processgroup

A process group is a collection of one or more processes in Unix-like operating systems that share a common process group ID (PGID). Process groups are used to manage processes as a unit for signals and for terminal job control, allowing a group of related processes to respond together to user input and system events.

Process groups are organized within a session. A session may contain multiple process groups, and one process group is designated as the foreground group for the session, which receives input from the terminal. Each process within the group has the same PGID, and the process whose ID equals the PGID is typically the group leader. Signals targeted at a process group—such as interrupts or stops—can be sent to the entire group.

Creation and manipulation of process groups are done through several system calls. A process can form or join a group with setpgid; a new session with setsid creates a new session and a new process group. The kill command can target a process group by using a negative PGID, delivering a signal to all processes in that group. For example, a shell uses process groups to implement job control, allowing foreground and background jobs to receive signals independently.

In practice, process groups are exploited by shells to implement pipelines and interactive job control, where a single group can represent a chain of processes connected by pipes. They are distinct from Linux control groups (cgroups), which manage resources rather than signaling. While similar concepts exist in other platforms, POSIX defines the core semantics of process groups, their relationship to sessions, and the key operations to manipulate them.