BASH

seeLnd · 01-09-2013, 01:53 AM

I show the command first:

Code:

see@linux-ub7v:~> setsid sleep 5 & echo "$! setsid"; sleep 1; ps -eo pid,args | grep sleep | grep -v grep
12498 setsid
12502 sleep 5

see@linux-ub7v:~> killall sleep

see@linux-ub7v:~> bash -c 'setsid sleep 5 & echo "$! setsid"; sleep 1; ps -eo pid,args | grep sleep | grep -v grep'
12507 setsid
12507 sleep 5

see@linux-ub7v:~> setsid --version
setsid from util-linux 2.21.2

The first commands were executed by an interactive shell, we can see there are the different pid between setsid and sleep,
but the third commands, I put them into a non-interactive shell, and then, we can see there are the same pid of setsid and sleep.
I guess setsid in first command may be "fork -> exec sleep by child -> parent exit" and setsid in third command just do "exec sleep" directly.
If I am right, why does setsid have a different behavior between interactive shell and non-interactive shell ?

Thanks in advance!

Ztcoracat · 01-29-2013, 12:11 AM

Hi!

Welcome to Linux Questions!

A shell running a script is always a non-interactive shell.
Also a non-interactive shell won't stop execution to ask anyone and can be run unattended by
the system.

It also doesn't have an associated terminal for output.

http://tldp.org/LDP/abs/html/intandnonint.html
http://www.linuxquestions.org/questi...shells-680599/
http://tldp.org/LDP/abs/html/

Hope this helps.

mina86 · 01-29-2013, 06:59 AM

The answer lays in setsid's source code. Specifically this part:

Code:

	if (getpgrp() == getpid()) {
		switch (fork()) {
		case -1:
			err(EXIT_FAILURE, _("fork"));
		case 0:
			/* child */
			break;
		default:
			/* parent */
			return 0;
		}
	}

setsid will fork if process group is equal to process ID which means that the process is group leader. This happens when you run the command interactively because bash, as part of its job control, creates a new process group for the setsid command. When the command is run in a non-interactive fashion, setsid command is part of the same process group as the whole shell and thus it's not a process group leader and it does not fork.