Shell script – waiting until specified day/time

I participated in a project in which job scheduling logic was completely implemented in shell script (it was a small project initially, and then it has grown in time). One of tasks was to enable scheduler script to wait until a specified time to run a job. Surprisingly, search in Internet did not bring anything that would look simple and elegant enough. There were bunch of quite complex solutions which did not really address my requirements and would be too hard to maintain for support teams in future… I came up with my own solution for this after reading Linux manuals, which I wanted to share.

It appears, ‘date’ utility is very flexible in terms of definition and representation of time. For example, user can supply target date/time in a string, using words like ‘Today 9am” or “Thursday 12pm” etc. In combination with ‘sleep’ command, I was able to achieve the desired functionality with required precision (seconds). I ended up writing this simple bash function:

#-[ wait_until ]------------------------------------------
# Waits until a given day/time
# $1 - target time/date in 'date' command format, e.g. "Today 13:00"
wait_until()
{
echo "Waiting until $1..."

TIME_TARGET=$( date -d "$1" +%s )
TIME_NOW=$( date +%s )

if [ $TIME_NOW -lt $TIME_TARGET ]; then

        TIME_WAIT=$( expr $TIME_TARGET - $TIME_NOW )
        echo "Waiting time: ${TIME_WAIT} seconds"
        sleep ${TIME_WAIT}s

else
        echo "Target time is already in past. No waiting required..."
fi
}

Use is very simple:

wait_until "Today 1pm"
#... some code ...
wait_until "Tomorrow 6am"
#... some code ...

etc…

Hope it helps someone who will be looking for this solution in Internet, just like I did before I had to write it :)