Process ID (PID)

A number identifying a running process.

Backend Engineer · Junior Data Engineer · Junior

Also known as: process ID, process id, PID

A process ID (PID) is the number the operating system assigns to each running process. It’s how you refer to one specific process when you want to inspect, signal or stop it.

echo $$              # the PID of the current shell
ps aux | grep nginx  # list processes and their PIDs
pgrep -f myapp       # find PIDs by name
pidof nginx

kill 4321            # ask process 4321 to terminate (SIGTERM)
kill -9 4321         # force it (SIGKILL): last resort

import os
os.getpid()          # this process's PID
os.getppid()         # its parent's PID

Details

Every process has a parent, whose PID is the PPID. A process starts as a child of the one that launched it (process fork).
PID 1 is the first process the system starts. It adopts orphaned processes, and on Linux is init or systemd.
PIDs are reused after a process ends, once the numbers wrap around. Don’t store a PID and assume it still refers to the same program later.
Inside a container, the main process is PID 1 in its own PID namespace. It must handle signals and reap child processes properly, or docker stop hangs (namespaces, graceful shutdown).

Common uses

Stopping a runaway or stuck program with kill (signals).
PID files: some services write their PID to a file such as /var/run/app.pid so scripts can signal them.
Finding what’s using a port: lsof -i :8080 prints the PID.
Debugging: attach tools (strace -p PID, a debugger) to a running process (strace).

Prefer a gentle kill (SIGTERM) first, so the program can clean up. Use kill -9 only when it ignores that. See managing processes.