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Daniel Janus’s blog

Today’s lesson: Mind the symlinks

11 June 2008

Probably every day I keep learning new things, without even realizing it most of the time. The vast majority of them are minor or even tiny tidbits of knowledge; but even these might be worth noting down from time to time, especially when they are tiny pitfalls I’d fallen into and spent a couple of minutes getting out. By sharing them, I might hopefully prevent someone else for slipping and falling in.

So here’s a simple Unix question: If you enter a subdirectory of the current directory and back to .., where will you end up? The most obvious answer is, of course, “in the original directory”, and is mostly correct. But is it always? Let’s see.

nathell@breeze:~$ pwd
nathell@breeze:~$ cd foobar
nathell@breeze:~/foobar$ cd ..
nathell@breeze:~$ pwd

So the hypothesis seems to be right. But let’s try doing this in Python, just for the heck of it:

nathell@breeze:~$ python
Python 2.5.2 (r252:60911, Apr 21 2008, 11:12:42)
[GCC 4.2.3 (Ubuntu 4.2.3-2ubuntu7)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> print os.getcwd()
>>> os.chdir("foobar")
>>> os.chdir("..")
>>> print os.getcwd()

Whoa, hang on! What’s that /var doing there? Of course the one thing I didn’t tell you is that foobar is not really a directory, but rather a symlink pointing to one (/var/log in this case).

The corollary is that the shell builtin cd is not the same as Unix chdir() (it is easily checked that both Perl and C exhibit the same behaviour). In fact, the shell builtin has an oft-forgotten command-line switch, -P, which causes it to follow physical instead of logical path structure.

On a closing note: I have somewhat neglected the blog throughout the previous month, but I hope to revive it soon. It is not unlikely that such irregularities will recur.