Shell from zero: a number is still text until you ask

cents=450 stores the characters 4, 5, and 0. echo "$cents" prints them. Nothing has added anything. $(( ... )) is the request to read that text as a whole number and do arithmetic. Two lattes are 900 cents. The result is text again, digits you can print.

Arithmetic is a mode, not a type#

cents=450 stores three characters. Nothing about the variable becomes numeric. $((cents * 2)) is a different parsing mode: names inside it are read as integers, the arithmetic runs, and the result is converted back to text for echo. You write cents there, not $cents. The spaces around * are allowed in that mode and forbidden around the = of an assignment. A non-numeric value becomes 0 in this mode, quietly. If cents were the word latte, $((cents * 2)) would print 0. $((5 / 2)) is 2, because the arithmetic is integer only.

1. cents450
2. cents * 2900

cents=450
echo $((cents * 2))

900

The double parentheses are the arithmetic#

cents * 2 inside $(( )) is 900, the cents for two lattes. You do not write $cents inside the arithmetic. The name cents is enough. Outside, you need $cents or "$cents". The spaces around * are fine here. They are not fine around the = of an assignment.

cents=latte and then $((cents * 2)) does not produce a useful product. The shell treats a value that is not a number as 0 in this context, so the print is 0. That quiet zero is why you confirm the contents before you calculate. Print "$cents" when a total looks wrong.

Integer arithmetic only#

$((5 / 2)) is 2. The fraction is dropped, the same tip as in the Java lesson. The shell's $(( )) does not have floats. A price in cents can live here as a whole number. 450 cents doubled is 900 cents, which is $9.00 when you format it. A price in dollars and cents belongs in cents until you format it. The next lesson wraps that formatting in a function you can call by name.