A nohup trick or two

We've all been frequent users of nohup throughout our system administrator careers. Today I'll share a trick or two about nohup which you may or may not already know.

I'll be using the below script as my command to run during the demonstrations.

root@sandbox:/# cat hup.bash
#!/bin/bash

while true

do
echo "printting endlessly"
date
sleep 5
done

It's a simple infinite while loop print the string "printing endlessly" and the date every 5 seconds.

Trick 1 - Redirect output of command to a file other than nohup.out:

When we run a command following the convention nohup command & the output of the command if any gets redirected to a file named nohup.out created in the directory from where the command was run. This is fine when running one command on one server but if we are running a command like this on multiple servers in a loop then we might need separate output files for each server.

To accomplish what I just mentioned you could use nohup as follows:

root@sandbox:/# nohup ./hup.bash &> $(hostname)_$(date '+%d-%m-%y').txt &
[1] 1131
root@sandbox:/# ls -l sandbox_01-10-17.txt
-rw-r--r--   1 root     root          98 Oct  1 21:05 sandbox_01-10-17.txt
root@sandbox:/# tail -5 !$
tail -5 sandbox_01-10-17.txt
Sun Oct  1 21:05:07 IST 2017
printting endlessly
Sun Oct  1 21:05:12 IST 2017
printting endlessly
Sun Oct  1 21:05:17 IST 2017

The output of the command run via nohup got redirected to the file name I porivded and not to nohup.out.

Trick 2 - Nohup an already running process:

For the second trick I'll demonstrate how we can use nohup on a process which is already running.

Let's run the hup.bash script again in the background.

root@sandbox:/# ./hup.bash &

printting endlessly

Sun Oct  1 21:08:31 IST 2017

[1] 1234

root@sandbox:/# printting endlessly

Sun Oct  1 21:08:36 IST 2017

printting endlessly

Sun Oct  1 21:08:41 IST 2017

printting endlessly

Sun Oct  1 21:08:46 IST 2017

Although I ran the script in the background the stdout is being redirected to the tereminal but notice it got frozen after a few runs. That is because I ran nohup on the process id in a separate terminal window as shown below:

root@sandbox:/# nohup -p 1234

Sending output to nohup.out

root@sandbox:/# tail nohup.out

printting endlessly

Sun Oct  1 21:07:50 IST 2017

printting endlessly

Sun Oct  1 21:07:55 IST 2017

printting endlessly

Sun Oct  1 21:08:00 IST 2017

printting endlessly

Sun Oct  1 21:08:51 IST 2017

printting endlessly

Sun Oct  1 21:08:56 IST 2017

The output of all subsequent iterations gets directed to nohup.out.

I hope you'll find these tricks useful and I thank you for reading.