.
format STDOUT =
@<<<<<<<<<<<<<<<<<< @2222222 @<<<<<<@>>>> @>>>> @<<<<<<<<<<<<<<<<<
$name, $login, $office,$uid,$gid, $home
.
@<<<<<<<<<<<<<<<<<<<<<<< @222 @>>>>>>>>>>>>>>>>>>>>>>>
$system, $%, $date
------------------------------------------------------------------
.
format STDOUT =
Subject: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$subject
Index: @<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$index, $description
Priority: @<<<<<<<<<< Date: @<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$priority, $date, $description
From: @<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$from, $description
Assigned to: @<<<<<<<<<<<<<<<<<<<<<< ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$programmer, $description
~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$description
~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$description
~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$description
~ ^<<<<<<<<<<<<<<<<<<<<<<<<<<<<
$description
~ ^<<<<<<<<<<<<<<<<<<<<<<<...
$description
.
It is possible to intermix prints with writes on the same
output channel, but you'll have to handle $- (lines left on
the page) yourself.
If you are printing lots of fields that are usually blank,
you should consider using the reset operator between
records. Not only is it more efficient, but it can prevent
the bug of adding another field and forgetting to zero it.
Interprocess Communication
The IPC facilities of perl are built on the Berkeley socket
mechanism. If you don't have sockets, you can ignore this
section. The calls have the same names as the corresponding
system calls, but the arguments tend to differ, for two rea-
sons. First, perl file handles work differently than C file
descriptors. Second, perl already knows the length of its
strings, so you don't need to pass that information. Here
is a sample client (untested):
($them,$port) = @ARGV;
$port = 2345 unless $port;
$them = 'localhost' unless $them;
$SIG{'INT'} = 'dokill';
$sockaddr = 'S n a4 x8';
chop($hostname = `hostname`);
($name, $aliases, $proto) = getprotobyname('tcp');
($name, $aliases, $port) = getservbyname($port, 'tcp')
unless $port =~ /^\d+$/;
($name, $aliases, $type, $len, $thisaddr) =
gethostbyname($hostname);
($name, $aliases, $type, $len, $thataddr) = gethostbyname($them);
$this = pack($sockaddr, &AF_INET, 0, $thisaddr);
$that = pack($sockaddr, &AF_INET, $port, $thataddr);
socket(S, &PF_INET, &SOCK_STREAM, $proto) 22 die "socket: $!";
bind(S, $this) 22 die "bind: $!";
connect(S, $that) 22 die "connect: $!";
select(S); $2 = 1; select(stdout);
if ($child = fork) {
while (<>) {
print S;
}
sleep 3;
do dokill();
}
else {
while (<S>) {
print;
}
}
And here's a server:
($port) = @ARGV;
$port = 2345 unless $port;
require 'sys/socket.ph';
$sockaddr = 'S n a4 x8';
($name, $aliases, $proto) = getprotobyname('tcp');
($name, $aliases, $port) = getservbyname($port, 'tcp')
unless $port =~ /^\d+$/;
$this = pack($sockaddr, &AF_INET, $port, "\0\0\0\0");
select(NS); $2 = 1; select(stdout);
socket(S, &PF_INET, &SOCK_STREAM, $proto) 22 die "socket: $!";
bind(S, $this) 22 die "bind: $!";
for (;;) {
print "Listening again\n";
($addr = accept(NS,S)) 22 die $!;
print "accept ok\n";
($af,$port,$inetaddr) = unpack($sockaddr,$addr);
@inetaddr = unpack('C4',$inetaddr);
print "$af $port @inetaddr\n";
while (<NS>) {
print;
print NS;
}
}
Predefined Names
The following names have special meaning to perl. I could
have used alphabetic symbols for some of these, but I didn't
want to take the chance that someone would say reset
"a-zA-Z" and wipe them all out. You'll just have to suffer
along with these silly symbols. Most of them have reason-
able mnemonics, or analogues in one of the shells.
$_ The default input and pattern-searching space. The
following pairs are equivalent:
while (<>) {... # only equivalent in while!
while ($_ = <>) {...
