Asynchronous shell commands

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...

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.

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).

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

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)

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.