Member Avatar for neoanomally

I am trying to figure out how to use fork() with exec(). I read through the perl docs on how it works, but I can't figure out how to implement it.

For example if I wanted to fork a custom sh script:

if (this_condition is true) {
    $mypid = fork(); ##?? not sure how this actually works
    exec("my_custom.sh > my_custom_output.txt"); ## is this affected by the fork or what do I do?
}

Every example I've seen creates a $mypid = fork(), but I don't know how it actually works and if this is what I'm really wanting to do do.

Thank you!

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  • Latest Post Latest Post by kordaff

Dani AI

Generated

, think of fork as cloning your Perl process. After the clone, both parent and child keep running the same code, but with different return values from fork. Building on ’s point: check three cases explicitly. In the child, $pid == 0. In the parent, $pid > 0 (the child’s PID). If fork failed, $pid is undef (use defined $pid to test this; do not compare to undef).

If you want to run the shell script in parallel and keep your Perl parent alive, exec should happen only in the child. Also, do redirection in Perl rather than depending on the shell, so you avoid quoting surprises and potential injection.

use POSIX qw(:sys_wait_h);

my $pid = fork // die "fork failed: $!";

if ($pid == 0) {
    # child: send output to a file, then replace process image
    open my $out, '>', 'my_custom_output.txt' or die "open: $!";
    open STDOUT, '>&', $out or die "dup STDOUT: $!";
    open STDERR, '>&', $out or die "dup STDERR: $!";   # optional

    exec './my_custom.sh' or die "exec failed: $!";     # never returns on success
}

# parent: do other work, then reap to avoid a zombie
waitpid($pid, 0) > 0 or die "waitpid failed: $!";

Notes you can apply right away:

  • If you insist on shell metacharacters like > in one string, use exec '/bin/sh', '-c', 'my_custom.sh > my_custom_output.txt', but prefer the list form above for safety.
  • If you do not waitpid, the child can become a zombie; alternatively, set $SIG{CHLD} = 'IGNORE' if you truly do not care about the exit status.
  • Want the parent to continue running Perl after the child finishes? Use system in the child only if you need Perl to regain control there; exec replaces the child completely.
Member Avatar for kordaff

The child process gets a pid of 0, the parent process gets a new pid that is not equal to zero. You'll probably only want to run the exec once, in either the child process from fork, or in the parent. All the code after fork() runs in each process, unless it was unsuccessful then pid will be undefined.

Put your exec where you want it to run, in the child process section, or in the parent process section:

if (this_condition is true) 
{
$mypid = fork(); ##?? not sure how this actually works

if (! $mypid)
{
# put your exec here to run in the child process.
}
elsif (undef $mypid)
{
# fork() failed.
}
else
{
# put exec here to run in the parent process
}
# you may want to exit one or both processes somewhere in 
this block too...
}
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