By popular demand, I have also produced a single-page version of this document, suitable for printing. I consider it inferior to the original document, and I do not particularly endorse its use. I do request that people not attempt to mirror it.
After locating your script, perl compiles it to an internal form. If the script is syntactically correct, it is executed.
A single-character option may be combined with the following option, if any. This is particularly useful when invoking a script using the #! construct which only allows one argument. Example:
#!/usr/bin/perl -spi.bak # same as -s -p -i.bak ...Options include:
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.
perl -ane 'print pop(@F), "\n";'is equivalent to
while (<>) {
@F = split(' ');
print pop(@F), "\n";
}
#!/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 $oldargv to know when
the filename has changed.
It does, however, use ARGVOUT for the selected filehandle.
Note that
STDOUT
is restored as the default output filehandle after the loop.
You can use eof to locate the end of each input file, in case you want to append to each file, or reset line numbering (see example under eof).
perl -lpe 'substr($_, 80) = ""'Note that the assignment $\ = $/ is done when the switch is processed, so the input record separator can be different than the output record separator if the -l switch is followed by a -0 switch:
gnufind / -print0 | perl -ln0e 'print "found $_" if -p'This sets $\ to newline and then sets $/ to the null character.
while (<>) {
... # your script goes here
}
Note that the lines are not printed by default.
See
-p
to have lines printed.
Here is an efficient way to delete all files older than a week:
gfind . -mtime +7 -print | perl -nle 'unlink;'This is faster than using the -exec switch of find because you don't have to start a process on every filename found.
while (<>) {
... # your script goes here
} continue {
print;
}
Note that the lines are printed automatically.
To suppress printing use the
-n
switch.
A
-p
overrides a
-n
switch.
#!/usr/bin/perl -s
if ($xyz) { print "true\n"; }
#!/usr/bin/perl eval "exec /usr/bin/perl -S $0 $*" if $running_under_some_shell;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 politely 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 following:
eval '(exit $?0)' && eval 'exec /usr/bin/perl -S $0 ${1+"$@"}'
& eval 'exec /usr/bin/perl -S $0 $argv:q'
if 0;
Perl has three data types: scalars, arrays of scalars, and associative arrays of scalars. Normal arrays are indexed by number, and associative arrays by string.
The interpretation of operations and values in perl sometimes depends on the requirements of the context around the operation or value. There are three major contexts: string, numeric and array. Certain operations return array values in contexts wanting an array, and scalar values otherwise. (If this is true of an operation it will be mentioned in the documentation for that operation.) Operations which return scalars don't care whether the context is looking for a string or a number, but scalar variables and values are interpreted as strings or numbers as appropriate to the context. A scalar is interpreted as TRUE in the boolean sense if it is not the null string or 0. Booleans returned by operators are 1 for true and 0 or '' (the null string) for false.
There are actually two varieties of null string: defined and undefined. Undefined null strings are returned when there is no real value for something, such as when there was an error, or at end of file, or when you refer to an uninitialized variable or element of an array. An undefined null string may become defined the first time you access it, but prior to that you can use the defined() operator to determine whether the value is defined or not.
References to scalar variables always begin with '$', 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
$#days # last index of array @days
but entire arrays or array slices are denoted by '@':
@days # ($days[0], $days[1],\|... $days[n])
@days[3,4,5] # same as @days[3.\|.5]
@days{'a','c'} # same as ($days{'a'},$days{'c'})
and entire associative arrays are denoted by '%':
%days # (key1, val1, key2, val2 ...)
Any of these eight constructs may serve as an lvalue, that is, may be assigned to. (It also turns out that an assignment is itself an lvalue in certain contexts--see examples under s, tr and chop.) Assignment to a scalar evaluates the righthand side in a scalar context, while assignment to an array or array slice evaluates the righthand side in an array context.
You may find the length of array @days by evaluating "$#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 efficiency 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 assigning 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:
scalar(@whatever) == $#whatever - $[ + 1;If you evaluate an associative array in a scalar context, it returns a value which is true if and only if the array contains any elements. (If there are any elements, the value returned is a string consisting of the number of used buckets and the number of allocated buckets, separated by a slash.)
Multi-dimensional arrays are not directly supported, but see the discussion of the $; variable later for a means of emulating multiple subscripts with an associative array. You could also write a subroutine to turn multiple subscripts into a single subscript.
Every data type has its own namespace. You can, without fear of conflict, use the same name for a scalar variable, an array, an associative array, a filehandle, a subroutine name, and/or a label. Since variable and array references always start with '$', '@', or '%', the "reserved" words aren't in fact reserved with respect to variable names. (They ARE reserved with respect to labels and filehandles, however, which don't have an initial special character. 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
4_294_967_296
String literals are delimited by either single or double quotes.
