出现问题：
RLException: [simple_launch] is neither a launch file in package [learning_launch] nor is [learning_launch] a launch file name
The traceback for the exception was written to the log file
bruce@bruce-desktop:~$ cd catkin_ws/
bruce@bruce-desktop:~/catkin_ws$ roslaunch learning_launch simple_launch//错误提示信息
RLException: [simple_launch] is neither a launch file in package [learning_launch] nor is [learning_launch] a launch file name
The traceback for the exception was written to the log file导致问题，有两种可能性：
可能性1解决办法：
这是因为setup.bash文件没有进一步说明包的来源，解决方法是
source catkin_ws/devel/setup.bash
可能性2解决办法：
可能是输入的命令有错误
正确输入：是 simple.launch 不是simple_launch
bruce@bruce-desktop:~/catkin_ws$
bruce@bruce-desktop:~/catkin_ws$ roslaunch learning_launch simple.launch//正确运作
... logging to /home/bruce/.ros/log/27d53722-26f1-11ea-a1aa-b827eb0500fa/roslaunch-bruce-desktop-7788.log
Checking log directory for disk usage. This may take awhile.
Press Ctrl-C to interrupt
Done checking log file disk usage. Usage is <1GB.started roslaunch server http://bruce-desktop:42021/SUMMARY