BG(1POSIX) POSIX Programmer's Manual BG(1POSIX)
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 util-
ity shall resume suspended jobs from the current environment (see Sec-
tion 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, Exam-
ples.
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 Con-
trol 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 vari-
ables that are unset or null. (See the Base Definitions vol-
ume of POSIX.1-2017, Section 8.2, Internationalization Vari-
ables 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 er-
ror.
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 expect-
ing no terminal input and its output has been redirected to non-termi-
nal 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 sus-
pended jobs. In the following examples:
... | xargs bg
(bg)
each bg operates in a different environment and does not share its par-
ent shell's understanding of jobs. For this reason, bg is generally im-
plemented 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 Ko-
rnShell. The standard developers examined the characteristics of the C
shell versions of these utilities and found that differences exist. De-
spite widespread use of the C shell, the KornShell versions were se-
lected for this volume of POSIX.1-2017 to maintain a degree of unifor-
mity 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 -- Por-
table Operating System Interface (POSIX), The Open Group Base Specifi-
cations 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.ker-
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 BG(1POSIX)
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