PGRP

A process group (referred to as a pgrp in some contexts) is a collection of one or more processes that can be managed as a unit for job control and signal delivery in Unix-like operating systems. Each process group has a process group ID (PGID), and the PGID is typically the ID of the group’s leader process.

Process groups exist within a larger structure called a session. A session can contain multiple process groups, and a terminal is associated with exactly one controlling terminal per session. The terminal designates one process group as the foreground process group, whose members can read from and write to the terminal. Other process groups in the session run in the background.

Creation and management of process groups commonly involve system calls such as setpgid and setsid. The setpgid system call can form a new process group or move a process into an existing one. The setsid call creates a new session and makes the calling process the session leader, forming a new process group and taking control of a controlling terminal.

Signaling is a key aspect of process groups. The kill system call may target a whole group by using a negative PGID, delivering the signal to all processes in that group. Signals sent to the foreground process group by the terminal influence all processes in that group.

In practice, pgrps are used to implement job control in shells. The shell can start, stop, and resume jobs by manipulating their process groups, including moving jobs between foreground and background and handling terminal I/O and signal delivery accordingly. Functions such as getpgid, getpgrp, and setpgid are used by programs to query and modify their own or other processes’ group memberships.