Add Make An Executable Ruby Script as a Ruby TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1224 TILs and counting..._
+_1225 TILs and counting..._
 
 ---
 
@@ -1001,6 +1001,7 @@ _1224 TILs and counting..._
 - [Limit Split](ruby/limit-split.md)
 - [List The Running Ruby Version](ruby/list-the-running-ruby-version.md)
 - [Listing Local Variables](ruby/listing-local-variables.md)
+- [Make An Executable Ruby Script](ruby/make-an-executable-ruby-script.md)
 - [Map With Index Over An Array](ruby/map-with-index-over-an-array.md)
 - [Mock Method Chain Calls With RSpec](ruby/mock-method-chain-calls-with-rspec.md)
 - [Mocking Requests With Partial URIs Using Regex](ruby/mocking-requests-with-partial-uris-using-regex.md)

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

@@ -0,0 +1,38 @@
+# 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
+```