bg

Linux bg command is used to run jobs in the background, if you have a long running scripts or application, you can run it in the background with the help of this command.

DEFINITION
bg — run jobs in the background
USAGE
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 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  cur‐
       rent  shell  execution  environment'',  as if it had been started as an asynchronous list;

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 Defini‐
                 tions volume of POSIX.1‐2008, 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.1‐2008, Section 8.2, Internationalization Variables for
                 the precedence of internationalization variables used to determine the values of locale cat‐
                 egories.)

       LC_ALL    If  set  to a non-empty string value, override the values of all the other internationaliza‐
                 tion 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
APPLICATION USAGE
       A job is generally suspended by typing the SUSP character (<control>‐Z on most systems); see the  Base
       Definitions volume of POSIX.1‐2008, 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
bobby@letusstudy:~$ ping letusstudy.org &
[2] 2385611
bobby@letusstudy:~$ PING letusstudy.org (134.122.21.22) 56(84) bytes of data.
64 bytes from letusstudy.org (134.122.21.22): icmp_seq=1 ttl=128 time=68.6 ms
64 bytes from letusstudy.org (134.122.21.22): icmp_seq=2 ttl=128 time=69.5 ms
^Z
[2]+  Stopped                 ping letusstudy.org
bobby@letusstudy:~$ jobs
[2]+  Stopped                 ping letusstudy.org
bobby@letusstudy:~$ 
# In this example, we used firefox to run in background. In real world, you can actually test with long running scripts or jobs. 
# firefox browser will open in background
bobby@letusstudy:~$ firefox &
[3] 2411011

#jobs command will list all the background jobs
bobby@letusstudy:~$ jobs
[2]+  Stopped                 ping letusstudy.org
[3]-  Running                 firefox &

#jobs -l is used to list with processid, so that you can kill them when they are no longer needed.
bobby@letusstudy:~$ jobs -l
[2]+ 2385611 Stopped                 ping letusstudy.org
[3]- 2411011 Running                 firefox &
bobby@letusstudy:~$ 

 

RATIONALE
The extensions to the shell specified in this volume of POSIX.1‐2008 have mostly been based on fea‐ tures 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.1‐2008 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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