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What's a simple way to run a command, say, 8 hours from now? I can think of this way:

nohup bash -c "sleep 28800 ; ./mycommand.sh" &

Is there a more "proper" way?

You can use the at command. The at execute commands at a later time. The at utility shall read commands from standard input and group them together as an at-job, to be executed at a later time.

Usually, at is installed by default in Ubuntu, but if your release doesn't include it, install via:

sudo apt-get install at

For more information, options, examples, and others see the Ubuntu Manpage Repository

Example:

at now +8 hours -f ~/myscript.sh

You can also use convenient shorthands, like tomorrow or noon, as in

echo "tweet fore" | at teatime 

Warning: This will run the command to the left of the pipe immediately but only present its output later.
The example also demonstrates how you can pipe actions into at. at -c is the way you can examine scheduled actions, which you can conveniently list with their number, as with:

at -c 3

Yes, you can set a cron job.

For example if now the time is 14:39:00 and today is friday, 30 august, you can add the following cron job (to be executed after 8 hours) in your crontab file using crontab -e command:

39 22 30 8 5  /path/to/mycommand.sh

More about:

• https://help.ubuntu.com/community/CronHowto


• http://en.wikipedia.org/wiki/Cron

Use the Gnome-based GUI for cron, at, and the like:

The introduction of the CronHowto suggests using the gnome-schedule gui, which is much nicer than typing all the garbage into the terminal (esp. for "average" Ubuntu users who are not "power" *nix/bsd users.)

Run it by using the Unity Dash (or other applications menu) to look for Scheduled Tasks or running gnome-schedule.

On Gnome-based Ubuntu systems Gnome Scheduled tasks tool (from the gnome-schedule
package) in Applications --> System Tools provides a graphical interface with prompting
for using Cron. The project website is at http://gnome-schedule.sourceforge.net/; the
software is installable from the Software Center or by typing

sudo apt-get install gnome-schedule

in a terminal.

Using gnome-schedule, for a script in your home directory, a new "at" command would be set up using this type of window:

I figured out a quick and dirty way to do this before I found out at was a thing.

Say you want your script to run at noon:

until [[ "$(date)" =~ "12:00:" ]]; do

    sleep 10

done

./mycommand.sh

Say you want it to run tomorrow at noon (today is Nov 24 for me):

until [[ "$(date)" =~ "25 12:00:" ]]; do 

    sleep 30

done

./mycommand.sh