bash Cookbook

Doing Arithmetic in Your Shell Script

Problem

You need to do some simple arithmetic in your shell script.

Solution

Use $(( )) or let for integer arithmetic expressions.

COUNT=$((COUNT + 5 + MAX * 2))
let COUNT+=5+MAX*2

Discussion

As long as you keep to integer arithmetic, you can use all the standard (i.e., C-like) operators inside of $(()) for arithmetic. There is one additional operator—you can use ** for raising to a power, as in MAX=$((2**8)), which yields 256.

Spaces are not needed nor are they prohibited around operators and arguments (though ** must be together) within a $(( )) expression. But you must not have spaces around the equals sign, as with any bash variable assignment. If you wrote:

COUNT = $((COUNT + 5)) # not what you think!

then bash will try to run a program named COUNT and its first argument would be an equal sign, and its second argument would be the number you get adding 5 to the value of $COUNT. Remember not to put spaces around the equal sign.

Another oddity to these expressions is that the $ that we normally put in front of a shell variable to say we want its value (as in $COUNT or $MAX ) is not needed inside the double parentheses. For example, $((COUNT +5 MAX * 2)) needs no dollar sign on the shell variables—in effect, the outer $ applies to the entire expression.

We do need the dollar sign, though, if we are using a positional parameter (e.g.,$2) to distinguish it from a numeric constant (e.g., “2”). Here’s an example:

COUNT=$((COUNT + $2 + OFFSET))

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