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



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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(1)
