The Linux Programming Interface

Process Groups, Sessions, and Job Control 701

Figure 34-1 shows the process group and session relationships between the various

processes resulting from the execution of the following commands:

$ echo $$ Display the PID of the shell

400

$ find / 2> /dev/null | wc -l & Creates 2 processes in background group

[1] 659

$ sort < longlist | uniq -c Creates 2 processes in foreground group

At this point, the shell (bash), find, wc, sort, and uniq are all running.

Figure 34-1: Relationships between process groups, sessions, and the controlling terminal

34.2 Process Groups

Each process has a numeric process group ID that defines the process group to

which it belongs. A new process inherits its parent’s process group ID. A process

can obtain its process group ID using getpgrp().

PID = 400

PPID = 399

bash

PGID = 400

PPID = 400

find

PGID = 658

SID = 400

PID = 659

PPID = 400

wc

PGID = 658

SID = 400

PPID = 400

sort

PGID = 660

SID = 400

PID = 661

PPID = 400

uniq

PGID = 660

SID=400

Process

group 660

Session 400

session

leader

background

process groups

foreground

process group

controlling process

process group leaders

Controlling terminal

Foreground PGID = 660

Controlling SID = 400

SID = 400

PID = 658 PID = 660

Process

group 658

Process

group 400

#include <unistd.h>

pid_t getpgrp(void);

Always successfully returns process group ID of calling process