exec-test.rb

#!/usr/bin/env ruby

def fork_test(command)
  pid = Kernel.fork do
    exec command
  end

  puts "parent sees pid: #{pid}"
  Process.waitpid(pid)
end

puts "Only single quotes:"
fork_test(%{ruby -e 'puts Process.pid'})

puts "With double quotes:"
fork_test(%{ruby -e "puts Process.pid"})

puts "With braces quotes:"
fork_test(%{ruby -e 'puts "#{Process.pid}"'})

linux-1.txt

> ruby exec-test.rb 
Only single quotes:
parent sees pid: 26766
26766
With double quotes:
parent sees pid: 26767
26767
With braces quotes:
parent sees pid: 26768
26765
> ruby -v
ruby 1.8.6 (2008-08-08 patchlevel 286) [i686-linux]

linux-2.txt

> ruby exec-test.rb 
Only single quotes:
parent sees pid: 18425
18425
With double quotes:
parent sees pid: 18426
18426
With braces quotes:
parent sees pid: 18427
18424
> ruby -v
ruby 1.8.7 (2009-06-12 patchlevel 174) [x86_64-linux], MBARI 0x6770, Ruby Enterprise Edition 2009.10

osx.txt

> ruby exec-test.rb
Only single quotes:
parent sees pid: 33453
33453
With double quotes:
parent sees pid: 33454
33454
With braces quotes:
parent sees pid: 33455
33452
> ruby -v
ruby 1.8.7 (2008-08-11 patchlevel 72) [universal-darwin10.0]