Glossary
Process
A running program, with its own ID, memory and environment.
A process is one running instance of a program. Every process has a numeric PID, an owner, a working directory, an environment, and three open streams. Processes form a tree: each is started by a parent and inherits a copy of the parent's environment and working directory at launch — never the other way round.
That one-way inheritance explains a surprising amount: why cd must be built into the shell, why a script cannot change the directory of the terminal that ran it, and why an exported variable set inside a script vanishes when it ends.
See it in practice
$ ps -o pid,ppid,user,cmd -p $$Shows your own shell process with its parent. $$ expands to the current shell’s PID — handy inside scripts.
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.
PID (Process ID)The number that identifies a running process.
Environment variableA named value processes inherit when they start.