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Subtracting strings (that are numbers) in a shell script
#!/bin/bash

current_remote_dir_size=789M
new_remote_dir_size=802M
new_size=`echo ${new_remote_dir_size} | grep -o [0-9]*`
current_size=`echo ${current_remote_dir_size} | grep -o [0-9]*`

echo "${new_size}-${current_size}"

-

$ echo $(( 5 + 8 ))
13

$ echo "${new_size}-${current_size}"
802-789
$ echo $(( ${new_size}-${current_size} ))
13

current_remote_dir_size=789M
current_size=${current_remote_dir_size%[A-Z]}
echo $current_size  # ==> 789

new_remote_dir_size=802M
new_size=${new_remote_dir_size%[A-Z]}
echo $new_size      # ==> 802

echo $((${new_remote_dir_size/M/}-${current_remote_dir_size/M/}))