BG(1P)                                              POSIX Programmer's Manual                                             BG(1P)



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (con-
       sult 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 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 Variables for the precedence of international-
              ization 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 stan-
              dard 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 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 envi-
       ronment 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.

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,  Copy-
       right (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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                        BG(1P)