Perl Guide

Perl

Table of Contents:
\	%	#	Assignment
Command-Line	Environment	Examples	Format
Functions	Io	Looping	Math
Operators	Pattern	Precedence	Random
Stacks	Subroutines	Switch	Text
Unix	Variables	Windows

\
\\	backslash	\"	double-quote	\a	bell
\b	backspace	\cC	control-'C'	\e	escape
\E	terminate \L or \U	\f	formfeed	\l	lowercase next char
\L	lowercase 'til \E	\n	newline	\r	carriage return
\t	tab	\u	uppercase next char	\U	uppercase 'til \E
\v	vertical tab	\177	octal 'delete'	\ooo	octal char
\xhh	hex char

%%: %
%s: string
%2d: 2-space integer
%02d: 2-space integer (leading zeros)
%2c: 2-space character
%6.2f: 6-space, 2-decimal floating point number

#: comment line(everything after is ignored)
#!/usr/local/bin/perl: has to be the first line of each (unix) perl program
use diagnostics;: explain warnings in greater detail
use strict;: generates errors for unsafe variable reference & subroutine constructs.

Assignment

$a = $b = $c;: this works from right to left. $b = $c & then $a = $b.
+= -= *= /= %= **= .=(string concatenation)

Command-Line

-c: check syntax only. do not run.
-d: perl debugger
-e: specify a line of perl code on the command line
-i: ammounts to changing a file in place.; perl -i.old -p -e "s/foo/bar/g" wiffle.bat; (take each line of wiffle.bat (-p), store the original in (wiffle.old)(-i), replace all foo with bar (-e) and write result(-p) to the original file(-i))
-p: assume 'while (<>) { }' around the line of command-line code
-P: run through the c pre-processor before perl compiles it.
-T: don't evaluate expressions given on the command line. Useful with doing web work.
-w: prints warnings only about dubious constructs
-W: prints all warnings
-0: use the NULL character as a record separator
-W0: combined -W and -0
.pl: perl library file
.pm: perl module file

Environment

%ENV: environment variables associative array.
$ENV{'PATH'}: returns path

Format

example: . format someformatname = fieldline value1, value2, value3 fieldline value1, value2 . . format ADDRESSLABEL = ==================================== | @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< | $name | @<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< | $address | @<<<<<<<<<<<<<<<<<<<<<, @< @<<<< | $city, $state, $zip ==================================== .

Using the ADDRESSLABEL format:

     open(ADDRESSLABEL,">file-to-write-to");
     ($name,$address,$city,$state,$zip) = split(/:/);
     #read in the fields
     write ADDRESSLABEL; #this will write appropriately

STDOUT

to write the label to STDOUT, name the label such.

@<<<<<<

left justified field data

@>>>>>>

right-justified field data

@||||||

centered field data

^<<<<<<

line continuation (break on word boundaries)

preceed with ~ to ignore of no data left

preceed with ~~ to go until no data is left

@###.##

number format

      to include special signs (-/$/etc...) use the sub:
      sub cool
      {
       local($n,$width) = @_;
       #back off for negative stuff;2
       $width -= 2; 
       @n = sprintf("%.2f",$n);
       if ($n < 0)
       {
        #negative numbers get brackets
        sprintf("[%$width.2f]", -$n);
       }
       else
       {
        #positive #'s get spaces
        sprintf(" %$width.2f]", $n);
       }
      }

@*

if the variable has \n's in it, this has to be between the fieldline and its associated variables

Functions

crypt PLAINTEXT,SALT

-encrypt data. Useful for password entry.

