In a Unix or Unix-like operating system, job control refers to controlling a process group as a job via a shell. Control features include suspend, resume, and terminate, and more advanced features can be performed by sending a signal to a job. Job control allows a user to manage processing in the Unix-based multiprocessing environment, and is distinct from general computing job control.
POSIX specifies job control commands – modeled after the Korn shell job control commands.[1]. Command bg resumes the foreground job paused via Control+Z as a background job. Command fg causes the background job to run in the foreground; either the job specified or the one most recently sent the background if none specified. Command wait pauses the interactive session for the specified background jobs to complete or for all background jobs of the active shell if none is specified.[2]
Job control was first implemented in the C shell by Jim Kulp,[3] then at IIASA in Austria, making use of features of the 4.1BSD kernel. The KornShell, developed at Bell Labs, adopted it and it was later incorporated into the SVR4 version of the Bourne shell, and exists in most modern Unix shells.
Overview
In a shell, a user has one foreground job but can run multiple jobs in the background. Many operations (i.e. listing files) are relatively quick so the user can wait for a response with little down time and some operations (i.e. editing) require interaction only possible via a foreground process. The user interactively enters a command and the implementation of the command (often a program) controls input and output such that the user cannot access the shell until the command completes or the user interrupts it (i.e. via Control+C).
However, a user may wish to perform an operation while using the shell for another purpose. An operation that is running but not using the interactive input and output is running in the background. The single operation that is using the interactive input and output is running in the foreground. Job control is the facility to control how operations run as foreground or background. A user can start a process in the background, send a running processes to the background, bring a background process to the foreground, and suspend or terminate a process.
A job maps to a single shell command execution which may result in multiple processes starting and completing. Multi-process operations come about because a process may create child processes, and a single shell command may consist of a pipeline of multiple communicating processes. For example, the following command line selects lines containing the text "title", sorts them alphabetically, and displays the result in a terminal pager: grep title somefile.txt | sort | less. This creates at least three processes: one for grep, one for sort, and one for less. Job control allows the shell to control these related processes as one entity, and when a user issues the appropriate key combination (usually Control+Z), the entire group of processes gets suspended.
Jobs are managed by the operating system as a single process group, and the job is the shell's internal representation of such a group. This is defined in POSIX as:[4]
A set of processes, comprising a shell pipeline, and any processes descended from it, that are all in the same process group.
A job can be referred to by a handle[a] called the job control job ID or simply job ID, which is used by shell builtins to refer to the job. A job ID begin with the % character; %n identifies job n, while %% identifies the current job. Other job IDs are specified by POSIX.[5] In informal usage the number may be referred to as the "job number" or "job ID", and Bash documentation refers to the (%-prefixed) job ID as the jobspec.[6]
Job control and job IDs are typically only used in interactive use, where they simplify referring to process groups; in scripting PGIDs are used instead, as they are more precise and robust, and indeed job control is disabled by default in bash scripts.
Implementation
Typically, the shell keeps a list of jobs in a job table. Recall that a job corresponds to a process group, which consists of all the members of a pipeline and their descendants. The jobs command will list the background jobs existing in the job table, along with their job number and job state (stopped or running). When a session ends when the user logs out (exits the shell, which terminates the session leader process), the shell process sends SIGHUP to all jobs, and waits for the process groups to end before terminating itself.
The disown command can be used to remove jobs from the job table, so that when the session ends the child process groups are not sent SIGHUP, nor does the shell wait for them to terminate. They thus become orphan processes, and may be terminated by the operating system, though more often this is used so the processes are adopted by init (the kernel sets their parent process to init) and continue executing as daemons. Alternatives to prevent jobs from being terminated include nohup and using a terminal multiplexer.
A job running in the foreground can be stopped by typing the suspend character (Ctrl-Z). This sends the "terminal stop" signal (SIGTSTP) to the process group. By default, SIGTSTP causes processes receiving it to stop, and control is returned to the shell. However, a process can register a signal handler for or ignore SIGTSTP. A process can also be paused with the "stop" signal (SIGSTOP), which cannot be caught or ignored.
A job running in the foreground can be interrupted by typing the interruption character (Ctrl-C). This sends the "interrupt" signal (SIGINT), which defaults to terminating the process, though it can be overridden.
A stopped job can be resumed as a background job with the bg builtin, or as the foreground job with fg. In either case, the shell redirects I/O appropriately, and sends the SIGCONT signal to the process, which causes the operating system to resume its execution. In Bash, a program can be started as a background job by appending an ampersand (&) to the command line; its output is directed to the terminal (potentially interleaved with other programs' output), but it cannot read from the terminal input.
A background process that attempts to read from or write to its controlling terminal is sent a SIGTTIN (for input) or SIGTTOU (for output) signal. These signals stop the process by default, but they may also be handled in other ways. Shells often override the default stop action of SIGTTOU so that background processes deliver their output to the controlling terminal by default.
In Bash-compatible shells, the kill builtin (not /bin/kill) can signal jobs by job ID as well as by process group ID – sending a signal to a job sends it to the whole process group, and jobs specified by a job ID should be killed by prefixing %. kill can send any signal to a job; however, if the intent is to rid the system of the processes the signals SIGKILL and SIGTERM (the default) are probably the most applicable.
Notes
- ^ A job ID is an abstract reference by the shell to a resource (a process group) managed externally, by the operating system, hence is a handle.
References
- ^ – Shell and Utilities Reference, The Single UNIX Specification, Version 5 from The Open Group; – Shell and Utilities Reference, The Single UNIX Specification, Version 5 from The Open Group.
- ^ Kerrisk, Michael (Feb 2, 2025). "wait(1p) — Linux manual page". man7.org. Retrieved May 13, 2025.
- ^ Foreword by Bill Joy in Anderson, Gail; Paul Anderson (1986). The UNIX C Shell Field Guide. Prentice-Hall. p. xvii. ISBN 0-13-937468-X.
- ^ IEEE Std 1003.1-2001, Section 3.201, Job
- ^ IEEE Std 1003.1-2001, Section 3.203, Job Control Job ID
- ^ 7.1 Job Control Basics
Further reading
- Marshall Kirk McKusick and George V. Neville-Neil (2004-08-02). "FreeBSD Process Management: Process Groups and Sessions". The Design and Implementation of the FreeBSD Operating System. Addison Wesley. ISBN 0-201-70245-2.
External links
- "Job Control", Bash Reference Manual