バージョンを指定したRailsのインストールは、下記。
gem install rails -v インストールするバージョン
インストールされているRailsのバージョンを表示するコマンドは下記。
rails -v
例： Rails 5.2.2.1をインストールする
zen:~$ gem install rails -v 5.2.2.1 Fetching thread_safe-0.3.6.gem Fetching tzinfo-1.2.5.gem Fetching i18n-1.6.0.gem Fetching activesupport-5.2.2.1.gem Fetching mini_portile2-2.4.0.gem Fetching nokogiri-1.10.1.gem Fetching concurrent-ruby-1.1.5.gem Fetching rails-dom-testing-2.0.3.gem Fetching rails-html-sanitizer-1.0.4.gem Fetching loofah-2.2.3.gem Fetching builder-3.2.3.gem Fetching actionview-5.2.2.1.gem Fetching rack-2.0.6.gem Fetching crass-1.0.4.gem Fetching rack-test-1.1.0.gem Fetching erubi-1.8.0.gem Fetching actionpack-5.2.2.1.gem Fetching activemodel-5.2.2.1.gem Fetching activejob-5.2.2.1.gem Fetching arel-9.0.0.gem Fetching activerecord-5.2.2.1.gem Fetching mini_mime-1.0.1.gem Fetching mail-2.7.1.gem Fetching globalid-0.4.2.gem Fetching nio4r-2.3.1.gem Fetching actionmailer-5.2.2.1.gem Fetching websocket-extensions-0.1.3.gem Fetching websocket-driver-0.7.0.gem Fetching actioncable-5.2.2.1.gem Fetching mimemagic-0.3.3.gem Fetching marcel-0.3.3.gem Fetching activestorage-5.2.2.1.gem Fetching thor-0.20.3.gem Fetching rails-5.2.2.1.gem Fetching method_source-0.9.2.gem Fetching sprockets-3.7.2.gem Fetching sprockets-rails-3.2.1.gem Fetching railties-5.2.2.1.gem Successfully installed concurrent-ruby-1.1.5 Successfully installed thread_safe-0.3.6 Successfully installed tzinfo-1.2.5 HEADS UP! i18n 1.1 changed fallbacks to exclude default locale. But that may break your application. Please check your Rails app for 'config.i18n.fallbacks = true'. If you're using I18n (>= 1.1.0) and Rails (< 5.2.2), this should be 'config.i18n.fallbacks = [I18n.default_locale]'. If not, fallbacks will be broken in your app by I18n 1.1.x. For more info see: https://github.com/svenfuchs/i18n/releases/tag/v1.1.0 Successfully installed i18n-1.6.0 Successfully installed activesupport-5.2.2.1 Successfully installed mini_portile2-2.4.0 Building native extensions. This could take a while... Successfully installed nokogiri-1.10.1 Successfully installed rails-dom-testing-2.0.3 Successfully installed crass-1.0.4 Successfully installed loofah-2.2.3 Successfully installed rails-html-sanitizer-1.0.4 Successfully installed erubi-1.8.0 Successfully installed builder-3.2.3 Successfully installed actionview-5.2.2.1 Successfully installed rack-2.0.6 Successfully installed rack-test-1.1.0 Successfully installed actionpack-5.2.2.1 Successfully installed activemodel-5.2.2.1 Successfully installed arel-9.0.0 Successfully installed activerecord-5.2.2.1 Successfully installed mini_mime-1.0.1 Successfully installed mail-2.7.1 Successfully installed globalid-0.4.2 Successfully installed activejob-5.2.2.1 Successfully installed actionmailer-5.2.2.1 Building native extensions. This could take a while... Successfully installed nio4r-2.3.1 Successfully installed websocket-extensions-0.1.3 Building native extensions. This could take a while... Successfully installed websocket-driver-0.7.0 Successfully installed actioncable-5.2.2.1 Successfully installed mimemagic-0.3.3 Successfully installed marcel-0.3.3 Successfully installed activestorage-5.2.2.1 Successfully installed method_source-0.9.2 Successfully installed thor-0.20.3 Successfully installed railties-5.2.2.1 Successfully installed sprockets-3.7.2 Successfully installed sprockets-rails-3.2.1 Successfully installed rails-5.2.2.1 Parsing documentation for concurrent-ruby-1.1.5 Installing ri documentation for concurrent-ruby-1.1.5 Parsing documentation for thread_safe-0.3.6 Installing ri documentation for thread_safe-0.3.6 Parsing documentation for tzinfo-1.2.5 Installing ri documentation for tzinfo-1.2.5 Parsing documentation for i18n-1.6.0 