How can I run Perl system commands in the background?

Question

#!/usr/bin/env perl
use warnings; use strict;
use 5.012;
use IPC::System::Simple qw(system);

system( \'xterm\', \'-geometry\', \'80x25-5-5\', \'-bg\', \'green\', \'         


        

Answer 1

This is not purely an explanation for Perl. The same problem is under C and other languages.

First understand what the system command does:

1. Forks
2. Under the child process call exec
3. The parent process is waiting for forked child process to finish

It does not matter if you pass multiple arguments or one argument. The difference is, with multiple arguments, the command is executed directly. With one argument, the command is wrapped by the shell, and finally executed as:

/bin/sh -c your_command_with_redirections_and_ambersand

When you pass a command as some_command par1 par2 &, then between the Perl interpreter and the command is the sh or bash process used as a wrapper, and it is waiting for some_command finishing. Your script is waiting for the shell interpreter, and no additional waitpid is needed, because Perl's function system does it for you.

When you want to implement this mechanism directly in your script, you should:

1. Use the fork function. See example: http://users.telenet.be/bartl/classicperl/fork/all.html
2. Under the child condition (if), use the exec function. Your user is similar to system, see the manual. Notice, exec causes the child process program/content/data cover by the executed command.
3. Under the parent condition (if, fork exits with non-zero), you use waitpid, using pid returned by the fork function.

This is why you can run the process in the background. I hope this is simple.

The simplest example:

if (my $pid = fork) { #exits 0 = false for child process, at this point is brain split
  # parent ($pid is process id of child)
  # Do something what you want, asynchronously with executed command
  waitpid($pid);  # Wait until child ends
  # If you don't want to, don't wait. Your process ends, and then the child process will be relinked
  # from your script to INIT process, and finally INIT will assume the child finishing.
  # Alternatively, you can handle the SIGCHLD signal in your script
}
else {
  # Child
  exec('some_command arg1 arg2'); #or exec('some_command','arg1','arg2');
  #exit is not needed, because exec completely overwrites the process content
}