====== Program execution from Perl ======

===== system() =====

With the ''**system()**'' function a process is forked to execute the system command, and the main program waits for the child process to return. The output is displayed to the screen (STDIN, STDOUT, STDERR are connected to the parent process?). The function returns the exit code and which signal, if any, the process died from.

<code perl>
$cmd = "tar zcf '$opt_f' -C '$tmp_dir' .";
print "$cmd\n" if ($debug);
$ret = system($cmd);
print "system() funct returned " . ($ret)        . "\n";
print "child died on signal "    . ($ret & 0xff) . "\n";
print "child exit code was "     . ($ret >> 8)   . "\n";
</code>

==== Executing a chain of commands ====

The following is a more complicated example. A ''**local_command**'' is executed, its output is piped through ''**ssh**'' to a remote host, where a ''**remote_command**'' is launched:

<code perl>
$cmd  = "local_command";
$cmd .= " | ssh -o StrictHostKeyChecking=no -p ${REMOTE_PORT} ${REMOTE_USER}\@${REMOTE_HOST}";
$cmd .= " remote_command";
$ret = system($cmd);
print "system() funct returned " . ($ret)        . "\n";
print "child died on signal "    . ($ret & 0xff) . "\n";
print "child exit code was "     . ($ret >> 8)   . "\n";
</code>

  * **STDOUT** of **local_command** is piped to **STDIN** of **remote_command**
  * **STDERR** of **local_command** is piped to **STDERR** of **Perl**
  * **STDOUT** and **STDERR** of **ssh** are piped to **Perl**
  * **STDOUT** and **STDERR** of **remote_command** are piped to **Perl**
  * The **child exit code** is the exit code of ''**ssh**'' (which, in turns, should be the exit code of ''**remote_command**'')

If an **error** occurrs in ''**local_command**'', it gets unnoticed unless ''**remote_command**'' fails too.

===== back quotes operator =====

Also known as the ''**%%qx//%%**'' operator. You can actually use any set of delimiters, not just the slashes or the parenthesis. It returns the STDOUT of the executed command, that can be captured into an array. The STDERR of the executed command is connected to STDERR of the perl program. The exit code of the executed command can be retrieved with the ''**$?**'' special variable (''**$CHILD_ERROR**'' variable). A process is forked to execute the system command and the main perl program waits for the child process to return. The ''**$?**'' variable is actually the exit code of the ''wait()'' system call, it contains the exit code and which signal, if any, the process died from.

<code perl>
$cmd = "du -s /tmp";
@output = `$cmd`;
$ret = $?;
print "back quotes returned " . ($ret)        . "\n";
print "child died on signal " . ($ret & 0xff) . "\n";
print "child exit code was "  . ($ret >> 8)   . "\n";
</code>

===== fork() =====

If you want to execute an external program and let the main program to continue, you have to fork(). This is equivalent to the ''fork()'' Unix system call; after the call you have two identical process running, the only difference is the value returned by the function: 0 in the child, the PID of the child in the parent:

<code perl>
if (($pid = fork()) == 0) {
    # Here it is the child running.
    $cmd = "ls -l";
    system($cmd);
    exit(0);
}
# Here it is che parent running.
print "Child process is running with PID $pid\n";
</code>

====== Exiting from a Perl program ======

===== die() =====

With the ''**die()**'' function:

<code perl>
die("Statement failed with error ${code}. Stopped") if ($code != 0);
</code>

The argument of the ''die()'' function will be printed to STDERR. If the string does not end with a newline, the the program name and line number where the error occurred will be appended:

<code>
Statement failed with error 1. Stopped at ./perl_example line 59.
</code>

The exit code is the current value of ''**$!**'' (''**[[http://www.rigacci.org/docs/biblio/online/perlqr/ch2.htm#OS_ERROR|$OS_ERROR]]**'') which may have been set by a previous function. If ''$!'' has a value of zero, ''**$?**'' (''**[[http://www.rigacci.org/docs/biblio/online/perlqr/ch2.htm#CHILD_ERROR|$CHILD_ERROR]]**'') will be returned instead. If ''$?'' is zero, it exits with an exit value of 255.

===== exit() =====

This function evaluates the expression given as an argument and exits the program with that value as the exit code. The default value for the exit code is 0 if no argument is supplied. If an END block has been defined, it will be called. Also, object destructors may be called before the process truly ends.