Execute command after seconds (without sleep)

Question

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

Answer 1

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.

Answer 2

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.

Answer 3

This is how you would start apache web server after 4 hours. I fully understand that you want it without the command sleep. But I used a different approach about keeping the command running even after logging-out using the command nohup. Don't forget to add the & character at the end. It's part of the whole command below. It helps in making the command run detached from the command line prompt.

nohup sleep 14400 && service apache2 start &