Bash: Jobs & Signals

So far, every spell you've cast has held the stage: it runs, you wait, you get your prompt back. But the shell is secretly a juggler — every command is a process, processes can be suspended, backgrounded, foregrounded, and signalled. This is the chapter where your terminal stops being a queue and becomes a workshop.

The ampersand: cast it and walk away

End a command with & and the shell hands it to the background — it runs while you keep working:

sleep 300 &            # a spell that does nothing for five minutes. Career-relevant.
# [1] 4821

That output: job number 1, process ID 4821. Your prompt returns immediately while the spell grinds away. See the roster anytime:

jobs
# [1]+  Running                 sleep 300 &

Ctrl-Z and the suspend dance

The move that changes how you use the terminal. You're deep in vim (or less or man — anything full-screen) and need the shell right now. Ctrl+Z:

^Z
[1]+  Stopped                 vim grimoire.md

The process is frozen mid-thought — not killed, paused, like a photo of a juggler between throws. Then:

fg          # bring the most recent job back to the foreground — resume where you froze it
bg          # send a stopped job to the background, running
jobs        # the roster, always

The full dance you'll do daily: Ctrl+Z → do a thing → fg → back to work. No new window, no re-opening files, no finding your place. The wizard's alt-tab.

Kill a background job through the roster (nicer than PID hunting):

kill %1      # job 1, as named by jobs

Signals: the escalating negotiation

Here's the truth under all of it: Ctrl+C was never magic. It's just your keyboard sending a signal — the operating system's inter-process mail system. Every process lives by signal rules:

kill sends them by name or number:

kill 4821          # TERM: "please terminate" — always try this first
kill -TERM 4821    # same thing, spelled out
kill -9 4821       # KILL: the last resort. No cleanup, no goodbye, no mercy.

The etiquette, and it's real etiquette: TERM first, KILL last. TERM lets a program flush buffers, close files, remove lockfiles — all the "orphaned process" horror stories begin with someone reaching for -9 first and leaving lockfiles and half-written data in their wake. -9 is the sword under the glass. It exists so you have it.

Find PIDs without scrolling history:

pgrep -l node          # find PIDs by name (list with names)
pkill -f build.sh      # signal every process matching a pattern. Crowd control.

pkill is glorious and dangerous — it signals everything matching. Check with pgrep first, always. Measure twice, signal once.

nice: manners for CPU hogs

When a heavy spell must run now but shouldn't stomp everyone else:

nice -n 10 ./crunch_data.sh        # lower priority (nicer to others; the name IS the doc)
renice -n 5 -p 4821                # re-nice a running process

Nice values run from -20 (greedy) to 19 (saintly). Default 0. A nice-19 backup job yields the CPU to anything that wants it — the considerate house guest of process scheduling.

nohup: the spell that outlives its summoner

Dark secret, revealed: background jobs are still children of your terminal session. Close the terminal — or lose an SSH connection — and they die with it. All that grinding, gone, because your laptop went to sleep.

The counter-spell is nohup ("no hangup" — a name from an era when hanging up a modem was a leading cause of death for long jobs):

nohup ./long_ritual.sh > ritual.log 2>&1 &

Now the process ignores the hangup signal and keeps grinding after you log off, writing its output to ritual.log (the > … 2>&1 from part 05, closing both streams into the log). Check progress from anywhere:

tail -f ritual.log

(Cron — the true "run things forever without a terminal" — arrives in part 14. nohup is the "run this once, survive this session" tool.)

Your quest

• sleep 300 &, then jobs. Then kill %1. You've managed your first minion.
• Open vim (yes, really), Ctrl+Z, run three commands, fg. Feel the dance click.
• yes > /dev/null & (a process that screams forever into a void), nice -n 19 yes > /dev/null & — compare them in top, then pkill yes before your fan takes flight
• Start nohup sleep 600, close the terminal entirely, reopen, and confirm it's still alive with pgrep sleep. Your first immortal spell.

Next: part 13 — The Custom Grimoire, where we edit .bashrc and make all of today's favourite spells permanent — aliases, functions, and a prompt that tells you things before you ask.

Filed under Wizardy of Bash — part 12 of 15 · Previous: Defensive Wizardry · Next: The Custom Grimoire