path: root/perl.man.2

''' Beginning of part 2
''' $Header: perl.man.2,v 2.0.1.1 88/06/28 16:31:49 root Exp $
'''
''' $Log:	perl.man.2,v $
''' Revision 2.0.1.1  88/06/28  16:31:49  root
''' patch1: fixed some quotes
''' patch1: clarified semantics of study
''' patch1: added example of y with short second string
''' patch1: added example of unlink with <*>
''' 
''' Revision 2.0  88/06/05  00:09:30  root
''' Baseline version 2.0.
''' 
'''
.Ip "goto LABEL" 8 6
Finds the statement labeled with LABEL and resumes execution there.
Currently you may only go to statements in the main body of the program
that are not nested inside a do {} construct.
This statement is not implemented very efficiently, and is here only to make
the sed-to-perl translator easier.
Use at your own risk.
.Ip "hex(EXPR)" 8 2
Returns the decimal value of EXPR interpreted as an hex string.
(To interpret strings that might start with 0 or 0x see oct().)
.Ip "index(STR,SUBSTR)" 8 4
Returns the position of SUBSTR in STR, based at 0, or whatever you've
set the $[ variable to.
If the substring is not found, returns one less than the base, ordinarily -1.
.Ip "int(EXPR)" 8 3
Returns the integer portion of EXPR.
.Ip "join(EXPR,LIST)" 8 8
.Ip "join(EXPR,ARRAY)" 8
Joins the separate strings of LIST or ARRAY into a single string with fields
separated by the value of EXPR, and returns the string.
Example:
.nf
    
    $_ = join(\|':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);

.fi
See
.IR split .
.Ip "keys(ASSOC_ARRAY)" 8 6
Returns a normal array consisting of all the keys of the named associative
array.
The keys are returned in an apparently random order, but it is the same order
as either the values() or each() function produces (given that the associative array
has not been modified).
Here is yet another way to print your environment:
.nf

.ne 5
	@keys = keys(ENV);
	@values = values(ENV);
	while ($#keys >= 0) {
		print pop(keys),'=',pop(values),"\en";
	}

or how about sorted by key:

.ne 3
	foreach $key (sort keys(ENV)) {
		print $key,'=',$ENV{$key},"\en";
	}

.fi
.Ip "kill LIST" 8 2
Sends a signal to a list of processes.
The first element of the list must be the (numerical) signal to send.
Returns the number of processes successfully signaled.
.nf

	$cnt = kill 1,$child1,$child2;
	kill 9,@goners;

.fi
If the signal is negative, kills process groups instead of processes.
(On System V, a negative \fIprocess\fR number will also kill process groups,
but that's not portable.)
.Ip "last LABEL" 8 8
.Ip "last" 8
The
.I last
command is like the
.I break
statement in C (as used in loops); it immediately exits the loop in question.
If the LABEL is omitted, the command refers to the innermost enclosing loop.
The
.I continue
block, if any, is not executed:
.nf

.ne 4
	line: while (<stdin>) {
		last line if /\|^$/;	# exit when done with header
		.\|.\|.
	}

.fi
.Ip "length(EXPR)" 8 2
Returns the length in characters of the value of EXPR.
.Ip "link(OLDFILE,NEWFILE)" 8 2
Creates a new filename linked to the old filename.
Returns 1 for success, 0 otherwise.
.Ip "local(LIST)" 8 4
Declares the listed (scalar) variables to be local to the enclosing block,
subroutine or eval.
(The \*(L"do 'filename';\*(R" operator also counts as an eval.)
This operator works by saving the current values of those variables in LIST
on a hidden stack and restoring them upon exiting the block, subroutine or eval.
The LIST may be assigned to if desired, which allows you to initialize
your local variables.
Commonly this is used to name the parameters to a subroutine.
Examples:
.nf

.ne 13
	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;
	}

.fi
.Ip "localtime(EXPR)" 8 4
Converts a time as returned by the time function to a 9-element array with
the time analyzed for the local timezone.
Typically used as follows:
.nf

.ne 3
    ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst)
       = localtime(time);

.fi
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.
.Ip "log(EXPR)" 8 3
Returns logarithm (base e) of EXPR.
.Ip "next LABEL" 8 8
.Ip "next" 8
The
.I next
command is like the
.I continue
statement in C; it starts the next iteration of the loop:
.nf

.ne 4
	line: while (<stdin>) {
		next line if /\|^#/;	# discard comments
		.\|.\|.
	}

.fi
Note that if there were a
.I 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.
.Ip "oct(EXPR)" 8 2
Returns the decimal value of EXPR interpreted as an octal string.
(If EXPR happens to start off with 0x, interprets it as a hex string instead.)
The following will handle decimal, octal and hex in the standard notation:
.nf

	$val = oct($val) if $val =~ /^0/;

.fi
.Ip "open(FILEHANDLE,EXPR)" 8 8
.Ip "open(FILEHANDLE)" 8
.Ip "open FILEHANDLE" 8
Opens the file whose filename is given by EXPR, and associates it with
FILEHANDLE.
If FILEHANDLE is an expression, its value is used as the name of the
real filehandle wanted.
If EXPR is omitted, the scalar variable of the same name as the FILEHANDLE
contains the filename.
If the filename begins with \*(L">\*(R", the file is opened for output.
If the filename begins with \*(L">>\*(R", the file is opened for appending.
If the filename begins with \*(L"|\*(R", the filename is interpreted
as a command to which output is to be piped, and if the filename ends
with a \*(L"|\*(R", the filename is interpreted as command which pipes
input to us.
(You may not have a command that pipes both in and out.)
Opening '\-' opens stdin and opening '>\-' opens stdout.
Open returns 1 upon success, '' otherwise.
Examples:
.nf
    
