Process groups and sessions form a two-level hierarchical relationship between processes: a process group is a collection of related processes, and a session is a collection of related process groups. The meaning of the term related in each case will become clear in the course of this chapter.
Process groups and sessions are abstractions defined to support shell job control, which allows interactive users to run commands in the foreground or in the background. The term job is often used synonymously with the term process group.
This chapter describes process groups, sessions, and job control.
A process group is a set of one or more processes sharing the same process group identifier (PGID). A process group ID is a number of the same type (pid_t) as a process ID. A process group has a process group leader, which is the process that creates the group and whose process ID becomes the process group ID of the group. A new process inherits its parent’s process group ID.
A process group has a lifetime, which is the period of time beginning when the leader creates the group and ending when the last member process leaves the group. A process may leave a process group either by terminating or by joining another process group. The process group leader need not be the last member of a process group.
A session is a collection of process groups. A process’s session membership is determined by its session identifier (SID), which, like the process group ID, is a number of type pid_t. A session leader is the process that creates a new session and whose process ID becomes the session ID. A new process inherits its parent’s session ID.
All of the processes in a session share a single controlling terminal. The controlling terminal is established when the session leader first opens a terminal device. A terminal may be the controlling terminal of at most one session.
At any point in time, one of the process groups in a session is the foreground process group for the terminal, and the others are background process groups. Only processes in the foreground process group can read input from the controlling terminal. When the user types one of the signal-generating terminal characters on the controlling terminal, a signal is sent to all members of the foreground process group. These characters are the interrupt character (usually Control-C), which generates SIGINT; the quit character (usually Control-\), which generates SIGQUIT; and the suspend character (usually Control-Z), which generates SIGTSTP.
As a consequence of establishing the connection to (i.e., opening) the controlling terminal, the session leader becomes the controlling process for the terminal. The principal significance of being the controlling process is that the kernel sends this process a SIGHUP signal if a terminal disconnect occurs.
Note
By inspecting the Linux-specific /proc/PID/stat files, we can determine the process group ID and session ID of any process. We can also determine the device ID of the process’s controlling terminal (expressed as a single decimal integer containing both major and minor IDs) and the process ID of the controlling process for that terminal. See the proc(5) manual page for further details.
The main use of sessions and process groups is for shell job control. Looking at a specific example from this domain helps clarify these concepts. For an interactive login, the controlling terminal is the one on which the user logs in. The login shell becomes the session leader and the controlling process for the terminal, and is also made the sole member of its own process group. Each command or pipeline of commands started from the shell results in the creation of one or more processes, and the shell places all of these processes in a new process group. (These processes are initially the only members of that process group, although any child processes that they create will also be members of the group.) A command or pipeline is created as a background process group if it is terminated with an ampersand (&). Otherwise, it becomes the foreground process group. All processes created during the login session are part of the same session.
Note
In a windowing environment, the controlling terminal is a pseudoterminal, and there is a separate session for each terminal window, with the window’s startup shell being the session leader and controlling process for the terminal.
Process groups occasionally find uses in areas other than job control, since they have two useful properties: a parent process can wait on any of its children in a particular process group (The waitpid() System Call), and a signal can be sent to all of the members of a process group (Sending Signals: kill()).
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 -cCreates 2 processes in foreground group
At this point, the shell (bash), find, wc, sort, and uniq are all running.
