TremaでOpenFlowプログラミング

上記サイトを参考にTremaのインストールを終え、実行すると以下のようなエラーが出ました。
command.rbを色々いじってみても結果は変わらず分かりません。お助け頂けると嬉しいです。
環境は、CentOS 7です。

[root@localhost hello_trema]# ./bin/trema run ./lib/hello_trema.rb -c trema.conf
/opt/rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/trema-0.10.1/lib/trema/command.rb:153:in `create_pid_file': HelloTrema is already running. (RuntimeError)
    from /opt/rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/trema-0.10.1/lib/trema/command.rb:29:in `run'
    from /opt/rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/trema-0.10.1/bin/trema:68:in `block (2 levels) in <module:App>'
    from /opt/rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/gli-2.13.4/lib/gli/command_support.rb:126:in `call'
    from /opt/rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/gli-2.13.4/lib/gli/command_support.rb:126:in `execute'
    from /opt/rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/gli-2.13.4/lib/gli/app_support.rb:296:in `block in call_command'
    from /opt/rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/gli-2.13.4/lib/gli/app_support.rb:309:in `call'
    from /opt/rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/gli-2.13.4/lib/gli/app_support.rb:309:in `call_command'
    from /opt/rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/gli-2.13.4/lib/gli/app_support.rb:83:in `run'
    from /opt/rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/trema-0.10.1/bin/trema:291:in `<module:App>'
    from /opt/rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/trema-0.10.1/bin/trema:14:in `<module:Trema>'
    from /opt/rbenv/versions/2.2.3/lib/ruby/gems/2.2.0/gems/trema-0.10.1/bin/trema:11:in `<top (required)>'
    from ./bin/trema:29:in `load'
    from ./bin/trema:29:in `<main>'