BG(P) POSIX Programmer's Manual BG(P)NAMEbg - run jobs in the background
SYNOPSISbg [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 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 Asynchronous Lists .
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 IEEE Std 1003.1-2001, Section 3.203, 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
IEEE Std 1003.1-2001, 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 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 iden‐
tified 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
IEEE Std 1003.1-2001, 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 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
implemented as a shell regular built-in.
EXAMPLES
None.
RATIONALE
The extensions to the shell specified in this volume of
IEEE Std 1003.1-2001 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 char‐
acteristics 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 IEEE Std 1003.1-2001 to main‐
tain 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
Asynchronous Lists , fg , kill() , jobs , wait()COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .
IEEE/The Open Group 2003 BG(P)
