Process::Status

        # Process::Status < Object

(from ruby core)
---
Process::Status encapsulates the information on the status of a running
or terminated system process. The built-in variable `$?` is either `nil`
or a Process::Status object.

    fork { exit 99 }   #=> 26557
    Process.wait       #=> 26557
    $?.class           #=> Process::Status
    $?.to_i            #=> 25344
    $? >> 8            #=> 99
    $?.stopped?        #=> false
    $?.exited?         #=> true
    $?.exitstatus      #=> 99

Posix systems record information on processes using a 16-bit integer. 
The lower bits record the process status (stopped, exited, signaled) and
the upper bits possibly contain additional information (for example the
program's return code in the case of exited processes). Pre Ruby 1.8,
these bits were exposed directly to the Ruby program. Ruby now
encapsulates these in a Process::Status object. To maximize
compatibility, however, these objects retain a bit-oriented interface.
In the descriptions that follow, when we talk about the integer value of
*stat*, we're referring to this 16 bit value.
---
# Class methods:

    wait

# Instance methods:

    &
    ==
    >>
    coredump?
    exited?
    exitstatus
    inspect
    pid
    signaled?
    stopped?
    stopsig
    success?
    termsig
    to_i
    to_s