Read from .ruby-version if version is unset or set to '.ruby-version'

* Fixes https://github.com/eregon/use-ruby-action/issues/4

.github/workflows/test.yml

@@ -7,7 +7,7 @@ jobs:
       matrix:
         os: [ ubuntu-16.04, ubuntu-18.04, macos-latest, windows-latest ]
         # Use various version syntaxes here for testing
-        ruby: [ 2.3, ruby-2.4, 2.5, ruby-2.6.5, 2.7.0, jruby, truffleruby ]
+        ruby: [ .ruby-version, 2.3, ruby-2.4, 2.5, ruby-2.6.5, 2.7.0, jruby, truffleruby ]
         exclude:
         - os: windows-latest
           ruby: 2.3

.ruby-version

@@ -0,0 +1 @@
+ruby-2.6.5

README.md

@@ -53,6 +53,8 @@ jobs:
     - run: ruby -v
 ```
 
+You can leave `ruby-version` unset to use the project's `.ruby-version` file.
+
 ### Matrix
 
 This matrix tests all stable releases of MRI, JRuby and TruffleRuby on Ubuntu and macOS.
@@ -82,6 +84,7 @@ jobs:
 * short version like `2.6`, automatically using the latest release matching that version (`2.6.5`)
 * version only like `2.6.5`, assumes MRI for the engine
 * engine only like `truffleruby`, uses the latest stable release of that implementation
+* `.ruby-version` reads from the project's `.ruby-version` file
 
 ### Bundler
 

action.yml

@@ -6,8 +6,8 @@ branding:
   icon: download
 inputs:
   ruby-version:
-    description: 'Engine and version to use, see the syntax in the README'
-    required: true
+    description: 'Engine and version to use, see the syntax in the README. Reads from .ruby-version if unset.'
+    default: '.ruby-version'
 outputs:
   ruby-prefix:
     description: 'The prefix of the installed ruby'

dist/index.js

@@ -1428,6 +1428,11 @@ async function getLatestReleaseTag() {
 }
 
 async function getRubyEngineAndVersion(rubyVersion) {
+  if (rubyVersion === '.ruby-version') { // Read from .ruby-version
+    rubyVersion = fs.readFileSync('.ruby-version', 'utf8').trim()
+    console.log(`Using ${rubyVersion} as input from file .ruby-version`)
+  }
+
   let engine, version
   if (rubyVersion.match(/^\d+/)) { // X.Y.Z => ruby-X.Y.Z
     engine = 'ruby'

index.js

@@ -47,6 +47,11 @@ async function getLatestReleaseTag() {
 }
 
 async function getRubyEngineAndVersion(rubyVersion) {
+  if (rubyVersion === '.ruby-version') { // Read from .ruby-version
+    rubyVersion = fs.readFileSync('.ruby-version', 'utf8').trim()
+    console.log(`Using ${rubyVersion} as input from file .ruby-version`)
+  }
+
   let engine, version
   if (rubyVersion.match(/^\d+/)) { // X.Y.Z => ruby-X.Y.Z
     engine = 'ruby'