Perl Network Programming Cheat Sheet

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The Perl Network Programming Cheat Sheet covers:

Perl help
Comparison operators
Command-line arguments
Special variables
Run mode options
References
Variables
Arrays
Socket programming with Server-side & Client-side examples
Relevant Perl functions

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What’s included in this cheat sheet

The following categories and items have been included in the cheat sheet:

Perl help

Install Package	$ sudo apt-get install perl-doc
perldoc perldoc	Look up Perl documentation in Pod format
perldoc perltoc	Perl documentation Table of Contents (ToC)
perldoc perl	Basics language interpreter
perldoc perlfunc	List of built-in Perl functions
perldoc -f	Help with a specific function
perldoc perlop	List of Perl operators and precedence
perldoc perlmodlib	For constructing perl modules and finding existing ones
perldoc perllocal	Locally installed modules list (if any)
perldoc	Documentation for specific module

Comparison operators

Arithmetic	Strings
Less than	<	lt
Greater than	>	gt
Less than or equal	<=	le
Greater than or equal	>=	ge
Equality	==	eq
Inequality	!=	ne

Boolean operators
Description	Example
C-style Logical AND operator	&&	($a && $b) is false
Logical AND operator	and	($a and $b) is false
C-style Logical OR operator	\|\|	($a \|\| $b) is true
Logical OR operator	or	($a or $b) is true
C-style Logical NOT operator	!	!($a) is false
Logical NOT operator	not	not($a) is false

Quotes ($foo = 5;)
Single-quotes - Literal data enclosing	$bar = 'it is worth $foo';	it is worth $foo
Double-quotes - Interpolated data enclosing	$bar = "it is worth $foo";	it is worth 5
Escape characters	$bar = "it is \"worth\" $foo"; $bar = 'it is \'worth\' $foo';	it is "worth" 5 it is 'worth' $foo
Without quotes	$bar = q(it is 'worth' $foo); $bar = qq(it is "worth" $foo);	it is 'worth' $foo it is "worth" 5

Assoc	Operators	Description
left	terms and list operators	See below.
left	->	Infix dereference operator
	=++	Auto-increment (magical on strings).
	--	Auto-decrement.
right	**	Exponentiation.
right	\	Reference to an object (unary).
right	! ~	Unary negation, bitwise complement.
right	+ -	Unary plus, minus.
left	= ~	Binds a scalar expression to a pattern match.
left	! ~	Same, but negates the result.
left	* / % x	Multiplication, division, modulo, repetition.
left	+ - .	Addition, subtraction, concatenation
left	>> <<	Bitwise shift right, bitwise shift left.
	named unary operators	E.g. sin, chdir, -f, -M.
	< > <= >= lt gt le ge	Numerical relational operators. String relational operators.
	== != <=> eq ne cmp	Numerical equal, not equal, compare. Stringwise equal, not equal, compare. Compare operators return -1 (less), 0 (equal) or 1 (greater).
left	&	Bitwise AND.
left	\| ˆ	Bitwise OR, exclusive OR.
left	&&	Logical AND.
left	\|\|	Logical OR.
	..	In scalar context, range operator. In array context, enumeration.
right	? :	Conditional (if ? then : else) operator.
right	= += -= *= etc.	Assignment operators.
left	,	Comma operator, also list element separator.
left	=>	Same, enforces the left operand to be a string.
	list operators (rightward)	See below.
right	not	Low precedence logical NOT.
left	and	Low precedence logical AND.
left	or xor	Low precedence logical OR, exclusive OR.

Command-line Arguments

-0[octal/hexadecimal]	Specifies the input record separator ($/ ) as an octal or hexadecimal number
-a	Turns on autosplit mode when used with a -n or -p
-c	Causes Perl to check the syntax of the program and then exit without executing it
-C [number/list]	The -C flag controls some of the Perl Unicode features.
-d, -dt	Runs the program under the Perl debugger
-Dletters, -Dnumber	Sets debugging flags (only if your perl binary has been built with debugging enabled)
-e commandline	May be used to enter one line of program
-E commandline	Like -e, except it implicitly enables all optional features (in the main compilation unit)
-f	Disable executing $Config{sitelib}/sitecustomize.pl at startup
-F pattern	Specifies the pattern to split on for -a (regex // , "" , or '')
-h	Prints summary if the options
-m	-mmodule executes use module (); before executing your program
-M	-Mmodule executes use module ; before executing your program
-n	Input loop in the script without line printing
-p	Input loop in the script with line printing
-S	Makes Perl use the PATH environment variable to search for the program
-T	Turns on taint so you can test
-v	Prints the version and patchlevel of your perl executable
-V	Prints summary of the major perl configuration values and the current values of @INC
-w	Prints warnings about dubious constructs
-xdirectory	Tells Perl that the program is embedded in a larger chunk of unrelated text

