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linuxunixfilesystemssymlinkcd

Path ambiguity through symbolic links

I have noticed a strange behavior in UNIX systems:

I'm standing in /noob/
I have a symbolic link to a folder (A@ -> /B/C/D/A)
I enter the folder via my symlink (cd A) 
pwd says /noob/A/
In /B/C/D/A/ i have a file abc which I can see now.
I want to copy it to /noob/
I type cp abc ..
I type cd ..
I end up in /noob/ which is empty - but the file ended up in /B/C/D/ ???

How come this ambiguity as to where cp and cd points when given .. as argument? I find it confusing. Can anyone explain it in terms I'll understand? (=simple)

All the best, and please forgive a UNIX-noob a stupid question. Lasse

Solution

  • First let's have a look at how cd command does behave by looking at the help menu. What we are looking for is option -L (the default behavior) and option -P

    $ help cd cd: cd [-L|[-P [-e]] [-@]] [dir]
Change the shell working directory.

...
...

Options:
    -L      force symbolic links to be followed: resolve symbolic links in
    DIR after processing instances of `..'
    -P      use the physical directory structure without following symbolic
    links: resolve symbolic links in DIR before processing instances
    of `..'
...
...

    Important section

    The default is to follow symbolic links, as if `-L' were specified.
`..' is processed by removing the immediately previous pathname component
back to a slash or the beginning of DIR.

Exit Status:
...

    As you can see the default behavior of cd is not what you think it is since he will manipulate the $PWD variable accessed by pwd command in his own way, at each step you can run pwd command or do an echo $PWD to see how it reacts with the different cd commands hereunder.

    Let's play with cd command:

    We start from the following folder, with a sym link:

    [/home/arobert/test/noob] >
ls -ltra
total 8
drwxrwxr-x 5 arobert arobert 4096  5月 11 09:48 ..
lrwxrwxrwx 1 arobert arobert   26  5月 11 09:48 A -> /home/arobert/link/B/C/D/A
drwxrwxr-x 2 arobert arobert 4096  5月 11 10:03 .

    USAGE EXAMPLES:

    [/home/arobert/test/noob] >
cd A
[/home/arobert/test/noob/A] >
cd ..
[/home/arobert/test/noob] >
cd -L A
[/home/arobert/test/noob/A] >
cd ..
[/home/arobert/test/noob] >
cd -P A
[/home/arobert/link/B/C/D/A] >
cd -P ..
[/home/arobert/link/B/C/D] >
cd /home/arobert/test/noob/
[/home/arobert/test/noob] >
cd A
[/home/arobert/test/noob/A] >
cd -P ..
[/home/arobert/link/B/C/D] >

    Now let's play with readlink and cp command:

    Let's say we have entered the symlink that points to A -> /home/arobert/link/B/C/D/A in which we have a file a

    [/home/arobert/test/noob/A] >
ls -ltra
total 8
drwxrwxr-x 3 arobert arobert 4096  5月 11 09:55 ..
-rw-rw-r-- 1 arobert arobert    0  5月 11 10:10 a
drwxrwxr-x 2 arobert arobert 4096  5月 11 10:10 .

    from this folder let's look at where does point . and .. by using readlink -f command: 

    [/home/arobert/test/noob/A] >
readlink -f .
/home/arobert/link/B/C/D/A

[/home/arobert/test/noob/A] >
readlink -f ..
/home/arobert/link/B/C/D

    By consequence, when you run from the location /home/arobert/test/noob/A equivalent to /home/arobert/link/B/C/D/A the command cp a .. the file will be moved to /home/arobert/link/B/C/D as .. points to it.

    What you can do now:

    • Use absolute path with your cp command to avoid bad surprise.
    • Call the command from /home/arobert/test/noob/ directory using

    For example: 

    [/home/arobert/test/noob] >
cp A/a .

    as readlink -f . points to the correct folder

    [/home/arobert/test/noob] >
readlink -f .
/home/arobert/test/noob

    Result:

    [/home/arobert/test/noob] >
ls -ltra
total 8
drwxrwxr-x 5 arobert arobert 4096  5月 11 09:48 ..
lrwxrwxrwx 1 arobert arobert   26  5月 11 09:48 A -> /home/arobert/link/B/C/D/A
-rw-rw-r-- 1 arobert arobert    0  5月 11 10:13 a
drwxrwxr-x 2 arobert arobert 4096  5月 11 10:13 .