Event

When I try to start the server with rails s, I get an error and the server cannot start. Error: "port3000 is already in use"

`initialize': Address already in use - bind(2) for "127.0.0.1" port 3000 (Errno::EADDRINUSE)

Cause

The port3000 you want to use is being used by an old process in the past and is preventing the new process from starting the server.

Remedy: Eliminate the batting of the used port

Check the process (PID) where the port number where the error is occurring is used

mao-no-MacBook-Air:hello_world_sample mao$ lsof -i:3000
COMMAND   PID USER   FD   TYPE             DEVICE SIZE/OFF NODE NAME
ruby    30406  mao   12u  IPv4 0x982d8086b787a0bb      0t0  TCP localhost:hbci (LISTEN)
ruby    30406  mao   13u  IPv6 0x982d8086b93685bb      0t0  TCP localhost:hbci (LISTEN)

List files open with "lsof" Specify option "-i: port number"

Delete the started process

mao-no-MacBook-Air:hello_world_sample mao$ kill 30406

Reboot the server and check that there are no errors

mao-no-MacBook-Air:hello_world_sample mao$ rails s
/Users/mao/.rbenv/versions/2.7.1/lib/ruby/2.7.0/x86_64-darwin17/stringio.bundle: warning: already initialized constant StringIO::VERSION
=> Booting Puma
=> Rails 6.0.3.2 application starting in development 
=> Run `rails server --help` for more startup options
Puma starting in single mode...
* Version 4.3.5 (ruby 2.7.1-p83), codename: Mysterious Traveller
* Min threads: 5, max threads: 5
* Environment: development
* Listening on tcp://127.0.0.1:3000
* Listening on tcp://[::1]:3000
Use Ctrl-C to stop

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