bg - run jobs in the background
bg [job_id ...]
If job control is enabled (see the description of set -m), the bg utility shall resume suspended jobs from the current environment (see Shell Execution Environment ) by running them as background jobs. If the job specified by job_id is already a running background job, the bg utility shall have no effect and shall exit successfully.
Using bg to place a job into the background shall cause its process ID to become "known in the current shell execution environment", as if it had been started as an asynchronous list; see Asynchronous Lists .
The following operand shall be supported:
The following environment variables shall affect the execution of bg:
The output of bg shall consist of a line in the format:
"[%d] %s\n", <job-number>, <command>
where the fields are as follows:
The standard error shall be used only for diagnostic messages.
The following exit values shall be returned:
If job control is disabled, the bg utility shall exit with an error and no job shall be placed in the background.
The following sections are informative.
A job is generally suspended by typing the SUSP character (<control>-Z on most systems); see the Base Definitions volume of IEEE Std 1003.1-2001, Chapter 11, General Terminal Interface. At that point, bg can put the job into the background. This is most effective when the job is expecting no terminal input and its output has been redirected to non-terminal files. A background job can be forced to stop when it has terminal output by issuing the command:
A background job can be stopped with the command:
kill -s stop job ID
The bg utility does not work as expected when it is operating in its own utility execution environment because that environment has no suspended jobs. In the following examples:
... | xargs bg (bg)
each bg operates in a different environment and does not share its parent shell's understanding of jobs. For this reason, bg is generally implemented as a shell regular built-in.
The extensions to the shell specified in this volume of IEEE Std 1003.1-2001 have mostly been based on features provided by the KornShell. The job control features provided by bg, fg, and jobs are also based on the KornShell. The standard developers examined the characteristics of the C shell versions of these utilities and found that differences exist. Despite widespread use of the C shell, the KornShell versions were selected for this volume of IEEE Std 1003.1-2001 to maintain a degree of uniformity with the rest of the KornShell features selected (such as the very popular command line editing features).
The bg utility is expected to wrap its output if the output exceeds the number of display columns.
Asynchronous Lists , fg , kill() , jobs , wait()
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Created 1996-2022 by Maxim Chirkov
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