How-to: Job control

Job control refers to the ability to selectively stop (suspend) the execution of processes and continue (resume) their execution at a later point. A user typically employs this facility via an interactive interface supplied jointly by the system’s terminal driver and Bash.

The shell associates a job with each pipeline. It keeps a table of currently executing jobs, which can be listed with the jobs command. When Bash starts a job asynchronously, it prints a line that looks like:

[1] 25647

indicating that this job is job number 1 and that the process ID of the last process in the pipeline associated with this job is 25647. All of the processes in a single pipeline are members of the same job. Bash uses the job abstraction as the basis for job control.

To facilitate the implementation of the user interface to job control, the Operating System maintains the notion of a current terminal process group ID. Members of this process group (processes whose process group ID is equal to the current terminal process group ID) receive keyboard-generated signals such as SIGINT. These processes are said to be in the foreground. Background processes are those whose process group ID differs from the terminal’s; such processes are immune to keyboard-generated signals. Only foreground processes are allowed to read from or write to the terminal. Background processes which attempt to read from (write to) the terminal are sent a SIGTTIN (SIGTTOU) signal by the terminal driver, which, unless caught, suspends the process.

If the Operating System on which Bash is running supports job control, Bash contains facilities to use it. Typing the suspend character (typically '^Z', Control-Z) while a process is running causes that process to be stopped and returns control to Bash.

Typing the delayed suspend character (typically '^Y', Control-Y) causes the process to be stopped when it attempts to read input from the terminal, and control to be returned to Bash. The user then manipulates the state of this job, using the bg command to continue it in the background, the fg command to continue it in the foreground, or the kill command to kill it.

A '^Z' takes effect immediately, and has the additional side effect of causing pending output and typeahead to be discarded.

There are a number of ways to refer to a job in the shell. The character '%' introduces a job name.

Job number n can be referred to as '%n'. The symbols '%%' and '%+' refer to the shell’s notion of the current job, which is the last job stopped while it was in the foreground or started in the background. The previous job can be referenced using '%-'. In output pertaining to jobs (e.g., the output of the jobs command), the current job is always flagged with a '+', and the previous job with a '-'.

A job can also be referred to using a prefix of the name used to start it, or using a substring that appears in its command line. For example, '%ce' refers to a stopped ce job. Using '%?ce', on the other hand, refers to any job containing the string 'ce' in its command line. If the prefix or substring matches more than one job, Bash reports an error.

Simply naming a job can be used to bring it into the foreground: '%1' is a synonym for 'fg %1', bringing job 1 from the background into the foreground. Similarly, '%1 &' resumes job 1 in the background, equivalent to 'bg %1'

The shell learns immediately whenever a job changes state. Normally, Bash waits until it is about to print a prompt before reporting changes in a job’s status so as to not interrupt any other output. If the the '-b' option to the set builtin is enabled, Bash reports such changes immediately.

If an attempt is made to exit Bash is while jobs are stopped, the shell prints a message warning that there are stopped jobs. The jobs command can then be used to inspect their status. If a second attempt to exit is made without an intervening command, Bash does not print another warning, and the stopped jobs are terminated.

When job control is not active, the kill and wait builtins do not accept jobspec arguments. They must be supplied process IDs.

Related Linux commands

bg - Resume the suspended job jobspec in the background.
cron - Daemon to execute scheduled commands.
fg - Resume the suspended job jobspec in the foreground.
jobs - list the active jobs.
disown - Remove job from the table of active jobs.
suspend - Suspend the execution of this shell.
wait - Wait until the child process exits.
PowerShell equivalent: Scheduler cmdlets

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