Jobs and Signals

Background jobs

Normally a command runs in the foreground: it has the terminal, and you wait for the prompt to come back. Put & at the end and it runs in the background, and you get the prompt back at once ¹:

me@linuxbox:~$ sleep 600 &
[1] 2381
me@linuxbox:~$ jobs
[1]+  Running                 sleep 600 &

The shell prints a job number in brackets and the process ID. jobs lists the jobs started from this shell ¹.

Three keys and commands move jobs around ¹:

• Ctrl-Z pauses (stops) the foreground job and gives you the prompt.
• bg resumes a stopped job in the background.
• fg brings a job to the foreground. Pick one with its job number: fg %1.

me@linuxbox:~$ rsync -a ~/photos /mnt/backup/      # oops, this will take a while
^Z
[1]+  Stopped                 rsync -a ~/photos /mnt/backup/
me@linuxbox:~$ bg
[1]+ rsync -a ~/photos /mnt/backup/ &
me@linuxbox:~$                                       # prompt back; rsync carries on

A background job still prints to your terminal, which can be messy; redirect its output to a file when you start it: long-job > job.log 2>&1 &. And a background job that tries to read from the keyboard is stopped until you bring it to the foreground ¹.

Signals

How do you tell a program to stop? You send it a signal, a small message the kernel delivers. Ctrl-C and Ctrl-Z are signals; so is everything kill sends ¹. A program can catch most signals and react, for example by saving its work before quitting.

The signals worth knowing ¹:

kill -l lists them all.

kill, killall, and pkill

kill sends a signal to a process ID or a job number ¹:

me@linuxbox:~$ kill 2381          # TERM: please stop
me@linuxbox:~$ kill %1            # the same, by job number
me@linuxbox:~$ kill -9 2381       # KILL: last resort
me@linuxbox:~$ kill -HUP 812      # signals by name or number

Always try plain kill first. kill -9 gives the program no chance to save, clean up temporary files, or finish writing, so it’s for programs that ignore TERM ¹.

To signal programs by name ¹:

me@linuxbox:~$ killall firefox        # every process named exactly firefox
me@linuxbox:~$ pkill -f backup.sh     # every process whose command line contains backup.sh
me@linuxbox:~$ pgrep -a python        # see what pkill would match, first

You can signal only your own processes; other users’ need sudo ¹. In top and htop, k sends a signal too.

Surviving a closed terminal

Closing a terminal sends HUP to the jobs running in it, and most programs quit on HUP ¹. To keep a long job alive after you close the window, start it with nohup, which makes it ignore HUP ¹:

me@linuxbox:~$ nohup ./big-backup.sh > backup.log 2>&1 &

(Without your own redirection, nohup saves the output to nohup.out.) For anything you’ll want to come back to and watch, a terminal multiplexer like tmux is better: it keeps whole sessions alive and lets you reattach, even over SSH. The Linux course covers it.

Priority: nice and renice

Each process has a niceness from -20 (greediest) to 19 (most polite); the default is 0. A nicer process gets the CPU only after less nice ones. Ordinary users can only make their own processes nicer; only root can raise priority ¹:

me@linuxbox:~$ nice -n 19 tar czf archive.tar.gz ~/photos    # start it polite
me@linuxbox:~$ renice -n 19 -p 2381                          # make a running one polite

Niceness 19 means “only when nothing else wants the CPU”: perfect for big compressions and video conversions while you keep working ¹.

Shutting down

Shutting down stops every process in order and makes sure everything is written to disk ¹:

me@linuxbox:~$ sudo shutdown -h now       # power off
me@linuxbox:~$ sudo reboot                # restart

Under WSL, there’s nothing to power off: close the window, or from PowerShell run wsl --shutdown to stop all distributions.

Your turn

So

& starts a job in the background; Ctrl-Z pauses the foreground one, and bg and fg move jobs between the two. Programs are stopped with signals: Ctrl-C sends INT, kill sends TERM, and kill -9 sends KILL only as a last resort. nohup survives a closed terminal, and nice keeps big jobs out of your way.