What's the difference between nohup and ampersand

linux bash nohup
Yarkee picture Yarkee · Mar 24, 2013 · Viewed 160.4k times · Source

Both nohup myprocess.out & or myprocess.out & set myprocess.out to run in the background. After I shutdown the terminal, the process is still running. What's the difference between them?

Answer

nemo picture nemo · Mar 24, 2013

nohup catches the hangup signal (see man 7 signal) while the ampersand doesn't (except the shell is confgured that way or doesn't send SIGHUP at all).

Normally, when running a command using & and exiting the shell afterwards, the shell will terminate the sub-command with the hangup signal (kill -SIGHUP <pid>). This can be prevented using nohup, as it catches the signal and ignores it so that it never reaches the actual application.

In case you're using bash, you can use the command shopt | grep hupon to find out whether your shell sends SIGHUP to its child processes or not. If it is off, processes won't be terminated, as it seems to be the case for you. More information on how bash terminates applications can be found here.

There are cases where nohup does not work, for example when the process you start reconnects the SIGHUP signal, as it is the case here.

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