I want to start writing in shell script but dont know what to download

At first, type

Code:

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uname -a

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in your terminal.
If you get an error, you're under Windows. Otherwise, the answer will tell you on what kernel you're running.
Then, your last line should be

Code:

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echo "$a + $b = " $((a+b))

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Because your code only ask to show the value of the $c variable, which you didn't calculate at first...