Primary Unit

I find bc trips me up when I try to calculations at the shell, so I wrapped it.

There are two hazards for me:

1. Failure to handle expressions without a final newline.
2. Integer division.

Final newline missing

No 1 often occurs when I produce a list of numbers in BBEdit, usually extracted and transformed via regexes.

For example, we get a PDF receipt from Sainbury’s for our online shopping, and to work out my share I like to edit the text from pdftotext in BBEdit, then run the line totals through a pipeline to calculate the total.

That used to look like this:

$ pbpaste | paste -s -d '+' - | bc
609

pbpaste pastes the macOS system clipboard, paste joins those lines with +, and bc (should!) print the total.

Except this happens if you’re missing a final newline:

$ pbpaste | paste -s -d '+' - | bc
(standard_in) 1: parse error

Integer division

And secondly, with the default settings, bc does integer division:

$ bc <<< 9/2
4

Which can be fixed by setting scale

$ bc <<< 'scale=2; 9/2'
4.50

(<<< starts a “here-string”, which you can learn more about at man -P "less -p '<<<'" zshmisc, which will take you directly to the right part of the zshmisc man page. Search the bash man page if you’re using bash, but it’s significantly more terse than the zsh explanation.)

Wrapping bc

So I wrote a simple wrapper script, calc:

#!/usr/local/bin/zsh
set -euo pipefail

# Read expression from $1 or stdin.
if [ $# -ge 1 ]; then
    expression=$1
else
    read expression || true  # Force successful exit
fi

# Default scale to 3 (.123) if $2 is not given.
bc <<< "scale=${2:-3}; $expression"

The conditional in the middle just handles being called in a pipe, with the expression coming on standard input, or having the expression provided as the first argument. The || true is necessary after read as read only exits with a success code if it encounters EOF, and since I have -e set in the file, that would cause the whole script to exit.

I set the scale to 3 by default (showing 3 decimal places), but you can configure that with the second argument.

Using a here-string for bc’s input means that it should work whether or not there is a final newline.

The use of read also allows the use of calc by itself to print the result of one expression, for example:

$ calc
(2 + 2) / (10 / 2)↩
.800

Wrapping summation

I use the “sum this list of numbers” pipeline fairly often, and it’s easy to pull out into its own command. I’m just using an alias, which I’ve added to my .zshrc:

alias ∑="paste -s -d '+' - | calc"

sum is already taken, so ∑ seemed appropriate. On macOS you can type it with ⌥w.

So my pipeline from before just becomes pbpaste | ∑.