UNIX for Dummies Questions & Answers

hello i am new to scripting and Linux in general. i am going to school for sys admin and i will need at least some knowledge on scripting. i have been so far ok with learning basic scripting now i am stuck. with a assignment that i was given it was create a script that will tell you if a number is odd or even, i got it to work but don't know why. the script is this:

Code:

#!/bin/bash
echo "please tell me a number between 0 and 20"
read num
even=$(( $num % 2 ))
if [ $even -eq  0 ]
    then echo "you have a even number"
    else echo "you have a odd number"
fi

my question is what does this command do

Code:

even=$(( $num % 2 ))

does it take the number and break it down to the lowest denominator (0 or 1) or does it do something else.
also what doe the % mean in scripting. i looked it up and found out its called that modulo (remainder) but i do not know what that means.
I am not trying to get an easy grade but rather i am trying to learn this because i have intro to programming next quarter and if I learn this now it will help me in the future.
one final thing. if anyone could tell me of a good beginner bash script "cheat sheet" so i can understand the scripting commands. i saw the Unix commands tab but i did not know which one to pick.

thank you

In standards conforming shells $((expression)) is known as an Arithmetic Expansion. The expression is evaluated, and the value of that expression is returned as the output from the expansion. The modulo operator (%) is a binary operator that returns the integer remainder obtained when dividing the first given operand by the second given operand. For example $((5 % 2)) will return 1 because 5 divided by 2 is 2 with the remainder 1 and $((6 % 2)) will return 0 because 6 divided by 2 is 3 with the remainder 0.

For more details on the expressions that can be processed in an arithmetic expansion, look at the bash man page on your system and search for Arithmetic Expansion.

thank you for answering my question. i will look into arithmetic expansion.