Server is already running in Rails

TL;DR Just Run this command to Kill it

sudo kill -9 $(lsof -i :3000 -t)

Root Cause:
Because PID is locked in a file and web server thinks that if that file exists then it means it is already running. Normally when a web server is closed that file is deleted, but in some cases, proper deletion doesn’t happen so you have to remove the file manually
New Solutions

when you run rails s

=> Booting WEBrick

=> Rails 4.0.4 application starting in development on http://0.0.0.0:3000

=> Run rails server -h for more startup options

=> Ctrl-C to shutdown server

A server is already running. Check /your_project_path/tmp/pids/server.pid.
Exiting

So place your path shown here /your_project_path/tmp/pids/server.pid

and remove this server.pid file:

rm /your_project_path/tmp/pids/server.pid

OR Incase you’re server was detached then follow below guidelines:

If you detached you rails server by using command “rails -d” then,

Remove rails detached server by using command

ps -aef | grep rails

OR by this command

sudo lsof -wni tcp:3000

then

kill -9 pID

OR use this command

To find and kill process by port name on which that program is running. For 3000 replace port on which your program is running.

sudo kill -9 $(lsof -i :3000 -t)

Old Solution:

rails s -p 4000 -P tmp/pids/server2.pid

Also you can find this post for more options
Rails Update to 3.2.11 breaks running multiple servers

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