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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 dif-
       fer (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 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 sus-
              pended 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  prece-
              dence 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  exam-
              ple, 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 writ-
              ten 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  utili-
              ties, 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 back-
       ground.

       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  Defini-
       tions  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.

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, Stan-
       dard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base  Specifica-
       tions  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 Stan-
       dard,  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 .



IEEE/The Open Group                  2003                               BG(1P)