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PROLOG | NAME | SYNOPSIS | DESCRIPTION | OPTIONS | OPERANDS | STDIN | INPUT FILES | ENVIRONMENT VARIABLES | ASYNCHRONOUS EVENTS | STDOUT | STDERR | OUTPUT FILES | EXTENDED DESCRIPTION | EXIT STATUS | CONSEQUENCES OF ERRORS | APPLICATION USAGE | EXAMPLES | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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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.1‐2017, 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.1‐2017, 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.
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.1‐2017, 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.
None.
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.
None.
Section 2.9.3.1, Examples, fg(1p), kill(1p), jobs(1p), wait(1p)
The Base Definitions volume of POSIX.1‐2017, 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 1003.1-2017, 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(1P)
Pages that refer to this page: fg(1p), jobs(1p)
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HTML rendering created 2023-12-22 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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