linux - Use a symbolic link to create a new symbolic link

I have 2 directories (FOO, LALA). I create a symbolic link in one of them like this

ln -s /etc/apache2 FOO/apache2

Afterwards I want to create another symlink, pointing to the same directory, but not using the initial directory but the symlink created. So I try

ln -s FOO/apache2 LALA/apache2

The problem is that the second link is invalid.

ls FOO/apache2

The above works and lists files in /etc/apache2

ls LALA/apache2

But this one doesn't. It just lists the link itself.

If I add a slash at the end of each ls

ls FOO/apache2/

Works as well.

ls LALA/apache2/

I get ls: cannot access LALA/apache2/: No such file or directory/

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If instead of

ln -s FOO/apache2 LALA/apache2

I do

cd LALA ; ln -s ../FOO/apache2 apache2

Then the symlink is valid

Any idea what's the difference?

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Initial question from here
https://stackoverflow.com/questions/10656666/use-a-symbolic-link-to-create-a-new-symbolic-link

Answer

The difference is that relative symlinks are resolved at access time, not creation time.

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In your first example, if the link's name is /somedir/LALA/apache2 and its target is FOO/apache2, it will be resolved to /somedir/LALA/FOO/apache2 when accessing it.

link name:                link target:    absolute target:
"/somedir/LALA/apache2" + "FOO/apache2" = "/somedir/LALA/FOO/apache2"

In the second example, the link's name is same – /somedir/LALA/apache2 – but its target, ../FOO/apache2, is now resolved as:

link name:                link target:       absolute target:
"/somedir/LALA/apache2" + "../FOO/apache2" = "/somedir/LALA/../FOO/apache2"
                                           → "/somedir/FOO/apache2"

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This means that you must always give "../FOO/apache2" as the target to ln -s when creating the second link, regardless of what directory you are currently in.

Note: In the latest version of coreutils, the ln command has a new option -r/--relative, which does the job for you. For example, ln -r -s FOO/apache2 LALA/apache2 would work correctly.

For older versions, you could hack something with realpath --strip --relative-to...
I wrote sym for my own use.