# bg

## PROLOG



           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.
    


## NAME



           bg — run jobs in the background
    


## 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 _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_.
    


## 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 POSIX.1‐2017, _Section 3.204_ ,
                     _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 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_.
    


## 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.
    


## 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
           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.
    


## EXAMPLES



           None.
    


## RATIONALE



           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.
    


## FUTURE DIRECTIONS



           None.
    


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