bundle exec unicorn_railsコマンドでのエラー

## はじめに
プログラミング初心者による備忘録です。

今回はEC2内でrailsサーバーを立ち上げようとした時に発生したエラーと解決できた方法を残します。

## 問題とエラーメッセージ

EC2内で以下のコマンドを実行。
```
$ bundle exec unicorn_rails -c config/unicorn.rb -E production -D
```
すると以下のようなエラーが発生。
```
Your RubyGems version (3.0.3) has a bug that prevents `required_ruby_version` from working for Bundler. Any scripts that use `gem install bundler` will break as soon as Bundler drops support for your Ruby version. Please upgrade RubyGems to avoid future breakage and silence this warning by running `gem update --system 3.2.3`
bundler: failed to load command: unicorn_rails (/home/ec2-user/.rbenv/versions/2.6.5/bin/unicorn_rails)
Traceback (most recent call last):
	25: from /home/ec2-user/.rbenv/versions/2.6.5/bin/bundle:23:in `<main>'
	24: from /home/ec2-user/.rbenv/versions/2.6.5/bin/bundle:23:in `load'
	23: from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/bundler-2.4.3/exe/bundle:33:in `<top (required)>'
	22: from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/bundler-2.4.3/lib/bundler/friendly_errors.rb:117:in `with_friendly_errors'
	21: from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/bundler-2.4.3/exe/bundle:45:in `block in <top (required)>'
	20: from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/bundler-2.4.3/lib/bundler/cli.rb:28:in `start'
	19: from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/bundler-2.4.3/lib/bundler/vendor/thor/lib/thor/base.rb:485:in `start'
	18: from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/bundler-2.4.3/lib/bundler/cli.rb:34:in `dispatch'
	17: from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/bundler-2.4.3/lib/bundler/vendor/thor/lib/thor.rb:392:in `dispatch'
	16: from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/bundler-2.4.3/lib/bundler/vendor/thor/lib/thor/invocation.rb:127:in `invoke_command'
	15: from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/bundler-2.4.3/lib/bundler/vendor/thor/lib/thor/command.rb:27:in `run'
	14: from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/bundler-2.4.3/lib/bundler/cli.rb:491:in `exec'
	13: from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/bundler-2.4.3/lib/bundler/cli/exec.rb:23:in `run'
	12: from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/bundler-2.4.3/lib/bundler/cli/exec.rb:58:in `kernel_load'
	11: from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/bundler-2.4.3/lib/bundler/cli/exec.rb:58:in `load'
	10: from /home/ec2-user/.rbenv/versions/2.6.5/bin/unicorn_rails:23:in `<top (required)>'
	 9: from /home/ec2-user/.rbenv/versions/2.6.5/bin/unicorn_rails:23:in `load'
	 8: from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/unicorn-5.4.1/bin/unicorn_rails:209:in `<top (required)>'
	 7: from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/unicorn-5.4.1/bin/unicorn_rails:209:in `new'
	 6: from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/unicorn-5.4.1/lib/unicorn/http_server.rb:77:in `initialize'
	 5: from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/unicorn-5.4.1/lib/unicorn/http_server.rb:77:in `new'
	 4: from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/unicorn-5.4.1/lib/unicorn/configurator.rb:77:in `initialize'
	 3: from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/unicorn-5.4.1/lib/unicorn/configurator.rb:84:in `reload'
	 2: from /home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/unicorn-5.4.1/lib/unicorn/configurator.rb:84:in `instance_eval'
	 1: from config/unicorn.rb:7:in `reload'
/home/ec2-user/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/unicorn-5.4.1/lib/unicorn/configurator.rb:592:in `working_directory': config_file=config/unicorn.rb would not be accessible in working_directory=/var/www/current (ArgumentError)
master failed to start, check stderr log for details

```

## 原因
参照サイト様
[デプロイ後、unicornの起動時にworking_directoryが参照されない](https://teratail.com/questions/229963)

`working_directory': config_file=config/unicorn.rb would not be accessible in working_directory=/var/www/current (ArgumentError)`
この部分がエラーの原因を直接教えてくれています。

`config_file=config/unicorn.rb`
を探しているが
`working_directory=/var/www/current`
にありませんよ？と言っているわけです。

要するに
`/var/www/current/config/unicorn.rb`を探しています。
しかし実際は
`/var/www/<アプリ名>/current/config/unicorn.rb`にあります。
なので、このパスを指定している部分を治す必要があります。

## 解決方法
EC2アプリ内で以下のコマンドを実行
```
$ vi config/unicorn.rb
```
`config/unicorn.rb`が開かれますので

`app_path = File.expand_path('../../../', __FILE__)`
を
`app_path = File.expand_path('../../', __FILE__)`
に修正します。

これでパスの指定は問題ありません。

## あとがき
受講スクールの記述を鵜呑みにして自分で考えていませんでした。
反省して、記述する内容はしっかり理解したものを実行していきます。