Linux: Start Command In Background

Syntax

You can put a task (such as command or script) in a background by appending a & at the end of the command line. The & operator puts command in the background and free up your terminal. The command which runs in background is called a job. You can type other command while background command is running. The syntax is:

command &
script-name &
/path/to/command arg1 arg2 &
command-1 | command-2 arg1 &
command-1 | command-2 -arg1 -arg2 >/path/to/output &

Examples

Put the ls command in the background, enter:
$ ls *.py > output.txt &
Put the following find command in a background by putting a ‘&’ at the end of the command line:

find . -iname "*.mp3" > myplaylist.txt &

Sample outputs:

Fig.01: Linux background job in action (click to enlarge)

How do I see jobs running in the background?

Type the following command:
jobs
Sample outputs:

[1]-  Running                 find / -iname "*.c" 2> /dev/null > /tmp/output.txt &
[2]+  Running                 grep -R "hostNamed" / 2> /dev/null > /tmp/grep.txt &

Where,

• [1] and [2] are job IDS.

To see process IDs for JOB IDs in addition to the normal information pass the -l option:
jobs -l
Sample outputs:

[1]-  7307 Running                 find / -iname "*.c" 2> /dev/null > /tmp/output.txt &
[2]+  7324 Running                 grep -R "hostNamed" / 2> /dev/null > /tmp/grep.txt &

To see process IDs only, enter:
jobs -p
Sample outputs:

7307
7324

How do I kill the jobs running in the background?

Use the kill command to kill process either gracefully or forcefully. The syntax is:

kill PID
kill -15 PID
kill -9 PID
killall process-Name-Here
killall -15 process-Name-Here
killall -9 process-Name-Here

See how to use killall command under Linux operating system for more information.

How do I bring process running in the background to the foreground?

The syntax is:
%JOB-ID
OR
fg JOB-ID
First, list the current jobs with jobs command, enter:
jobs -l
Sample outputs:

[1]-  7307 Running                 find / -iname "*.c" 2> /dev/null > /tmp/output.txt &
[2]+  7324 Running                 grep -R "hostNamed" / 2> /dev/null > /tmp/grep.txt &

To bring the job id #2 to the foreground, enter:
%2
OR use fg command:
fg 2
Sample outputs:

grep -R "hostNamed" / 2> /dev/null > /tmp/grep.txt

To send back this job in the background hit CTRL-Z i.e. while holding the CTRL key, press z key. This will suspend the current foreground job. Type the following command to send back the job in the background:
%2 &
OR use bg command:
bg
The grep command job is now running in the background.

Summary of all useful commands

Note: n == Job id (use jobs command to see job id)..

See also:

• Putting jobs in background from the Linux shell scripting tutorial.
• Command examples pages: jobs command, bg command, and fg command
• Man pages: ksh(1)