ROS： roscore cannot run as another roscore/master is already running.

在ubuntu系统打开命令终端，输入roscore，以启动ros节点，但是提示如下错误：

roscore cannot run as another roscore/master is already running. 
Please kill other roscore/master processes before relaunching.
The ROS_MASTER_URI is http://wm-OptiPlex-302016348B0335:11311/
The traceback for the exception was written to the log file

原因

另一个master或roscore已经打开，关闭即可。

解决方法：

终端输入 killall -9 roscore && killall -9 rosmaster