Home DRY by reading .ruby-version in your Gemfile

DRY by reading .ruby-version in your Gemfile

Most Ruby projects have a .ruby-version file that is used by different version managers to automatically switch Ruby version based on the content of that file. Very useful if you have many projects with different Ruby versions.

But the same Ruby version is also sometimes declared in the project’s Gemfile.

You might have something like

Your .ruby-version


Your Gemfile

ruby "3.2.0"

Both are the same values but on a different file, if you need to update your Ruby version, you would need to update both which is a bit tedious and prone to forgetting.

When you somehow got a mismatched version on those files you might get the following error:

Your Ruby version is 3.2.0, but your Gemfile specified 3.1.0

How do we improve it?

Gemfile is just another Ruby script that is executed when running bundle, you can just treat it as a simple Ruby script.

In your Gemfile you can do:

ruby File.read(File.join(__dir__, ".ruby-version")).strip

I have seen on other sources that they are using a shorter version such as:

ruby File.read(".ruby-version").strip

The problem with this is that bundle stops working if you navigate into a directory.

An example on where this is necessary is on a React Native app, where the Gemfile and .ruby-version is on the root directory but you have to run cd ios; bundle exec pod install, that code raises the following error.

[!] There was an error parsing `Gemfile`: No such file or directory @ rb_sysopen - .ruby-version. Bundler cannot continue.

 #  from /Users/davidangulo/myapp/Gemfile:5
 #  -------------------------------------------
 >  ruby File.read(".ruby-version").strip
 #  -------------------------------------------

Because .ruby-version is no longer present in my current directory which is ios.

To make bundle work on any subdirectory, I used File.join(__dir__, ".ruby-version"), where __dir__ is the current directory of the Gemfile, this assumes that the Gemfile and .ruby-version are always in the same directory. So instead of executing in the current directory it executes based on the Gemfiles directory.

See: https://github.com/facebook/react-native/pull/35410


We have now DRYed our Ruby version declaration and only needs to be changed in one place instead of multiple.

You can use the shorter version if you don’t need to run bundle on a subdirectory.

Use the longer version to make it path agnostic and works everywhere on your project.

This post is licensed under CC BY 4.0 by the author.

Get the filesize of base64 file

Pretty print JSON in JavaScript