The wrong way to find yesterdays date:
I had a command that was used to see if. It used some arithmetic operators to subtract 1 from the current day. That would give us yesterdays day which we could then use to check if a backup was created then.
day=$(date +%d) ; date=$(($day - 1)) ; echo "yesterday date is $date"
It worked great, unless you happened to be on the 8th or 9th of the month. Looks like bash is interpreting 08 and 09 in octal format. https://stackoverflow.com/questions/24777597/value-too-great-for-base-error-token-is-08
-bash: 08: value too great for base (error token is "08")
The better way
Fortunately there is an easier and more reliable way to do this. Using the date command, you can specify yesterday and it will print out yesterdays date.
date --date=yesterday +%d
Much easier to use.
Some more info.