Subtracting strings (that are numbers) in a shell script

Question

I made a shell script that finds the size of a directory, returning it in human readable format (e.g., 802M or 702K). I want to calculate the difference between the sizes.

Here's my shell script so far:

#!/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}"

But the output of the script is just

-

How can I make the subtraction work?

Answer 1

You can do basic integer math in bash by wrapping the expression in $(( and )).

$ echo $(( 5 + 8 ))
13

In your specific case, the following works for me:

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

Your 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.

Answer 2

You don't need to call out to grep to strip letters from your strings:

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

See Shell Parameter Expansion in the bash manual.

Answer 3

All you need is:

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

• $((a+b - 4)) can be used for arithmetic expressions
• ${string/M/} replaces M in string with nothing. See man bash, Section String substitution, for more possibilities and details.