gem commands

* help
gem -h
gem environment
gem help example
gem help platforms
gem help search

* list up gem pack(first char is 'r')
gem list r

* inst/uninst/update
gem insall rails
gem update erubis
gem uninsall sshexpect

* search with :-d detail :-r remote
gem search -r  -d  expect
gem search -r  -d  httpclient
gem search -r  -d  sshexpect
gem search evil --remote

* start RDoc presentation web server
gem server --port 8808 --no-daemon