/^Subject:/
$_ =~ /^Subject:/
y/a-z/A-Z/
$_ =~ y/a-z/A-Z/
chop
chop($_)
(Mnemonic: underline is understood in certain opera-
tions.)
$. The current input line number of the last filehandle
that was read. Readonly. Remember that only an
explicit close on the filehandle resets the line
number. Since <> never does an explicit close, line
numbers increase across ARGV files (but see examples
under eof). (Mnemonic: many programs use . to mean
the current line number.)
If set to a value longer than one character, only
the first character is used. (Mnemonic: / is used
to delimit line boundaries when quoting poetry.)
$, The output field separator for the print operator.
Ordinarily the print operator simply prints out the
comma separated fields you specify. In order to get
behavior more like awk, set this variable as you
would set awk's OFS variable to specify what is
printed between fields. (Mnemonic: what is printed
when there is a , in your print statement.)
$" This is like $, except that it applies to array
values interpolated into a double-quoted string (or
similar interpreted string). Default is a space.
(Mnemonic: obvious, I think.)
$\ The output record separator for the print operator.
Ordinarily the print operator simply prints out the
comma separated fields you specify, with no trailing
newline or record separator assumed. In order to
get behavior more like awk, set this variable as you
would set awk's ORS variable to specify what is
printed at the end of the print. (Mnemonic: you set
$\ instead of adding \n at the end of the print.
Also, it's just like /, but it's what you get "back"
from perl.)
$# The output format for printed numbers. This vari-
able is a half-hearted attempt to emulate awk's OFMT
variable. There are times, however, when awk and
perl have differing notions of what is in fact
numeric. Also, the initial value is %.20g rather
than %.6g, so you need to set $# explicitly to get
awk's value. (Mnemonic: # is the number sign.)
$% The current page number of the currently selected
output channel. (Mnemonic: % is page number in
nroff.)
$= The current page length (printable lines) of the
currently selected output channel. Default is 60.
(Mnemonic: = has horizontal lines.)
$- The number of lines left on the page of the
currently selected output channel. (Mnemonic:
lines_on_page - lines_printed.)
$~ The name of the current report format for the
currently selected output channel. (Mnemonic:
brother to $^.)
$2 If set to nonzero, forces a flush after every write
or print on the currently selected output channel.
Default is 0. Note that STDOUT will typically be
line buffered if output is to the terminal and block
buffered otherwise. Setting this variable is useful
primarily when you are outputting to a pipe, such as
when you are running a perl script under rsh and
want to see the output as it's happening.
(Mnemonic: when you want your pipes to be piping
hot.)
$$ The process number of the perl running this script.
(Mnemonic: same as shells.)
$? The status returned by the last pipe close, backtick
(``) command or system operator. Note that this is
the status word returned by the wait() system call,
so the exit value of the subprocess is actually ($?
>> 8). $? & 255 gives which signal, if any, the
process died from, and whether there was a core
dump. (Mnemonic: similar to sh and ksh.)
$& The string matched by the last pattern match (not
counting any matches hidden within a BLOCK or eval
enclosed by the current BLOCK). (Mnemonic: like &
in some editors.)
$` The string preceding whatever was matched by the
last pattern match (not counting any matches hidden
within a BLOCK or eval enclosed by the current
BLOCK). (Mnemonic: ` often precedes a quoted
string.)
$' The string following whatever was matched by the
last pattern match (not counting any matches hidden
within a BLOCK or eval enclosed by the current
BLOCK). (Mnemonic: ' often follows a quoted
string.) Example:
$_ = 'abcdefghi';
/def/;
print "$`:$&:$'\n"; # prints abc:def:ghi
$+ The last bracket matched by the last search pattern.
This is useful if you don't know which of a set of
alternative patterns matched. For example:
/Version: (.*)2Revision: (.*)/ && ($rev = $+);
(Mnemonic: be positive and forward looking.)
matches. Pattern matches on strings containing mul-
tiple newlines can produce confusing results when $*
is 0. Default is 0. (Mnemonic: * matches multiple
things.) Note that this variable only influences the
interpretation of ^ and $. A literal newline can be
searched for even when $* == 0.
