Perl - Process Management

Perl - Process Management

You can use Perl in various ways to create new processes as per your requirements. This tutorial will list down few important and most frequently used methods of creating and managing Perl processes.

·        You can use special variables $$ or $PROCESS_ID to get current process ID.

·        Every process created using any of the mentioned methods, maintains its own virtual environment with-in %ENV variable.

·        The exit() function always exits just the child process which executes this function and the main process as a whole will not exit unless all running child-processes have exited.

·        All open handles are dup()-ed in child-processes, so that closing any handles in one process does not affect the others.

Backstick Operator

This simplest way of executing any Unix command is by using backstick operator. You simple put your command inside backstick operator, which will result in execution of the command and returns its result which can be stored as follows:

#!/usr/bin/perl

@files = `ls -l`;

foreach $file (@files){

   print $file;

}

1;

When above code is executed, it lists down all the files and directories available in the current directory:

drwxr-xr-x 3 root root 4096 Sep 14 06:46 9-14

drwxr-xr-x 4 root root 4096 Sep 13 07:54 android

-rw-r--r-- 1 root root  574 Sep 17 15:16 index.htm

drwxr-xr-x 3  544  401 4096 Jul  6 16:49 MIME-Lite-3.01

-rw-r--r-- 1 root root   71 Sep 17 15:16 test.pl

drwx------ 2 root root 4096 Sep 17 15:11 vAtrJdy

The system() Function

You can also use system() function to execute any Unix command, whose output will go to the output of the perl script. By default, it is the screen, i.e. STDOUT, but you can redirect it to any file by using redirection operator >:

#!/usr/bin/perl

system( "ls -l")

1;

When above code is executed, it lists down all the files and directories available in the current directory:

drwxr-xr-x 3 root root 4096 Sep 14 06:46 9-14

drwxr-xr-x 4 root root 4096 Sep 13 07:54 android

-rw-r--r-- 1 root root  574 Sep 17 15:16 index.htm

drwxr-xr-x 3  544  401 4096 Jul  6 16:49 MIME-Lite-3.01

-rw-r--r-- 1 root root   71 Sep 17 15:16 test.pl

drwx------ 2 root root 4096 Sep 17 15:11 vAtrJdy

Be careful when your command contains shell environmental variables like $PATH or $HOME. Try following three scenarios:

#!/usr/bin/perl

$PATH = "I am Perl Variable";

system('echo $PATH');  # Treats $PATH as shell variable

system("echo $PATH");  # Treats $PATH as Perl variable

system("echo \$PATH"); # Escaping $ works.

1;

When above code is executed, it produces following result depending on what is set in shell variable $PATH.

/usr/local/bin:/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/sbin

I am Perl Variable

/usr/local/bin:/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/sbin

The fork() Function

Perl provides a fork() function that corresponds to the Unix system call of the same name. On most Unix-like platforms where the fork() system call is available, Perl's fork() simply calls it. on some platforms such as Windows where the fork() system call is not available, Perl can be built to emulate fork() at the interpreter level.

The fork() function is used to clone a current process. This call create a new process running the same program at the same point. It returns the child pid to the parent process, 0 to the child process, or undef if the fork is unsuccessful.

You can use exec() function within a process to launch the requested executable, which will be executed in a separate process area and exec() will wait for it to complete before exiting with the same exit status as that process.

#!/usr/bin/perl

if(!defined($pid = fork())) {

   # fork returned undef, so unsuccessful

   die "Cannot fork a child: $!";

}elsif ($pid == 0) {

   print "Printed by child process\n";

   exec("date") || die "can't exec date: $!";

 

} else {

   # fork returned 0 nor undef

   # so this branch is parent

   print "Printed by parent process\n";

   $ret = waitpid($pid, 0);

   print "Completed process id: $ret\n";

}

1;

When above code is executed, it produces following result:

Printed by parent process

Printed by child process

Tue Sep 17 15:41:08 CDT 2013

Completed process id: 17777

The wait() and waitpid() can be passed a pseudo-process ID returned by fork(). These calls will properly wait for the termination of the pseudo-process and return its status. If you fork without ever waiting on your children using waitpid() function, you will accumulate zombies. On Unix systems, you can avoid this by setting $SIG{CHLD} to "IGNORE" as follows:

#!/usr/bin/perl

local $SIG{CHLD} = "IGNORE";

if(!defined($pid = fork())) {

   # fork returned undef, so unsuccessful

   die "Cannot fork a child: $!";

}elsif ($pid == 0) {

   print "Printed by child process\n";

   exec("date") || die "can't exec date: $!";

 

} else {

   # fork returned 0 nor undef

   # so this branch is parent

   print "Printed by parent process\n";

   $ret = waitpid($pid, 0);

   print "Completed process id: $ret\n";

}

1;

When above code is executed, it produces following result:

Printed by parent process

Printed by child process

Tue Sep 17 15:44:07 CDT 2013

Completed process id: -1

The kill() Function

Perl kill('KILL', (Process List)) function can be used to terminate a pseudo-process by passing it the ID returned by fork().