Installing ri documentation for i18n-1.6.0 Parsing documentation for activesupport-5.2.2.1 Installing ri documentation for activesupport-5.2.2.1 Parsing documentation for mini_portile2-2.4.0 Installing ri documentation for mini_portile2-2.4.0 Parsing documentation for nokogiri-1.10.1 Installing ri documentation for nokogiri-1.10.1 Parsing documentation for rails-dom-testing-2.0.3 Installing ri documentation for rails-dom-testing-2.0.3 Parsing documentation for crass-1.0.4 Installing ri documentation for crass-1.0.4 Parsing documentation for loofah-2.2.3 Installing ri documentation for loofah-2.2.3 Parsing documentation for rails-html-sanitizer-1.0.4 Installing ri documentation for rails-html-sanitizer-1.0.4 Parsing documentation for erubi-1.8.0 Installing ri documentation for erubi-1.8.0 Parsing documentation for builder-3.2.3 Installing ri documentation for builder-3.2.3 Parsing documentation for actionview-5.2.2.1 Installing ri documentation for actionview-5.2.2.1 Parsing documentation for rack-2.0.6 Installing ri documentation for rack-2.0.6 Parsing documentation for rack-test-1.1.0 Installing ri documentation for rack-test-1.1.0 Parsing documentation for actionpack-5.2.2.1 Installing ri documentation for actionpack-5.2.2.1 Parsing documentation for activemodel-5.2.2.1 Installing ri documentation for activemodel-5.2.2.1 Parsing documentation for arel-9.0.0 Installing ri documentation for arel-9.0.0 Parsing documentation for activerecord-5.2.2.1 Installing ri documentation for activerecord-5.2.2.1 Parsing documentation for mini_mime-1.0.1 Installing ri documentation for mini_mime-1.0.1 Parsing documentation for mail-2.7.1 Installing ri documentation for mail-2.7.1 Parsing documentation for globalid-0.4.2 Installing ri documentation for globalid-0.4.2 Parsing documentation for activejob-5.2.2.1 Installing ri documentation for activejob-5.2.2.1 Parsing documentation for actionmailer-5.2.2.1 Installing ri documentation for actionmailer-5.2.2.1 Parsing documentation for nio4r-2.3.1 Installing ri documentation for nio4r-2.3.1 Parsing documentation for websocket-extensions-0.1.3 Installing ri documentation for websocket-extensions-0.1.3 Parsing documentation for websocket-driver-0.7.0 Installing ri documentation for websocket-driver-0.7.0 Parsing documentation for actioncable-5.2.2.1 Installing ri documentation for actioncable-5.2.2.1 Parsing documentation for mimemagic-0.3.3 Installing ri documentation for mimemagic-0.3.3 Parsing documentation for marcel-0.3.3 Installing ri documentation for marcel-0.3.3 Parsing documentation for activestorage-5.2.2.1 Installing ri documentation for activestorage-5.2.2.1 Parsing documentation for method_source-0.9.2 Installing ri documentation for method_source-0.9.2 Parsing documentation for thor-0.20.3 Installing ri documentation for thor-0.20.3 Parsing documentation for railties-5.2.2.1 Installing ri documentation for railties-5.2.2.1 Parsing documentation for sprockets-3.7.2 Installing ri documentation for sprockets-3.7.2 Parsing documentation for sprockets-rails-3.2.1 Installing ri documentation for sprockets-rails-3.2.1 Parsing documentation for rails-5.2.2.1 Installing ri documentation for rails-5.2.2.1 Done installing documentation for concurrent-ruby, thread_safe, tzinfo, i18n, activesupport, mini_portile2, nokogiri, rails-dom-testing, crass, loofah, rails-html-sanitizer, erubi, builder, actionview, rack, rack-test, actionpack, activemodel, arel, activerecord, mini_mime, mail, globalid, activejob, actionmailer, nio4r, websocket-extensions, websocket-driver, actioncable, mimemagic, marcel, activestorage, method_source, thor, railties, sprockets, sprockets-rails, rails after 40 seconds 38 gems installed zen:~$ zen:~$ rails -v Rails 5.2.2.1