Add Add Linux As A Bundler Platform as a ruby til

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
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@@ -816,6 +816,7 @@ _1056 TILs and counting..._
 ### Ruby
 
 - [A Shorthand For Rerunning Failed Tests With RSpec](ruby/a-shorthand-for-rerunning-failed-tests-with-rspec.md)
+- [Add Linux As A Bundler Platform](ruby/add-linux-as-a-bundler-platform.md)
 - [Are They All True?](ruby/are-they-all-true.md)
 - [Assert About An Object's Attributes With RSpec](ruby/assert-about-an-objects-attributes-with-rspec.md)
 - [Assoc For Hashes](ruby/assoc-for-hashes.md)

ruby/add-linux-as-a-bundler-platform.md

@@ -0,0 +1,34 @@
+# Add Linux As A Bundler Platform
+
+With a locally developed Ruby project on Mac OSX, I have `darwin` specified as
+the _platform_ in the `Gemfile.lock`.
+
+```ruby
+PLATFORMS
+  x86_64-darwin-19
+```
+
+When setting up CI for my project on a linux container, I'd get an error like
+this:
+
+> Your bundle only supports platforms ["x86_64-darwin-19"] ...
+
+This platform incompatability can be solved with by adding linux as a platform
+and re-resolving the lock file. This is done with [`bundle
+lock`](https://bundler.io/v2.0/man/bundle-lock.1.html) and the `--add-platform`
+flag.
+
+```bash
+$ bundle lock --add-platform x86_64-linux
+```
+
+If all existing gems work with the new linux platform, the command will succeed
+and the updated `Gemfile.lock` will have the following `PLATFORMS` section.
+
+```ruby
+PLATFORMS
+  x86_64-darwin-19
+  x86_64-linux
+```
+
+[source](https://github.com/rubygems/rubygems/issues/4269#issuecomment-759591430)