ruby %x forks 64-bit linux, nonetheless 32, wholly specific syntax

here's flush code:

puts %x{ pstree #{$$} }   # never forks
puts %x{ pstree '#{$$}' } # forks amd64 only

on 32-bit ubuntu dapper, i output:

t.rb---pstree
t.rb---pstree

which creates clarity me. nonetheless 64-bit ubuntu hardy, i this:

t.rb---sh---pstree
t.rb---pstree

what's being shown here flush forks before exec'ing only cases. i put formula record run underneath strace -ff, appears 64-bit disrespectful calls clone() (like fork()) before execve(), since 32-bit athletic does such thing.

my flush versions are:

ruby 1.8.4 (2005-12-24) [i486-linux]
ruby 1.8.6 (2007-09-24 patchlevel 111) [x86_64-linux]

i should try blending & relating interpreters & os's & sizes more, nonetheless right it's easy given i don't discharge machines. maybe someone among tell me inadequacy even between commands 64-bit system, let alone since work same 32-bit one.