Where should I expect to get using ./symlink/../

Couling · 09-14-2010, 04:08 AM

Hi

I've got a question about the behaviour of symlinks after being supprised by the result of something i tried yesterday.

Example directory structure:

Code:

a
   link --> /b/b_child1
b
   b_child_1
   b_child_2

If my current working directroy was set to /a/link and then i use "cd .." then i will get to /a

I'd allways thought this was ubiquitous, but on my debian box, I found this to be the exception. So for example, it is possible to:

Code:

touch /a/link/../b_child_2/new_file

With the result that /b/b_child_2/new_file is created.

How wide spread is this behaviour?
That is will i expect to see the same thing only on Debian or on any Linux or across the whole of POSIX?

Thanks very much for your time.

frankbell · 09-14-2010, 09:08 PM

Your examples are so abstract that I'm not sure I understand the question. Would you be kind enough to make them more concrete?

The best explanation I have so far found for symlinks is in the Slackbook:

http://www.slackbook.org/html/filesy...ure-links.html

Couling · 09-15-2010, 04:06 AM

Sorry for being unclear. Sadly the link you gave makes no reference to the use of .. with symbolic links (which is the cause of my confusion).

I'll try to demonstrate what I'm talking about with some actual commands:

Set up some directories

Code:

~ $ mkdir test_dir
~ $ cd test_dir/
~/test_dir $ mkdir a b b/c
~/test_dir $ ln -s ../b/c a/link
~/test_dir $ find .
.
./b
./b/c
./a
./a/link

Test 1 (This is how I previously thought links allways behaved)

Code:

~/test_dir $ cd a/link/..
~/test_dir/a $ touch file_1
~/test_dir/a $ cd ~/test_dir/
~/test_dir $ find .
.
./b
./b/c
./a
./a/file_1
./a/link

cd moved to ~/test_dir/a and the file was consiquently there.

Test 2 (Links usually act like this at least on my Debian box)

Code:

~/test_dir $ touch a/link/../file_2
~/test_dir $ find .
.
./b
./b/file_2
./b/c
./a
./a/file_1
./a/link

touch created the file in b not a.

It seems that cd is the exception, all other programs I test perform as in Test 2.

Before a couple of days ago i'd never seen this before and it has the potential to leave a few security holes open.

Is the result of Test 2 part of the POSIX standard or is it a linux specific thing?