.ne 3
	$article = 100;
	open article || die "Can't find article $article";
	while (<article>) {\|.\|.\|.

	open(LOG, '>>/usr/spool/news/twitlog'\|);	# (log is reserved)

	open(article, "caeser <$article |"\|);		# decrypt article

	open(extract, "|sort >/tmp/Tmp$$"\|);		# $$ is our process#

.ne 7
	# 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 $filename\en";
			last;		# note block inside sub
		}
		while (<$input>) {		# note the use of indirection
			if (/^#include "(.*)"/) {
				do process($1,$input);
				next;
			}
			.\|.\|.		# whatever
		}
	}}

.fi
You may also, in the Bourne shell tradition, specify an EXPR beginning
with \*(L">&\*(R", 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.
Here is a script that saves, redirects, and restores stdout and stdin:
.nf

.ne 21
	#!/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\en";	# this works for
	print stderr "stderr 1\en";	# subprocesses too

	close(stdout);
	close(stderr);

	open(stdout,">&saveout");
	open(stderr,">&saveerr");

	print stdout "stdout 2\en";
	print stderr "stderr 2\en";

.fi
If you open a pipe on the command \*(L"-\*(R", i.e. either \*(L"|-\*(R" or \*(L"-|\*(R",
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.
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 equivalent:
.nf

.ne 5
	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;

.fi
Explicitly closing the filehandle causes the parent process to wait for the
child to finish, and returns the status value in $?.
.Ip "ord(EXPR)" 8 3
Returns the ascii value of the first character of EXPR.
.Ip "pop ARRAY" 8 6
.Ip "pop(ARRAY)" 8
Pops and returns the last value of the array, shortening the array by 1.
Has the same effect as
.nf

	$tmp = $ARRAY[$#ARRAY]; $#ARRAY--;

.fi
.Ip "print FILEHANDLE LIST" 8 9
.Ip "print LIST" 8
.Ip "print" 8
Prints a string or a comma-separated list of strings.
FILEHANDLE may be a scalar variable name, in which case the variable contains
the name of the filehandle, thus introducing one level of indirection.
If FILEHANDLE is omitted, prints by default to standard output (or to the
last selected output channel\*(--see select()).
If LIST is also omitted, prints $_ to stdout.
To set the default output channel to something other than stdout use the select operation.
.Ip "printf FILEHANDLE LIST" 8 9
.Ip "printf LIST" 8
Equivalent to a \*(L"print FILEHANDLE sprintf(LIST)\*(R".
.Ip "push(ARRAY,LIST)" 8 7
Treats ARRAY (@ is optional) as a stack, and pushes the values of LIST
onto the end of ARRAY.
The length of ARRAY increases by the length of LIST.
Has the same effect as
.nf

    for $value (LIST) {
	    $ARRAY[$#ARRAY+1] = $value;
    }

.fi
but is more efficient.
.Ip "redo LABEL" 8 8
.Ip "redo" 8
The
.I redo
command restarts the loop block without evaluating the conditional again.
The
.I continue
block, if any, is not executed.
If the LABEL is omitted, the command refers to the innermost enclosing loop.
This command is normally used by programs that want to lie to themselves
about what was just input:
.nf

.ne 16
	# 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;
	}

.fi
.Ip "rename(OLDNAME,NEWNAME)" 8 2
Changes the name of a file.
Returns 1 for success, 0 otherwise.
.Ip "reset EXPR" 8 3
Generally used in a
.I continue
block at the end of a loop to clear variables and reset ?? searches
so that they work again.
The expression is interpreted as a list of single characters (hyphens allowed
for ranges).
All variables and arrays beginning with one of those letters are reset to
their pristine state.
If the expression is omitted, one-match searches (?pattern?) are reset to
match again.
Always returns 1.
Examples:
.nf

.ne 3
    reset 'X';	\h'|2i'# reset all X variables
    reset 'a-z';\h'|2i'# reset lower case variables
    reset;	\h'|2i'# just reset ?? searches

.fi
Note: resetting "A-Z" is not recommended since you'll wipe out your ARGV and ENV
arrays.
.Ip "s/PATTERN/REPLACEMENT/gi" 8 3
Searches a string for a pattern, and if found, replaces that pattern with the
replacement text and returns the number of substitutions made.
Otherwise it returns false (0).
The \*(L"g\*(R" is optional, and if present, indicates that all occurences
of the pattern are to be replaced.
The \*(L"i\*(R" is also optional, and if present, indicates that matching
is to be done in a case-insensitive manner.
Any delimiter may replace the slashes; if single quotes are used, no
interpretation is done on the replacement string.
If no string is specified via the =~ or !~ operator,
the $_ string is searched and modified.
(The string specified with =~ must be a scalar variable, an array element,
or an assignment to one of those, i.e. an lvalue.)
If the pattern contains a $ that looks like a variable rather than an
end-of-string test, the variable will be interpolated into the pattern at
run-time.
See also the section on regular expressions.
Examples:
.nf

    s/\|\e\|bgreen\e\|b/mauve/g;		# don't change wintergreen

    $path \|=~ \|s|\|/usr/bin|\|/usr/local/bin|;

    s/Login: $foo/Login: $bar/; # run-time pattern

    s/\|([^ \|]*\|) *\|([^ \|]*\|)\|/\|$2 $1/;	# reverse 1st two fields

    ($foo = $bar) =~ s/bar/foo/;

