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The following diagram from APUE leads me to wonder: in a process session, does the process group of the session leader contain only the session leader and no other process?

  • Any process created by fork() will inherit the same process group and session from its parent. So can the session leader fork arbitrary number of processes into its own process group?

  • For example, is a shell (when it is a session leader) the only process in its process group? If yes, is it because when the shell forks a child process, the child is initially in the same process group as the shell, but then immediately starts a new process group by calling setpgid() ?

Thanks.

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No, there's no such restriction. If it was the case, commands that don't implement job control (in practice, only shells do) wouldn't be able to fork a process (as child processes inherit the process group) when started as xterm -e that-command for instance.

Even when the session leader is an interactive shell with job control enabled, you can have other processes in its group.

Running:

xterm -e 'sleep 1000 & exec zsh'

And in that xterm:

 PID PGID SID TTY TIME CMD 14003 14003 14003 pts/20 00:00:00 zsh 14004 14003 14003 pts/20 00:00:00 sleep 14012 14012 14003 pts/20 00:00:00 ps

Most commands run from an interactive shell are run in separate process groups, but it's not the case for all.

For instance, in bash:

$ exec 3< <(sleep 1000) $ ps -j PID PGID SID TTY TIME CMD 13913 13913 13913 pts/19 00:00:00 bash 14136 13913 13913 pts/19 00:00:00 bash 14137 13913 13913 pts/19 00:00:00 sleep 14138 14138 13913 pts/19 00:00:00 ps

Or the processes started as part of prompt expansions:

$ PS1=$'$(ps -j)\n$ ' PID PGID SID TTY TIME CMD 14212 14212 14212 pts/18 00:00:00 bash 14292 14212 14212 pts/18 00:00:00 ps $
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  • Thanks. Do you happen to know why bash makes some child processes into new process groups, but not some other child processes? Commented Dec 4, 2018 at 14:26
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All external commands are run in a process group different from the one of the shell. The shell is a session leader and thus that is the only process in its process group.

All commands which belong to the same pipeline are in the same process group.

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  • Thanks. Is it correct that proc1 and proc2 may run external commands, but why are they in the same group in the diagram, contradicting to "All external commands are run in a different process group" ? Commented May 6, 2018 at 14:48
  • @Tim Probably a misunderstanding. I have improved the wording. Commented May 6, 2018 at 15:00
  • Thanks. I have updated my post. Hope that I clarify my problem. Commented Dec 4, 2018 at 12:56

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