Home

Differences between nohup and the ampersand

There are many cases when small differences between environments can bite you. This is one into which I have ran recently. What is the difference between these two commands?

1 ~ $ nohup myprocess.out &
2 ~ $ myprocess.out &

The answer is the same as usual - it depends.

nohup catches the hangup signal while the ampersand does not.

What is the hangup signal?

SIGHUP - hangup detected on controlling terminal or death of controlling process (value: 1).

Normally, when running a command using & and exiting the shell afterwards, the shell will terminate the sub-command with the hangup signal (like 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.

Fine, but like in this case there are always ‘buts’. There is no difference between these launching methods when the shell is configured in a way where it does not send SIGHUP at all.

In case you are using bash, you can use the command specified below to find out whether your shell sends SIGHUP to its child processes or not:

~ $ shopt | grep hupon

And moreover - there are cases where nohup does not work. For example, when the process you start reconnects the NOHUP signal (it is done inside, on the application code level).

In the described case, lack of differences bit me when inside a custom service launching script there was a call to a second script which sets up and launches the proper application without a nohup command.

On one Linux environment everything worked smoothly, on a second one the application quit as soon as the second script exited (detecting that case, of course took me much more time then you might think :stuck_out_tongue:).

After adding nohup as a launching method to second script, application keeps running even if the scripts will exit and this behavior became consistent on both environments.

References

  1. man 7 signal
Home