     $pwd = (getpwuid($<))[1];
     $salt = substr($pwd, 0, 2);
     system "stty -echo";
     print "Password: ";
     chop($word = );
     print "\n";
     system "stty echo";
     if(crypt($word, $salt) ne $pwd)
     { die "Sorry...\n"; }
     else { print "ok\n"; }

defined EXPR

returns boolean value wether EXPR has a real value or not.

to distinguish between undefined null scalar & defined null scalar.

do 'stat.pl';

execute contents of file 'stat.pl' as a perl script. primarily used for including subroutines from a perl library. superceedes 'eval'.

fcntl FILEHANDLE,FUNCTION,SCALAR

implements fcntl(2) function. Will probably need 'use Fcntl;'

     use Fcntl;
     fcntl($filehandle, F_GETLK, $packed_return_buffer);

see also: flock, ioctl

rename

rename($old,$new); or system("mv",$old,$new);

single-character input

     my $BSD_STYLE = 1;
     sub getsingle()
     {
      if ($BSD_STYLE)
      { system "stty cbreak /dev/tty 2>&1"; }
      else
      { system "stty", '-icanon', 'eol', "\001"; }
      my $tmp = getc(STDIN);
      if ($BSD_STYLE)
      { system "stty -cbreak /dev/tty 2>&1"; }
      else
      { system "stty", 'icanon', 'eol', '^@'; # ascii null }
      print "\n";
      return $tmp;
     }

standard filehandles

STDIN keyboard ($a = <STDIN>;)

$a = <STDIN>;
while ($x = <STDIN>) { ... }
while (<STDIN>) { ... }

STDOUT: screen
STDERR: error output

open(FILEHANDLE,"<filename");

open a file for reading (returns T/F)

open(FILEHANDLE,">filename");

open a file for writing (returns T/F)

open(FILEHANDLE,">>filename");

open a file for appending (returns T/F)

open(FILEHANDLE,"+<filename");

open a file for reading & writing

open(FILEHANDLE,"+>filename");

open a file for reading & writing

open(FILEHANDLE,"|name");

command to pipe output to

open(FILEHANDLE,"name|");

command that pipes into us

close(FILEHANDLE);

opendir

closedir(DIRHANDLE)

print OUTFILE "text";

print "text\n","text2\n";

while () { print OUT $_; }

print a file by line

translate '~' to a filename

     $filename =~ s{
     ^ ~             # find a leading tilde
     (               # save this in $1
      [^/]        # a non-slash character
          *     # repeated 0 or more times (0 means me)
           )
     }{
       $1
       ? (getpwnam($1))[7]
       : ( $ENV{HOME} || $ENV{LOGDIR} )
        }ex;

read a file by paragraphs

use $\. set it to '""' to eliminate empty paragraphs or '\n\n' to accept empty paragraphs.

File Tests: if (-x $filename) { }

-r	readable	-w	writable
-x	executable	-o	owned by user
-e	exists	-z	exists & 0 size
-s	size in bytes	-f	plain file
-d	directory	-l	symlink
-k	sticky-bit set	-T	text file
-B	binary file	-M	modification age (days)
-A	access age in days	-C	inode-modification age

stat(FILEHANDLE);

returns size 13 array of file info ($def,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,$atime,$mtime,$ctime,$blksize,$blocks) (see also: stat(2))

if (-r $filevar && -w _)

tests the file twice with 1 system call

test existance on open

unless (open(FILE,"<$FILENAME")) { print " "; exit; }

Looping

do { <block> } while( <condition> );
for ( initial; while condition; inc/dec ) { }
foreach $i (@list) { }: $i is local to the loop, is restored when exited.; changing $i locally changes the @list elements!
foreach (@list) { }: $_ is the effective loop variable
last;: exit current loop
next;: skip directly to the next instance in the loop
redo;: pass control directly to innermost loop (or LABEL)
until (condition) { }
while (condition) { }: while (<>) { print $_; } (/bin/cat)

Math

atan2(Y,X): arctangent of Y/X in the range of -PI to PI
cos(EXPR): cosine of EXPR in radians. takes $_ if ommited
eval '$answer = $a / $b'; warn $@ if $@;: 1/0 non-fatal
exp EXPR;: exponential.