They work much like shell quotes:
double-quoted string literals are subject to backslash and variable
substitution; single-quoted strings are not (except for \' and \e).
The usual backslash rules apply for making characters such as newline, tab,
etc., as well as some more exotic forms:
\t tab \n newline \r return \f form feed \b backspace \a alarm (bell) \e escape \033 octal char \x1b hex char \c[ control char \l lowercase next char \u uppercase next char \L lowercase till \E \U uppercase till \E \E end case modificationYou 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. (The DATA filehandle may read data only from the main script, but not from any required file or evaluated string.) The two control characters ^D and ^Z are synonyms for __END__.
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 otherwise 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 interpretation 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 identifier. (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 surrounding whitespace) on the terminating line.
print <<EOF; # same as above The price is $Price. EOF print <<"EOF"; # same as above 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. barArray 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 were a member of LIST. Thus arrays
lose their identity in a LIST--the list
(@foo,@bar,&SomeSub)
contains all the elements of @foo followed by all the elements 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 command, 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 each line of output. (You can set $/ to use a different line terminator.) The command is executed each time the pseudo-literal is evaluated. The status value of the command 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 happens. If (and only if) the input symbol is the only thing inside the conditional of a while loop, the value is automatically assigned to the variable "$_". (This may seem like an odd thing to you, but you'll use the construct in almost every perl script you write.) Anyway, the following lines are equivalent to each other:
while ($_ = <STDIN>) { 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
}
is equivalent to the following Perl-like pseudo code:
unshift(@ARGV, '-') if $#ARGV < $[;
while ($ARGV = shift) {
open(ARGV, $ARGV);
while (<ARGV>) {
... # code for each line
}
}
except that it isn't as cumbersome to say, and will actually work.
It really does shift array ARGV and put the current filename into
variable ARGV.
It also uses filehandle ARGV internally--<> is just a synonym for
<ARGV>, which is magical.
(The pseudo code above doesn't work because it treats <ARGV> as non-magical.)
You can modify @ARGV before the first <> as long as the array ends up containing the list of filenames you really want. 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, $_;
}
is equivalent to
open(foo, "echo *.c | tr -s ' \t\r\f' '\\012\\012\\012\\012'");
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>;
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 uninitialized 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 control over which files and which lines you look at. (Actually, I'm lying--it is possible to do an implicit loop with either the -n or -p switch.)
A declaration can be put anywhere a command can, but has no effect on the execution of the primary sequence of commands--declarations all take effect at compile time. Typically all the declarations are put at the beginning or the end of the script.
Perl is, for the most part, a free-form language. (The only exception to this is format declarations, for fairly obvious reasons.) Comments are indicated by the # 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.
The following compound commands may be used to control flow:
if (EXPR) BLOCK if (EXPR) BLOCK else BLOCK if (EXPR) BLOCK elsif (EXPR) BLOCK ... else BLOCK LABEL while (EXPR) BLOCK LABEL while (EXPR) BLOCK continue BLOCK LABEL for (EXPR; EXPR; EXPR) BLOCK LABEL foreach VAR (ARRAY) BLOCK LABEL BLOCK continue BLOCKNote that, unlike C and Pascal, these are defined in terms of BLOCKs, not statements. This means that the curly brackets are required--no dangling statements allowed. If you want to write conditionals without curly brackets there are several other ways to do it. The following all do the same thing:
if (!open(foo)) { die "Can't open $foo: $!"; }
die "Can't open $foo: $!" unless open(foo);
open(foo) || die "Can't open $foo: $!"; # foo or bust!
open(foo) ? '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 ambiguity 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. 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:
for ($i = 1; $i < 10; $i++) {
...
}
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" keyword 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 modifying 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 control 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
are already several ways to write the equivalent.
In addition to the above, you could write
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.)
Any simple statement may optionally be followed by a single modifier, just before the terminating semicolon. The possible modifiers are:
if EXPR unless EXPR while EXPR until EXPRThe if and unless modifiers have the expected semantics. The while and until modifiers also have the expected semantics (conditional evaluated first), except when applied to a do-BLOCK or a do-SUBROUTINE 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.)
SUBROUTINE may also be a single scalar variable, in which case the name of the subroutine to execute is taken from the variable.
As an alternate (and preferred) 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:
if (defined &$var) { &$var($parm); undef &$var; }
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:
do $foo; # eval a file do $foo(); # call a subroutineNote that inclusion of library routines is better done with the "require" operator.
line: while (<STDIN>) {
last line if /^$/; # exit when done with header
...
}
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.
