number calculation in bash

computation efficiency rank:
(()) == let > expr > bc

1.(()) and let is bash buildin command, which can execute efficiently
2.expr and bc is system command, drain memory and low efficiency.
3.(()) let and expr can only do integer computation.
4.bc can do float computation.

(())

value=$((3*(5+2)))
((value++))

let

let "value=3*(5+2)"
let value++

expr

value=`expr 3 \* \( 5 + 2\)`
value=`expr $value+1`

bc

value=`echo "3*(5+2)" | bc`
value=`echo "$value+1" | bc `