How do you launch a background process and get pid?

bt101 has asked for the wisdom of the Perl Monks concerning the following question:

Hi - I need to lauch a background process and get the pid. I had a look at a lot of post and they say to use fork and exec as follows:

#!/usr/bin/perl
use strict;
use warnings;

my $pid = fork;
if ((defined $pid) && ($pid == 0))
{
    exec("sleep 30 >> /dev/null 2>&1 &");
    exit;
}

print "child pid [$pid]\n";
[download]

However it does not give me the correct pid of the sleep program:

$ perl tst.pl
child pid [23367]
$ ps ax | grep sleep
23368 pts/7    S      0:00 sleep 30
23371 pts/7    S+     0:00 grep --colour=auto sleep
[download]

How can I a launch a program and get its pid?

Comment on How do you launch a background process and get pid? Select or Download Code

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Re: How do you launch a background process and get pid? by graff (Chancellor) on Apr 18, 2016 at 09:24 UTC
Just to clarify what AM said above: because you are using both redirection and backgrounding in the string passed to "exec", a shell process has to be launched to execute that command line, and the shell exits. If you don't background it, the shell will keep running for as long as the sleep, and your "`ps ax \| grep sleep`" step will show three lines of output - the additional line would be the shell process that is running "sleep 30". Apart from that, you should be aware that the "exit" statement in your "if" block is never reached -- as stated in the perlfunc manpage, the "exec" call never returns. If you remove both backgrounding and redirection from the command line string, your parent perl process will get the pid for the child sleep process, because "exec" won't have to invoke a shell to run the command -- i.e. you could do either this: `exec( "sleep 30" );` [download] or this: `exec( "sleep", "30" );` [download]	[reply] [d/l] [select]
Re^2: How do you launch a background process and get pid? by bt101 (Acolyte) on Apr 18, 2016 at 16:33 UTC
Thanks. I found this thread for suppressing the output:http://www.perlmonks.org/?node_id=53870 The code now looks like this: `#!/usr/bin/perl use strict; use warnings; my $pid = fork; if ((defined $pid) && ($pid == 0)) { open(STDOUT, "/dev/null"); # suppressing output open(STDERR, "/dev/null"); # suppressing output exec("sleep", "30"); exit; } print "child pid [$pid]\n";` [download]	[reply] [d/l]
Re^3: How do you launch a background process and get pid? by afoken (Chancellor) on Apr 18, 2016 at 17:12 UTC
`open(STDOUT, "/dev/null"); # suppressing output open(STDERR, "/dev/null"); # suppressing output` [download] Yes, this suppresses output, but accidentally. You open STDOUT and STDERR for input. That may confuse your child process. What you really want is the three-argument form of open, used for output: `open STDOUT,'>','/dev/null'; open STDERR,'>','/dev/null';` [download] Alexander -- Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)	[reply] [d/l] [select]
Re: How do you launch a background process and get pid? by Anonymous Monk on Apr 18, 2016 at 03:48 UTC
You are execing a shell that launches sleep. The pid looks correct.	[reply]
Re^2: How do you launch a background process and get pid? by Anonymous Monk on Apr 18, 2016 at 03:51 UTC
Because it takes a shell to do all that redirection.	[reply]