Special Variables

$	Default variable
$/	The input record separator, newline by default
$\	The output record separator for the print operator
$(	The real GID (Group ID) of this process
$)	The effective GID (Group ID) of this process
$&	The string matched by the last successful pattern match
$`	The string preceding whatever was matched by the last successful pattern match
$'	The string following whatever was matched by the last successful pattern match
$ARGV	Contains the name of the current file when reading from <>
@ARGV	The array @ARGV contains the command-line arguments intended for the script
%ENV	The hash %ENV contains your current environment
@_	Within a subroutine the array @_ contains the parameters passed to that subroutine
@INC	Contains the list of places that the do EXPR , require, or use constructs look for their library files
$~	The name of the current report format for the currently selected output channel
$^	The name of the current top-of-page format for the currently selected output channel
$^A	The current value of the write() accumulator for format() lines
$^L	What formats output as a form feed. The default is \f
$^T	The time at which the program began running, in seconds since the epoch (beginning of 1970)
$^X	The name used to execute the current copy of Perl
$!	Each element of %! has a true value only if $! is set to that value - %ERRNO
$@	The Perl error from the last eval operator, i.e. the last exception that was caught
$?	The status returned by the last pipe close, backtick (`` ) command, successful call to wait() or waitpid(), or from the system() operator
$.	Current line number for the last filehandle accessed.
$%	The current page number of the currently selected output channel
$=	The current page length (printable lines) of the currently selected output channel. The default is 60
$-	The number of lines left on the page of the currently selected output channel
$\|	If set to nonzero, forces a flush right away and after every write or print on the currently selected output channel
$0	Contains the name of the program being executed
$+	The text matched by the highest used capture group of the last successful search pattern

Run mode options

-e	Single line of script
-w	Warnings
-c	Checks syntax
-n	Input loop without printing
-p	Printing an input loop
-a	Automatic split
-M	Load a module
-U	Unsafe operations mode
-v	Version and patch level of script

References

\	reference
[ ]	arrayref
{ }	hashref
\( )	List of refs

Variables

$var	Default variable
$var[20]	21st element of array @var
$p = \@var	Now $p is a reference to @var
$$p[20]	21st element of array referenced by $p
$var[-1]	Last element of array @var
$var[$x][$y]	$y-th element of $x-th element of array @var
$var{’JAN’}	A value from ‘hash’ %var
$p = \%var	Now $p is a reference to hash %var
$$p{’JAN’}	A value from hash referenced by $p
$#var	Last index of array @var
@var	The entire array
@var[5,6,7]	A slice of array @var
@var{’X’,’Y’}	A slice of %var; same as ($var{’X’},$var{’Y’})
%var	The entire hash;
$var{’a’,1,...}	Emulates a multidimensional array;

Arrays

@arr = (1..3);	Array initialization
$i = @arr;	Number of elements in the array
@arr = split(/-/,$text);	Split the string into $text
push(@arr, $s);	Append $s to @arr
$arr = pop(@arr);	Removes the last element in the array
chop(@arr);	Removes the last character in the array
Special variables	@ARGV array which stores all the command line arguments @ENV hash of program’s environment

