address already in use ruby
#get list of process running on tcp:3000
lsof -wni tcp:3000
#you will get something like this
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
ruby 2401 ubuntu 10u IPv4 805038 0t0 TCP 127.0.0.1:3000 (LISTEN)
ruby 2401 ubuntu 15u IPv6 805039 0t0 TCP [::1]:3000 (LISTEN)
#now kill process.
#by running this command ->$ kill -9 <PID>
kill -9 2401
#now try to start your server again.