Processes: ps, kill, and the background

Programs become processes

The day this lesson matters: a program freezes your terminal, or something on a server is stuck eating 100% CPU, or a port is "already in use" and you need to find the program holding it. Process tools let you see and stop running programs without rebooting the machine.

A process is a running program. Open three terminals and run bash in each: one program, three processes. The OS gives every process a PID (process ID number).

The inspection tools:

• ps: list your processes. ps aux lists everything running on the machine, one process per line (great for piping into grep).
• kill PID: politely ask a process to quit (it sends a signal named TERM).
• kill -9 PID: force-quit, no questions (signal KILL). Last resort.

And the keyboard shortcuts you'll use daily in a real terminal: Ctrl+C stops the program currently hogging your prompt, and Ctrl+Z pauses it and drops it into the background.

Foreground and background

Normally the shell waits for each command to finish before giving you the prompt back: the command runs in the foreground. End a command with & and it runs in the background instead, so you keep working while it churns:

sleep 60 &

The shell tracks these as jobs: jobs lists them, fg brings one back to the foreground, and wait pauses the script until background jobs finish. Inside a script, the special variable $! holds the PID of the most recent background job, which is exactly what you need to kill it later.