Glossary
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 nginxPrints 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.
killTerminate processes politely or forcibly — and understand what -9 really does.
systemctlStart, stop, enable and inspect services — the control panel of modern Linux.
ProcessA running program, with its own ID, memory and environment.