Processes

By the end of this lesson you'll be able to see every program running on your machine, find a stuck app by its PID, stop it cleanly (or force it as a last resort), and run long jobs in the background that survive after you log out.

Part of the free Cli course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

What You'll Learn

1️⃣ Processes, PIDs & ps

A process is just a running program — your browser, a server, even the shell you're typing in. The operating system gives each one a unique PID (Process ID), a number you use to refer to it. The ps command lists processes. On its own it shows only yours in the current terminal; the classic ps aux shows every process with useful columns like %CPU and %MEM . Pipe it into grep to find one program by name.

2️⃣ Live Monitoring with top & htop

ps gives you a single snapshot. When you want to watch what's happening — which process is eating your CPU right now — use top , a live monitor that refreshes every couple of seconds. Inside top , press P to sort by CPU, M to sort by memory, and q to quit. htop is a friendlier, colour version with scrolling and mouse support — install it once and you'll never go back.

🎯 Your Turn: Find a Process's PID

Fill in the blanks marked ___ using the hints, then run it. You'll start a throwaway timer and track down its PID — the exact skill you need before you can stop anything.

3️⃣ Stopping Processes: kill & Signals

To stop a process you send it a signal — a short message the kernel delivers. kill PID sends SIGTERM (signal 15): a polite "please shut down" that lets the program save its work, close files, and free up ports. Only if a process ignores that and truly hangs do you reach for kill -9 PID , which sends SIGKILL (signal 9) — the kernel destroys it instantly, with no chance to clean up. Don't know the PID? killall name targets processes by exact name, and pkill -f "pattern" matches anywhere in the command line.

SIGTERM vs SIGKILL — get the order right

Always try kill PID (SIGTERM) first and give it a second or two. Reaching straight for kill -9 can corrupt files, leave locks behind, or strand a database mid-write, because the program never gets to finish what it was doing. kill -9 is the emergency brake, not the parking brake.

4️⃣ Foreground, Background & nohup

When you run a command normally it runs in the foreground and holds your terminal hostage until it finishes. Add a trailing & to run it in the background so you keep working. jobs lists this shell's background and paused jobs; fg pulls one to the foreground and bg resumes a paused one in the background. Press Ctrl+Z to pause whatever's in the foreground. One catch: a plain background job dies when you close the terminal (it receives SIGHUP ). Start it with nohup — "no hangup" — and it survives logout.

Pro Tip

nohup ... & is fine for a quick task, but for a real server you want it to restart on crash and start on boot. Reach for a process manager: pm2 for Node apps, or systemd services on Linux. tmux and screen are also great for keeping interactive sessions alive over SSH.

🎯 Your Turn: Background a Job, Then Reclaim It

Fill in the blanks, then run it. You'll send a timer to the background, confirm it's there with jobs , and pull it back to the foreground.

Common Errors (and the fix)

• Reaching for kill -9 first: SIGKILL gives the process no chance to save data or release ports, which can corrupt files. Always try kill PID (SIGTERM) first; use -9 only when it genuinely won't stop.
• Killing the wrong PID: PIDs get reused, and a quick glance is easy to misread. Confirm with ps -p PID or ps aux | grep name before you kill. Killing the wrong number can take down something important.
• Background job vanishes after logout: a plain command & receives SIGHUP and dies when the terminal closes. Start it with nohup command & so it keeps running.
• bash: kill: (1842) - No such process : that PID has already exited (or you mistyped it). Re-run ps aux | grep name to get the current PID.
• Operation not permitted : you're trying to kill a process owned by another user (often root). Re-run with sudo kill PID — but be doubly sure of the PID first.

📋 Quick Reference

Command

What it does

ps aux

List every running process with details

ps aux | grep name

Find a process by name (read its PID)

top / htop

Live, updating process monitor

kill PID

Graceful stop (SIGTERM) — try this first

kill -9 PID

Force kill (SIGKILL) — last resort

killall name / pkill -f pat

Kill by name / by command-line pattern

command &

Run in the background

jobs / fg %1 / bg %1

List jobs / foreground / background a job

nohup command &

Keep a job running after logout

Signal

Number

Meaning

SIGTERM

15

Polite "shut down" — the default for kill; allows cleanup

SIGKILL

9

Forced kill — cannot be caught or ignored; no cleanup

SIGHUP

1

"Hang up" — sent on terminal close; also "reload config"

SIGINT

2

Interrupt from the keyboard (Ctrl+C)

SIGSTOP

19

Pause a process (Ctrl+Z sends the related SIGTSTP)

SIGCONT

18

Resume a paused process

Frequently Asked Questions

Mini-Challenge: Launch, Inspect & Stop a Server

No blanks this time — just a brief and an outline. Write the commands yourself, run them in your terminal, and check the result against the expected note. This is the exact loop you'll run whenever a server misbehaves.

🎉 Lesson Complete!

• ✅ Every running program is a process with a unique PID
• ✅ ps aux lists processes; top / htop watch them live
• ✅ kill PID (SIGTERM) is graceful; kill -9 (SIGKILL) is a forced last resort
• ✅ killall and pkill -f stop processes by name or pattern
• ✅ & , jobs , fg , and bg move work between foreground and background
• ✅ nohup command & keeps a job alive after you log out
• ✅ Next lesson: Permissions and Users — chmod , chown , and managing access

Practice quiz

Continue this course

• Previous: Shell Scripting Basics
• Next: Permissions and Users