# a simpleminded Pascal comment stripper
# (warning: assumes no { or } in strings)
line: while (<STDIN>) {
while (s|({.*}.*){.*}|$1 |) {}
s|{.*}| |;
if (s|{.*| |) {
$front = $_;
while (<STDIN>) {
if (/}/) { # end of comment?
s|^|$front{|;
redo line;
}
}
}
print;
}
Here's what perl has that C doesn't:
print '-' x 80; # print row of dashes print '-' x80; # illegal, x80 is identifier print "\t" x ($tab/8), ' ' x ($tab%8); # tab over @ones = (1) x 80; # an array of 80 1's @ones = (5) x @ones; # set all elements to 5
In a scalar context, .. returns a boolean value. The operator is bistable, like a flip-flop, and emulates the line-range (comma) operator of sed, awk, and various editors. 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 test the right operand and become false on the same evaluation it became true (as in awk), but it still returns true once. If you don't want it to test the right operand till the next evaluation (as in sed), use three dots (...) instead of two.) The right operand is not evaluated while the operator is in the "false" state, and the left operand is not evaluated while the operator is in the "true" state. The precedence is a little lower than || and &&. The value returned is either the null string for false, or a sequence number (beginning with 1) for true. The sequence number is reset for each range encountered. The final sequence number in a range has the string 'E0' appended to it, which doesn't affect its numeric value, but gives you something to search for if you want to exclude the endpoint. You can exclude the beginning point by waiting for the sequence number to be greater than 1. If either 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
-r File is readable by effective uid/gid. -w File is writable by effective uid/gid. -x File is executable by effective uid/gid. -o File is owned by effective uid. -R File is readable by real uid/gid. -W File is writable by real uid/gid. -X File is executable by real uid/gid. -O File is owned by real uid. -e File exists. -z File has zero size. -s File has non-zero size (returns size). -f File is a plain file. -d File is a directory. -l File is a symbolic link. -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 substitution.
Saying -exp($foo) still works as expected, however--only single letters
following a minus are interpreted as file tests.
The -T and -B switches work as follows. The first block or so of the file is examined for odd characters such as strange control codes or metacharacters. 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 considered 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.
If any of the file tests (or either stat operator) are given the special filehandle consisting of a solitary underline, then the stat structure of the previous file test (or stat operator) is used, saving a system call. (This doesn't work with -t, and you need to remember that lstat and -l will leave values in the stat structure for the symbolic link, not the real file.)
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:
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 substitution. Backslash interpretation also happens at compile time. You can say
'Now is the time for all' . "\n" . 'good men to come to.'and this all reduces to one string internally.
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'The autodecrement is not magical.
The range operator (in an array context) makes use of the 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 specified 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 operator, with a precedence depending only on whether it is a LIST operator or not. LIST operators have lowest precedence. All other unary operators have a precedence greater than relational operators but less than arithmetic operators. See the section on Precedence.
For operators that can be used in either a scalar or array context, failure is generally indicated in a scalar context by returning the undefined value, and in an array context by returning the null list. Remember though that there is no general rule for converting a list into a scalar. Each operator decides which sort of scalar it would be most appropriate to return. Some operators return the length of the list that would have been returned in an array context. Some operators return the first value in the list. Some operators return the last value in the list. Some operators return a count of successful operations. In general, they do what you want, unless you want consistency.
($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).
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) =
localtime(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 localtime(time).
$val = oct($val) if $val =~ /^0/;If EXPR is omitted, uses $_.
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. v A short in "VAX" (little-endian) order. V A long in "VAX" (little-endian) order. 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 will gobble up that many values from the LIST. A * for the repeat count means to use however many items are left. The "a" and "A" types gobble just one value, but pack it as a string of length count, padding 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 interchange has been made. This means that packed floating 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.
sub substr {
local($what,$where,$howmuch) = @_;
unpack("x$where a$howmuch", $what);
}
and then there's
sub ord { unpack("c",$_[0]); }
In addition, you may prefix a field with a %<number> to indicate that
you want a <number>-bit checksum of the items instead of the items themselves.
Default is a 16-bit checksum.
For example, the following computes the same number as the System V sum program:
while (<>) {
$checksum += unpack("%16C*", $_);
}
$checksum %= 65536;
Vectors created with vec() can also be manipulated with the logical operators |, & and ^, which will assume a bit vector operation is desired when both operands are strings. This interpretation is not 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 *.
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.
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.
If the code to be executed doesn't vary, you may use the eval-BLOCK form to trap run-time errors without incurring the penalty of recompiling each time. The error, if any, is still returned in $@. Evaluating a single-quoted string (as EXPR) has the same effect, except that the eval-EXPR form reports syntax errors at run time via $@, whereas the eval-BLOCK form reports syntax errors at compile time. The eval-EXPR form is optimized to eval-BLOCK the first time it succeeds. (Since the replacement side of a substitution is considered a single-quoted string when you use the e modifier, the same optimization occurs there.) Examples:
# make divide-by-zero non-fatal
eval { $answer = $a / $b; }; warn $@ if $@;
# optimized to same thing after first use
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
eval { $answer = };
# a run-time error
eval '$answer ='; # sets $@
$foo = q!I said, "You said, 'She said it.'"!;
$bar = q('This is it.');
$today = qx{ date };
$_ .= qq
*** The previous line contains the naughty word "$&".\n
if /(ibm|apple|awk)/; # :-)
The following deletes all the values of an associative array:
foreach $key (keys %ARRAY) {
delete $ARRAY{$key};
}
(But it would be faster to use the
reset
command.
