Ctrl-Z + bg

I used to use the trick of hitting Ctrl-Z when a long running process in running in the terminal and then entering bg to background it.

nohup

But, I have a new favorite way to background using nohup. Lets say I am running a program like one that calculates pi called pi, for example. I can enter…

nohup pi &

…and it keeps the process running even if I kill the parent process, as in  the shell that I launched it from, by closing it. All of the output from the program winds up in the directory in a file called nohup.out.

Stopping the backgrounded program

If I really want to stop the program, I can do it via kill.

First find the process ID (PID) using…

ps aux | grep pi

…what I get is this…

erick 2315 89.5 0.6 10884 8116 pts/1 R+ 18:24 0:10 pi 10000000

The PID is the first number, in this case 2315. So I will use…

kill 2315

…and this process will be killed gracefully by sending a SIGTERM to ask the process to stop, just like a Ctrl-C would do from the command prompt.

Forcing a backgrounded program to stop

If a process won’t stop when you issue a plain kill (SIGTERM) you an send a sigspec, specifying the type of kill set to KILL to force it to quit…

kill -s KILL 2315

…will shut the program down immediately without saving to disc or whatever the program might do to clean up ( might not free up memory that was MALLOC’d for instance ) when it shuts down.

You can see the difference if you run a process in one terminal and use kill in another.

kill 2315 gives you…

erick@erick-laptop:~/pi$ pi 100000000
 Terminated

where

kill -s KILL 2315 gives you…

erick@erick-laptop:~/pi$ pi 100000000
 Killed