expr EXPERSSION returns 1 when the
EXPERSSION is empty or 0. Thus, you cannot really use it to do arithmetic.
For example, you may find your script doing
expr 1 - 1 returns as error.
To be fair, man page
expr actually mentions this. However, instead of a dedicate section EXIT STATUSES, it is hidden in the second last paragraph of DESCRIPTION.
So do yourself a favour, use BASH arithmetic like