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BG(1P) POSIX Programmer's Manual BG(1P)
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
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
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.
None.
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.1‐2008, Section 3.204, Job
Control Job ID.
Not used.
None.
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.1‐2008, 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.
Default.
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.
The standard error shall be used only for diagnostic messages.
None.
None.
The following exit values shall be returned:
0 Successful completion.
>0 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. 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.
None.
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, 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.1‐2008 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.
None.
Section 2.9.3.1, Examples, fg(1p), kill(1p), jobs(1p), wait(1p)
The Base Definitions volume of POSIX.1‐2008, Section 3.204, Job
Control Job ID, Chapter 8, Environment Variables, Chapter 11, 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 .
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 2013 BG(1P)
Pages that refer to this page: fg(1p), jobs(1p)