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.
bg — run jobs in the background
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 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.
bobby@letusstudy:~$ ping letusstudy.org &  2385611 bobby@letusstudy:~$ PING letusstudy.org (184.108.40.206) 56(84) bytes of data. 64 bytes from letusstudy.org (220.127.116.11): icmp_seq=1 ttl=128 time=68.6 ms 64 bytes from letusstudy.org (18.104.22.168): icmp_seq=2 ttl=128 time=69.5 ms ^Z + Stopped ping letusstudy.org bobby@letusstudy:~$ jobs + 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 &  2411011 #jobs command will list all the background jobs bobby@letusstudy:~$ jobs + Stopped ping letusstudy.org - 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 + 2385611 Stopped ping letusstudy.org - 2411011 Running firefox & bobby@letusstudy:~$
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.
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edi‐ tion, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 ap‐ plied.) 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.unix.org/online.html . Any typographical or formatting errors that appear in this page are most likely to have been intro‐ duced 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 .