The double parenthesis construct in bash allows for arithmetic expansion. Using this in the simplest format, we can easily carry out integer arithmetic. This becomes a replacement for the let built-in. The following examples show the use of the let command and the double parenthesis to achieve the same result:
$ a=(( 2 + 3 ))$ let a=2+3
In both cases, the a parameter is populated with the sum of 2 + 3. If you want to write it on a shell script, you need to add a dollar sign before the parentheses:
#!/bin/bashecho $(( 2 + 3 ))