Saying undef %ARRAY is faster yet.)
while (($key,$value) = each %ENV) {
print "$key=$value\n";
}
See also keys() and values().
@foo = grep(!/^#/, @bar); # weed out commentsNote 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.
$_ = join(':',
$login,$passwd,$uid,$gid,$gcos,$home,$shell);
See
split.
@keys = keys %ENV;
@values = values %ENV;
while ($#keys >= 0) {
print pop(@keys), '=', pop(@values), "\n";
}
or how about sorted by key:
foreach $key (sort(keys %ENV)) {
print $key, '=', $ENV{$key}, "\n";
}
$tmp = $ARRAY[$#ARRAY--];If there are no elements in the array, returns the undefined value.
for $value (LIST) {
$ARRAY[++$#ARRAY] = $value;
}
but is more efficient.
In the interests of efficiency the normal calling code for subroutines is bypassed, with the following effects: the subroutine may not be a recursive subroutine, 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 reference so don't modify $a and $b.
Examples:
# sort lexically
@articles = sort @files;
# same thing, but with explicit sort routine
@articles = sort {$a cmp $b} @files;
# same thing in reversed order
@articles = sort {$b cmp $a} @files;
# sort numerically ascending
@articles = sort {$a <=> $b} @files;
# sort numerically descending
@articles = sort {$b <=> $a} @files;
# sort using explicit subroutine name
sub byage {
$age{$a} <=> $age{$b}; # presuming integers
}
@sortedclass = sort byage @class;
sub reverse { $b cmp $a; }
@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
push(@a,$x,$y)\h'|3.5i'splice(@a,$#a+1,0,$x,$y)
pop(@a)\h'|3.5i'splice(@a,-1)
shift(@a)\h'|3.5i'splice(@a,0,1)
unshift(@a,$x,$y)\h'|3.5i'splice(@a,0,0,$x,$y)
$a[$x] = $y\h'|3.5i'splice(@a,$x,1,$y);
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)) { ... }
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);(When assigning to a list, if LIMIT is omitted, perl supplies a LIMIT one larger than the number of variables in the list, to avoid unnecessary work. For 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 substring 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.
unshift(ARGV, '-e') unless $ARGV[0] =~ /^-/;
$cnt = chmod 0755, 'foo', 'bar'; chmod 0755, @executables;
$cnt = chown $uid, $gid, 'foo', 'bar'; chown $uid, $gid, @filenames;Here's an example that looks up non-numeric uids in the passwd file:
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;
}
$rin = $win = $ein = ''; vec($rin,fileno(STDIN),1) = 1; vec($win,fileno(STDOUT),1) = 1; $ein = $rin | $win;If you want to select on many filehandles you might wish to write a subroutine:
sub fhbits {
local(@fhlist) = split(' ',$_[0]);
local($bits);
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 fractional. NOTE: not all implementations are capable of returning the $timeleft. If not, they always return $timeleft equal to the supplied $timeout.
($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";
}
(This only works on machines for which the device number is negative under NFS.)
$symlink_exists = (eval 'symlink("","");', $@ eq '');
$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.
#!/usr/bin/perl $now = time; utime $now, $now, @ARGV;
open(OUTPUT, '|sort >foo'); # pipe to sort ... # print stuff to output close OUTPUT; # wait for sort to finish open(INPUT, 'foo'); # get sort's resultsFILEHANDLE may be an expression whose value gives the real filehandle name.
Values assigned to the associative array prior to 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 preallocating that number of garbage entries in the associative 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);
# 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);
}
}
require "fcntl.ph"; # probably /usr/local/lib/perl/fcntl.phfirst 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 processing 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).
