I've had to look this up a few times, so thought it might be helpful to write down my solution, at least on Mac.
When you start a webserver the process will start listening and bind to a specific port. Sometimes, we'll start a webserver in a terminal or from an IDE then close the terminal or close the IDE and lose the ability to kill a given process. This is annoying because when you go to start a server on the same port you'll get an error like:
❯ ruby server.rb /Users/me/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/thin-2.2.16.stripe/lib/thin/server.rb:104: warning: constant ::Fixnum is deprecated == Sinatra (v2.1.0) has taken the stage on 4242 for development with backup from Thin >> Thin web server (v1.2.8 codename Does It Offend You, Yeah?) >> Maximum connections set to 65536 >> Listening on localhost:4242, CTRL+C to stop >> Stopping ... == Sinatra has ended his set (crowd applauds) Traceback (most recent call last): 11: from /Users/me/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/sinatra-2.1.0/lib/sinatra/main.rb:45:in `block in <module:Sinatra>' 10: from /Users/me/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/sinatra-2.1.0/lib/sinatra/base.rb:1499:in `run!' 9: from /Users/me/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/sinatra-2.1.0/lib/sinatra/base.rb:1565:in `start_server' 8: from /Users/me/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/rack-2.2.3/lib/rack/handler/thin.rb:22:in `run' 7: from /Users/me/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/thin-2.2.16.stripe/lib/thin/server.rb:159:in `start' 6: from /Users/me/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/thin-2.2.16.stripe/lib/thin/backends/base.rb:71:in `start' 5: from /Users/me/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/eventmachine-1.2.7/lib/eventmachine.rb:195:in `run' 4: from /Users/me/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/eventmachine-1.2.7/lib/eventmachine.rb:195:in `run_machine' 3: from /Users/me/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/thin-2.2.16.stripe/lib/thin/backends/base.rb:63:in `block in start' 2: from /Users/me/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/thin-2.2.16.stripe/lib/thin/backends/tcp_server.rb:16:in `connect' 1: from /Users/me/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/eventmachine-1.2.7/lib/eventmachine.rb:531:in `start_server' /Users/me/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/eventmachine-1.2.7/lib/eventmachine.rb:531:in `start_tcp_server': no acceptor (port is in use or requires root privileges) (RuntimeError)
After a short time debugging, I always land on this StackOverflow post which shows how to use
lsof on mac which lists open files.
-i tcp:3000 where 3000 is the port that the server was started on will filter the output to the running webserver process that is listening on port 3000.
Once you've found the running process with
lsof -i tcp:4242:
❯ lsof -i tcp:4242 COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME ruby 53769 me 19u IPv6 0xae667db68e1c760f 0t0 TCP localhost:4242 (LISTEN)
You can kill the process with
kill -9 <pid> in this case:
kill -9 53769
Which stops the process and allows you to again try restarting your server.