Convert absolute symlink to relative symlink with simple Linux command

Question

I have a complete sub-filesystem inside a path /home/user/system containing the standard Linux structure with directories /bin, /home, /root, /usr, /var, /etc,...

This sub-filesystem contain symbolic links, either relative or absolute. The relative symlinks are just fine, they stay within the sub-filesystem under /home/user/system. But absolute symlinks are problematic, as they point to a target outside of the sub-filesystem.

As an example we assume an absolute symlink as follows (seen inside the sub-filesystem):

/usr/file1 -> /usr/lib/file1

In the overall filesystem we have a link at /home/user/system/usr/file1 that now point to a file /usr/lib/file1 outside the sub-filesystem, instead of a file /home/user/system/usr/lib/file1 inside the sub-filesystem.

I would like to have a simple script, preferably a single command line (rsync, chroot, find, ...) that converts every absolute symlink to a relative one.

In the given example, that relative link would become

/usr/file1 -> ../usr/lib/file1

Answer 1

With the symlinks utility by Mark Lord (offered by many distributions; if yours doesn't have it, build it from source):

chroot /home/user/system symlinks -cr .

Alternatively, on systems that have a readlink command and a -lname predicate to find (warning: untested code):

cd /home/user/system &&
find . -lname '/*' -exec ksh -c '
  for link; do
    target=$(readlink "$link")
    link=${link#./}
    root=${link//+([!\/])/..}; root=${root#/}; root=${root%..}
    rm "$link"
    ln -s "$root${target#/}" "$link"
  done
' _ {} +

Answer 2

Pure bash & coreutils, changes symlinks to relative without unnecessary ../s in path:

find . -type l | while read l; do
    target="$(realpath "$l")"
    ln -fs "$(realpath --relative-to="$(dirname "$(realpath -s "$l")")" "$target")" "$l"
done

You can change:

• find . to find /path/to/directory to convert symlinks in that directory
• ln -fs to echo ln -fs for a dry run

Explanation:

• target="$(realpath "$l")" - finds absolute path to symlink target
• ln -fs - creates symlink (-s), forcing (-f) rewrite of existing
• realpath -s "$l" - finds absolute path to symlink itself
• dirname "$(realpath -s "$l")" - finds absolute path to directory containing the symlink
• realpath --relative-to="$(dirname "$(realpath -s "$l")")" "$target" - finds path of target relative to symlink, in other words: converts absolute to relative path

Answer 3

Transforming absolute in relative links is supported by sshfs which mounts remote directories via ssh.

The command is: There

sudo sshfs <remote_user>@<remote_ip_address>:/ /home/<host_user>/mntpoint/ -o transform_symlinks -o allow_other

The command, especially the <placeholders>, shall be adapted to the specific environment.

Answer 4

The accepted answer is great, but the chroot is only required due to the specific requirement of this question that want to re-interpret differently the symlinks. For those of you that simply want to update absolute symlinks into a relative symlinks (for instance to allow copy/paste of folders without breaking the symlinks), just install the tiny program symlinks, and run:

symlinks -cr .

where . is the path of the directory, r is for recursive change, and c turns asbolute symlinks into normal symlinks. Without any option, it will list the absolute symlinks.

Answer 5

find -lname "$PWD/*" | while read link ;do  
   target=$(readlink $link);
   relative_target=$(realpath $target --relative-to=$link/.. --no-symlinks --canonicalize-missing);
   ln --force --symbolic --no-target-directory $relative_target $link
done

"$PWD/*" stands to convert only absolute links which point inside current workdir

Answer 6

Here is a pure sh solution.

cd /home/user/system &&
find . -lname '/*' |
while read l ; do
  echo ln -sf $(echo $(echo $l | sed 's|/[^/]*|/..|g')$(readlink $l) | sed 's/.....//') $l
done |
sh