$LOCK_SH = 1;
$LOCK_EX = 2;
$LOCK_NB = 4;
$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();
require "ioctl.ph"; # probably /usr/local/lib/perl/ioctl.phfirst 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 manipulating the values of structures used by ioctl(). The following example sets the erase character to DEL.
require 'ioctl.ph';
$sgttyb_t = "ccccs"; # 4 chars and a short
if (ioctl(STDIN,$TIOCGETP,$sgttyb)) {
@ary = unpack($sgttyb_t,$sgttyb);
$ary[2] = 127;
$sgttyb = pack($sgttyb_t,@ary);
ioctl(STDIN,$TIOCSETP,$sgttyb)
|| die "Can't ioctl: $!";
}
The return value of ioctl (and fcntl) is as follows:
if OS returns:\h'|3i'perl returns: -1\h'|3i' undefined value 0\h'|3i' string "0 but true" anything else\h'|3i' that numberThus 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(...)) || ($retval = -1); printf "System returned %d\n", $retval;
$article = 100;
open article || die "Can't find article $article: $!\n";
while (<article>) {...
open(LOG, '>>/usr/spool/news/twitlog');
# (log is reserved)
open(article, "caesar <$article |");
# decrypt article
open(extract, "|sort >/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
and
STDERR:
#!/usr/bin/perl open(SAVEOUT, ">&STDOUT"); open(SAVEERR, ">&STDERR"); open(STDOUT, ">foo.out") || die "Can't redirect stdout"; open(STDERR, ">&STDOUT") || die "Can't dup stdout"; select(STDERR); $| = 1; # make unbuffered select(STDOUT); $| = 1; # make unbuffered print STDOUT "stdout 1\n"; # this works for print STDERR "stderr 1\n"; # subprocesses too close(STDOUT); close(STDERR); open(STDOUT, ">&SAVEOUT"); open(STDERR, ">&SAVEERR"); print STDOUT "stdout 2\n"; print STDERR "stderr 2\n";If you open a pipe on the command "-", i.e. either "|-" or "-|", then there is an implicit fork done, and the return value of open is the pid of the child within the parent process, and 0 within the child process. (Use defined($pid) to determine if the open was successful.) The filehandle behaves normally for the parent, but i/o to that filehandle is piped from/to the STDOUT/ STDIN of the child process. In the child process the filehandle isn't opened--i/o happens from/to the new STDOUT or STDIN. Typically this is used like the normal piped open when you want to exercise more control over just how the pipe command gets executed, such as when 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, "|tr '[a-z]' '[A-Z]'"); open(FOO, "|-") || exec 'tr', '[a-z]', '[A-Z]'; open(FOO, "cat -n '$file'|"); open(FOO, "-|") || 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 operation which may do a fork, unflushed buffers remain unflushed in both processes, which means you may need to set $| to avoid duplicate output.
The filename that is passed to open will have leading and trailing whitespace deleted. In order to open a file with arbitrary weird characters in it, it's necessary to protect any leading and trailing whitespace thusly:
$file =~ s#^(\s)#./$1#;
open(FOO, "< $file\0");
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); $| = 1; select($oldfh);
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 writing 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 format is the name of the filehandle with "_TOP" appended, but it may be dynamicallly set to the format of your choice by assigning the name to the $^ variable while the filehandle is selected. 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.
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 followed by the optional letter 'i', the matching is done in a case-insensitive manner. PATTERN may contain references to scalar variables, which will be interpolated (and the pattern recompiled) every time the pattern search is evaluated. (Note that $) and $| 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 recompilations, and is useful when the value you are interpolating won't change over the life of the script. If the PATTERN evaluates to a null string, the most recent successful regular expression is used instead.
If used in a context that requires an array value, a pattern match returns an array consisting of the subexpressions matched by the parentheses in the 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 $Etc.
The conditional is true if any variables were assigned, i.e. if the pattern
matched.
The "g" modifier specifies global pattern matching--that is, matching as many times as possible within the string. How it behaves depends on the context. In an array context, it returns a list of all the substrings matched by all the parentheses in the regular expression. If there are no parentheses, it returns a list of all the matched strings, as if there were parentheses around the whole pattern. In a scalar context, it iterates through the string, returning TRUE each time it matches, and FALSE when it eventually runs out of matches. (In other words, it remembers where it left off last time and restarts the search at that point.) It presumes that you have not modified the string since the last match. Modifying the string between matches may result in undefined behavior. (You can actually get away with in-place modifications via substr() that do not change the length of the entire string. In general, however, you should be using s///g for such modifications.) Examples:
# array context
($one,$five,$fifteen) = (\`uptime\` =~ /(\d+\.\d+)/g);
# scalar context
$/ = ""; $* = 1;
while ($paragraph = <>) {
while ($paragraph =~ /[a-z]['")]*[.!?]+['")]*\s/g) {
$sentences++;
}
}
print "$sentences\n";
s/\bgreen\b/mauve/g; # don't change wintergreen
$path =~ s|/usr/bin|/usr/local/bin|;
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
(Note the use of $ instead of \ in the last example. See section
on regular expressions.)
