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: resolve symbolic links in
        DIR after processing instances of `..'
        -P    use the physical directory structure without following symbolic
        links: resolve symbolic links in DIR before processing instances
        of `..'
        -e    if the -P option is supplied, and the current working directory
        cannot be determined successfully, exit with a non-zero status
        -@  on systems that support it, present a file with extended attributes
            as a directory containing the file attributes

    The default is to follow symbolic links, as if `-L' were specified.
    `..' is processed by removing the immediately previous pathname component
    back to a slash or the beginning of DIR.

    Exit Status:
    Returns 0 if the directory is changed, and if $PWD is set successfully when
    -P is used; non-zero otherwise.

这个命令的作用是改变工作目录。

cd ~ 进入用户主目录

cd . 当前目录

cd .. 上一级目录

cd - 上一次所在目录

当仅实用"-"一个选项时，当前工作目录将被切换到环境变量"OLDPWD"所表示的目录。

cd 相对路径或者绝对路径

-p 如果要切换到的目标目录是一个符号连接，直接切换到符号连接指向的目标目录

-L 如果要切换的目标目录是一个符号的连接，直接切换到字符连接名代表的目录，而非符号连接所指向的目标目录。

cd ..\/.. 返回上两级目录；

cd !$ 把上个命令的参数作为cd参数使用。