NAME

bg — run jobs in the background

SYNOPSIS

[UP] bg [job_id...]

DESCRIPTION

If job control is enabled (see the description of set -m), the shell is interactive, and the current shell execution environment (see 2.13 Shell Execution Environment) is not a subshell environment, the bg utility shall resume suspended jobs from the current shell execution environment by running them as background jobs, as described in 2.11 Job Control; it may also do so if the shell is non-interactive or the current shell execution environment is a subshell environment. If the job specified by job_id is already a running background job, the bg utility shall have no effect and shall exit successfully.

OPTIONS

None.

OPERANDS

The following operand shall be supported:

job_id

Specify the job to be resumed as a background job. If no job_id operand is given, the most recently suspended job shall be used. The format of job_id is described in XBD 3.182 Job ID .

STDIN

Not used.

INPUT FILES

None.

ENVIRONMENT VARIABLES

The following environment variables shall affect the execution of bg:

LANG

Provide a default value for the internationalization variables that are unset or null. (See XBD 8.2 Internationalization Variables for the precedence of internationalization variables used to determine the values of locale categories.)

LC_ALL

If set to a non-empty string value, override the values of all the other internationalization variables.

LC_CTYPE

Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte as opposed to multi-byte characters in arguments).

LC_MESSAGES

Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard error.

NLSPATH

^[XSI] Determine the location of messages objects and message catalogs.

ASYNCHRONOUS EVENTS

Default.

STDOUT

The output of bg shall consist of a line in the format:

"[%d] %s\n", <job-number>, <command>

where the fields are as follows:

<job-number>

A number that can be used to identify the job to the wait, fg, and kill utilities. Using these utilities, the job can be identified by prefixing the job number with '%'.

<command>

The associated command that was given to the shell.

STDERR

The standard error shall be used only for diagnostic messages.

OUTPUT FILES

None.

EXTENDED DESCRIPTION

None.

EXIT STATUS

The following exit values shall be returned:

 0

Successful completion.

>0

An error occurred.

CONSEQUENCES OF ERRORS

If job control is disabled, the bg utility shall exit with an error and no job shall be placed in the background.

APPLICATION USAGE

This utility is required to be intrinsic. See 1.7 Intrinsic Utilities for details.

A job is generally suspended by typing the SUSP character (<control>-Z on most systems); see XBD 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:

stty tostop

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.

EXAMPLES

None.

RATIONALE

The extensions to the shell specified in this volume of POSIX.1-2024 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 POSIX.1-2024 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.

The bg and fg utilities are not symmetric as regards the list of process IDs known in the current shell execution environment. Whereas fg removes a process ID from this list, bg has no need to add one to this list when it resumes execution of a suspended job in the background, because this has already been done by the shell when the suspended background job was created (see 2.11 Job Control).

FUTURE DIRECTIONS

None.

SEE ALSO

2.9.3.1 Asynchronous AND-OR Lists, fg, kill, jobs, wait

XBD 3.182 Job ID, 8. Environment Variables, 11. General Terminal Interface

CHANGE HISTORY

First released in Issue 4.

Issue 6

This utility is marked as part of the User Portability Utilities option.

The JC margin marker on the SYNOPSIS is removed since support for Job Control is mandatory in this version. This is a FIPS requirement.

Issue 7

SD5-XCU-ERN-97 is applied, updating the SYNOPSIS.

Issue 8

Austin Group Defect 854 is applied, adding a note to the APPLICATION USAGE section that this utility is required to be intrinsic.

Austin Group Defect 1122 is applied, changing the description of NLSPATH .

Austin Group Defect 1254 is applied, updating the DESCRIPTION to account for the addition of 2.11 Job Control and adding a paragraph to RATIONALE.