For example, here is a loop which inserts index producing entries before any line containing a certain pattern:
while (<>) {
study;
print ".IX foo\n" if /\bfoo\b/;
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 undefining $/ to input entire files as one record, this can be very fast, often faster than specialized programs 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;
undef $/;
eval $search; # this screams
$/ = "\n"; # put back to normal input delim
foreach $file (sort keys(%seen)) {
print $file, "\n";
}
If the c modifier is specified, the SEARCHLIST character 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 SEARCHLIST, 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/; \h'|3i'# canonicalize to lower case
$cnt = tr/*/*/; \h'|3i'# count the stars in $_
$cnt = tr/0-9//; \h'|3i'# count the digits in $_
tr/a-zA-Z//s; \h'|3i'# bookkeeper -> bokeper
($HOST = $host) =~ tr/a-z/A-Z/;
y/a-zA-Z/ /cs; \h'|3i'# change non-alphas to single space
tr/\200-\377/\0-\177/;\h'|3i'# delete 8th bit
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 message will cause it to make better sense when the string "at foo line 123" is appended. Suppose you are running script "canasta".
die "/etc/games is no good"; die "/etc/games is no good, stopped"; produce, respectively /etc/games is no good at canasta line 123. /etc/games is no good, stopped at canasta line 123.See also exit.
exec '/bin/echo', 'Your arguments are: ', @ARGV; exec "sort $outfile | uniq";
If you don't really want to execute the first argument, but want to lie to the program you are executing about its own name, you can specify the program you actually want to run 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
$ans = <STDIN>; exit 0 if $ans =~ /^[Xx]/;See also die. If EXPR is omitted, exits with 0 status.
$cnt = kill 1, $child1, $child2; kill 9, @goners;If the signal is negative, kills process groups instead of processes. (On System V, a negative process number will also kill process groups, but that's not portable.) You may use a signal name in quotes.
require 'syscall.ph'; # may need to run h2ph syscall(&SYS_write, fileno(STDOUT), "hi there\n", 9);
($user,$system,$cuser,$csystem) = times;
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 supporting either the waitpid (2) or wait4 (2) system calls. However, waiting for a particular pid with FLAGS of 0 is implemented everywhere. (Perl emulates the system call by remembering the status values of processes that have exited but have not been harvested by the Perl script yet.)
# An internet sockaddr $sockaddr = 'S n a4 x8'; $hersockaddr = getpeername(S); ($family, $port, $heraddr) = unpack($sockaddr,$hersockaddr);
# An internet sockaddr $sockaddr = 'S n a4 x8'; $mysockaddr = getsockname(S); ($family, $port, $myaddr) = unpack($sockaddr,$mysockaddr);
$semop = pack("sss", $semnum, -1, 0);
die "Semaphore trouble: $!\n" unless semop($semid, $semop);
To signal the semaphore, replace "-1" with "1".
($package,$filename,$line) = caller;With EXPR, returns some extra information that the debugger uses to print a stack trace. The value of EXPR indicates how many call frames to go back before the current one.
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.
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 $ARGV[0] eq '-d';
QUICKSTART:
do Getopt('f');
sub RANGEVAL {
local($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 ($base12) {
# (NOTE: not claiming this is efficient!)
local(%digits) = (%digits,'t',10,'e',11);
do parse_num();
}
Note that local() is a run-time command, and so gets executed every time
through a loop, using up more stack storage each time until it's all
released at once when the loop is exited.
sub require {
local($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.
The file must return true as the last statement to indicate successful
execution of any initialization code, so it's customary to end
such a file with "1;" unless you're sure it'll return true otherwise.
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.)
undef $foo;
undef $bar{'blurfl'};
undef @ary;
undef %assoc;
undef &mysub;
return (wantarray ? () : undef) if $they_blew_it;
return wantarray ? () : undef;
($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...(If the entry doesn't exist you get a null list.)
Within a scalar context, you get the name, unless the function was a lookup by name, in which case you get the other thing, whatever it is. (If the entry doesn't exist you get the undefined value.) For example:
$uid = getpwnam $name = getpwuid $name = getpwent $gid = getgrnam $name = getgrgid $name = getgrent etc.The $members value returned by getgr... is a space separated list of the login names of the members of the group.
