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Doing Calculations on the Command Line with the bc Tool and Your Shell


Here's how you can add, subtract, multiply and divide numbers without leaving your terminal.

Quick Jump: Shell Script | bc Tool | Demo Video

Once in a while you may find yourself wanting to do a quick calculation on the command line or within a shell script. I typically reach for the bc tool for this but your shell also has a few ways to do basic calculations.

Shell Script

Here’s a couple of examples using $(( ... )) which is POSIX compliant, you can use let or expr too but they tend to be less portable:

$ echo $(( 2 + 2 ))

$ echo $(( 3 * 3 ))

$ echo $(( 4 % 3 ))

$ value=$(( 5 - 3 )) && echo $(( value += 1 ))

Where this tends to fall short is when you want to deal with decimals.

For example with sh or bash:

$ echo $(( 2 * 2.25 ))
bash: 2 * 2.25 : syntax error: invalid arithmetic operator (error token is ".25 ")

Note, if you’re using zsh the above will work because it handles decimals, but at least for me while I do use zsh, I tend to use sh or bash in my scripts so I wouldn’t depend on zsh.

bc Tool

Quite a few systems will have bc installed by default, but if not you can install it with your package manager of choice such as apt-get install bc, here’s a few examples:

$ bc <<< "2 + 3 - 4"

$ bc <<< 2+3-4

$ bc <<< "4 % 3"

$ value="$(bc <<< "3 * 5")" && echo "${value}"

$ bc <<< 1/4

$ bc -l <<< 1/4

$ bc <<< "scale=3; 1/4"

By adding the -l flag, that will configure bc to handle decimals which defaults to 20 decimal places. Alternatively you can set scale=x where x is how many decimal places you want. When you don’t supply either option it rounds down to 0 decimal places.

The bc tool can do a whole lot more, I suggest skimming man bc for more details but if you’re in a pinch to do some basic math on the command line it’s quite handy.

I used it in my invoice script and you can use it to calculate arbitrary expressions or anytime it makes sense!

The video below demos some of the commands.

Demo Video


  • 0:09 – Using functionality built into your shell
  • 1:47 – Using the bc tool
  • 3:01 – Handling decimals

When was the last time you did math on the command line? Let me know below.

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