$ gem install selenium
selenium install - installs selenium
selenium - runs the selenium server
If you are on OSX, you have better option with homebrew help:
brew install selenium-server-standalone
Standalone selenium server is implemented as launchd agent.
To have launchd start selenium-server-standalone at login, create soft link:
ln -sfv /usr/local/opt/selenium-server-standalone/*.plist ~/Library/LaunchAgents
Then load selenium-server-standalone agent:
launchctl load ~/Library/LaunchAgents/homebrew.mxcl.selenium-server-standalone.plist
It will run selenium server on port 4444.
If you don't want to use agent, use java directly:
java -jar /usr/local/opt/selenium-server-standalone/selenium-server-standalone-2.35.0.jar -p 4444