$0 Contains the name of the file containing the perl
script being executed. (Mnemonic: same as sh and
ksh.)
$<digit>
Contains the subpattern from the corresponding set
of parentheses in the last pattern matched, not
counting patterns matched in nested blocks that have
been exited already. (Mnemonic: like \digit.)
$[ The index of the first element in an array, and of
the first character in a substring. Default is 0,
but you could set it to 1 to make perl behave more
like awk (or Fortran) when subscripting and when
evaluating the index() and substr() functions.
(Mnemonic: [ begins subscripts.)
$] The string printed out when you say "perl -v". It
can be used to determine at the beginning of a
script whether the perl interpreter executing the
script is in the right range of versions. If used
in a numeric context, returns the version +
patchlevel / 1000. Example:
# see if getc is available
($version,$patchlevel) =
$] =~ /(\d+\.\d+).*\nPatch level: (\d+)/;
print STDERR "(No filename completion available.)\n"
if $version * 1000 + $patchlevel < 2016;
or, used numerically,
warn "No checksumming!\n" if $] < 3.019;
(Mnemonic: Is this version of perl in the right
bracket?)
$; The subscript separator for multi-dimensional array
emulation. If you refer to an associative array
element as
$foo{$a,$b,$c}
it really means
$foo{join($;, $a, $b, $c)}
which means
($foo{$a},$foo{$b},$foo{$c})
Default is "\034", the same as SUBSEP in awk. Note
that if your keys contain binary data there might
not be any safe value for $;. (Mnemonic: comma (the
syntactic subscript separator) is a semi-semicolon.
Yeah, I know, it's pretty lame, but $, is already
taken for something more important.)
$! If used in a numeric context, yields the current
value of errno, with all the usual caveats. (This
means that you shouldn't depend on the value of $!
to be anything in particular unless you've gotten a
specific error return indicating a system error.) If
used in a string context, yields the corresponding
system error string. You can assign to $! in order
to set errno if, for instance, you want $! to return
the string for error n, or you want to set the exit
value for the die operator. (Mnemonic: What just
went bang?)
$@ The perl syntax error message from the last eval
command. If null, the last eval parsed and executed
correctly (although the operations you invoked may
have failed in the normal fashion). (Mnemonic:
Where was the syntax error "at"?)
$< The real uid of this process. (Mnemonic: it's the
uid you came FROM, if you're running setuid.)
$> The effective uid of this process. Example:
$< = $>; # set real uid to the effective uid
($<,$>) = ($>,$<); # swap real and effective uid
(Mnemonic: it's the uid you went TO, if you're run-
ning setuid.) Note: $< and $> can only be swapped on
machines supporting setreuid().
$( The real gid of this process. If you are on a
machine that supports membership in multiple groups
simultaneously, gives a space separated list of
groups you are in. The first number is the one
returned by getgid(), and the subsequent ones by
getgroups(), one of which may be the same as the
first number. (Mnemonic: parentheses are used to
GROUP things. The real gid is the group you LEFT,
if you're running setgid.)
groups you are in. The first number is the one
returned by getegid(), and the subsequent ones by
getgroups(), one of which may be the same as the
first number. (Mnemonic: parentheses are used to
GROUP things. The effective gid is the group that's
RIGHT for you, if you're running setgid.)
Note: $<, $>, $( and $) can only be set on machines
that support the corresponding set[re][ug]id() rou-
tine. $( and $) can only be swapped on machines
supporting setregid().
$: The current set of characters after which a string
may be broken to fill continuation fields (starting
with ^) in a format. Default is " \n-", to break on
whitespace or hyphens. (Mnemonic: a "colon" in poe-
try is a part of a line.)
$ARGV contains the name of the current file when reading
from <>.
@ARGV The array ARGV contains the command line arguments
intended for the script. Note that $#ARGV is the
generally number of arguments minus one, since
$ARGV[0] is the first argument, NOT the command
name. See $0 for the command name.
@INC The array INC contains the list of places to look
for perl scripts to be evaluated by the "do EXPR"
command or the "require" command. It initially con-
sists of the arguments to any -I command line
switches, followed by the default perl library,
probably "/usr/local/lib/perl", followed by ".", to
represent the current directory.
