Unix Tools - Shell Arithmetic

Use to perform arithmetic operations.
Syntax:
expr op1 math-operator op2
Examples:

$ expr 1 + 3
$ expr 2 - 1
$ expr 10 / 2
$ expr 20 % 3
$ expr 10 \* 3
$ echo `expr 6 + 3`

Note:
expr 20 %3 - Remainder read as 20 mod 3 and remainder is 2. expr 10 \* 3 - Multiplication use \* and not * since its wild card.
For the last statement not the following points
(1) First, before expr keyword we used ` (back quote) sign not the (single quote i.e. ') sign. Back quote is generally found on the key under tilde (~) on PC keyboard OR to the above of TAB key.

(2) Second, expr is also end with ` i.e. back quote.
(3) Here expr 6 + 3 is evaluated to 9, then echo command prints 9 as sum
(4) Here if you use double quote or single quote, it will NOT work For e.g.
$ echo "expr 6 + 3" # It will print expr 6 + 3
$ echo 'expr 6 + 3' # It will print expr 6 + 3