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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 .