pgroups

Pgoups (process groups) are a mechanism used by POSIX-compliant Unix-like systems to manage related processes as a unit. Each process group has a unique process group ID (PGID), and all members of the group share that PGID. A process can become the leader of a new group by calling setpgid or by creating a new session with setsid.

Process groups are organized within sessions. A session can contain one or more process groups, and a session has a controlling terminal. The foreground process group of a session is the group that receives input from the terminal. Background groups are typically not allowed to read from the terminal, and attempts to do so may generate signals such as SIGTTIN or SIGTTOU.

Signals in relation to process groups are a core feature. Signals can be directed to a single process, to all members of a process group, or to a session in some cases. A common convention is to target a whole group by using a negative PID value with the kill command, which results in the signal being delivered to every process in that group.

In practice, shells use process groups to implement job control. When a user starts a job or a pipeline, the shell creates a dedicated process group for that job. The shell can move the group between foreground and background, bring it to the foreground with commands like fg, or suspend it with Ctrl-Z. Signals can then be delivered to the entire group as a unit.

Pgoups are standard in POSIX systems and are implemented in Linux and most BSD-derived systems, with variations in behavior across platforms. Windows uses a different mechanism (job objects) for similar management tasks. See also: process, session, signals, job control, kill.