根据
help cd，
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 `..'
换句话说，
-L是指使用 逻辑 结构，而
-P实际上使用 物理 目录结构。
逻辑结构是这样的，
$ tree aa└── b └── symlink -> ..
去的时候实际的物理结构
a/b/symlink是
a
如果要使用 real
..，则还必须使用
cd -P：
The -P option says to use the physical directory structure instead of following symbolic links (see also the -P option to the set builtin command); the -L option forces symbolic links to be followed.
一个例子，
$ cd$ cd a/b/symlink # physical location is at a/$ cd .. # now is at a/b$ cd symlink # goes back to a/b/symlink$ cd -P .. # follow physical path (resolve all symlinks)$ pwd -P# -P is optional here to show effect of cd ../home/sarnold$
