bg

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 (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.

SEE ALSO
       Section 2.9.3.1, Examples, fg, kill, jobs, wait

       The Base Definitions volume of POSIX.1‐2017, Section 3.204, Job Control
       Job ID, Chapter 8, Environment Variables, Chapter 11, General Terminal
       Interface

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       Any typographical or formatting errors that appear in this page are most
       likely to have been introduced during the conversion of the source files
       to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                   2017                                BG(1P)