Compare A Variable Regularaly Using Linux Scripts and Cron

I’m trying to check if a number differs from what it was last time it was checked, in this case checking a number every minute, using Linux scripts and cron.

eg:

newNum = getNum()

if oldNum != newNum: run some code

oldNum = newNum

(repeat every minute using crontab)

But the problem I am having is that the variables aren’t accessible between scripts and using source (eg. source script.sh) runs the script again, hence getting the latest version, not the one from a minute ago.

The best I’ve got is running a first script which gets the current number, then sleeps for a minute, then runs a second script which is essentially the first two lines of the code above.

eg:

oldNum = getNum()

sleep 60

export oldNum

script2.sh 

This seems inefficient to me and I’d like to know if there is a better solution if possible.

Answer

You could cache the previous number in a file:

number_cache=/path/to/cache_file

# read the previous number
oldNum=$(< "$number_cache" )
# acquire the new number
newNum=$(getNum)

if [[ "$oldNum" -eq "$newNum" ]]; then
    do_something
fi

# cache the new number
printf "%dn" "$newNum" > "$number_cache"