.fi
(Note the use of $ instead of \|\e\| in the last example.  See section
on regular expressions.)
.Ip "seek(FILEHANDLE,POSITION,WHENCE)" 8 3
Randomly positions the file pointer for FILEHANDLE, just like the fseek()
call of stdio.
FILEHANDLE may be an expression whose value gives the name of the filehandle.
Returns 1 upon success, 0 otherwise.
.Ip "select(FILEHANDLE)" 8 3
Sets the current default filehandle for output.
This has two effects: first, a
.I write
or a
.I print
without a filehandle will default to this FILEHANDLE.
Second, references to variables related to output will refer to this output
channel.
For example, if you have to set the top of form format for more than
one output channel, you might do the following:
.nf

.ne 4
    select(report1);
    $^ = 'report1_top';
    select(report2);
    $^ = 'report2_top';

.fi
Select happens to return TRUE if the file is currently open and FALSE otherwise,
but this has no effect on its operation.
FILEHANDLE may be an expression whose value gives the name of the actual filehandle.
.Ip "shift(ARRAY)" 8 6
.Ip "shift ARRAY" 8
.Ip "shift" 8
Shifts the first value of the array off and returns it,
shortening the array by 1 and moving everything down.
If ARRAY is omitted, shifts the ARGV array.
See also unshift(), push() and pop().
Shift() and unshift() do the same thing to the left end of an array that push()
and pop() do to the right end.
.Ip "sleep EXPR" 8 6
.Ip "sleep" 8
Causes the script to sleep for EXPR seconds, or forever if no EXPR.
May be interrupted by sending the process a SIGALARM.
Returns the number of seconds actually slept.
.Ip "sort SUBROUTINE LIST" 8 7
.Ip "sort LIST" 8
Sorts the LIST and returns the sorted array value.
Nonexistent values of arrays are stripped out.
If SUBROUTINE is omitted, sorts in standard string comparison order.
If SUBROUTINE is specified, gives the name of a subroutine that returns
a -1, 0, or 1, depending on how the elements of the array are to be ordered.
In the interests of efficiency the normal calling code for subroutines
is bypassed, with the following effects: the subroutine may not be a recursive
subroutine, and the two elements to be compared are passed into the subroutine
not via @_ but as $a and $b (see example below).
SUBROUTINE may be a scalar variable name, in which case the value provides
the name of the subroutine to use.
Examples:
.nf

.ne 4
	sub byage {
	    $age{$a} < $age{$b} ? -1 : $age{$a} > $age{$b} ? 1 : 0;
	}
	@sortedclass = sort byage @class;

.ne 9
	sub reverse { $a lt $b ? 1 : $a gt $b ? -1 : 0; }
	@harry = ('dog','cat','x','Cain','Abel');
	@george = ('gone','chased','yz','Punished','Axed');
	print sort @harry;
		# prints AbelCaincatdogx
	print sort reverse @harry;
		# prints xdogcatCainAbel
	print sort @george,'to',@harry;
		# prints AbelAxedCainPunishedcatchaseddoggonetoxyz

.fi
.Ip "split(/PATTERN/,EXPR)" 8 8
.Ip "split(/PATTERN/)" 8
.Ip "split" 8
Splits a string into an array of strings, and returns it.
If EXPR is omitted, splits the $_ string.
If PATTERN is also omitted, splits on whitespace (/[\ \et\en]+/).
Anything matching PATTERN is taken to be a delimiter separating the fields.
(Note that the delimiter may be longer than one character.)
Trailing null fields are stripped, which potential users of pop() would
do well to remember.
A pattern matching the null string (not to be confused with a null pattern)
will split the value of EXPR into separate characters at each point it
matches that way.
For example:
.nf

	print join(':',split(/ */,'hi there'));

.fi
produces the output 'h:i:t:h:e:r:e'.

The pattern /PATTERN/ may be replaced with an expression to specify patterns
that vary at runtime.
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.
.sp
Example:
.nf

.ne 5
	open(passwd, '/etc/passwd');
	while (<passwd>) {
.ie t \{\
		($login, $passwd, $uid, $gid, $gcos, $home, $shell) = split(\|/\|:\|/\|);
'br\}
.el \{\
		($login, $passwd, $uid, $gid, $gcos, $home, $shell)
			= split(\|/\|:\|/\|);
'br\}
		.\|.\|.
	}

.fi
(Note that $shell above will still have a newline on it.  See chop().)
See also
.IR join .
.Ip "sprintf(FORMAT,LIST)" 8 4
Returns a string formatted by the usual printf conventions.
The * character is not supported.
.Ip "sqrt(EXPR)" 8 3
Return the square root of EXPR.
.Ip "stat(FILEHANDLE)" 8 6
.Ip "stat(EXPR)" 8
Returns a 13-element array giving the statistics for a file, either the file
opened via FILEHANDLE, or named by EXPR.
Typically used as follows:
.nf

