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Kalecser
Asked: 2010-02-24 08:23:28 +0800 CST

How to list symbolic link chains?

  • 772

Given this example:

mkdir a
ln -s a b
ln -s b c
ln -s c d

If I execute:

ls -l d

It will show:

d -> c

Is there a way for ls or any other linux command to show d -> c -> b -> a instead?

linux shell symbolic-link

4 Answers

  1. Best Answer

    Just use namei:

    $ namei d
f: d
 l d -> c
   l c -> b
     l b -> a
       d a
    • 82

  2. readlink -e <link>

    readlink [OPTION]... FILE

    • -e, --canonicalize-existing
      canonicalize by following every symlink in every component of the given name recursively, all components must exist 
    $ mkdir testlink
$ cd testlink
pjb@pjb-desktop:~/testlink$ ln -s c b
pjb@pjb-desktop:~/testlink$ ln -s b a
pjb@pjb-desktop:~/testlink$ ls -l 
total 0
lrwxrwxrwx 1 pjb pjb 1 2010-02-23 08:48 a -> b
lrwxrwxrwx 1 pjb pjb 1 2010-02-23 08:48 b -> c
pjb@pjb-desktop:~/testlink$ echo foo > c
pjb@pjb-desktop:~/testlink$ cat a
foo
pjb@pjb-desktop:~/testlink$ readlink -e a
/home/pjb/testlink/c

    note: readlink a by itself returns b

    note #2: together with find -l, a utility to list the chains could easily be written in perl, but also has to be smart enough to detect loops

    readlink will not output anything if you have a loop. This is better than getting stuck, I suppose. 

    pjb@pjb-desktop:~/testlink$ ln -sf a c
pjb@pjb-desktop:~/testlink$ ls -l 
total 0
lrwxrwxrwx 1 pjb pjb 1 2010-02-23 08:48 a -> b
lrwxrwxrwx 1 pjb pjb 1 2010-02-23 08:48 b -> c
lrwxrwxrwx 1 pjb pjb 1 2010-02-23 09:03 c -> a
pjb@pjb-desktop:~/testlink$ readlink -e a
pjb@pjb-desktop:~/testlink$ # (note: no output)
    • 21

  3. Here is a recursive function in Bash:

    chain() { export chain; local link target; if [[ -z $chain ]]; then chain="$1"; fi; link=$(stat --printf=%N $1); while [[ $link =~ \-\> ]]; do target="${link##*\`}"; target="${target%\'}"; chain+=" -> $target"; chain "$target"; return; done; echo "$chain"; unset chain; }

    On multiple lines:

    chain() {
    export chain
    local link target
    if [[ -z $chain ]]
    then
        chain="$1"
    fi
    link=$(stat --printf=%N "$1")
    while [[ $link =~ \-\> ]]
    do
        target="${link##*\`}"
        target="${target%\'}"
        chain+=" -> $target"
        if [[ ! $target =~ / && $1 =~ / ]]
        then
            target="${1%/*}/$target"
        fi
        chain "$target"
        return
    done
    echo "$chain"
    unset chain
}

    Examples:

    $ chain d
d -> c -> b -> a
$ chain c
c -> b -> a
$ chain a
a

    It requires stat(1) which may not be present on some systems.

    It will fail if names contain backticks, single quotes, or "->". It gets stuck in a loop with symlink loops (this could be solved using an associative array in Bash 4). It exports a variable called "chain" without regard to whether it's already in use.

    There may be other problems with it.

    Edit:

    Fixed a problem with some relative symlinks. Some still don't work, but the version below doesn't require the target of the link to exist.

    Added a version that uses readlink:

    chain ()
{
    export chain;
    local target;
    if [[ -z $chain ]]; then
        chain="$1";
    fi;
    target=$(readlink "$1");
    while [[ $target ]]; do
        chain+=" -> $target";
        if [[ ! $target =~ / && $1 =~ / ]]
        then
            target="${1%/*}/$target"
        fi
        chain "$target";
        return;
    done;
    echo "$chain";
    unset chain
}
    • 4

  4. You could just postprocess the output of namei with something like awk or grep to get just the lines you want:

    namei d | awk '$1=="l"'

    or

    namei d | egrep -e "->"
    • 0