How to stop a process in Linux

The program is still on the machine

The window closed and the computer is still slow. The program did not leave. On Linux, every running program is a process, and it keeps the machine busy until it is stopped.

Closing the window does not always stop the process. Its number stays on the list.

The number in the PID column

ps aux lists what is running. The PID column is that process's number. The program name sits at the end of the line. That name confirms the number belongs to the stuck program.

aluno@desafio:~$ ps aux
USER    PID  %CPU COMMAND
aluno   4821 98.2 python app.py
aluno   2204  0.1 bash

python app.py is 4821 and it is using almost the whole processor. bash is the terminal itself, at 2204. The number you want is the stuck program's number.

Ask it to stop

kill 4821 asks that process to leave. The request gives it time to close a file and let go of what it had open. The terminal writes nothing when it accepts the request.

Then you run ps aux again. If the 4821 line is gone, the program left.

aluno@desafio:~$ kill 4821
aluno@desafio:~$ ps aux
USER    PID  %CPU COMMAND
aluno   2204  0.1 bash

-9 comes last

If ps still shows the same 4821, the process ignored the request. Then kill -9 4821 comes in. This request forces it out immediately. The process cannot save what it had open.

That is why -9 waits for the first try. kill without the extra number already works when the program can still say goodbye.

aluno@desafio:~$ ps aux
USER    PID  %CPU COMMAND
aluno   4821 98.2 python app.py
aluno@desafio:~$ kill -9 4821

The number that is already gone

The process can leave on its own between one command and the next. Then kill answers No such process. The number is no longer on the list.

aluno@desafio:~$ kill 4821
bash: kill: (4821) - No such process

This notice is about the number. The process is already gone. If the answer is Operation not permitted, the process belongs to another user. Who can act on each thing is the subject of permissions.