Issue
nohup command & can keep a command running after closing the terminal.
I want to play all mp3 music in directory Music.
ls Music/*mp3 |xargs -d "\n" mplayer
ls Music/*mp3 can list all mp3 files in directory Music, send it via pipe with xargs, -d "\n" is to treat blanks in the filenames.
I want to redirect all stdout and stderr into /dev/null, and run it in the background.
ls Music/*mp3 |xargs -d "\n" mplayer > /dev/null 2>&1 &
This works fine, but I want it running after closing the terminal.
nohup ls Music/*mp3 | xargs -d "\n" mplayer > /dev/null 2>&1 &
ls Music/*mp3 | xargs -d "\n" nohup mplayer > /dev/null 2>&1 &
ls Music/*mp3 | nohup xargs -d "\n" mplayer > /dev/null 2>&1 &
When I close the terminal, the process running in the background ends.
Why does nohup not take effect in any of the above three commands? How can I make it take effect?
disown can solve the issue, but I would like a solution using nohup.
ls Music/*mp3 | xargs -d "\n" mplayer > /dev/null 2>&1 & disown
Solution
nohup command means "run command and ignore HUP signals".
So before we can effectively use nohup we need to ask: when and how is SIGHUP sent? As the Bash manual says, "Before exiting, an interactive shell resends the SIGHUP to all jobs, running or stopped.". It goes on to say that the proper way to suppress this behavior is with disown. I realize you are asking about nohup, but it's worth calling out that disown is the intended and easier way to accomplish what you want. Note that disown is not equivalent to nohup.
The reason nohup is tricky to work with here is because it applies to a single process, whereas & creates a background job of a whole command pipeline, which can consist of multiple processes. This means you need to nohup each command in the pipeline in order to ensure that the individual commands don't receive a SIGHUP, e.g.:
$ nohup ls Music/*mp3 2>/dev/null | nohup xargs -d "\n" nohup mplayer &> /dev/null &
This should work, though I haven't tested it with these specific commands. If it doesn't, it's likely another process that you aren't starting directly is still receiving a SIGHUP. This is harder to address, which is exactly why we have disown.
manishg's suggestion is also reasonable; by moving the pipeline into a separate process you can nohup that process, which should in turn prevent a SIGHUP from reaching its children when your shell closes.
All that said, you don't need ls and xargs here in the first place; find can be used to similar effect and will simplify reasoning about the command. Try:
$ nohup find Music -maxdepth 1 -name '*mp3' -exec mplayer {} + &> /dev/null &
Answered By - dimo414