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How can I run a command after X seconds without sleep. Or with sleep but without the shell just wait for the response of that command?

I used this but it didn't work

sleep 5 ; ls > a.txt

I need to run it in background. I try not to have to run it in a script I try to run ls after 5 seconds, and shell does not just wait for the end of the sleep

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Why does it not work without the shell? Which error do you get? Are you passing it the full path? Do you know which part parses the ; ? – Hennes Apr 4 '13 at 17:30
    
in this way the shell will block 5 seconds and then run the command., the shell to be stalled I do not need that. – Martin Apr 4 '13 at 17:37
    
Then you either need to start it in a script in the background, or in a sub-shell. – Hennes Apr 4 '13 at 17:39
up vote 22 down vote accepted

A more concise way of writing what Hennes suggested is

(sleep 5; echo foo) & 

Alternatively, if you need more than a few seconds, you could use at. There are three ways of giving a command to at:

  1. Pipe it:

    $ echo "ls > a.txt" | at now + 1 min
    warning: commands will be executed using /bin/sh
    job 3 at Thu Apr  4 20:16:00 2013
    
  2. Save the command you want to run in a text file, and then pass that file to at:

    $ echo "ls > a.txt" > cmd.txt
    $ at now + 1 min < cmd.txt
    warning: commands will be executed using /bin/sh
    job 3 at Thu Apr  4 20:16:00 2013
    
  3. You can also pass at commands from STDIN:

    $ at now + 1 min
    warning: commands will be executed using /bin/sh
    at> ls
    

    Then, press CtrlD to exit the at shell. The ls command will be run in one minute.

You can give very precise times in the format of [[CC]YY]MMDDhhmm[.ss], as in

$ at -t 201412182134.12 < script.sh

This will run the script script.sh at 21:34 and 12 seconds on the 18th of December 2014. So, in theory, you could use at to run something five seconds in the future. However, that is kinda like using a tank to swat a fly, and Hennes's suggestion is better.

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I kept messing with $(sleep 5; echo foo) & I am not sure why I thought I needed a $ in front of that, but with my syntax wrong I just posted the longer (and more understandable) version. ( command_for_in_new_shell ) is shorter though. – Hennes Apr 4 '13 at 19:23
    
@Hennes and I blatantly ripped off your very good idea (which I upvoted). All I did was reformat it to be shorter, all credit for the actual thinking bits is yours :). – terdon Apr 5 '13 at 14:02

You can work around it by starting a shell or a script in the background.

Example:

/bin/sh -c "sleep 5 ; echo foo" &
[1] 63791
>                               Active shell prompt here

foo                             5 second later output appears.
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