Operators

Numbers

+ - * / % (always double)

** (power)

< <= == >= > !=

++ -- (also works with floats)

<=> (returns -1(less),0(equal),1(more)

Strings

subject	strings	numbers
concatenation	.
equality	eq	==
not equal	ne	!=
less than	<	<
greater than	>	>
less than/equal	<=	<=
greater than/equal	>=	>=

<>

@ARGV array

cmp

returns -1(less),0(equal),1(more). numbers & strings are 'truncated'.

repetition operator

'"fred" x 3' is "fredfredfred"

" "

de-compose variables within

' '

literal string (doesn't change variables

Pattern

grep

if (/abc/) { print "$_"; } - single line

=~ (with string other than $_)

in front of the slashes

replace

not

tr/searchlist/replacementlist/

translates all occurrences of the characters found in the searchlist with the corresponding character in the replacement list

match-operator optional switches

g	replace all (cannot be used with tr)
i	case insensitive
c	complement the searchlist
d	delete found but unreplaced characters
s	squash duplicate replaced characters
o	evaluate the regular expression one time only

matching inside the slashes

^	start of string	$	end of string	\b	word boundry
\B	not word boundry	\d	digits	\D	not-digits
\f	form feed	\t	tab	\n	newline
\r	carriage return	\w	alphanumeric	\W	not alphanumeric
\s	whitespace	\S	not whitespace	a\|b	match only one of a or b

atoms (Regular Expressions)

.

any character except \n (b.b = bob ! bb)

[ab]

any of the included characters (^[Bb] = Bob,bob ! Rbob)

[a-c]

same as [abc]

atom quantifiers

*	0 or more	+	1 or more
?	0 or 1	{n}	'n' instances
{n,}	at least 'n' instances	{n,m}	at least n, at most m

after the match

\1, \2, \3: the 'words' from a previous match
$&: part that matched $' (the part before)
$': part after

examples

warn "has nondigits" if /\D/;

warn "not a whole number" unless /^\d+$/;

warn "not an integer" unless /^-?\d+$/; # reject +3

warn "not an integer" unless /^[+-]?\d+$/;

warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2

warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;

warn "not a C float" unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;

s/\..*//; (remove filename extensions)

Precedence Lowest precedence on top

associativity	operator
none	the 'list' operators
left	,
right	+= ('assignment' operators)
right	?: (trinary if/then/else)
none	.. (range operator, list constructor)
left	\|\| (or)
left	&& (and)
left	\| ^ (bitwise or & exclusive or)
left	& (bitwise and)
none	== != <=> eq ne cmp ('equal' operators)
none	< <= > >= lt le gt ge ('not-equal' operators)
none	named unary operators
none	file test operators
left	<< >> (bit shift)
left	+ - .(string concatenation)
left	* / % x(string repetition)
left	=~(match) !~(doesn't match)
right	** (exponentiation)
right	! ~(bitwise not) -
none	++ --

Random

bring in a random line from a file: srand;; rand($.) < 1 && ($line = $_) while <>;

Stacks

push(@mylist,3,4,5);: adds 3,4,5 to the end of @mylist
$a = pop(@mylist);: removes last member of @mylist to $a & returns undef if empty
unshift(@list,3,4,5);: adds 3,4,5 to beginning of @list
$x = shift(@mylist);: removes first member of @mylist to $x & returns undef if empty

