Is SystemExit a special kind of Exception?

Question

How does SystemExit behave differently from other Exceptions? I think I understand some of the reasoning about why it wouldn't be good to raise a proper Exception. For example, you wouldn't want something strange like this to happen:

begin
  exit
rescue => e
  # Silently swallow up the exception and don't exit
end

But how does the rescue ignore SystemExit? (What criteria does it use?)

Answer 1

When you write rescue without one or more classes, it is the same as writing:

begin
  ...
rescue StandardError => e
  ...
end

There are Exceptions that do not inherit from StandardError, however. SystemExit is one of these, and so it is not captured. Here is a subset of the hierarchy in Ruby 1.9.2, which you can find out yourself:

BasicObject
  Exception
    NoMemoryError
    ScriptError
      LoadError
        Gem::LoadError
      NotImplementedError
      SyntaxError
    SecurityError
    SignalException
      Interrupt
    StandardError
      ArgumentError
      EncodingError
        Encoding::CompatibilityError
        Encoding::ConverterNotFoundError
        Encoding::InvalidByteSequenceError
        Encoding::UndefinedConversionError
      FiberError
      IOError
        EOFError
      IndexError
        KeyError
        StopIteration
      LocalJumpError
      NameError
        NoMethodError
      RangeError
        FloatDomainError
      RegexpError
      RuntimeError
      SystemCallError
      ThreadError
      TypeError
      ZeroDivisionError
    SystemExit
    SystemStackError
    fatal

You can thus capture just SystemExit with:

begin
  ...
rescue SystemExit => e
  ...
end

...or you can choose to capture every exception, including SystemExit with:

begin
  ...
rescue Exception => e
  ...
end

Try it yourself:

begin
  exit 42
  puts "No no no!"
rescue Exception => e
  puts "Nice try, buddy."
end
puts "And on we run..."

#=> "Nice try, buddy."
#=> "And on we run..."

---

Note that this example will not work in (some versions of?) IRB, which supplies its own exit method that masks the normal Object#exit.

In 1.8.7:

method :exit
#=> #<Method: Object(IRB::ExtendCommandBundle)#exit>

In 1.9.3:

method :exit
#=> #<Method: main.irb_exit>