%INC The associative array INC contains entries for each
filename that has been included via "do" or
"require". The key is the filename you specified,
and the value is the location of the file actually
found. The "require" command uses this array to
determine whether a given file has already been
included.
$ENV{expr}
The associative array ENV contains your current
environment. Setting a value in ENV changes the
environment for child processes.
$SIG{expr}
The associative array SIG is used to set signal
handlers for various signals. Example:
close(LOG);
exit(0);
}
$SIG{'INT'} = 'handler';
$SIG{'QUIT'} = 'handler';
...
$SIG{'INT'} = 'DEFAULT'; # restore default action
$SIG{'QUIT'} = 'IGNORE'; # ignore SIGQUIT
The SIG array only contains values for the signals
actually set within the perl script.
Packages
Perl provides a mechanism for alternate namespaces to pro-
tect packages from stomping on each others variables. By
default, a perl script starts compiling into the package
known as "main". By use of the package declaration, you can
switch namespaces. The scope of the package declaration is
from the declaration itself to the end of the enclosing
block (the same scope as the local() operator). Typically
it would be the first declaration in a file to be included
by the "require" operator. You can switch into a package in
more than one place; it merely influences which symbol table
is used by the compiler for the rest of that block. You can
refer to variables and filehandles in other packages by pre-
fixing the identifier with the package name and a single
quote. If the package name is null, the "main" package as
assumed.
Only identifiers starting with letters are stored in the
packages symbol table. All other symbols are kept in pack-
age "main". In addition, the identifiers STDIN, STDOUT,
STDERR, ARGV, ARGVOUT, ENV, INC and SIG are forced to be in
package "main", even when used for other purposes than their
built-in one. Note also that, if you have a package called
"m", "s" or "y", the you can't use the qualified form of an
identifier since it will be interpreted instead as a pattern
match, a substitution or a translation.
Eval'ed strings are compiled in the package in which the
eval was compiled in. (Assignments to $SIG{}, however,
assume the signal handler specified is in the main package.
Qualify the signal handler name if you wish to have a signal
handler in a package.) For an example, examine perldb.pl in
the perl library. It initially switches to the DB package
so that the debugger doesn't interfere with variables in the
script you are trying to debug. At various points, however,
it temporarily switches back to the main package to evaluate
various expressions in the context of the main package.
are referring to when you use the *name notation. In fact,
the following have the same effect (in package main, any-
way), though the first is more efficient because it does the
symbol table lookups at compile time:
local(*foo) = *bar;
local($_main{'foo'}) = $_main{'bar'};
You can use this to print out all the variables in a pack-
age, for instance. Here is dumpvar.pl from the perl
library:
package dumpvar;
sub main'dumpvar {
($package) = @_;
local(*stab) = eval("*_$package");
while (($key,$val) = each(%stab)) {
{
local(*entry) = $val;
if (defined $entry) {
print "\$$key = '$entry'\n";
}
if (defined @entry) {
print "\@$key = (\n";
foreach $num ($[ .. $#entry) {
print " $num\t'",$entry[$num],"'\n";
}
print ")\n";
}
if ($key ne "_$package" && defined %entry) {
print "\%$key = (\n";
foreach $key (sort keys(%entry)) {
print " $key\t'",$entry{$key},"'\n";
}
print ")\n";
}
}
}
}
Note that, even though the subroutine is compiled in package
dumpvar, the name of the subroutine is qualified so that its
name is inserted into package "main".
Style
Each programmer will, of course, have his or her own prefer-
ences in regards to formatting, but there are some general
guidelines that will make your programs easier to read.
consider picking the most readable one. For instance
open(FOO,$foo) 22 die "Can't open $foo: $!";
is better than
die "Can't open $foo: $!" unless open(FOO,$foo);
because the second way hides the main point of the
statement in a modifier. On the other hand
print "Starting analysis\n" if $verbose;
is better than
$verbose && print "Starting analysis\n";
since the main point isn't whether the user typed -v or
not.
