Add Make An Executable Ruby Script as a Ruby TIL

ruby/make-an-executable-ruby-script.md

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+# Make An Executable Ruby Script
+
+In a unix environment with Ruby available, I can make a Ruby script. To do this
+I stick some code in a Ruby file, like `database_url.rb`.
+
+```ruby
+result = `heroku pg:credentials:url DATABASE_URL --app my-app`
+puts result.split("\n")[2].strip
+```
+
+And then execute that file with `ruby`:
+
+```bash
+$ ruby database_url.rb
+```
+
+I can instead make an executable file that doesn't need to be explicitly
+invoked with the `ruby` command. To do this, I need to prefix my file with a
+[shebang](https://unix.stackexchange.com/a/87600/5916) for
+[`ruby`](https://devcenter.heroku.com/articles/ruby-binstub-shebang).
+
+And I'll even just call the file `database_url` now, no file suffix.
+
+```ruby
+#!/usr/bin/env ruby
+
+result = `heroku pg:credentials:url DATABASE_URL --app my-app`
+puts result.split("\n")[2].strip
+```
+
+When executed, this script will see the first line and understand that it needs
+to execute the rest of the script using `ruby` as the interpreter.
+
+Like any other executable, you can call it as is, like so:
+
+```ruby
+$ database_url
+```