When you execute a Unix job in the background ( using &, bg command), and logout from the session, your process will get killed. You can avoid this using several methods — executing the job with nohup, or making it as batch job using at, batch or cron command.
This quick tip is for beginners. If you’ve been using nohup for a while, leave us a comment and tell us under what situations you use nohup.
In this quick tip, let us review how to make your process running even after you logout, using nohup.
Nohup stands for no hang up, which can be executed as shown below.
nohup syntax:
# nohup command-with-options &
Nohup is very helpful when you have to execute a shell-script or command that take a long time to finish. In that case, you don’t want to be connected to the shell and waiting for the command to complete. Instead, execute it with nohup, exit the shell and continue with your other work.
Explanation about nohup.out file
By default, the standard output will be redirected to nohup.out file in the current directory. And the standard error will be redirected to stdout, thus it will also go to nohup.out. So, your nohup.out will contain both standard output and error messages from the script that you’ve executed using nohup command.
Instead of using nohup.out, you can also redirect the output to a file using the normal shell redirections.
Example: Printing lines to both standard output & standard error
while(true) do echo "standard output" echo "standard error" 1>&2 sleep 1; done
Execute the script without redirection
$ nohup sh custom-script.sh & [1] 12034 $ nohup: ignoring input and appending output to `nohup.out' $ tail -f nohup.out standard output standard error standard output standard error ..
Execute the script with redirection
$ nohup sh custom-script.sh > custom-out.log & [1] 11069 $ nohup: ignoring input and redirecting stderr to stdout $ tail -f custom-out.log standard output standard error standard output standard error ..
If you log-out of the shell and login again, you’ll still see the custom-script.sh running in the background.
$ ps aux | grep sathiya sathiya 12034 0.0 0.1 4912 1080 pts/2 S 14:10 0:00 sh custom-script.sh
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I use nohup in combination with the watch command to move filtered syslog files to a particular directory, but only when I need them for a little while. Rather than use a cron job I set a watch command up for x amount of seconds and run a script to pull the correct logs, filter them and submit them to an ftp or email output. using nohup I can allow this process to run even while I am not logged into my station and close it at a later time (i.e a few hours or minutes) – this is the way I use the nohup command most often.
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