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 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 .
The following operand shall be supported:
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.
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 .
The output of bg shall consist of a line in the format:
"[%d] %s\n", <job-number>, <command>
where the fields are as follows:
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 '%' .
The associated command that was given to the shell.
The standard error shall be used only for diagnostic messages.
The following exit values shall be returned:
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.
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 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
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 editing features).
The bg utility is expected to wrap its output if the output exceeds the number of display columns.