Behavior of cd/bash on symbolic links

Question

Assume I have the folders ~/a/b in my home folder, and the folder b contains a symbolic link to '..' named 'symlink'. Then I perform the following actions in bash:

hm@mach:~$ cd a/b/symlink
hm@mach:~/a/b/symlink$ pwd -P
/home/hm/a
hm@mach:~/a/b/symlink$ cd ..
hm@mach:~/a/b$ pwd -P
/home/hm/a/b

pwd -P prints the current working directory, dereferencing all symbolic links. Why is the working directory /home/hm/a/b at the end, and not /home/hm?

Grochow

Answer 1

According to 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 `..'

In other words, -L means using the logical structure, whereas -P uses the actually physical directory structure.

The logical structure is like this,

$ tree a
a
└── b
    └── symlink -> ..

The actual physical structure when you go to a/b/symlink is,

a

If you want to use the real .., then you must also use 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.

An example,

$ 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
$ 

Yeryerevan

FYI, you can use set -o physical or set -P to make this behavior permanent. – Cerebellum

.. I should mention that the implementation is probably more in line with @geekosaur's description; I wouldn't expect overwriting OLDPWD with your own value would actually influence cd .. behavior. – Yeryerevan

I agree that I don't think OLDPWD actually does anything here. I tested it by "cd"ing down a symlink chain three levels deep and then doing "cd .." a few times. It worked fine. If OLDPWD was used, it wouldn't work. OLDPWD only remembers one directory, and couldn't be used to go back more than one directory. Thus, OLDPWD isn't used. – Gleeson

The reference to OLDPWD in your answer is irrelevant. OLDPWD is used by cd - (not by cd ..). – Bistoury

@Leon, even in the case of having traversed a symlink to a directory? – Yeryerevan

@Yeryerevan Yes. cd - is a shortcut for cd "$OLDPWD" (i.e. change to the previous working directory) whereas cd .. means change to the parent directory). Previous and parent directories are completely different things, though they may coincide in certain situations. – Bistoury

You can revert back to logical with set +o physical – Mydriasis

Agree. OLDPWD is irrelevant. It's confusing and I just tried on my machine. I think without -P, it just uses the logical directory structure. Could someone edit it? – Negative

Answer 2

bash keeps track of the logical current directory path, as shown in your prompt, and interprets things like cd .. according to that. This makes things a little more consistent if you only use such paths in cd (or pushd), at the cost of unexpected things happening if you then expect the same thing to happen with paths in command arguments (or inside commands; emacs and vim have their own configurable rules for symlink handling, but most commands rely on the kernel to deal with it).

Attrition