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However, sometimes the user will wish to carry out a task while using the terminal for another purpose. A task that is running but is not receiving input from the terminal is said to be running "in the background", while the single task that is receiving input from the terminal is "in the
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maps the (shell) concept of a single shell command to the (operating system) concept of the possibly many processes that the command entails. Multi-process tasks come about because processes may create additional child processes, and a single shell command may consist of a
551:(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.
254:
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.
485:) 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 (
112:
shell). Most tasks (directory listing, editing files, etc.) can easily be accomplished by letting the program take control of the terminal and returning control to the shell when the program exits – formally, by attaching to
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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
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foreground". Job control is a facility developed to make this possible, by allowing the user to start processes in the background, send already running processes into the background, bring
564:) 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
64:. Basic job control features are the suspending, resuming, or terminating of all processes in the job/process group; more advanced features can be performed by sending
536:) 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.
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of multiple communicating processes. For example, a command to select lines containing the text "title", sort these alphabetically, and display the result in a
117:
to the shell, which reads or writes from the terminal, and catching signals sent from the keyboard, like the termination signal resulting from pressing
248:. 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
181:. Job control allows the shell to control these related processes as one entity, and when a user issues the appropriate key combination (usually
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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.
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command will list the background jobs existing in the job table, along with their job number and job state (stopped or running). When a
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A set of processes, comprising a shell pipeline, and any processes descended from it, that are all in the same process group.
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The POSIX standard specifies two commands for resuming suspended jobs in the background and foreground, respectively
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31:
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Here "task" is a non-technical term for "some activity", while "process" and "job" are technical terms.
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can send any signal to a job; however, if the intent is to rid the system of the processes the signals
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279:, developed at Bell Labs, adopted it and it was later incorporated into the SVR4 version of the
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417:. Recall that a job corresponds to a process group, which consists of all the members of a
8:
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A job running in the foreground can be interrupted by typing the interruption character (
196:, and the job is the shell's internal representation of such a group. This is defined in
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Please expand the section to include this information. Further details may exist on the
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signal to the process, which causes the operating system to resume its execution. In
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A job running in the foreground can be stopped by typing the suspend character (
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to all jobs, and waits for the process groups to end before terminating itself.
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This article is about job control on Unix system. For the general term, see
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502:), which defaults to terminating the process, though it can be overridden.
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532:, a program can be started as a background job by appending an ampersand (
458:(the kernel sets their parent process to init) and continue executing as
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to the job. Job control is of particular interest in Unix due to its
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A background process that attempts to read from or write to its
339:. These were modeled after the Korn shell job control commands.
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generally, which is frequently applied to sequential execution (
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462:. Alternatives to prevent jobs from being terminated include
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identifies the current job. Other job IDs are specified by
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A stopped job can be resumed as a background job with the
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into the foreground, and suspend or terminate processes.
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Jobs are managed by the operating system as a single
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189:), the entire group of processes gets suspended.
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580:(the default) are probably the most applicable.
413:Typically, the shell keeps a list of jobs in a
165:This creates at least three processes: one for
271:in Austria, making use of features of the 4.1
729: – Shell and Utilities Reference,
713: – Shell and Utilities Reference,
228:to refer to the job. Job IDs begin with the
381:. Unsourced material may be challenged and
754:and George V. Neville-Neil (2004-08-02).
401:Learn how and when to remove this message
283:, and exists in most modern Unix shells.
263:Job control was first implemented in the
433:(exits the shell, which terminates the
14:
796:
678:Anderson, Gail; Paul Anderson (1986).
520:. In either case, the shell redirects
496:). This sends the "interrupt" signal (
489:), which cannot be caught or ignored.
379:adding citations to reliable sources
346:
290:
477:). This sends the "terminal stop"
72:, and should be distinguished from
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437:process), the shell process sends
115:standard input and standard output
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651:Section 3.203, Job Control Job ID
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514:, or as the foreground job with
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686:. Prentice-Hall. p. xvii.
554:In Bash-compatible shells, the
27:Control of jobs by a Unix shell
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208:A job can be referred to by a
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731:The Single UNIX Specification
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524:appropriately, and sends the
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421:and their descendants. The
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10:
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29:
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108:(it may or may not be a
96:operating systems via a
32:job control (computing)
752:Marshall Kirk McKusick
662:7.1 Job Control Basics
649:IEEE Std 1003.1-2001,
637:IEEE Std 1003.1-2001,
306:is missing information
206:
788:Bash Reference Manual
267:by Jim Kulp, then at
202:
153:greptitlesomefile.txt
52:refers to control of
541:controlling terminal
468:terminal multiplexer
375:improve this section
131:background processes
429:ends when the user
224:, which is used by
764:. Addison Wesley.
639:Section 3.201, Job
214:job control job ID
733:, Version 4 from
717:, Version 4 from
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391:February 2020
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466:and using a
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281:Bourne shell
275:kernel. The
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308:about wait.
232:character;
212:called the
88:When using
74:job control
50:job control
625:References
543:is sent a
320:April 2021
216:or simply
171:, one for
562:/bin/kill
415:job table
362:does not
312:talk page
277:KornShell
94:Unix-like
43:Unix-like
18:Bg (Unix)
798:Category
674:Bill Joy
584:See also
431:logs out
419:pipeline
287:Commands
250:jobspec.
240:, while
143:pipeline
98:terminal
84:Overview
590:C shell
578:SIGTERM
574:SIGKILL
549:SIGTTOU
545:SIGTTIN
526:SIGCONT
512:builtin
487:SIGSTOP
483:SIGTSTP
460:daemons
427:session
383:removed
368:sources
265:C shell
259:History
183:Control
119:Control
66:signals
768:
690:
499:SIGINT
494:Ctrl-C
479:signal
475:Ctrl-Z
447:disown
439:SIGHUP
220:job ID
210:handle
596:Notes
534:&
464:nohup
269:IIASA
246:POSIX
198:POSIX
161:less
147:pager
110:login
106:shell
58:shell
56:by a
804:Unix
766:ISBN
688:ISBN
576:and
570:kill
557:kill
530:Bash
456:init
444:The
423:jobs
366:any
364:cite
335:and
200:as:
178:less
173:sort
168:grep
157:sort
100:(or
90:Unix
54:jobs
41:and
39:Unix
676:in
522:I/O
377:by
273:BSD
138:job
92:or
80:).
37:In
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