Subtracting strings (that are numbers) in a shell script
You can do basic integer math in bash by wrapping the expression in $(( and )).
$ echo $(( 5 + 8 ))13In your specific case, the following works for me:
$ echo "${new_size}-${current_size}"802-789$ echo $(( ${new_size}-${current_size} ))13Your output at the end is a bit odd. Check that the grep expression actually produces the desired output. If not, you might need to wrap the regular expression in quotation marks.
You don't need to call out to grep to strip letters from your strings:
current_remote_dir_size=789Mcurrent_size=${current_remote_dir_size%[A-Z]}echo $current_size # ==> 789new_remote_dir_size=802Mnew_size=${new_remote_dir_size%[A-Z]}echo $new_size # ==> 802See Shell Parameter Expansion in the bash manual.