SYNOPSIS

bg [job_id ...]

DESCRIPTION

If job control is enabled (see the description of set -m), the bg utility 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.

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.

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 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.

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 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 editing features).

The bg utility is expected to wrap its output if the output exceeds the number of display columns.

FUTURE DIRECTIONS

None.

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 .