Using Just to run a subset of test files

2026-10-15

I now spend a lot of time in the terminal, whether to run Git, Air, or Jarl. Lately I’ve been using Just to run custom commands to lint, format, and test code.

Just is an easy-to-install command runner. It allows one to define commands in a justfile and run them with just <command name>. For instance, our justfile could define a lint command that would run a custom Jarl command:

lint:
  jarl check . --output-format concise

and then we could call just lint in the terminal.

I was already using Just in tidypolars, a project whose test suite has thousands of tests that usually take about two to three minutes to run, but @Yousa-Mirage gave the nice suggestion to add the following rule in justfile:

test filter="":
    Rscript -e 'devtools::test(filter = commandArgs(TRUE)[[1]])' {{ quote(filter) }}

This uses the filter argument in devtools::test() to only run test files whose name contain the value of filter. However, running this in the R console is a bit annoying as I either need to write it in full, or to navigate the console history and modify the value of filter.

Now, I can go to the terminal and run just test foo, just test bar, etc. This is very useful if your test suite takes more than a few seconds to run1This works even better if you have autocompletion in your terminal..

Couldn’t you simply use Make instead of Just?

Probably. I just never had to write Makefiles and discovered Just via a co-developer.

I’m pretty sure Make would also work fine for this simple example, but the Just docs list many differences and improvements over Make. The goal of this post was just to showcase a small quality-of-life improvement in my workflow.