command not found error( UBUNTU下很奇怪的错误）

今天发现服务器上出现了这个错误。 (today I met this error on my personal server) 

GOOGLE一下，没搞定（参考 http://www.thattommyhall.com/2012/12/12/sorry-command-not-found-has-crashed/ )  (this post doesn't help me) 

后来看到这个帖子 (http://blog.csdn.net/miyuehu/article/details/12039347)受到了启发，觉得需要从 没找到的命令 入手，所以发现是rbenv, gem, bundle都失效了。 于是查看 $PATH, 发现 $PATH 设置的不对，里面包含了一段错误的PATH。 改掉之后就好了。 ( I got some clues from the link of this paragraph, and found that my rbenv and its related command are broken.  and further more, my $PATH variable is broken . so I fixed this problem by correcting the $PATH in my ~/.bashrc file) 

root@li539-250:/opt# echo $PATH  # there's no /opt/mongodb... folder. 
/root/.rbenv/shims:/root/.rbenv/bin:/opt/mongodb-linux-i686-2.4.5/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games

after correction:
$ echo $PATH
/root/.rbenv/shims:/root/.rbenv/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games