How to do math in shell environments (bash, zsh, ...)

I was kneedeep into CI/CD scripting, when I hit a wall. You can't just do simple math like adding or subtracting integers in a shell environment? How could I have not discovered this fact before?

As it turns out, simple operations like summing and subtracting two variables need some serious shell knowledge.

Here's the problem.

#!/bin/sh

a=1
b=2
c=$a+$b
echo $c # 1+2

When evaluating c, $a and $b are resolved to their values (1 and 2), but the + is treated as a string. All three parts are strings, and now the result is 1+2. Ufff.

After researching this problem, I learned that there are multiple ways to do math in shell environments. Let's have a look!

Let's start with my first Google result — expr.

#!/bin/sh

a=1
b=2
c=`expr $a + $b`

echo $c #3

Backticks and expr worked, but Shellcheck wasn't happy about it.

I didn't spend time on "legacy backticks" and moved on quickly.

I learned that let and declare let you evaluate arithmetic expressions in a single line. Fancy!

#!/bin/sh

a=1
b=2
let c=a+b
declare -i d=a+b

echo $c # 3
echo $d # 3

The let and declare approach also worked fine, but I didn't find these one-liners good to read.

And more importantly, again, Shellcheck complained. let isn't available in sh shells, and there seems to be a better option in bash.

So here's the better and final option: the winner is arithmetic expansion ($(( ))). It's a fancy syntax to mark parts of your shell code as an arithmetic operation. In plain English, anything between the parentheses is considered math. 💪

#!/bin/sh

a=1
b=2
c=$((a+b))

echo $c # 3

Again, this script worked fine, but this time Shellcheck was also happy about it. Success!

Let's look at two quick arithmetic expansion facts I discovered.

Many online examples show variables with the $ character. It's optional when you want to perform an arithmetic operation.

#!/bin/sh

a=1
b=2
c=$((a+b))
d=$(($a+$b))

echo $c # 3
echo $d # 3

Also, variables that don't represent an integer are ignored in your calculations. They don't throw an error.

#!/bin/sh

a=hello
b=3
c=$((a+b))
echo $c # 3

And that's it! If you want to do math in your shell scripts, $(( )) is your friend.

Sure, I learned how to add numbers in a shell environment today, but can I now say I know what I'm doing when writing these scripts? Absolutely not.

My Google research unveiled all these different approaches. How would I have known what's best? I'd have used the first solution that worked. Luckily, Shellcheck showed me the way.

So there's another hidden lesson here today: linters are great software. Use them!