Similarly, just because an operator lets you assume
default arguments doesn't mean that you have to make use
of the defaults. The defaults are there for lazy sys-
tems programmers writing one-shot programs. If you want
your program to be readable, consider supplying the
argument.
Along the same lines, just because you can omit
parentheses in many places doesn't mean that you ought
to:
return print reverse sort num values array;
return print(reverse(sort num (values(%array))));
When in doubt, parenthesize. At the very least it will
let some poor schmuck bounce on the % key in vi.
Even if you aren't in doubt, consider the mental welfare
of the person who has to maintain the code after you,
and who will probably put parens in the wrong place.
2. Don't go through silly contortions to exit a loop at the
top or the bottom, when perl provides the "last" opera-
tor so you can exit in the middle. Just outdent it a
little to make it more visible:
last line if $foo;
next line if /^#/;
statements;
}
3. Don't be afraid to use loop labels--they're there to
enhance readability as well as to allow multi-level loop
breaks. See last example.
4. For portability, when using features that may not be
implemented on every machine, test the construct in an
eval to see if it fails. If you know what version or
patchlevel a particular feature was implemented, you can
test $] to see if it will be there.
5. Choose mnemonic identifiers.
6. Be consistent.
Debugging
If you invoke perl with a -d switch, your script will be run
under a debugging monitor. It will halt before the first
executable statement and ask you for a command, such as:
h Prints out a help message.
T Stack trace.
s Single step. Executes until it reaches the
beginning of another statement.
n Next. Executes over subroutine calls, until it
reaches the beginning of the next statement.
f Finish. Executes statements until it has fin-
ished the current subroutine.
c Continue. Executes until the next breakpoint is
reached.
c line Continue to the specified line. Inserts a one-
time-only breakpoint at the specified line.
<CR> Repeat last n or s.
l min+incr List incr+1 lines starting at min. If min is
omitted, starts where last listing left off. If
incr is omitted, previous value of incr is used.
l List next window.
- List previous window.
w line List window around line.
l subname List subroutine. If it's a long subroutine it
just lists the beginning. Use "l" to list more.
/pattern/ Regular expression search forward for pattern;
the final / is optional.
?pattern? Regular expression search backward for pattern;
the final ? is optional.
L List lines that have breakpoints or actions.
S Lists the names of all subroutines.
t Toggle trace mode on or off.
b line condition
Set a breakpoint. If line is omitted, sets a
breakpoint on the line that is about to be exe-
cuted. If a condition is specified, it is
evaluated each time the statement is reached and
a breakpoint is taken only if the condition is
true. Breakpoints may only be set on lines that
begin an executable statement.
b subname condition
Set breakpoint at first executable line of sub-
routine.
d line Delete breakpoint. If line is omitted, deletes
the breakpoint on the line that is about to be
executed.
D Delete all breakpoints.
a line command
Set an action for line. A multi-line command
may be entered by backslashing the newlines.
A Delete all line actions.
< command Set an action to happen before every debugger
prompt. A multi-line command may be entered by
backslashing the newlines.
entered by backslashing the newlines.
V package List all variables in package. Default is main
package.
! number Redo a debugging command. If number is omitted,
redoes the previous command.
! -number Redo the command that was that many commands
ago.
H -number Display last n commands. Only commands longer
than one character are listed. If number is
omitted, lists them all.
q or ^D Quit.
command Execute command as a perl statement. A missing
semicolon will be supplied.
p expr Same as "print DB'OUT expr". The DB'OUT
filehandle is opened to /dev/tty, regardless of
where STDOUT may be redirected to.
If you want to modify the debugger, copy perldb.pl from the
perl library to your current directory and modify it as
necessary. (You'll also have to put -I. on your command
line.) You can do some customization by setting up a .perldb
file which contains initialization code. For instance, you
could make aliases like these:
$DB'alias{'len'} = 's/^len(.*)/p length($1)/';
$DB'alias{'stop'} = 's/^stop (at2in)/b/';
$DB'alias{'.'} =
's/^\./p "\$DB\'sub(\$DB\'line):\t",\$DB\'line[\$DB\'line]/';
Setuid Scripts
Perl is designed to make it easy to write secure setuid and
setgid scripts. Unlike shells, which are based on multiple
substitution passes on each line of the script, perl uses a
more conventional evaluation scheme with fewer hidden
"gotchas". Additionally, since the language has more
built-in functionality, it has to rely less upon external
(and possibly untrustworthy) programs to accomplish its pur-
poses.
