Since Lua 5.2 (just released) the pipe:close() method returns the exit status of the command. For more information see http://www.lua.org/manual/5.2/manual.html#pdf-file:close

You can also get the exit status using Lua 5.1 but it's definitely nasty (io.popen('your-command; echo $?') and the last line of output is the exit status).