Note that using kill('KILL', (Process List)) on a pseudo-process() may typically cause memory leaks, because the thread that implements the pseudo-process does not get a chance to clean up its resources.

You can use kill() function to send any other signal to target processes, for example following will send SIGINT to a process IDs 104 and 102:

#!/usr/bin/perl

kill('INT', 104, 102);

1;

Perl - Embedded Documentation

You can embed Pod (Plain Old Text) documentation in your Perl modules and scripts. Following is the rule to use embedded documentation in your Perl Code:

Start your documentation with an empty line, a =head1 command at the beginning, and end it with a =cut command and an empty line.

Perl will ignore the Pod text you entered in the code. Following is a simple example of using embedded documentation inside your Perl code:

#!/usr/bin/perl

print "Hello, World\n";

=head1 Hello, World Example

This example demonstrate very basic syntax of Perl.

=cut

print "Hello, Universe\n";

When above code is executed, it produces following result:

Hello, World

Hello, Universe

If you're going to put your Pod at the end of the file, and you're using an __END__ or __DATA__ cut mark, make sure to put an empty line there before the first Pod command as follows, otherwise without an empty line before the =head1, many translators wouldn't have recognized the =head1 as starting a Pod block.

#!/usr/bin/perl

print "Hello, World\n";

while(<DATA>){

  print $_;

}

__END__

=head1 Hello, World Example

This example demonstrate very basic syntax of Perl.

print "Hello, Universe\n";

When above code is executed, it produces following result:

Hello, World

=head1 Hello, World Example

This example demonstrate very basic syntax of Perl.

print "Hello, Universe\n";

Let's take one more example for the same code without reading DATA part:

#!/usr/bin/perl

print "Hello, World\n";

__END__

=head1 Hello, World Example

This example demonstrate very basic syntax of Perl.

print "Hello, Universe\n";

When above code is executed, it produces following result:

Hello, World

What is POD?

Pod is a simple-to-use markup language used for writing documentation for Perl, Perl programs, and Perl modules. There are various translators available for converting Pod to various formats like plain text, HTML, man pages, and more. Pod markup consists of three basic kinds of paragraphs:

·        Ordinary Paragraph: You can use formatting codes in ordinary paragraphs, for bold, italic, code-style , hyperlinks, and more.

·        Verbatim Paragraph: Verbatim paragraphs are usually used for presenting a codeblock or other text which does not require any special parsing or formatting, and which shouldn't be wrapped.

·        Command Paragraph: A command paragraph is used for special treatment of whole chunks of text, usually as headings or parts of lists. All command paragraphs start with =, followed by an identifier, followed by arbitrary text that the command can use however it pleases. Currently recognized commands are:

=pod

=head1 Heading Text

=head2 Heading Text

=head3 Heading Text

=head4 Heading Text

=over indentlevel

=item stuff

=back

=begin format

=end format

=for format text...

=encoding type

=cut

POD Examples

Consider the following POD:

=head1 SYNOPSIS

Copyright 2005 [TUTORIALSOPOINT].

=cut

You can use pod2html utility available on Linux to convert above POD into HTML, so it will produce following result:

Copyright 2005 [TUTORIALSOPOINT].

Next consider the following example:

=head2 An Example List

=over 4

=item * This is a bulleted list.

=item * Here's another item.

=back

=begin html

<p>

Here's some embedded HTML.  In this block I can

include images, apply <span style="color: green">

styles</span>, or do anything else I can do with

HTML.  pod parsers that aren't outputting HTML will

completely ignore it.

</p>

=end html

When you convert above POD into HTML using pod2html, it will produces following result:

An Example List

• This is a bulleted list.
• Here's another item.

Here's some embedded HTML. In this block I can include images, apply styles, or do anything else I can do with HTML. pod parsers that aren't outputting HTML will completely ignore it.