.ne 3
    ($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,
       $atime,$mtime,$ctime,$blksize,$blocks)
           = stat($filename);

.fi
.Ip "study(SCALAR)" 8 6
.Ip "study"
Takes extra time to study SCALAR ($_ if unspecified) in anticipation of
doing many pattern matches on the string before it is next modified.
This may or may not save time, depending on the nature and number of patterns
you are searching on, and on the distribution of character frequencies in
the string to be searched\*(--you probably want to compare runtimes with and
without it to see which runs faster.
Those loops which scan for many short constant strings (including the constant
parts of more complex patterns) will benefit most.
(The way study works is this: a linked list of every character in the string
to be searched is made, so we know, for example, where all the `k' characters
are.
From each search string, the rarest character is selected, based on some
static frequency tables constructed from some C programs and English text.
Only those places that contain this \*(L"rarest\*(R" character are examined.)
.Sp
For example, here is a loop which inserts index producing entries before an line
containing a certain pattern:
.nf

.ne 8
	while (<>) {
		study;
		print ".IX foo\en" if /\ebfoo\eb/;
		print ".IX bar\en" if /\ebbar\eb/;
		print ".IX blurfl\en" if /\ebblurfl\eb/;
		.\|.\|.
		print;
	}

.fi
In searching for /\ebfoo\eb/, 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.
.Sp
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 setting $/ 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:
.nf

.ne 12
	$search = 'while (<>) { study;';
	foreach $word (@words) {
	    $search .= "\e++$seen{\e$ARGV} if /\eb$word\eb/;\en";
	}
	$search .= "}";
	@ARGV = @files;
	$/ = "\e177";		# something that doesn't occur
	eval $search;		# this screams
	$/ = "\en";		# put back to normal input delim
	foreach $file (sort keys(seen)) {
	    print $file,"\en";
	}

.fi
.Ip "substr(EXPR,OFFSET,LEN)" 8 2
Extracts a substring out of EXPR and returns it.
First character is at offset 0, or whatever you've set $[ to.
.Ip "system LIST" 8 6
Does exactly the same thing as \*(L"exec LIST\*(R" except that a fork
is done first, and the parent process waits for the child process to complete.
Note that argument processing varies depending on the number of arguments.
The return value is the exit status of the program as returned by the wait()
call.
To get the actual exit value divide by 256.
See also exec.
.Ip "symlink(OLDFILE,NEWFILE)" 8 2
Creates a new filename symbolically linked to the old filename.
Returns 1 for success, 0 otherwise.
On systems that don't support symbolic links, produces a fatal error at
run time.
To check for that, use eval:
.nf

	$symlink_exists = (eval 'symlink("","");', $@ eq '');

.fi
.Ip "tell(FILEHANDLE)" 8 6
.Ip "tell" 8
Returns the current file position for FILEHANDLE.
FILEHANDLE may be an expression whose value gives the name of the actual
filehandle.
If FILEHANDLE is omitted, assumes the file last read.
.Ip "time" 8 4
Returns the number of seconds since January 1, 1970.
Suitable for feeding to gmtime() and localtime().
.Ip "times" 8 4
Returns a four-element array giving the user and system times, in seconds, for this
process and the children of this process.
.sp
    ($user,$system,$cuser,$csystem) = times;
.sp
.Ip "tr/SEARCHLIST/REPLACEMENTLIST/" 8 5
.Ip "y/SEARCHLIST/REPLACEMENTLIST/" 8
Translates all occurences of the characters found in the search list with
the corresponding character in the replacement list.
It returns the number of characters replaced.
If no string is specified via the =~ or !~ operator,
the $_ string is translated.
(The string specified with =~ must be a scalar variable, an array element,
or an assignment to one of those, i.e. an lvalue.)
For
.I sed
devotees,
.I y
is provided as a synonym for
.IR tr .
Examples:
.nf

    $ARGV[1] \|=~ \|y/A-Z/a-z/;	\h'|3i'# canonicalize to lower case

    $cnt = tr/*/*/;		\h'|3i'# count the stars in $_

    ($HOST = $host) =~ tr/a-z/A-Z/;

    y/\e001-@[-_{-\e177/ /;	\h'|3i'# change non-alphas to space

.fi
.Ip "umask(EXPR)" 8 3
Sets the umask for the process and returns the old one.
.Ip "unlink LIST" 8 2
Deletes a list of files.
Returns the number of files successfully deleted.
.nf

.ne 2
	$cnt = unlink 'a','b','c';
	unlink @goners;
	unlink <*.bak>;

.fi
Note: unlink will not delete directories unless you are superuser and the \-U
flag is supplied to perl.
.ne 7
.Ip "unshift(ARRAY,LIST)" 8 4
Does the opposite of a shift.
Or the opposite of a push, depending on how you look at it.
Prepends list to the front of the array, and returns the number of elements
in the new array.
.nf

	unshift(ARGV,'-e') unless $ARGV[0] =~ /^-/;

.fi
.Ip "utime LIST" 8 2
Changes the access and modification times on each file of a list of files.
The first two elements of the list must be the NUMERICAL access and
modification times, in that order.
Returns the number of files successfully changed.
The inode modification time of each file is set to the current time.
Example of a \*(L"touch\*(R" command:
.nf

.ne 3
	#!/usr/bin/perl
	$now = time;
	utime $now,$now,@ARGV;

