用法：
help cd
cd: cd [-L|[-P [-e]]] [dir]
Change the shell working directory.
Change the current directory to DIR. The default DIR is the value of the
HOME shell variable.
The variable CDPATH defines the search path for the directory containing
DIR. Alternative directory names in CDPATH are separated by a colon (:).
A null directory name is the same as the current directory. If DIR begins
with a slash (/), then CDPATH is not used.
If the directory is not found, and the shell option `cdable_vars' is set,
the word is assumed to be a variable name. If that variable has a value,
its value is used for DIR.
Options:
-L force symbolic links to be followed
-P use the physical directory structure without following symbolic
links
-e if the -P option is supplied, and the current working directory
cannot be determined successfully, exit with a non-zero status
The default is to follow symbolic links, as if `-L' were specified.
Exit Status:
Returns 0 if the directory is changed, and if $PWD is set successfully when
-P is used; non-zero otherwise.
1，回到当前用户的主目录
#cd ~
2，回到当前用户的主目录 同上条指令
#cd (不带参数)
3，回到上一次的工作目录
#cd -
4，回到根目录
#cd /
5，回到当前目录的父目录
#cd ..
6，把上个命令的参数作为cd参数使用。
#cd !$
7，.表示当前目录
#cd .
