Why change the owner of a symbolic link in linux

symlink

In linux it's possible to change the owner or the group owner of a symbolic link (symlink). I was wondering why someone would want to do that, since permissions of a symlink are not used when accessing a file through it.

I can only imagine one use case where it could be useful: to allow a user to delete a symlink in a directory with sticky bit.

Do you know other cases where it might be useful to change the owner or group owner of a symlink ?

Best Answer

Suppose root is working in a directory that Eve can write to. There's a file foo in this directory that needs to be changed to belong to Eve. So root types chown eve foo. But just before root hits Enter, Eve runs ln -sf /etc/passwd foo. Now /etc/passwd belongs to Eve! If root can run chown -h eve foo to make sure not to follow symlinks, then the most harm that can be done is that some other file in the same directory has been changed to belong to Eve.

lchown is also convenient when you're changing the owner of a directory tree. You don't need to worry about accidentally affecting a file outside the tree because you called chown on a symbolic link.

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Is chmod behavior wrong for symlinks

Making symlinks to binaries is fine; check /bin and /usr/bin and you'll almost certainly find a number of aliases. The problem here is using sudo without fully understanding the consequences. Fortunately there was no permanent harm and you learned an important lesson. When you're just making sure they're executable, use a+x or even u+x instead. As uther says, you'd probably be better off with an alias anyway.

The man page itself explains why this is the action -- the permissions on symlinks don't matter, only on the files they resolve to. More often than not, the user does want to change the attributes of the file the link points to.

Also, I just checked the chmod() syscall man pages for Linux and BSD. They both return errors for symbolic link loops, implying that this behavior of changing the file's perms and not the link's is determined and enforced at the kernel level.

Python – How to get ‘realpath’ to find the symbolic link

If you need to resolve (or investigate) a symlink you can use the platform independent bash library 'realpath-lib'. By default it emulates readlink and will work on Mac or Unix. It can be found on Github or Bitbucket and it's free.

But it sounds like you want to just do python2 (rather than ./python2) from your local (working) directory. It might be possible to do this with an alias in your .bashrc or otherwise you will need to add the working directory (that contains your symlink) to your PATH environment variable. This can also be done for the current session only or within the .bashrc file for future sessions. This could be a solution for just a specific user.

Another option that would work for all users would be to create the python2 symlink to /usr/bin/python in another directory on the path, say in /usr/local/bin. Perhaps something like:

sudo ln -s /usr/bin/python /usr/local/bin/python2

Then any user or script should find the python or python2 commands. Of course this option requires admin (root) privileges to install.

Best Answer

Related Solutions

Is chmod behavior wrong for symlinks

Python – How to get ‘realpath’ to find the symbolic link

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