If job control is enabled (see the description of set-m), the
bg utility shall resume suspended jobs from the current environment
(see ShellExecution 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 AsynchronousLists
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
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
IEEE Std 1003.1-2001, 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.
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:
A background job can be stopped with the command:
kill -s stopjob 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.
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 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
IEEE Std 1003.1-2001 to maintain a degree of uniformity with the
rest of the KornShell features selected (such as the very popular command line
The bg utility is expected to wrap its output if the output exceeds the
number of display columns.
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