Exercise: Juggle jobs in one terminal

Run three long commands at once from a single terminal: start, pause, resume, bring to the front and end them, and see which survive a logout.

Juggle jobs in one terminal

Start, pause, resume and stop jobs with &, Ctrl+Z, bg, fg and kill.
Distinguish job numbers from PIDs.
Keep a command running after logout with nohup.

Start two background jobs

On servera, start sleep 800 and sleep 801 as background jobs, then list them with PIDs.

[student@workstation ~]$ ssh student@servera
[student@servera ~]$ sleep 800 &
[1] 762
[student@servera ~]$ sleep 801 &
[2] 763
[student@servera ~]$ jobs -l
[1]-   762 Running                 sleep 800 &
[2]+   763 Running                 sleep 801 &

The numbers in brackets are job numbers; the others are PIDs.

Pause one in the foreground

Bring job 1 to the foreground, then press Ctrl+Z to pause it. Check the state with jobs, and with ps.

[student@servera ~]$ fg %1
sleep 800
^Z
[1]+  Stopped                 sleep 800
[student@servera ~]$ jobs
[1]+  Stopped                 sleep 800
[2]-  Running                 sleep 801 &
[student@servera ~]$ ps -o pid,stat,cmd -C sleep
    PID STAT CMD
    762 T    sleep 800
    763 S    sleep 801

T is the stopped state, S sleeping. The shell printed ^Z where you pressed the key.

Resume it in the background and end the other

[student@servera ~]$ bg %1
[1]+ sleep 800 &
[student@servera ~]$ kill %2
[2]+  Terminated              sleep 801
[student@servera ~]$ jobs
[1]+  Running                 sleep 800 &

Interrupt a foreground command

Run sleep 600 in the foreground (the prompt does not return). Press Ctrl+C. What came back?

[student@servera ~]$ sleep 600
^C
[student@servera ~]$

Ctrl+C sent SIGINT, the program ended, and the prompt returned. Unlike Ctrl+Z, the command is gone, not paused.

Survive a dropped terminal

Start sleep 802 and nohup sleep 700 in the background. Then simulate a closed terminal by sending SIGHUP to your own shell with kill -HUP $. Log in again and find out which of the two is still alive. Why?

[student@servera ~]$ sleep 802 &
[1] 4699
[student@servera ~]$ nohup sleep 700 &
[2] 4700
nohup: ignoring input and appending output to 'nohup.out'
[student@servera ~]$ kill -HUP $
Connection to servera closed.
[student@workstation ~]$ ssh student@servera
[student@servera ~]$ pgrep -a sleep
4700 sleep 700

A closed terminal or a dropped connection makes the shell receive SIGHUP, and it passes the hangup on to its jobs. sleep 802 ended; sleep 700 was protected by nohup. (Typing exit normally is different: the shell leaves its background jobs alone.)

Clean up

[student@servera ~]$ pkill sleep
[student@servera ~]$ rm -f nohup.out
[student@servera ~]$ exit