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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...

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5 Answers 5

Reset to default
147

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.

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7 Comments

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?
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.
@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.
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
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?
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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).
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8 Comments

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?
disown command and on OSX
This is the best answer. Most comprehensive.
Definitely the best answer, don't know why so little ups
@VladGanshin No.
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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

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6 Comments

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" :) )
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!
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.
What if I do this, but want to stop the background process?
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.
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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)

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Thanks, thats a cool little trick... Starting to really appreciate some of the shell goodness...
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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.

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2 Comments

@LadenkovVladislav I'm 95% certain you can install screen on RedHat. I have on CentOS.
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.

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