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.
bg — run jobs in the background
If job control is enabled (see the description of set −m
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
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 188.8.131.52
The following operand shall be supported:
- 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.1‐2008, Section 3.204, Job Control Job
The following environment variables shall affect the execution of bg
- Provide a default value for the internationalization
variables that are unset or null. (See the Base Definitions volume of
POSIX.1‐2008, Section 8.2, Internationalization
Variables for the precedence of internationalization variables used to
determine the values of locale categories.)
- If set to a non-empty string value, override the values of
all the other internationalization variables.
- 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).
Determine the locale that should be used to affect the format and contents
of diagnostic messages written to standard error.
- Determine the location of message catalogs for the
processing of LC_MESSAGES.
The output of bg
shall consist of a line in the format:
"[%d] %s\n", <job-number>, <command>
where the fields are as follows:
- 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
- The associated command that was given to the shell.
The standard error shall be used only for diagnostic messages.
The following exit values shall be returned:
- Successful completion.
- An error occurred.
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
POSIX.1‐2008, Chapter 11
, General Terminal Interface
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:
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:
operates in a different environment and does not share its parent
shell's understanding of jobs. For this reason, bg
implemented as a shell regular built-in.
The extensions to the shell specified in this volume of POSIX.1‐2008 have
mostly been based on features provided by the KornShell. The job control
features provided by bg
, 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.1‐2008 to maintain a degree of uniformity with the rest
of the KornShell features selected (such as the very popular command line
utility is expected to wrap its output if the output exceeds the
number of display columns.
The Base Definitions volume of POSIX.1‐2008, Section 3.204
Control Job ID
, Chapter 8
, Environment Variables
, General Terminal Interface
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2013 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
7, Copyright (C) 2013 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013
Technical Corrigendum 1 applied.) 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.unix.org/online.html .
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