For the gethost... functions, if the h_errno variable is supported in C, it will be returned to you via $? if the function call fails. The @addrs value returned by a successful call is a list of the raw addresses returned by the corresponding system library call. In the Internet domain, each address is four bytes long and you can unpack it by saying something like:
($a,$b,$c,$d) = unpack('C4',$addr[0]);
nonassoc print printf exec system sort reverse chmod chown kill unlink utime die return left , right = += -= *= etc. right ?: nonassoc .. left || left && left | ^ 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 + - . left * / % x left =~ !~ right ! ~ and unary minus right ** nonassoc ++ -- leftAs 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 || die; # (chdir $foo) || die chdir($foo) || die; # (chdir $foo) || die chdir ($foo) || die; # (chdir $foo) || die chdir +($foo) || die; # (chdir $foo) || diebut, because * is higher precedence than ||:
chdir $foo * 20; # chdir ($foo * 20) chdir($foo) * 20; # (chdir $foo) * 20 chdir ($foo) * 20; # (chdir $foo) * 20 chdir +($foo) * 20; # chdir ($foo * 20) rand 10 * 20; # rand (10 * 20) rand(10) * 20; # (rand 10) * 20 rand (10) * 20; # (rand 10) * 20 rand +(10) * 20; # rand (10 * 20)In the absence of parentheses, the precedence of list operators such as print, sort or chmod is either very high or very low depending on whether you look at the left side of operator or the right side of it. For example, in
@ary = (1, 3, sort 4, 2); print @ary; # prints 1324the commas on the right of the sort are evaluated before the sort, but the commas on the left are evaluated after. In other words, list operators tend to gobble up all the arguments that follow them, and then act like a simple term with regard to the preceding expression. Note that you have to be careful with parens:
# These evaluate exit before doing the print: print($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 ($foo), exit; # Or even this. Also note that print ($foo & 255) + 1, "\n";probably doesn't do what you expect at first glance.
sub NAME BLOCK
Any arguments passed to the routine come in as array @_, that is ($_[0], $_[1], ...). The array @_ is a local array, but its values are references to the actual scalar parameters. The return value of the subroutine is the value of the last expression evaluated, and can be either an array value or a scalar value. Alternately, a return statement may be used to specify the returned value and exit the subroutine. To create local variables see the local operator.
A subroutine is called using the do operator or the & operator.
Example:
sub MAX {
local($max) = pop(@_);
foreach $foo (@_) {
$max = $foo if $max < $foo;
}
$max;
}
...
$bestday = &MAX($mon,$tue,$wed,$thu,$fri);
Example:
# get a line, combining continuation lines
# 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
Example:
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 elements 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.
$+ 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)(.|\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 ? modifier to {0,1}. There is no limit to the size of n or m, but large numbers will chew up more memory.
You will note that all backslashed metacharacters in perl are alphanumeric, such as \b, \w, \n. Unlike some other regular expression languages, there are no backslashed symbols that aren't alphanumeric. So anything that looks like \\, \(, \), \<, \>, \{, or \} is always interpreted as a literal character, not a metacharacter. This makes it simple 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;
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:
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 | 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.
Picture fields that begin with ^ rather than @ are treated specially. The value supplied must be a scalar variable name which contains a text string. Perl puts as much text 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 STDOUT_TOP =
Passwd File
Name Login Office Uid Gid Home
------------------------------------------------------------------
.
format STDOUT =
@<<<<<<<<<<<<<<<<<< @||||||| @<<<<<<@>>>> @>>>> @<<<<<<<<<<<<<<<<<
$name, $login, $office,$uid,$gid, $home
.
# a report from a bug report form
format STDOUT_TOP =
Bug Reports
@<<<<<<<<<<<<<<<<<<<<<<< @||| @>>>>>>>>>>>>>>>>>>>>>>>
$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 $- (lines left on the page) yourself.
If you are printing lots of fields that are usually blank, you should consider using the reset operator between records. Not only is it more efficient, but it can prevent the bug of adding another field and forgetting to zero it.
($them,$port) = @ARGV;
$port = 2345 unless $port;
$them = 'localhost' unless $them;
$SIG{'INT'} = 'dokill';
sub dokill { kill 9,$child if $child; }
require 'sys/socket.ph';
$sockaddr = 'S n a4 x8';
chop($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) || die "socket: $!";
bind(S, $this) || die "bind: $!";
connect(S, $that) || die "connect: $!";
select(S); $| = 1; select(stdout);
if ($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); $| = 1; select(stdout);
socket(S, &PF_INET, &SOCK_STREAM, $proto) || die "socket: $!";
bind(S, $this) || die "bind: $!";
listen(S, 5) || die "connect: $!";
select(S); $| = 1; select(stdout);
for (;;) {
print "Listening again\n";
($addr = accept(NS,S)) || die $!;
print "accept ok\n";
($af,$port,$inetaddr) = unpack($sockaddr,$addr);
@inetaddr = unpack('C4',$inetaddr);
print "$af $port @inetaddr\n";
while (<NS>) {
print;
print NS;
}
}
while (<>) {... # only equivalent in while!
while ($_ = <>) {...
/^Subject:/
$_ =~ /^Subject:/
y/a-z/A-Z/
$_ =~ y/a-z/A-Z/
chop
chop($_)
(Mnemonic: underline is understood in certain operations.)
$_ = 'abcdefghi'; /def/; print "$\`:$&:$'\n"; # prints abc:def:ghi
/Version: (.*)|Revision: (.*)/ && ($rev = $+);
(Mnemonic: be positive and forward looking.)
