What causes ln: //: Is a directory?

Question

When linking a directory to root, I get this error:

$ ln -s ~/inbox/ /
$ ln: //: Is a directory

Bash autocompletes the directory path by adding a /. I've tried escaping without success.

$ ln -s ~/inbox /

works though. Why is this?

Solution

In pathname resolution, having a trailing / is equivalent to a trailing /. — in other words, ~/inbox/ is equivalent to ~/inbox/. in this context. So the ln command is trying to create a link called . in the root directory (and, unsurprisingly, failing).

Zsh removes the trailing / when you press Space after completing a directory (unless configured not to do so). I don't know if bash can be made to do this.

Comments

[jsbillings]

What OS is this? Is it GNU Coreutils, or the BSD utils?

[Mikel]

I don't think it's coreutils. My version puts the file name in quotes, e.g. ln: 'foo': hard link not allowed for directory.

[jippie]

I think it is good to explain what cd $(mktemp -d) does. Not everybody is familiar with this construction.

[Mikel]

It creates a new directory somewhere under /tmp. It makes sure that the files I'm using in my examples don't already exist so that everyone can get try running the commands and get the same results.

[Mikel]

Very interesting. Pathname resolution also says that symlinks should be resolved. But when you do ln symlink blah, blah should point to symlink, not what symlink points at.