恢复ubuntu环境变量的示例：
在终端输入ls命令后，显示如下结果:
root@wei:/etc# ls
Command 'ls' is available in '/bin/ls'
The command could not be located because '/bin' is not included in the PATH environment variable.
ls: command not found
解决方法：
直接在终端输入以下命令即可。
export PATH=/usr/bin:/bin
