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I'm trying to use a shell script to start a command. I don't care if/when/how/why it finishes. I want the process to start and run, but I want to be able to get back to my shell immediately...

Ian Dunn
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LorenVS
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    Related, see [Difference between nohup, disown and &](http://unix.stackexchange.com/q/3886) on the Unix and Linux Stack Exchange. – jww Jun 11 '16 at 23:53
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    For those looking for a more in-depth answer that talks about the differences between **`nohup`, `&` and `disown`**, [click here to scroll to the fourth answer](http://stackoverflow.com/a/37531889/1459669). – noɥʇʎԀʎzɐɹƆ Dec 27 '16 at 14:37

5 Answers5

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You can just run the script in the background:

$ myscript &

Note that this is different from putting the & inside your script, which probably won't do what you want.

Carl Norum
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    I knew it was going to be something easy, thanks a ton... Linux just isn't my thing, but I'm trying to get up to speed... Btw, will this work when combined with nohup? – LorenVS Mar 03 '10 at 01:08
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    What is the difference between putting the & on the command line and putting it in the script? I was not aware that they were different. – Jacob Sharf Jul 10 '13 at 22:41
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    @JacobSharf, give it a try and you'll see. If the `&` is inside the script and you don't have a `wait`, the background command will be killed when the script exits. – Carl Norum Jul 10 '13 at 22:47
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    oh. That explains a lot. I was recently noticing some effect that would be caused by that. Googling it actually lead me to this page. Thanks – Jacob Sharf Jul 10 '13 at 23:05
  • If `myscript` modifies the terminal environment, ex. it is a terminal initialization command that is not needed immediately and can be delayed, will it still modify the terminal environment? – noɥʇʎԀʎzɐɹƆ Oct 08 '16 at 16:02
  • The meaning of running in the background is, The default behavior of the kernel is Synchronous If we want to change it to Asynchronous then we use '&', Which makes the command run in an Asynchronous (parallel) way (that's why it looks to running in the background) – Abhijit Manepatil Jan 18 '23 at 11:15
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Everyone just forgot disown. So here is a summary:

  • & puts the job in the background.

    • Makes it block on attempting to read input, and
    • Makes the shell not wait for its completion.

  • disown removes the process from the shell's job control, but it still leaves it connected to the terminal.

    • One of the results is that the shell won't send it a SIGHUP(If the shell receives a SIGHUP, it also sends a SIGHUP to the process, which normally causes the process to terminate).
    • And obviously, it can only be applied to background jobs(because you cannot enter it when a foreground job is running).

  • nohup disconnects the process from the terminal, redirects its output to nohup.out and shields it from SIGHUP.

    • The process won't receive any sent SIGHUP.
    • Its completely independent from job control and could in principle be used also for foreground jobs(although that's not very useful).
    • Usually used with &(as a background job).
Ani Menon
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  • I can't find `disown` on Debian or OS X. I thought it was a program, but I seem to be mistaken. What is it? – jww Jun 11 '16 at 23:52
  • [disown command](http://www.cyberciti.biz/faq/unix-linux-disown-command-examples-usage-syntax/) and on [OSX](http://ss64.com/osx/disown.html) – Ani Menon Jun 12 '16 at 02:45
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    This is the best answer. Most comprehensive. – noɥʇʎԀʎzɐɹƆ Dec 27 '16 at 14:35
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    Definitely the best answer, don't know why so little ups – HeberLZ Jan 26 '17 at 03:55
  • There is also the widely available [`setsid`](http://man7.org/linux/man-pages/man1/setsid.1.html) to _immediately send the command in background_; the current session can be safely closed too. – bufh Jul 10 '19 at 15:00
  • Thanks. I still have doubt. Is it necessary to call `disown` if the command was run with `nohup` like `nohup my-script.sh > out 2>&1 &` – Vlad Ganshin May 08 '20 at 21:41
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    @VladGanshin No. – Ani Menon May 10 '20 at 07:15
  • @jww `disown` is a [shell builtin command](http://manpages.ubuntu.com/manpages/impish/en/man1/bash.1.html#shell%20builtin%20commands) – Mr. Tao Aug 22 '21 at 17:58
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nohup cmd

doesn't hangup when you close the terminal. output by default goes to nohup.out

You can combine this with backgrounding, 

nohup cmd &

and get rid of the output, 

nohup cmd > /dev/null 2>&1 &

you can also disown a command. type cmd, Ctrl-Z, bg, disown

Steve B.
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    Cool, crazy how everything combines, I think the ordering would get to me at first, but I suppose you could just memorize it ( what you wrote or "nohup cmd & > /dev/null 2>&1" :) ) – LorenVS Mar 03 '10 at 21:38
  • I stumbled upon this tonight. I've been fighting with a shell script for 2 days and this suggestion got things working. Thank you mucho! – JD Long May 20 '11 at 01:43
  • Awesome, this is very useful. When running in background mode, you can check the command's output every once in a while using `tail nohup.out`, this will display the last 10 lines of the command output. I use this for rsync backup jobs to see what file it's currently at. – Martin Hansen Nov 01 '15 at 18:32
  • What if I do this, but want to stop the background process? – Stephan Bijzitter Feb 02 '16 at 20:36
  • The shell prints a job identifier when you start a job precisely for this purpose. You can see your running jobs at any time with `jobs`. – tripleee Aug 13 '18 at 15:20
  • Discarding the output is a poor idea. Send it to a file so you can see what it's doing and react to any error messages. – tripleee Aug 13 '18 at 15:20
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Alternatively, after you got the program running, you can hit Ctrl-Z which stops your program and then type

bg

which puts your last stopped program in the background. (Useful if your started something without '&' and still want it in the backgroung without restarting it)

pajton
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    Thanks, thats a cool little trick... Starting to really appreciate some of the shell goodness... – LorenVS Mar 03 '10 at 21:37
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screen -m -d $command$ starts the command in a detached session. You can use screen -r to attach to the started session. It is a wonderful tool, extremely useful also for remote sessions. Read more at man screen.

mluthra
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  • @LadenkovVladislav I'm 95% certain you can install screen on RedHat. I have on CentOS. – BuvinJ Oct 28 '19 at 12:44
  • On Ubuntu, I wanted to emulate the Windows / batch "start" command. I.e. asynchronously launch a foreground (gui) program, and continue on through a shell script. This does exactly that. – BuvinJ Oct 28 '19 at 12:46