Subroutines

sleep(sec): sleeps the terminal for the specified number of seconds
read(handle,variable,integer - bytes of data);: reads 'integer' bytes from file 'handle' into a variable
seek(filehandle,position,start);: sets file pointer within a file.; handle: filehandle; position: number of bytes to move (relative to 'start'); start: (0=start,1=current position,2=end of file)
tell(filehandle);: returns position in a file (see: seek) & defaults to last file accessed
($s,$m,$h,$d,$mo,$y,$wd,$yd,$isdst)=localtime(time);$mo++;$y+=1900;: second, minute, hour, day, month(starts at 0. add 1), year(since 1900. add 1900), weekday(days since sunday(0-6), days since jan. 1 (0-365), daylight savings time (flag) (0/1)
time(): returns # of non-leap seconds since 00:00:00 UTC (January 1, 1970) (useful with tmp files)
local(?variable_name): creates a local variable in a subroutine
@a = reverse @a;: reverse @a in place
@y = sort(@y);: sort @y by 'ascii' value
chop($name);: removes the last character from 'name' returns the removed last character
chomp($name);: removes '\n' only.
chop(@list);: removes last character of last element
@array = split(/:/);: split on the ':' key.; @array = split (/:/,variable,split-column);
@array = split;: splits on whitespace like (/\s+/)
print substr ($_,0,1),"\n";: print first character of array
join($glue,@list);: joins elements of the list with the $glue
ord($string_or_char);: returns ascii value of first char
chr(##);: returns character of the ascii value
hex() oct()
sub subroutine { ; ; ; }: all vars. are global. the last item calculated or referred to in the subroutine is the return value.
&subroutine;: invoke the subroutine
&subroutine($x,$y,$z);: send arguments to the subroutine; in the subroutine, these vars. are given to @_ (local) elements are in the form $_[0], $_[1], etc.; retrieving a hash (send with %hash) use $$_[0]{"key"}.; return; returns.
Switch: if (condition) { } elsif (condition) { } else { } (braces are required); unless (condition) { } else { }

Text

Re-Format paragraphs

Use Text::Wrap (part of the standard perl distribution):

      use Text::Wrap;
      print wrap("\t", '  ', @paragraphs);

Unix

chdir EXPR: default is home directory
chmod: chmod 0755, $filename, 'filename2.txt';; chmod 0600, "filename";; chmod 0755, @executables;
chown $uid, $gid, $filename;: uid & gid must be numerical
system "shell_command"

Variables

false

0, "", undef, 1-1

true

1,"00", "1", "0.000"

$ARGV[0]

first command-line argument

$0

name of perl script being executed

$_

assumed variable, ie: loops, etc.

$.

current line number of latest filehandle read

$$

process id # of the running perl script

$|

zero=buffered output,non-zero=unbuffered output

$!

current system error number (errorno)

(1..5)

equal to (1,2,3,4,5)

$name (scalar)

($a,$b,$c) = (1,2,3);: ($a=1,$b=2,$c=3)
($a,$b) = ($b,$a);: (swap $a & $b)
($e,@fred) = @fred;: remove first of @fred -> $e. any excess values are discarded
$a = @fred;: $a = length of @fred
$#fred;: (last element index value)

@words (array)

arrays can be added into arrays

@fred = (1,2,3);

@fred = ("one","two","three");

@huh = 1; ($huh[0] = 1)

@all = (3,@fred,@huh); (= 3,1,2,3,1)

@fred[0,1] ($fred[0],$fred[1])

@fred[0..4] (fred 0, 1, 2, 3 & 4)

"".@a (# of elements)

@a = (each array element is a line)

$number = scalar @fred =# of elements

%words (hash/associative array - cannot be sorted)

%words=("a","z","b","y") (creates new whole hash)

$words{"a"} = "z"; (creates new %words)

%words{"c"} = "x"; (adds onto %words)

keys(%array); (list the keys (a,b,c))

values(%array); (list the values (z,y,x))

while (($first,$last) = each(@lastname))

delete $fred{"aaa"};

$numkeys = scalar keys $words (# of keys)

Examples

The Perl Online Faq
@files = sort (grep (!/^\./, readdir (DIRHANDLE)));: reads in & sorts files not beginning with '.' into @files
$foo =~ tr/[A-Z]/[a-z]/;: tolower() everything
$foo =~ /^[a-z]$/;: true if begins & ends w/same [a-z]

Windows

When sending windows directory commands through the shell that have 'spaces' in the filenames, you need to use the dos equivelant filename file~1. These can only be a maximum of 8 characters. This is handy when dealing with 'start menu' & 'program files'.