Add Replace The Current Process With An External Command as a ruby til.

README.md

@@ -131,6 +131,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 - [Percent Notation](ruby/percent-notation.md)
 - [Question Mark Operator](ruby/question-mark-operator.md)
 - [Rake Only Lists Tasks With Descriptions](ruby/rake-only-lists-tasks-with-descriptions.md)
+- [Replace The Current Process With An External Command](ruby/replace-the-current-process-with-an-external-command.md)
 - [Squeeze Out The Extra Space](ruby/squeeze-out-the-extra-space.md)
 - [Summing Collections](ruby/summing-collections.md)
 - [Uncaught Exceptions In Pry](ruby/uncaught-exceptions-in-pry.md)

ruby/replace-the-current-process-with-an-external-command.md

@@ -0,0 +1,29 @@
+# Replace The Current Process With An External Command
+
+Ruby's
+[`Kernel#exec`](http://ruby-doc.org/core-2.2.3/Kernel.html#method-i-exec)
+method can be used to run an external command. What differentiates it from
+executing commands with the likes of back ticks or `%x[]` is that instead of
+forking a child process, it replaces the current process.
+
+For instance, the following ruby script, when executed, will replace itself
+with an `irb` session.
+
+```ruby
+Kernel.exec('irb')
+```
+
+The external command will even benefit from the existing environment. For
+example, if I set the following environment variable
+
+```bash
+$ export GREETING=hello
+```
+
+and then execute a file containing
+
+```ruby
+Kernel.exec('echo $GREETING')
+```
+
+I can expect to see `hello` printed to stdout.