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In Unix and Unix-like operating systems, job control refers to control of jobs by a shell, especially interactively, where a "job" is a shell's representation for a process group. 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 signals to the job. Job control is of particular interest in Unix due to its multiprocessing, and should be distinguished from job control generally, which is frequently applied to sequential execution (batch processing).

This creates at least three processes: one for grep, one for sort, and one for 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 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 background processes into the foreground, and suspend or terminate processes.

The concept of a job 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 pipeline of multiple communicating processes. For example, a command to select lines containing the text "title", sort these alphabetically, and display the result in a 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:[1]

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[b] called the job control job ID or simply job ID, whi

A job can be referred to by a handle[b] called the job control job ID or simply job ID, which is used by shell builtins to refer to the job. Job IDs begin with the % character; %n identifies job n, while %% identifies the current job. Other job IDs are specified by POSIX.[2] 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.[3]

Job control

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.

Job control was first implemented in the C shell by Jim Kulp,[4] 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.

CommandsThe POSIX standard specifies two commands for resuming suspended jobs in the background and foreground, respectively bg and fg. These were modeled after the Korn shell job control commands.[5]

Implementation