In an unpatched 4.2 or 4.3bsd kernel, setuid scripts are
intrinsically insecure, but this kernel feature can be dis-
abled. If it is, perl can emulate the setuid and setgid
mechanism when it notices the otherwise useless setuid/gid
script feature, or put a C wrapper around the script.
When perl is executing a setuid script, it takes special
precautions to prevent you from falling into any obvious
traps. (In some ways, a perl script is more secure than the
corresponding C program.) Any command line argument,
environment variable, or input is marked as "tainted", and
may not be used, directly or indirectly, in any command that
invokes a subshell, or in any command that modifies files,
directories or processes. Any variable that is set within
an expression that has previously referenced a tainted value
also becomes tainted (even if it is logically impossible for
the tainted value to influence the variable). For example:
$foo = shift; # $foo is tainted
$bar = $foo,'bar'; # $bar is also tainted
$xxx = <>; # Tainted
$path = $ENV{'PATH'}; # Tainted, but see below
$abc = 'abc'; # Not tainted
system "echo $foo"; # Insecure
system "/bin/echo", $foo; # Secure (doesn't use sh)
system "echo $bar"; # Insecure
system "echo $abc"; # Insecure until PATH set
$ENV{'PATH'} = '/bin:/usr/bin';
$ENV{'IFS'} = '' if $ENV{'IFS'} ne '';
$path = $ENV{'PATH'}; # Not tainted
system "echo $abc"; # Is secure now!
open(FOO,"$foo"); # OK
open(FOO,">$foo"); # Not OK
open(FOO,"echo $foo2"); # Not OK, but...
open(FOO,"-2") 22 exec 'echo', $foo; # OK
$zzz = `echo $foo`; # Insecure, zzz tainted
unlink $abc,$foo; # Insecure
umask $foo; # Insecure
exec "echo $foo"; # Insecure
exec "echo", $foo; # Secure (doesn't use sh)
exec "sh", '-c', $foo; # Considered secure, alas
The taintedness is associated with each scalar value, so
some elements of an array can be tainted, and others not.
If you try to do something insecure, you will get a fatal
error saying something like "Insecure dependency" or
"Insecure PATH". Note that you can still write an insecure
if you reference a substring using $1, $2, etc, you knew
what you were doing when you wrote the pattern:
$ARGV[0] =~ /^-P(\w+)$/;
$printer = $1; # Not tainted
This is fairly secure since \w+ doesn't match shell meta-
characters. Use of .+ would have been insecure, but perl
doesn't check for that, so you must be careful with your
patterns. This is the ONLY mechanism for untainting user
supplied filenames if you want to do file operations on them
(unless you make $> equal to $<).
It's also possible to get into trouble with other operations
that don't care whether they use tainted values. Make judi-
cious use of the file tests in dealing with any user-
supplied filenames. When possible, do opens and such after
setting $> = $<. Perl doesn't prevent you from opening
tainted filenames for reading, so be careful what you print
out. The tainting mechanism is intended to prevent stupid
mistakes, not to remove the need for thought.
ENVIRONMENT
Perl uses PATH in executing subprocesses, and in finding the
script if -S is used. HOME or LOGDIR are used if chdir has
no argument.
Apart from these, perl uses no environment variables, except
to make them available to the script being executed, and to
child processes. However, scripts running setuid would do
well to execute the following lines before doing anything
else, just to keep people honest:
$ENV{'PATH'} = '/bin:/usr/bin'; # or whatever you need
$ENV{'SHELL'} = '/bin/sh' if $ENV{'SHELL'} ne '';
$ENV{'IFS'} = '' if $ENV{'IFS'} ne '';
AUTHOR
Larry Wall <lwall@jpl-devvax.Jpl.Nasa.Gov>
MS-DOS port by Diomidis Spinellis <dds@cc.ic.ac.uk>
FILES
/tmp/perl-eXXXXXX temporary file for -e commands.