Socket Programming with Server-side & Client-side examples

Server-side Method
`socket() call- socket( SOCKET, DOMAIN, TYPE, PROTOCOL ); bind() call -bind( SOCKET, ADDRESS ); listen() call- listen( SOCKET, QUEUESIZE ); accept() call - accept( NEW_SOCKET, SOCKET );`
Server-side Socket Script Example
#!/usr/bin/perl -w # Filename : serversocket.pl use strict; use Socket; # use port 7999 my $port = shift \|\| 7999; my $proto = getprotobyname('tcp'); my $server = "localhost"; # Host IP running the server # create a socket socket(SOCKET, PF_INET, SOCK_STREAM, $proto) or die "cannot open socket $!\n"; setsockopt(SOCKET, SOL_SOCKET, SO_REUSEADDR, 1) or die "cannot make reusable $!\n"; # bind to a port followed by listen bind( SOCKET, pack_sockaddr_in($port, inet_aton($server))) or die "cannot bind to port $port! \n"; listen(SOCKET, 5) or die "listen: $!"; print "SERVER socket started on port $port\n"; # accept a connection my $client_addr; while ($client_addr = accept(NEW_SOCKET, SOCKET)) { # send message to close connection my $name = gethostbyaddr($client_addr, AF_INET ); print NEW_SOCKET "new socket welcome"; print "Connection established $name\n"; close NEW_SOCKET; }
Client-side Method
connect() call -connect( SOCKET, ADDRESS );
Client-side Socket Script Example
!/usr/bin/perl -w # Filename : clientsocket.pl use strict; use Socket; # start host and port my $host = shift \|\| 'localhost'; my $port = shift \|\| 7999; my $server = "localhost"; # Host IP address of the server # create the socket and connect to the port socket(SOCKET,PF_INET,SOCK_STREAM,(getprotobyname('tcp'))[2]) or die "Cannot create a socket $!\n"; connect( SOCKET, pack_sockaddr_in($port, inet_aton($server))) or die "Cannot connect to port $port! \n"; my $line; while ($line = ) { print "$line\n"; } close SOCKET or die "close: $!";

Relevant Perl Functions

abs	Absolute value
accept	Accept an incoming socket connection
bind	Binds an address to a socket
binmode	Prepare binary files for input/output
chdir	Change current working directory
chmod	Changes the permissions on a file/list of files
chop	Remove the last character from a string
chown	Change the ownership of the file
close	Close file
closedir	Close directory
connect	Connect to a remote socket
crypt	One-way encryption
delete	Deletes a value from a hash
die	Raise an exception
dump	Create a core dump immediately
eof	End of file
eval	Compile and run code
exit	Terminate running s program
exp	Exponential
fork	Create a new process just like the existing one
gethostbyaddr	Get host record IP address
gethostbyname	Get host record given name
getlogin	Return who is logged in at this TTY
getnetbyname	Get networks record given name
getnetent	Get next networks record
getpeername	FInd the other end of a socket connection
getprotobyname	Get protocol record given name
getprotobynumber	Get protocol record numeric protocol
getprotoent	Get next protocols record
getpwent	Get next passwd record
getpwnam	Get passwd record given user login name
getpwuid	Get passwd record given user ID
getservbyname	Get services record given its name
getservbyport	Get services record given numeric port
getservent	Get next services record
getsockname	Retrieve the sockaddr for a given socket
getsockopt	Get socket options on a given socket
hex	Convert a string to a hexadecimal number
join	Join a list into a string using a separator
kill	Send a signal to a process or process group
length	Return the number of bytes in a string
listen	Register your socket as a server
m	Match a string with a regular expression pattern
mkdir	Create a directory
msgrcv	Receive a SysV IPC message from a message queue
msgsnd	Send a SysV IPC message to a message queue
my	Declare and assign a local variable (lexical scoping)
package	Declare a separate global namespace
print	Output a list to a filehandle
printf	Output redirect to a filehandle
push	Append one or more elements to an array
q	Singly quote a string
qq	Doubly quote a string
qr	Compile pattern
quotemeta	Quote regular expression magic characters
qw	Quote a list of words
qx	Backquote quote a string
rand	Retrieve the next pseudorandom number
read	Fixed-length buffered input from a filehandle
readdir	Get a directory from a directory handle
readline	Fetch a record from a file
readpipe	Run a system command and collect standard output
recv	Receive a message via a Socket
rename	Change a filename
return	Exit function early
rmdir	Remove a directory
send	Send a message via a socket
shift	Remove the first element of an array
shutdown	Terminate half of the socket connection
socket	Create a socket
sort	Sort a list of values
sqrt	Square root function
syscall	Execute an arbitrary system call

Perl network programming FAQs

Is Perl good for networking?

Perl shares a lot of characteristics with C and it includes daemon handling procedures, such as port binding and session management that enables the system to be used for network programming. Like the function library system of C, Perl can be extended by imported modules. This means that Perl can be infinitely extended with new functions or libraries of previously developed utilities.

Is Perl programming still used?

Perl is still widely used because there is a large programmer community with skills in Perl, which is always a good sign that a programming language is going to continue to be used for a long time. There are a large number of libraries of functions that are available for Perl that make the programming language suitable for a wide range of tasks.

Is Perl an Internet language?

Perl is useful for network programming and as the Internet is a network, Perl is also good for Internet programming. The useful features of Perl that make it great for Internet and Web programming are its ability to manage connections, handle files, and build interfaces.

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