.fi
.Ip "values(ASSOC_ARRAY)" 8 6
Returns a normal array consisting of all the values of the named associative
array.
The values are returned in an apparently random order, but it is the same order
as either the keys() or each() function produces (given that the associative array
has not been modified).
See also keys() and each().
.Ip "wait" 8 6
Waits for a child process to terminate and returns the pid of the deceased
process.
The status is returned in $?.
.Ip "write(FILEHANDLE)" 8 6
.Ip "write(EXPR)" 8
.Ip "write(\|)" 8
Writes a formatted record (possibly multi-line) to the specified file,
using the format associated with that file.
By default the format for a file is the one having the same name is the
filehandle, but the format for the current output channel (see
.IR select )
may be set explicitly
by assigning the name of the format to the $~ variable.
.sp
Top of form processing is handled automatically:
if there is insufficient room on the current page for the formatted 
record, the page is advanced, 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 \*(L"top\*(R", but it
may be set to the
format of your choice by assigning the name to the $^ variable.
.sp
If FILEHANDLE is unspecified, output goes to the current default output channel,
which starts out as stdout but may be changed by the
.I 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.
.Sh "Precedence"
Perl operators have the following associativity and precedence:
.nf

nonassoc\h'|1i'print printf exec system sort
\h'1.5i'chmod chown kill unlink utime
left\h'|1i',
right\h'|1i'=
right\h'|1i'?:
nonassoc\h'|1i'..
left\h'|1i'||
left\h'|1i'&&
left\h'|1i'| ^
left\h'|1i'&
nonassoc\h'|1i'== != eq ne
nonassoc\h'|1i'< > <= >= lt gt le ge
nonassoc\h'|1i'chdir die exit eval reset sleep
nonassoc\h'|1i'-r -w -x etc.
left\h'|1i'<< >>
left\h'|1i'+ - .
left\h'|1i'* / % x
left\h'|1i'=~ !~ 
right\h'|1i'! ~ and unary minus
nonassoc\h'|1i'++ --
left\h'|1i''('

.fi
Actually, 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 1324

the commas on the right of the sort are evaluated before the sort, but
the commas on the left are evaluated after.
In other words, list operators tend to gobble up all the arguments that
follow them, and then act like a simple term with regard to the preceding
expression.
.Sh "Subroutines"
A subroutine may be declared as follows:
.nf

    sub NAME BLOCK

.fi
.PP
Any arguments passed to the routine come in as array @_,
that is ($_[0], $_[1], .\|.\|.).
The return value of the subroutine is the value of the last expression
evaluated.
To create local variables see the \*(L"local\*(R" operator.
.PP
A subroutine is called using the
.I do
operator.
.nf

.ne 12
Example:

	sub MAX {
		local($max) = pop(@_);
		foreach $foo (@_) {
			$max = $foo \|if \|$max < $foo;
		}
		$max;
	}

	.\|.\|.
	$bestday = do MAX($mon,$tue,$wed,$thu,$fri);

.ne 21
Example:

	# get a line, combining continuation lines
	#  that start with whitespace
	sub get_line {
		$thisline = $lookahead;
		line: while ($lookahead = <stdin>) {
			if ($lookahead \|=~ \|/\|^[ \^\e\|t]\|/\|) {
				$thisline \|.= \|$lookahead;
			}
			else {
				last line;
			}
		}
		$thisline;
	}

	$lookahead = <stdin>;	# get first line
	while ($_ = get_line(\|)) {
		.\|.\|.
	}

.fi
.nf
.ne 6
Use array assignment to local list to name your formal arguments:

	sub maybeset {
		local($key,$value) = @_;
		$foo{$key} = $value unless $foo{$key};
	}

.fi
Subroutines may be called recursively.
.Sh "Regular Expressions"
The patterns used in pattern matching are regular expressions such as
those supplied in the Version 8 regexp routines.
(In fact, the routines are derived from Henry Spencer's freely redistributable
reimplementation of the V8 routines.)
In addition, \ew matches an alphanumeric character (including \*(L"_\*(R") and \eW a nonalphanumeric.
Word boundaries may be matched by \eb, and non-boundaries by \eB.
A whitespace character is matched by \es, non-whitespace by \eS.
A numeric character is matched by \ed, non-numeric by \eD.
You may use \ew, \es and \ed within character classes.
Also, \en, \er, \ef, \et and \eNNN have their normal interpretations.
Within character classes \eb represents backspace rather than a word boundary.
The bracketing construct \|(\ .\|.\|.\ \|) may also be used, in which case \e<digit>
matches the digit'th substring, where digit can range from 1 to 9.
(Outside of patterns, use $ instead of \e in front of the digit.
The scope of $<digit> extends to the end of the enclosing BLOCK, or to
the next pattern match with subexpressions.)
$+ returns whatever the last bracket match matched.
$& returns the entire matched string.
($0 normally returns the same thing, but don't depend on it.)
Alternatives may be separated by |.
Examples:
.nf
    
	s/\|^\|([^ \|]*\|) \|*([^ \|]*\|)\|/\|$2 $1\|/;	# swap first two words

.ne 5
	if (/\|Time: \|(.\|.\|):\|(.\|.\|):\|(.\|.\|)\|/\|) {
		$hours = $1;
		$minutes = $2;
		$seconds = $3;
	}

