I encontered error when I run roscore.

edit

I encontered error when I run roscore.

Roslaunch got a 'No such file or directory' error while attempting to run:

/opt/ros/fuerte/share/rosout/bin/rosout __name:=rosout __log:=/home/xcmai/.ros/log/6641ddba-1ef1-11e5-9ce7-782bcb967059/rosout-1.log

Please make sure that all the executables in this command exist and have
executable permission. This is often caused by a bad launch-prefix.

Can anyone know how to solve it?

Edit: Thank you, redskydeng.

I added ros environment variables, and then I ran roscore again. There was another error happening:

... logging to /home/epuck/.ros/log/b6a4138c-bfde-11d3-a726-8f981662d4a4/roslaunch-epuck1-987.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.

Traceback (most recent call last):
  File "/opt/ros/fuerte/bin/rosversion", line 4, in <module>
    import pkg_resources
  File "/usr/lib/python2.6/dist-packages/pkg_resources.py", line 2659, in <module>
    parse_requirements(__requires__), Environment()
  File "/usr/lib/python2.6/dist-packages/pkg_resources.py", line 546, in resolve
    raise DistributionNotFound(req)
pkg_resources.DistributionNotFound: rospkg==1.0.2
Invalid <param> tag: Cannot load command parameter [rosversion]: command [rosversion ros] returned with code [1]. 

Param xml is <param command="rosversion ros" name="rosversion"/>