# see if getc is available
($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?)
$foo{$a,$b,$c}
it really means
$foo{join($;, $a, $b, $c)}
But don't put
@foo{$a,$b,$c} # a slice--note the @
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.)
$< = $>; # set real uid to the effective uid ($<,$>) = ($>,$<); # swap real and effective uid(Mnemonic: it's the uid you went TO, if you're running setuid.) Note: $< and $> can only be swapped on machines supporting setreuid().
Note: $<, $>, $( and $) can only be set on machines that support the corresponding set[re][ug]id() routine. $( and $) can only be swapped on machines supporting setregid().
sub handler { # 1st argument is signal name
local($sig) = @_;
print "Caught a SIG$sig--shutting down\n";
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.
Only identifiers starting with letters are stored in the packages symbol table. All other symbols are kept in package "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 $SIG{}, however, assume the signal handler specified is in the main package. Qualify the signal handler name if you wish to have a signal handler in a package.) For an example, examine perldb.pl in the perl library. It initially switches to the DB package so that the debugger doesn't interfere with variables in the script you are trying to debug. At various points, however, it temporarily switches back to the main package to evaluate various expressions in the context of the main package.
The symbol table for a package happens to be stored in the associative array of that name prepended with an underscore. The value in each entry of the associative array is what you are referring to when you use the *name notation. In fact, the following have the same effect (in package main, anyway), though the first is more efficient because it does the symbol table lookups at compile time:
local(*foo) = *bar;
local($_main{'foo'}) = $_main{'bar'};
You can use this to print out all the variables in a package, for instance.
Here is dumpvar.pl from the perl library:
package dumpvar;
sub main'dumpvar {
($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".
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 systems 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.
line:
for (;;) {
statements;
last line if $foo;
next line if /^#/;
statements;
}
<dt>d line
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 (at|in)/b/';
$DB'alias{'.'} =
's/^\./p "\$DB\'sub(\$DB\'line):\t",\$DB\'line[\$DB\'line]/';
In an unpatched 4.2 or 4.3bsd kernel, setuid scripts are intrinsically insecure, but this kernel feature can be disabled. If it is, perl can emulate the setuid and setgid mechanism when it notices the otherwise useless setuid/gid bits on perl scripts. If the kernel feature isn't disabled, perl will complain loudly that your setuid script is insecure. You'll need to either disable the kernel setuid 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 $foo|"); # Not OK, but...
open(FOO,"-|") || 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 system call or exec, but only by explicitly doing something like the last example above. You can also bypass the tainting mechanism by referencing subpatterns--\c perl presumes that 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 taintedThis is fairly secure since \w+ doesn't match shell metacharacters. 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 judicious 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.
require 'perldb.pl'
Apart from these, perl uses no other 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 '';
Setuid scripts have additional constraints that can produce error messages such as "Insecure dependency". See the section on setuid scripts.
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 $;
Cerebral C programmers should take note of the following:
Seasoned sed programmers should take note of the following:
Sharp shell programmers should take note of the following:
On page 5, the examples which read
eval "/usr/bin/perlshould read
eval "exec /usr/bin/perl
On page 195, the equivalent to the System V sum program only works for very small files. To do larger files, use
undef $/;
$checksum = unpack("%32C*",<>) % 32767;
The descriptions of alarm and sleep refer to signal SIGALARM. These should refer to SIGALRM.
The -0 switch to set the initial value of $/ was added to Perl after the book went to press.
The -l switch now does automatic line ending processing.
The qx// construct is now a synonym for backticks.
$0 may now be assigned to set the argument displayed by ps (1).
The new @###.## format was omitted accidentally from the description on formats.
It wasn't known at press time that s///ee caused multiple evaluations of the replacement expression. This is to be construed as a feature.
(LIST) x $count now does array replication.
There is now no limit on the number of parentheses in a regular expression.
In double-quote context, more escapes are supported: \e, \a, \x1b, \c[, \l, \L, \u, \U, \E. The latter five control up/lower case translation.
The $/ variable may now be set to a multi-character delimiter.
There is now a g modifier on ordinary pattern matching that causes it to iterate through a string finding multiple matches.
All of the $^X variables are new except for $^T.
The default top-of-form format for FILEHANDLE is now FILEHANDLE_TOP rather than top.
The eval {} and sort {} constructs were added in version 4.018.
The v and V (little-endian) template options for pack and unpack were added in 4.019.
Perl is at the mercy of your machine's definitions of various 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. (This doesn't apply to sysread() and syswrite().)
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, and no component of your PATH may be longer than 255 if you use -S. A regular expression may not compile to more than 32767 bytes internally.
Perl actually stands for Pathologically Eclectic Rubbish Lister, but don't tell anyone I said that.