.fi
By default, the ^ character matches only the beginning of the string, and
.I perl
does certain optimizations with the assumption that the string contains
only one line.
You may, however, wish to treat a string as a multi-line buffer, such that
the ^ will match after any newline within the string.
At the cost of a little more overhead, you can do this by setting the variable
$* to 1.
Setting it back to 0 makes
.I perl
revert to its old behavior.
.PP
To facilitate multi-line substitutions, the . character never matches a newline.
In particular, the following leaves a newline on the $_ string:
.nf

	$_ = <stdin>;
	s/.*(some_string).*/$1/;

If the newline is unwanted, try one of

	s/.*(some_string).*\en/$1/;
	s/.*(some_string)[^\000]*/$1/;
	s/.*(some_string)(.|\en)*/$1/;
	chop; s/.*(some_string).*/$1/;
	/(some_string)/ && ($_ = $1);

.fi
.Sh "Formats"
Output record formats for use with the
.I write
operator may declared as follows:
.nf

.ne 3
    format NAME =
    FORMLIST
    .

.fi
If name is omitted, format \*(L"stdout\*(R" is defined.
FORMLIST consists of a sequence of lines, each of which may be of one of three
types:
.Ip 1. 4
A comment.
.Ip 2. 4
A \*(L"picture\*(R" line giving the format for one output line.
.Ip 3. 4
An argument line supplying values to plug into a picture line.
.PP
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 justfication,
right justification, or centering.
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.
.PP
The values are specified on the following line, in the same order as
the picture fields.
They must currently be either scalar variable names or literals (or
pseudo-literals).
Currently you can separate values with spaces, but commas may be placed
between values to prepare for possible future versions in which full expressions
are allowed as values.
.PP
Picture fields that begin with ^ rather than @ are treated specially.
The value supplied must be a scalar variable name which contains a text
string.
.I 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.
.PP
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.)
The tilde will be translated to a space upon output.
.PP
Examples:
.nf
.lg 0
.cs R 25

.ne 10
# a report on the /etc/passwd file
format top =
\&                        Passwd File
Name                Login    Office   Uid   Gid Home
------------------------------------------------------------------
\&.
format stdout =
@<<<<<<<<<<<<<<<<<< @||||||| @<<<<<<@>>>> @>>>> @<<<<<<<<<<<<<<<<<
$name               $login   $office $uid $gid  $home
\&.

.ne 29
# a report from a bug report form
format 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
\&.

.cs R
.lg
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.
.fi
.PP
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.
.Sh "Predefined Names"
The following names have special meaning to
.IR perl .
I could have used alphabetic symbols for some of these, but I didn't want
to take the chance that someone would say reset \*(L"a-zA-Z\*(R" and wipe them all
out.
You'll just have to suffer along with these silly symbols.
Most of them have reasonable mnemonics, or analogues in one of the shells.
.Ip $_ 8
The default input and pattern-searching space.
The following pairs are equivalent:
.nf

.ne 2
	while (<>) {\|.\|.\|.	# only equivalent in while!
	while ($_ = <>) {\|.\|.\|.

.ne 2
	/\|^Subject:/
	$_ \|=~ \|/\|^Subject:/

.ne 2
	y/a-z/A-Z/
	$_ =~ y/a-z/A-Z/

.ne 2
	chop
	chop($_)

.fi 
(Mnemonic: underline is understood in certain operations.)
.Ip $. 8
The current input line number of the last filehandle that was read.
Readonly.
Remember that only an explicit close on the filehandle resets the line number.
Since <> never does an explicit close, line numbers increase across ARGV files
(but see examples under eof).
(Mnemonic: many programs use . to mean the current line number.)
.Ip $/ 8
The input record separator, newline by default.
Works like awk's RS variable, including treating blank lines as delimiters
if set to the null string.
If set to a value longer than one character, only the first character is used.
(Mnemonic: / is used to delimit line boundaries when quoting poetry.)
.Ip $, 8
The output field separator for the print operator.
Ordinarily the print operator simply prints out the comma separated fields
you specify.
In order to get behavior more like awk, set this variable as you would set
awk's OFS variable to specify what is printed between fields.
(Mnemonic: what is printed when there is a , in your print statement.)
.Ip $\e 8
The output record separator for the print operator.
Ordinarily the print operator simply prints out the comma separated fields
you specify, with no trailing newline or record separator assumed.
In order to get behavior more like awk, set this variable as you would set
awk's ORS variable to specify what is printed at the end of the print.
(Mnemonic: you set $\e instead of adding \en at the end of the print.
Also, it's just like /, but it's what you get \*(L"back\*(R" from perl.)
.Ip $# 8
The output format for printed numbers.
This variable is a half-hearted attempt to emulate awk's OFMT variable.
There are times, however, when awk and perl have differing notions of what
is in fact numeric.
Also, the initial value is %.20g rather than %.6g, so you need to set $#
explicitly to get awk's value.
(Mnemonic: # is the number sign.)
.Ip $% 8
The current page number of the currently selected output channel.
(Mnemonic: % is page number in nroff.)
.Ip $= 8
The current page length (printable lines) of the currently selected output
channel.
Default is 60.
(Mnemonic: = has horizontal lines.)
.Ip $\- 8
The number of lines left on the page of the currently selected output channel.
(Mnemonic: lines_on_page - lines_printed.)
.Ip $~ 8
The name of the current report format for the currently selected output
channel.
(Mnemonic: brother to $^.)
.Ip $^ 8
The name of the current top-of-page format for the currently selected output
channel.
(Mnemonic: points to top of page.)
.Ip $| 8
If set to nonzero, forces a flush after every write or print on the currently
selected output channel.
Default is 0.
Note that stdout will typically be line buffered if output is to the
terminal and block buffered otherwise.
Setting this variable is useful primarily when you are outputting to a pipe,
such as when you are running a perl script under rsh and want to see the
output as it's happening.
(Mnemonic: when you want your pipes to be piping hot.)
.Ip $$ 8
The process number of the
.I perl
running this script.
(Mnemonic: same as shells.)
.Ip $? 8
The status returned by the last backtick (``) command or system operator.
Note that this is the status word returned by the wait() system
call, so the exit value of the subprocess is actually ($? >> 8).
$? & 255 gives which signal, if any, the process died from, and whether
there was a core dump.
(Mnemonic: similar to sh and ksh.)
.Ip $& 8 4
The string matched by the last pattern match.
(Mnemonic: like & in some editors.)
.Ip $+ 8 4
The last bracket matched by the last search pattern.
This is useful if you don't know which of a set of alternative patterns
matched.
For example:
.nf

