Making a calculator with bash and sed

Tuesday 22 March 2016 at 08:00 GMT

Here's an easy way to make a calculator REPL:

#!/usr/bin/env bash

while read line; do
    echo $(($line))
done

In Bash (and other shells), $((…)) is the syntax for an arithmetic expansion. Anything inside the parentheses is evaluated as most programming languages would, for integer maths (no floating-point magic here). So you can do simple maths:

$ echo $((2 + 2))
4

You can also use shell variables as you'd expect, except you don't need to use the $ to refer to them—it's implicit.

$ i=7
$ i=$((i + 1))
    # The above statement could also be written as:
    #   * `((i += 1))`
    #   * `let i='i + 1'`
$ echo $i
8

So, when we run our "program", we can even use variables, as long as we declare them outside.

$ apple_price=17
$ ./calculator
2 + 2
4
4 * 5
20
3 * apple_price
51

Unfortunately, we can't do the same with functions. My triangular number function works fine in the shell itself but can't be used as part of an arithmetic expression:

$ $ function triangular { echo $(($1 * ($1 + 1) / 2)); }
$ triangular 4
10
$ ./calculator
triangular 4
bash: triangular 4: syntax error in expression (error token is "4")

However, with a bit of regular expression magic in the script, we can replace triangular <n> with the expression itself. It's a hack and a half, but for prototyping, it works pretty well.

#!/usr/bin/env bash

triangular_replacement='s/triangular ([0-9]+)/\1 * (\1 + 1) \/ 2/g'
sed --regexp-extended "$triangular_replacement" | while read line; do
    echo $(($line))
done

(If you want a decent sed on Mac OS X, install gsed with brew install gnu-sed and use that instead.)

Let's try it.

$ ./calculator
triangular 3 + triangular 4

… Nothing happened. Let's try again:

10 + triangular 4

Still nothing.

Unfortunately, sed buffers when piping to something else. I was pairing with @sleepyfox a couple of weeks ago in a workshop, and it really confused us, so this is actually the original inspiration for this post. When I hit Ctrl+D to send the end-of-file character and tell read we're done here, sed flushes its buffer.

<Ctrl+D>
16
20

Both outputs happen at the same time. Not so helpful. So today, when figuring this one out, I came across the --unbuffered switch. Duh.

#!/usr/bin/env bash

triangular_replacement='s/triangular ([0-9]+)/\1 * (\1 + 1) \/ 2/g'
sed --unbuffered --regexp-extended "$triangular_replacement" | while read line; do
    echo $(($line))
done

Running it:

$ ./calculator
triangular 5 * triangular 6
315

Excellent.