Programs and processes

A program is the file on disk (or the install package) — the instructions. A process is one running instance of that program: memory, open files, and a process id (PID) the OS tracks until it exits.

Starting a program creates a process. Closing the window, quitting the app, or sending a stop signal ends that process. The program file stays on disk; only the running instance goes away.

One program can have many processes (ten browser tabs' helper processes, several python jobs). Operators ask "is it running?" about processes, not about whether the install folder still exists.

In a shell you often start a process by typing its command name. Later lessons cover finding and stopping processes; here the skill is naming the difference: install ≠ running, file ≠ PID.

Misses: thinking deleting the app while it runs always frees the PID instantly on every OS, or assuming two windows means two different programs when they are two processes of the same program.

# program on disk vs process in memory
# start → new process (gets a PID)
# quit / kill → process ends; program file remains
ps  # list processes (names + PIDs) on many Unix shells
# Task Manager / Activity Monitor show the same idea on GUIs