Jobs: pause, background & bring back

Some commands take a long time: a big download, a backup, a build. You don't have to sit and wait, or open a second terminal. The shell can pause a running command, let it keep running in the background while you do something else, and bring it back when you want it. This is called job control, and it's built into bash on every Linux.

Foreground, stopped, background

A normal command runs in the foreground: it owns your terminal until it finishes. Two key combos change that:

sleep 300          # waits 5 minutes…
^Z                 # Ctrl+Z
[1]+  Stopped                 sleep 300
jobs               # [1]+  Stopped   sleep 300
bg                 # let it continue, in the background:  [1]+ sleep 300 &
fg                 # bring it back to the foreground

The [1] is the job number. It's only for this shell, and you write it with a %: fg %1, bg %2, kill %1. The + marks the "current" job (the one fg and bg pick with no number), - the one before it.

Start in the background: &

sleep 200 &        # [1] 5905   ← job number and process ID (PID)
jobs -l            # with PIDs
ps                 # the same sleep shows up as a normal process

When a background job finishes, bash tells you just before your next prompt: [1]+ Done sleep 200. A background job that prints things still prints them into your terminal, so redirect its output (> out.txt 2>&1) if it's chatty.

Jobs are processes with a nickname

yes > /dev/null &   # a job that keeps one CPU at 100%
top                 # there it is at the top of the list
kill %1             # by job number…
kill 2981           # …or by PID: same thing

Job numbers are a convenience of your shell. Other terminals and other users only see PIDs. (More on processes and signals in Linux Sysadmin, lesson 6.)

Logging out kills your jobs (unless…)

When your SSH session ends, the shell sends its jobs a SIGHUP ("hang up"), and most programs quit. Two ways around it:

nohup ./report.sh &     # ignore the hang-up; output goes to nohup.out
disown %1               # already running? tell the shell to forget it

Practice: juggle some jobs 🤹

sleep really waits here, so it makes a good long-running job. There's also a slow script, report.sh, in your home folder.

Quick check