Jobs: &, Ctrl+Z, bg, fg and nohup

Foreground and background

Normally a command runs in the foreground: the shell waits until it's finished before you get the prompt back. Put & at the end and it runs in the background instead, so you can keep working. The shell prints its job number in square brackets, then its PID:

sleep 300 &
sleep 400 &
jobs

In the terminal

student@vulntech:~$ sleep 300 &
[1] 1601
student@vulntech:~$ sleep 400 &
[2] 1610
student@vulntech:~$ jobs
[1]-  Running                 sleep 300 &
[2]+  Running                 sleep 400 &

jobs lists the background jobs of this shell. The + marks the current job (the one fg and bg use if you don't name one), - the one before.

Ctrl+C and Ctrl+Z

Two keys control the program in the foreground. Ctrl+C interrupts it (the INT signal), which usually ends it. In the examples, ^C means you press Ctrl+C:

sleep 100
^C
echo Back at the prompt

In the terminal

student@vulntech:~$ sleep 100
^C
student@vulntech:~$ echo Back at the prompt
Back at the prompt

Ctrl+Z (^Z) pauses it instead, and gives you the prompt back. The job is Stopped: still there, but frozen. bg lets it carry on in the background:

sleep 200
^Z
jobs
bg
jobs

In the terminal

student@vulntech:~$ sleep 200
^Z
[1]+  Stopped                 sleep 200
student@vulntech:~$ jobs
[1]+  Stopped                 sleep 200
student@vulntech:~$ bg
[1]+ sleep 200 &
student@vulntech:~$ jobs
[1]+  Running                 sleep 200 &

Back to the foreground: fg

fg brings a job back to the foreground, and the shell waits for it again. Choose a job with % and its number. This one only sleeps for 3 seconds, so the prompt comes back by itself:

sleep 3 &
sleep 500 &
fg %1

In the terminal

student@vulntech:~$ sleep 3 &
[1] 1601
student@vulntech:~$ sleep 500 &
[2] 1610
student@vulntech:~$ fg %1
-bash: fg: job has terminated

Job numbers work with kill too, which saves looking up PIDs:

sleep 500 &
sleep 600 &
kill %1
jobs

In the terminal

student@vulntech:~$ sleep 500 &
[1] 1601
student@vulntech:~$ sleep 600 &
[2] 1610
student@vulntech:~$ kill %1
student@vulntech:~$ jobs
[1]-  Terminated              sleep 500
[2]+  Running                 sleep 600 &

Surviving the end of the session: nohup

When you close a terminal or log out, its jobs get the HUP (hang-up) signal and normally stop. nohup makes a command ignore it, and saves its output in nohup.out because the terminal will be gone:

nohup sleep 900 &
ls

In the terminal

student@vulntech:~$ nohup sleep 900 &
[1] 1601
student@vulntech:~$ ls
nohup.out

To keep a job you've already started, take it off the shell's list with disown. It keeps running, but jobs no longer knows about it:

sleep 800 &
disown
jobs
pgrep -a sleep

In the terminal

student@vulntech:~$ sleep 800 &
[1] 1601
student@vulntech:~$ disown
student@vulntech:~$ jobs
student@vulntech:~$ pgrep -a sleep
1601 sleep 800

Common mistake: Ctrl+Z doesn't end a program; it only pauses it. If you pressed it by accident, type fg to get your program back.

Remember: & runs in the background, Ctrl+Z pauses, bg and fg move jobs, Ctrl+C interrupts, nohup keeps a job alive after you leave.