Processes and Jobs: ps, top, kill and Background Tasks

Introduction

A web server that will not stop, a script stuck in a loop, a program using all the memory — sooner or later you need to see what is running and take control of it. This lesson covers processes: how to list them, run them in the background and stop them.

What Is a Process?

A process is a running copy of a program. Running ls creates a process that lives for a split second; a web server is a process that runs for months. Every process has a unique number, its PID (process ID), which is how you refer to it when you want to stop it.

Listing Processes: ps

ps on its own shows the processes started from your current terminal — usually just your shell (bash) and ps itself:

ps

ps aux lists every process on the system, with its owner, CPU and memory use. On a normal computer that is hundreds of lines, so you usually filter it with grep:

Background Jobs

Normally a command occupies your terminal until it finishes. Adding & runs it in the background so you can keep working. sleep simply waits for a number of seconds, which makes it a harmless way to practise. jobs lists what you started in the background from this terminal:

sleep 600 &
sleep 900 &
jobs

[1] and [2] are job numbers, local to your terminal. The + marks the job that fg (foreground) would bring back. Now the processes show up in the system list:

ps aux | grep -E "^USER|sleep [0-9]"

Finding and Stopping Processes

pgrep finds processes by name and prints their PIDs (-a adds the full command):

pgrep -a sleep

kill <PID> asks a process to stop. Despite the name, it sends a polite request (the TERM signal), which lets the program save its work and exit cleanly. Here Priya stops the older of the two:

kill $(pgrep -o -x sleep)
pgrep -a sleep

($(...) runs the command inside and uses its output — here the PID of the oldest sleep process.) pkill stops processes by name, so every matching process goes at once:

pkill sleep
pgrep -a sleep || echo "no sleep processes left"

If a frozen program ignores the polite request, kill -9 sends the KILL signal, which ends it immediately with no chance to clean up:

sleep 500 &
PID=$!
echo "started sleep with PID $PID"
kill -9 $PID

$! holds the PID of the last background command — very handy in scripts.

Live View: top and htop

top shows a live, constantly updating table of processes, sorted by CPU use — the command-line equivalent of Task Manager. Press q to quit. Its header summarises the whole system:

top -b -n 1 | head -n 5

(-b -n 1 prints a single snapshot instead of the interactive view.) The header shows uptime, load averages for the last 1, 5 and 15 minutes, task counts, CPU usage and memory. The recording above comes from a small container, which is why it lists only a few tasks — your own computer will show hundreds. Many people prefer htop, a friendlier, colourful version: install it with sudo apt install htop.

Keeping Jobs Running: nohup

Background jobs normally stop when you close the terminal or disconnect from a server. nohup (no hang-up) keeps a command running anyway and saves its output to a file:

nohup bash -c "sleep 1; echo backup finished" > backup.out 2>&1 &
sleep 2
cat backup.out

For anything that should run permanently, such as a web server, use a proper service manager instead (systemd, lesson 14) — it also restarts the program if it crashes.

Common Mistakes

Frequently Asked Questions

Key Takeaways

Summary

You can now see and control everything running on a Linux system. Next you will install new software — the Linux way, with a package manager.