ruby popen fds

exec.rb

open('/tmp/f','w'){|f| f << 'sekrit'}
f = open('/tmp/f')

exec("ruby -e 'p IO.open(#{f.fileno}).read'")
# Prints 'sekrit'

popen.rb

open('/tmp/f','w'){|f| f << 'sekrit'}
f = open('/tmp/f')

IO.popen("ruby -e 'p IO.open(#{f.fileno}).read'")
# Fails 'Bad file descriptor'