PID (Process ID)

The number that identifies a running process.

Every process gets a unique PID when it starts. It is how you refer to a process in commands like kill, and how the system tracks parent-child relationships (the parent's PID is the PPID).

PID 1 is special: it is the first process the kernel starts, and on modern systems it is systemd, which then launches everything else. Find PIDs with ps aux, pgrep name, or from a service's status output.

See it in practice

$ pgrep -f nginx

Prints the PIDs of matching processes, ready to feed into kill. Cleaner than ps aux | grep, which also matches itself.

Where you'll meet it

See what is running, find a process ID, and combine with grep to hunt down a program.

kill

Terminate processes politely or forcibly, and understand what -9 really does.

systemctl

Start, stop, enable and inspect services: the control panel of modern Linux.

Process

A running program, with its own ID, memory and environment.

Errors that come down to this concept: bind: Address already in use

Learn these concepts by typing them: the interactive course builds them up lesson by lesson in a real simulated shell.