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PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface
may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface
may not be implemented on Linux.
NAME
bg — run jobs in the background
SYNOPSIS
bg [job_id...]
DESCRIPTION
If job control is enabled (see the description of set -m), the bg utility shall resume suspended jobs
from the current environment (see Section 2.12, 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 Section 2.9.3.1,
Examples.
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 the Base Definitions
volume of POSIX.1‐2017, Section 3.204, Job Control 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 the
Base Definitions volume of POSIX.1‐2017, Section 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 Determine the location of message catalogs for the processing of LC_MESSAGES.
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.
The following sections are informative.
APPLICATION USAGE
A job is generally suspended by typing the SUSP character (<control>‐Z on most systems); see the Base
Definitions volume of POSIX.1‐2017, 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:
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.1‐2017 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.1‐2017 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.
FUTURE DIRECTIONS
None.
SEE ALSO
Section 2.9.3.1, Examples, fg, kill, jobs, wait
The Base Definitions volume of POSIX.1‐2017, Section 3.204, Job Control Job ID, Chapter 8, Environment
Variables, Chapter 11, General Terminal Interface
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original
IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document.
The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced
during the conversion of the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 BG(1POSIX)