What is a process?
A process is a running program; the operating system gives each one its own memory and an ID number.
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A process is one instance of a program that is currently executing. The program file on disk is the recipe; the process is the meal being cooked right now. Open the same browser twice and you have two processes from one program. Each gets a unique process ID (PID), its own memory, and a parent that started it.
You meet processes when something eats all the CPU, when a port is busy, or when a program hangs and must be killed. Tools such as ps, top and htop list them; kill sends them signals. Processes can spawn children, forming a tree whose root is systemd or init. Threads are lighter workers that live inside one process and share its memory.
ps aux | grep python
kill -15 12345See also: What is a daemon or service? and What is a shell?
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See also
- 0061 Fix: Address already in use (free a busy port) Your server cannot start because another process already listens on its port. This script shows which process holds the port and stops… Intermediate · Fixes an error · Linux, macOS Script Processes & ports · Shell scripting 15 Oct 2026
- 0320 Fix: address already in use (EADDRINUSE) Something is already listening on the port your program wants. Find the process with lsof or ss, stop it or pick another… Beginner · Fixes an error · Linux, macOS Article Networking & firewalls · Node.js · Processes & ports 15 Oct 2026
- 0326 Fix: too many open files A process hit its file descriptor limit, usually 1024. Raise it for the shell, the systemd unit or the user, fix a… Intermediate · Fixes an error · Linux, macOS Article Performance & tuning · Processes & ports · Services & systemd 15 Oct 2026
- 0334 Fix: port is already allocated docker run or docker compose up fails with Bind for 0.0.0.0:8080 failed: port is already allocated. Find what holds the port, another… Beginner · Fixes an error · Linux, macOS, Windows Article Docker fixes · Processes & ports 15 Oct 2026
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