To take the authentication with Sinatra a bit farther you may want to use  another service to do your authentication against. This is some sample code adapted from a Rails example. This code takes use of the linkedin gem from Wynn Netherland to do the authentication and also make some calls to the LinkedIn API. There are other gems that just do authentication for many services like Facebook and Twitter but for this sample I wanted to be able to make additional calls to the LinkedIn API.

require "rubygems"
require "haml"
require "sinatra"
require "linkedin"
enable :sessions
helpers do
def login?
if session[:atoken].nil?
return false
else
return true
end
end
def profile
unless session[:atoken].nil?
client = LinkedIn::Client.new(settings.api, settings.secret)
client.authorize_from_access(session[:atoken], session[:asecret])
return client.profile
end
end
def connections
unless session[:atoken].nil?
client = LinkedIn::Client.new(settings.api, settings.secret)
client.authorize_from_access(session[:atoken], session[:asecret])
return client.connections
end
end
end
configure do
# get your api keys at https://www.linkedin.com/secure/developer
set :api, "your_api_key"
set :secret, "your_secret"
end
get "/" do
haml :index
end
get "/auth" do
client = LinkedIn::Client.new(settings.api, settings.secret)
request_token = client.request_token(:oauth_callback => "http://#{request.host}:#{request.port}/auth/callback")
session[:rtoken] = request_token.token
session[:rsecret] = request_token.secret
redirect client.request_token.authorize_url
end
get "/auth/logout" do
session[:atoken] = nil
redirect "/"
end
get "/auth/callback" do
client = LinkedIn::Client.new(settings.api, settings.secret)
if session[:atoken].nil?
pin = params[:oauth_verifier]
atoken, asecret = client.authorize_from_request(session[:rtoken], session[:rsecret], pin)
session[:atoken] = atoken
session[:asecret] = asecret
end
redirect "/"
end
__END__
@@index
-if login?
%p Welcome #{profile.first_name}!
%a{:href => "/auth/logout"} Logout
%p= profile.headline
%br
%div= "You have #{connections.total} connections!"
-connections.all.each do |c|
%div= "#{c.first_name} #{c.last_name} - #{c.headline}"
-else
%a{:href => "/auth"} Login using LinkedIn
view raw sinatra_linkedin.rb hosted with ❤ by GitHub

If you know of any way to make this code better please comment or fork the Gist.