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A Process group is a collection of related processes. One important feature is that it is possible to send a signal to every process in the group. Typically when the shell creates a process to run an application, the process is placed into a new process group. As that application forks new processes, the processes are all members of the process group. Process groups are used for distribution of signals, and by terminals to arbitrate requests for their input and output:

Foreground Process Groups

  • A foreground process has read and write access to the terminal.
  • Every process in the foreground receives SIGINT SIGQUIT and SIGTSTP signals.
  • Usually consists of the processes forked by the shell and all the processes they fork.

Background Process Groups

  • A background process does not have read access to the terminal.
  • If a background process attempts to read from its controlling terminal its process group will be sent a SIGTTIN.

Example

Notice that cat, sort and uniq all have the same PGID (Process Group ID).

staz@tokra:~/log$ cat results | sort | uniq & ps -jf
[1] 1239
UID        PID  PPID  PGID   SID  C STIME TTY          TIME CMD
staz      1237 28341  1237 28341  0 17:55 pts/6    00:00:00 cat results
staz      1238 28341  1237 28341  0 17:55 pts/6    00:00:00 sort
staz      1239 28341  1237 28341  0 17:55 pts/6    00:00:00 uniq

The TIOCGPGRP? and TIOCSPGRP? calls described in termios(4)? are used to get/set the process group of the control terminal.

If a ProcessGroup has a ControllingTerminal, CLOCAL is not set and a hangup occurs, then the ProcessGroup is sent a SIGHUP.

If a ProcessGroup Leader exits, all children processes are sent a SIGHUP

If the exit of the process causes a process group to become orphaned, and if any member of the newly-orphaned process group is stopped, then a SIGHUP signal followed by a SIGCONT signal will be sent to each process in the newly-orphaned process group.

See Also