pgid

PGID stands for Process Group Identifier. In Unix-like operating systems, a process group is a collection of one or more processes that can be managed as a unit for job control and signal delivery. Each process belongs to exactly one process group, identified by the group's PGID. The group leader is the process whose PGID equals its own PID, so the group's PGID is also the leader’s PID.

By default, a new process inherits the PGID of its parent. A process can form a new group by calling setpgid(pid, pgid). If pid is zero, the call applies to the calling process; if pgid is zero, the process becomes the leader of a new group with PGID equal to its own PID. Setsid creates a new session and makes the calling process the session leader, which also establishes a new process group with PGID equal to the process’s PID and detaches from any controlling terminal.

PGIDs are central to terminal and job control. The terminal maintains a foreground process group; signals generated by the terminal, such as SIGINT or SIGTSTP, are delivered to all processes in the foreground group. Shells use process groups to manage jobs, allowing foreground and background execution, and to move or suspend entire groups with commands like fg, bg, or jobs.

Practical notes include how to inspect and manipulate PGIDs. The getpgid or getpgrp system calls return the PGID of a given process or the calling process, respectively. To signal an entire group, a program can use a negative PGID with many signaling interfaces (for example, kill(-pgid, SIGTERM)). PGID is a fundamental concept for process management, session handling, and terminal control in Unix-like systems.