    /Version: \|(.*\|)|Revision: \|(.*\|)\|/ \|&& \|($rev = $+);

.fi
(Mnemonic: be positive and forward looking.)
.Ip $* 8 2
Set to 1 to do multiline matching within a string, 0 to assume strings contain
a single line.
Default is 0.
(Mnemonic: * matches multiple things.)
.Ip $0 8
Contains the name of the file containing the
.I perl
script being executed.
The value should be copied elsewhere before any pattern matching happens, which
clobbers $0.
(Mnemonic: same as sh and ksh.)
.Ip $<digit> 8
Contains the subpattern from the corresponding set of parentheses in the last
pattern matched, not counting patterns matched in nested blocks that have
been exited already.
(Mnemonic: like \edigit.)
.Ip $[ 8 2
The index of the first element in an array, and of the first character in
a substring.
Default is 0, but you could set it to 1 to make
.I perl
behave more like
.I awk
(or Fortran)
when subscripting and when evaluating the index() and substr() functions.
(Mnemonic: [ begins subscripts.)
.Ip $! 8 2
If used in a numeric context, yields the current value of errno, with all the
usual caveats.
If used in a string context, yields the corresponding system error string.
You can assign to $! in order to set errno
if, for instance, you want $! to return the string for error n, or you want
to set the exit value for the die operator.
(Mnemonic: What just went bang?)
.Ip $@ 8 2
The error message from the last eval command.
If null, the last eval parsed and executed correctly.
(Mnemonic: Where was the syntax error \*(L"at\*(R"?)
.Ip $< 8 2
The real uid of this process.
(Mnemonic: it's the uid you came FROM, if you're running setuid.)
.Ip $> 8 2
The effective uid of this process.
Example:
.nf

	$< = $>;	# set real uid to the effective uid

.fi
(Mnemonic: it's the uid you went TO, if you're running setuid.)
.Ip $( 8 2
The real gid of this process.
If you are on a machine that supports membership in multiple groups
simultaneously, gives a space separated list of groups you are in.
The first number is the one returned by getgid(), and the subsequent ones
by getgroups(), one of which may be the same as the first number.
(Mnemonic: parens are used to GROUP things.
The real gid is the group you LEFT, if you're running setgid.)
.Ip $) 8 2
The effective gid of this process.
If you are on a machine that supports membership in multiple groups
simultaneously, gives a space separated list of groups you are in.
The first number is the one returned by getegid(), and the subsequent ones
by getgroups(), one of which may be the same as the first number.
(Mnemonic: parens are used to GROUP things.
The effective gid is the group that's RIGHT for you, if you're running setgid.)
.Sp
Note: $<, $>, $( and $) can only be set on machines that support the
corresponding set[re][ug]id() routine.
.Ip @ARGV 8 3
The array ARGV contains the command line arguments intended for the script.
Note that $#ARGV is the generally number of arguments minus one, since
$ARGV[0] is the first argument, NOT the command name.
See $0 for the command name.
.Ip @INC 8 3
The array INC contains the list of places to look for perl scripts to be
evaluated by the \*(L"do EXPR\*(R" command.
It initially consists of the arguments to any -I command line switches, followed
by the default perl library, probably \*(L"/usr/local/lib/perl\*(R".
.Ip $ENV{expr} 8 2
The associative array ENV contains your current environment.
Setting a value in ENV changes the environment for child processes.
.Ip $SIG{expr} 8 2
The associative array SIG is used to set signal handlers for various signals.
Example:
.nf

.ne 12
	sub handler {	# 1st argument is signal name
		local($sig) = @_;
		print "Caught a SIG$sig--shutting down\en";
		close(LOG);
		exit(0);
	}

	$SIG{'INT'} = 'handler';
	$SIG{'QUIT'} = 'handler';
	.\|.\|.
	$SIG{'INT'} = 'DEFAULT';	# restore default action
	$SIG{'QUIT'} = 'IGNORE';	# ignore SIGQUIT

.fi
.SH ENVIRONMENT
.I Perl
currently uses no environment variables, except to make them available
to the script being executed, and to child processes.
However, scripts running setuid would do well to execute the following lines
before doing anything else, just to keep people honest:
.nf

