roscore failing

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I finished the installation of kinetic version on my Ubuntu 16.04 LTS by doing this link text. But when I tried to check the correctness of my installation using roscore, it failed. The terminal prompt this:

... logging to /home/ming/.ros/log/710aa780-76d3-11e8-8629-240a64d121e7/roslaunch-ming-X550VC-2935.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.

Invalid <param> tag: Cannot load command parameter [rosversion]: no such command [rosversion roslaunch]. 

Param xml is <param command="rosversion roslaunch" name="rosversion"/>
The traceback for the exception was written to the log file

and the trackback info in the corresponding log file is:

Param xml is <param command="rosversion roslaunch" name="rosversion"/>
[roslaunch][ERROR] 2018-06-23 18:51:54,327: The traceback for the exception was written to the log file
[roslaunch][ERROR] 2018-06-23 18:51:54,329: Traceback (most recent call last):
File "/opt/ros/kinetic/lib/python2.7/dist-packages/roslaunch/__init__.py", line 306, in main
    p.start()
File "/opt/ros/kinetic/lib/python2.7/dist-packages/roslaunch/parent.py", line 268, in start
    self._start_infrastructure()
File "/opt/ros/kinetic/lib/python2.7/dist-packages/roslaunch/parent.py", line 217, in _start_infrastructure
    self._load_config()
File "/opt/ros/kinetic/lib/python2.7/dist-packages/roslaunch/parent.py", line 132, in _load_config
    roslaunch_strs=self.roslaunch_strs, verbose=self.verbose)
File "/opt/ros/kinetic/lib/python2.7/dist-packages/roslaunch/config.py", line 445, in load_config_default
    load_roscore(loader, config, verbose=verbose)
File "/opt/ros/kinetic/lib/python2.7/dist-packages/roslaunch/config.py", line 94, in load_roscore
    loader.load(f_roscore, config, core=True, verbose=verbose)
File "/opt/ros/kinetic/lib/python2.7/dist-packages/roslaunch/xmlloader.py", line 748, in load
    self._load_launch(launch, ros_config, is_core=core, filename=filename, argv=argv, verbose=verbose)
File "/opt/ros/kinetic/lib/python2.7/dist-packages/roslaunch/xmlloader.py", line 720, in _load_launch
    self._recurse_load(ros_config, launch.childNodes, self.root_context, None, is_core, verbose)
File "/opt/ros/kinetic/lib/python2.7/dist-packages/roslaunch/xmlloader.py", line 654, in _recurse_load
    default_machine, is_core, verbose)
File "/opt/ros/kinetic/lib/python2.7/dist-packages/roslaunch/xmlloader.py", line 664, in _recurse_load
    self._param_tag(tag, context, ros_config, verbose=verbose)
File "/opt/ros/kinetic/lib/python2.7/dist-packages/roslaunch/xmlloader.py", line 95, in call
    return f(*args, **kwds)
File "/opt/ros/kinetic/lib/python2.7/dist-packages/roslaunch/xmlloader.py", line 279, in _param_tag
    "Invalid <param> tag: %s. \n\nParam xml is %s"%(e, tag.toxml()))
XmlParseException: Invalid <param> tag: Cannot load command parameter [rosversion]: no such command [rosversion roslaunch].
Param xml is <param command="rosversion roslaunch" name="rosversion"/>

Need help!

And I sourced before running roscore. The output of export | grep ROS is this:

declare -x ROSLISP_PACKAGE_DIRECTORIES=""
declare -x ROS_DISTRO="kinetic"
declare -x ROS_ETC_DIR="/opt/ros/kinetic/etc/ros"
declare -x ROS_MASTER_URI="http://localhost:11311"
declare -x ROS_PACKAGE_PATH="/opt/ros/kinetic/share"
declare -x ROS_ROOT="/opt/ros/kinetic/share/ros"
declare -x ROS_VERSION="1"