Programs and processes

What is a process?

• A process is a running program. Your browser, your editor, and the shell itself are all processes.
• Each has a number — a PID (process ID) — that the system uses to keep track of it.
• (These commands talk to the real operating system, so they aren't part of our simulator — but here is what you'll use on a real machine.)

Seeing what is running

• ps lists your processes; ps aux lists all of them.
• top (or the nicer htop) shows a live view that updates every second — handy to spot a program using too much CPU or memory:

top

• Press q to quit top.

Stopping a process

• If a program is frozen, you can stop it. In the terminal, Ctrl-C stops the program running right now.
• For another process, find its PID with ps and then kill it:

kill 1234

• kill politely asks the process to stop; kill -9 forces it. Use force only as a last resort.

Now you try

• ps, top and kill talk to the real OS, so they are not in our simulator.
• Instead, a snapshot of ps output is saved in processes.txt. Use the file tools you already know — grep and wc — to find the CPU-hog and count the processes.

Common mistakes

• ps lists running processes; kill stops one by its PID.
• A trailing & runs a command in the background.