Processes and Job Control

A running program is a process, identified by a process ID (PID). Bash can run processes in the background, wait for them, and stop them.

Viewing processes

# illustrative
ps aux | grep nginx
pgrep -a python
top          # live view; press q to quit (or use htop)

Background jobs

Add & to run a command in the background, so you can keep working:

sleep 0.3 && echo "slow task finished" &
echo "started in the background"
wait
echo "all background jobs done"

started in the background
slow task finished
all background jobs done

• & starts the command and returns immediately.
• wait pauses until background jobs finish. wait $pid waits for one; $! holds the PID of the last background job.

Running tasks in parallel

task() { sleep "$2"; echo "$1 done"; }

task "download A" 0.6 &
task "download B" 0.2 &
task "download C" 0.4 &
wait
echo "All downloads finished"

download B done
download C done
download A done
All downloads finished

All three run at the same time, so the whole thing takes about 0.6 seconds (the longest task) instead of 1.2 seconds one after another — and each task reports as soon as it finishes, which is why B comes first.

Interactive job control

$ long-command        # running in the foreground
^Z                    # Ctrl+Z pauses it
[1]+  Stopped   long-command
$ bg                  # resume it in the background
$ jobs                # list jobs
$ fg %1               # bring job 1 back to the foreground

Stopping processes: signals

kill sends a signal to a process:

• kill PID sends SIGTERM — "please stop" (programs can clean up).
• kill -9 PID sends SIGKILL — stops it immediately, with no clean-up. Use it only when a program won't respond to a normal kill.
• pkill name signals processes by name.
• Ctrl + C sends SIGINT to the foreground program.

Keeping jobs running after logout

A job started from a terminal usually stops when you log out. Use nohup command &, a terminal multiplexer such as tmux or screen, or — for services — a proper service manager like systemd.