gistfile1.rb

class DirThreadsafe < Dir
  def self.chdir(target)
    Thread.current[:pwd] = target
  end
end


module Kernel
  alias :_backtick_original :"`"

  def `(value)
    if pwd = Thread.current[:pwd]
      value = "cd #{pwd} && #{value}"
    end
    _backtick_original value
  end
end


`mkdir foo`
`touch foo/foo_is_not_empty`

`mkdir bar`
`touch bar/bar_is_not_empty`

threads = []
threads << Thread.new do
  DirThreadsafe.chdir("foo")
  puts "foo: #{`ls`}"
end

threads << Thread.new do
  DirThreadsafe.chdir("bar")
  puts "bar: #{`ls`}"
end

threads.map(&:join)

puts "current: #{`ls`.inspect}"
# => foo: foo_is_not_empty
# => bar: bar_is_not_empty
# => current: "bar\nfoo\n"