SEE ALSO
a2p awk to perl translator
s2p sed to perl translator
DIAGNOSTICS
Compilation errors will tell you the line number of the
error, with an indication of the next token or token type
Setuid scripts have additional constraints that can produce
error messages such as "Insecure dependency". See the sec-
tion on setuid scripts.
TRAPS
Accustomed awk users should take special note of the follow-
ing:
* Semicolons are required after all simple statements in
perl. Newline is not a statement delimiter.
* Curly brackets are required on ifs and whiles.
* Variables begin with $ or @ in perl.
* Arrays index from 0 unless you set $[. Likewise string
positions in substr() and index().
* You have to decide whether your array has numeric or
string indices.
* Associative array values do not spring into existence
upon mere reference.
* You have to decide whether you want to use string or
numeric comparisons.
* Reading an input line does not split it for you. You
get to split it yourself to an array. And the split
operator has different arguments.
* The current input line is normally in $_, not $0. It
generally does not have the newline stripped. ($0 is
the name of the program executed.)
* $<digit> does not refer to fields--it refers to sub-
strings matched by the last match pattern.
* The print statement does not add field and record
separators unless you set $, and $\.
* You must open your files before you print to them.
* The range operator is "..", not comma. (The comma
operator works as in C.)
* The match operator is "=~", not "~". ("~" is the one's
complement operator, as in C.)
* The exponentiation operator is "**", not "^". ("^" is
the XOR operator, as in C.)
as a division operator--the tokener is in fact slightly
context sensitive for operators like /, ?, and <. And
in fact, . itself can be the beginning of a number.)
* Next, exit and continue work differently.
* The following variables work differently
Awk Perl
ARGC $#ARGV
ARGV[0] $0
FILENAME $ARGV
FNR $. - something
FS (whatever you like)
NF $#Fld, or some such
NR $.
OFMT $#
OFS $,
ORS $\
RLENGTH length($&)
RS $/
RSTART length($`)
SUBSEP $;
* When in doubt, run the awk construct through a2p and see
what it gives you.
Cerebral C programmers should take note of the following:
* Curly brackets are required on ifs and whiles.
* You should use "elsif" rather than "else if"
* Break and continue become last and next, respectively.
* There's no switch statement.
* Variables begin with $ or @ in perl.
* Printf does not implement *.
* Comments begin with #, not /*.
* You can't take the address of anything.
* ARGV must be capitalized.
* The "system" calls link, unlink, rename, etc. return
nonzero for success, not 0.
* Signal handlers deal with signal names, not numbers.
* The pattern matching metacharacters (, ), and 2 do not
have backslashes in front.
* The range operator is .. rather than comma.
Sharp shell programmers should take note of the following:
* The backtick operator does variable interpretation
without regard to the presence of single quotes in the
command.
* The backtick operator does no translation of the return
value, unlike csh.
* Shells (especially csh) do several levels of substitu-
tion on each command line. Perl does substitution only
in certain constructs such as double quotes, backticks,
angle brackets and search patterns.
* Shells interpret scripts a little bit at a time. Perl
compiles the whole program before executing it.
* The arguments are available via @ARGV, not $1, $2, etc.
* The environment is not automatically made available as
variables.
ERRATA AND ADDENDA
The Perl book, Programming Perl , has the following omis-
sions and goofs.
The -0 switch was added to Perl after the book went to
press.
The new @###.## format was omitted accidentally.
It wasn't known at press time that s///ee caused multiple
evaluations.
BUGS
Perl is at the mercy of your machine's definitions of vari-
ous operations such as type casting, atof() and sprintf().
If your stdio requires an seek or eof between reads and
writes on a particular stream, so does perl.
While none of the built-in data types have any arbitrary
size limits (apart from memory size), there are still a few
arbitrary limits: a given identifier may not be longer than
255 characters; sprintf is limited on many machines to 128
characters per field (unless the format specifier is exactly
Perl actually stands for Pathologically Eclectic Rubbish
Lister, but don't tell anyone I said that.
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