Rubyのgemパッケージマネージャの使い方とbundlerによる複数バージョンのgemの管理

gem -v # View the version of RubyGems software
gem help #Show RubyGem usage help
gem help example # List some examples of RubyGem commands
gem install [gemname] # Install the specified gem package, the program will first find the gem package from the local machine and install it, or install it from the remote gem if it is not available locally.
gem install -l [gemname] # install gem package from local machine only
gem install -r [gemname] # install gem package from remote only
gem install [gemname] --version=[ver] # Install the specified version of the gem package
gem uninstall [gemname] # Remove the specified gem package, note that this command will remove all installed versions
gem uninstall [gemname] --version=[ver] # Remove a specified version of gem
gem update --system # Update the RubyGems software itself
gem update [gemname] # update all|specified installed gem packages
gem list # View all installed gem packages on the local machine # Show RubyGem help

タオバオのミラーを交換
国内ネットワークの都合（ご存知ですよね）により、Amazon S3に保存されているrubygems.orgのリソースファイルへの接続に断続的に失敗することがあるようです。ここでは、デフォルトのミラーをタオバオのものに置き換える必要があります。

gem sources --remove https://rubygems.org/
gem sources -a https://ruby.taobao.org/
gem sources -l
*** CURRENT SOURCES ***
https://ruby.taobao.org
# Make sure only ruby.taobao.org

gem install bundler

mkdir app1; cd app1;
echo "source 'https://ruby.taobao.org/'" > Gemfile
echo "gem 'rails,'4.1.0'" >> Gemfile
bundle install

mkdir app2; cd app2;
echo "source 'https://ruby.taobao.org/'" > Gemfile
echo "gem 'rails,'3.2.13'" >> Gemfile
bundle install

bundle exec rails s
bundle exec rake db:create
...