跟着中國慕課網，按着上面的步驟，試了一下 roslaunch robot_sim_demo robot_spawn.launch，總是提示如下內容，簡直令人抓狂，找了各種方法都不行
[robot_spawn.launch] is neither a launch file in package [robot_sim_demo] nor is [robot_sim_demo] a launch file name
The traceback for the exception was written to the log file
最後試了刷新環境變量，終於可以了
source ~/catkin_ws/devel/setup.bash