perl ignore return codes

twistedpair · 02-14-2006, 11:05 AM

All,

I am making a system call in perl like this:

system ("/usr/bin/mycommad");

Without getting into specifics, I don't want the perl script that runs this to wait for a return code from "mycommand" before it continues with the rest of its code. I did some reading and saw something about %SIG{CHLD} and 'IGNORE' but I'm not sure if I am on the right track with that. Any ideas?

-Pair

Matir · 02-14-2006, 12:22 PM

It's just waiting for the system() call to return. You might just be able to do

Code:

system("command &");

or use fork() to launch a separate process.

puffinman · 02-14-2006, 01:17 PM

You definitely want to use fork and exec for this. One idiom would be

Code:

my $pid = fork;
if ($pid) {

  # the parent process, $pid contains the child pid
  print "$$: I'm the parent, child is $pid\n";
  # continue with other stuff

} elsif (defined $pid) {

  # the child process, $pid is zero
  print "$$: I'm the child\n";

  # run the program you want to replace this process
  exec "command";

} else {
  die "Fork error: $!\n";
}

Note that $$ is the special variable that contains the current process pid. Remember to use wait or waitpid to reap your dead children, or you'll have zombies until the parent exits (morbid terminology

. Also, if you want to control the input and output of the child process, look at IPC::Open2 or IPC::Open3. You can also open a socket or two before forking to do the IPC. Check out the Perl Cookbook for all this crap

.

twistedpair · 02-14-2006, 02:47 PM

Quote:

Originally Posted by puffinman

You definitely want to use fork and exec for this. One idiom would be

Code:

my $pid = fork;
if ($pid) {

  # the parent process, $pid contains the child pid
  print "$$: I'm the parent, child is $pid\n";
  # continue with other stuff

} elsif (defined $pid) {

  # the child process, $pid is zero
  print "$$: I'm the child\n";

  # run the program you want to replace this process
  exec "command";

} else {
  die "Fork error: $!\n";
}

Note that $$ is the special variable that contains the current process pid. Remember to use wait or waitpid to reap your dead children, or you'll have zombies until the parent exits (morbid terminology

. Also, if you want to control the input and output of the child process, look at IPC::Open2 or IPC::Open3. You can also open a socket or two before forking to do the IPC. Check out the Perl Cookbook for all this crap

.

Thank you for all the tips. Wouldn't I not want the wait? Exec calls just a normal binary which is coded to just run then stop and I actually don't want the perl script to wait for that binary to return. Is the above code supposed to do that, or is it made specifically to wait (assuming I use "wait"?

BTW, I have tried the:

system ("\usr\bin\mycommand &");

command, and it didn't work.

Am I just completely misunderstanding?

-Pair

puffinman · 02-14-2006, 03:41 PM

Well, if you don't care to track the progress of your child processes, then using wait would not be necessary. However, if you don't reap them when they are done, zombie processes will accumulate until the parent process exits. Read this for a detailed explanation and solutions for dealing with this problem.

Using the code I posted earlier, the parent process will not stop execution. Try the fork man page or read here for some info on process forking, it may make things a little clearer. That page talks about forks in C but it is very similar in perl, and forking is a feature of the operating system, not a particular language.

chrism01 · 02-14-2006, 04:54 PM

Actually, for this idiom:

Code:

system ("\usr\bin\mycommand &");

what you need is to detach it from the calling proc as well ie

Code:

system ("nohup \usr\bin\mycommand &");