jeremy@jeremy-desktop:~/Desktop$ X=3+4
jeremy@jeremy-desktop:~/Desktop$ echo $X
3+4

jeremy@jeremy-desktop:~/Desktop$ let X=3+4
jeremy@jeremy-desktop:~/Desktop$ echo $X
7
Integer math only
bc is the arbitrary precision binary calculator:
jeremy@jeremy-desktop:~/Desktop$ echo 3*1.34 | bc
4.02