Lua中拿到os.execute的输出结果

gistfile1.lua

-- from http://stackoverflow.com/questions/132397/get-back-the-output-of-os-execute-in-lua

function os.capture(cmd, raw)
  local f = assert(io.popen(cmd, 'r'))
  local s = assert(f:read('*a'))
  f:close()
  if raw then return s end
  s = string.gsub(s, '^%s+', '')
  s = string.gsub(s, '%s+$', '')
  s = string.gsub(s, '[\n\r]+', ' ')
  return s
end

How to get the output of os.execute(cmd)?
what is 'raw'?