autostart.lua

-- On rc.lua
function spawn_once(command, class, tag)
  -- create move callback
  local callback
  callback = function(c)
    if c.name == class then
      awful.client.movetotag(tag, c)
      client.disconnect_signal("manage", callback)
    end
  end
  client.connect_signal("manage", callback)
  -- now check if not already running!
  local findme = command
  local firstspace = findme:find(" ")
  
  if firstspace then
    findme = findme:sub(0, firstspace-1)
  end
  -- finally run it
  awful.util.spawn_with_shell("pgrep -u $USER -x " .. findme .. " > /dev/null || (" .. command .. ")")
end