.ne 3
    $ENV{'PATH'} = '/bin:/usr/bin';    # or whatever you need
    $ENV{'SHELL'} = '/bin/sh' if $ENV{'SHELL'};
    $ENV{'IFS'} = '' if $ENV{'IFS'};

.fi
.SH AUTHOR
Larry Wall <lwall@jpl-devvax.Jpl.Nasa.Gov>
.SH FILES
/tmp/perl\-eXXXXXX	temporary file for
.B \-e
commands.
.SH SEE ALSO
a2p	awk to perl translator
.br
s2p	sed to perl translator
.br
perldb	interactive perl debugger
.SH DIAGNOSTICS
Compilation errors will tell you the line number of the error, with an
indication of the next token or token type that was to be examined.
(In the case of a script passed to
.I perl
via
.B \-e
switches, each
.B \-e
is counted as one line.)
.SH TRAPS
Accustomed awk users should take special note of the following:
.Ip * 4 2
Semicolons are required after all simple statements in perl.  Newline
is not a statement delimiter.
.Ip * 4 2
Curly brackets are required on ifs and whiles.
.Ip * 4 2
Variables begin with $ or @ in perl.
.Ip * 4 2
Arrays index from 0 unless you set $[.
Likewise string positions in substr() and index().
.Ip * 4 2
You have to decide whether your array has numeric or string indices.
.Ip * 4 2
Associative array values do not spring into existence upon mere reference.
.Ip * 4 2
You have to decide whether you want to use string or numeric comparisons.
.Ip * 4 2
Reading an input line does not split it for you.  You get to split it yourself
to an array.
And split has different arguments.
.Ip * 4 2
The current input line is normally in $_, not $0.
It generally does not have the newline stripped.
($0 is initially the name of the program executed, then the last matched
string.)
.Ip * 4 2
The current filename is $ARGV, not $FILENAME.
NR, RS, ORS, OFS, and OFMT have equivalents with other symbols.
FS doesn't have an equivalent, since you have to be explicit about
split statements.
.Ip * 4 2
$<digit> does not refer to fields--it refers to substrings matched by the last
match pattern.
.Ip * 4 2
The print statement does not add field and record separators unless you set
$, and $\e.
.Ip * 4 2
You must open your files before you print to them.
.Ip * 4 2
The range operator is \*(L"..\*(R", not comma.
(The comma operator works as in C.)
.Ip * 4 2
The match operator is \*(L"=~\*(R", not \*(L"~\*(R".
(\*(L"~\*(R" is the one's complement operator.)
.Ip * 4 2
The concatenation operator is \*(L".\*(R", not the null string.
(Using the null string would render \*(L"/pat/ /pat/\*(R" unparseable,
since the third slash would be interpreted as a division operator\*(--the
tokener is in fact slightly context sensitive for operators like /, ?, and <.
And in fact, . itself can be the beginning of a number.)
.Ip * 4 2
Next, exit, and continue work differently.
.Ip * 4 2
When in doubt, run the awk construct through a2p and see what it gives you.
.PP
Cerebral C programmers should take note of the following:
.Ip * 4 2
Curly brackets are required on ifs and whiles.
.Ip * 4 2
You should use \*(L"elsif\*(R" rather than \*(L"else if\*(R"
.Ip * 4 2
Break and continue become last and next, respectively.
.Ip * 4 2
There's no switch statement.
.Ip * 4 2
Variables begin with $ or @ in perl.
.Ip * 4 2
Printf does not implement *.
.Ip * 4 2
Comments begin with #, not /*.
.Ip * 4 2
You can't take the address of anything.
.Ip * 4 2
ARGV must be capitalized.
.Ip * 4 2
The \*(L"system\*(R" calls link, unlink, rename, etc. return nonzero for success, not 0.
.Ip * 4 2
Signal handlers deal with signal names, not numbers.
.PP
Seasoned sed programmers should take note of the following:
.Ip * 4 2
Backreferences in substitutions use $ rather than \e.
.Ip * 4 2
The pattern matching metacharacters (, ), and | do not have backslashes in front.
.Ip * 4 2
The range operator is .. rather than comma.
.PP
Sharp shell programmers should take note of the following:
.Ip * 4 2
The backtick operator does variable interpretation without regard to the
presence of single quotes in the command.
.Ip * 4 2
The backtick operator does no translation of the return value, unlike csh.
.Ip * 4 2
Shells (especially csh) do several levels of substitution on each command line.
Perl does substitution only in certain constructs such as double quotes,
backticks, angle brackets and search patterns.
.Ip * 4 2
Shells interpret scripts a little bit at a time.
Perl compiles the whole program before executing it.
.Ip * 4 2
The arguments are available via @ARGV, not $1, $2, etc.
.Ip * 4 2
The environment is not automatically made available as variables.
.SH BUGS
.PP
You can't currently dereference arrays or array elements inside a
double-quoted string.
You must assign them to a scalar and interpolate that.
.PP
Associative arrays really ought to be first class objects.
.PP
Perl is at the mercy of the C compiler's definitions of various operations
such as % and atof().
In particular, don't trust % on negative numbers.
.PP
.I Perl
actually stands for Pathologically Eclectic Rubbish Lister, but don't tell
anyone I said that.
.rn }` ''