find all symbolic links in a directory tree pointing outside that tree

Question

I frequently move directory trees to other locations or copy their tarballs to other machines, and I would like to have a method to check whether any symlinks in a directory tree A point to locations outside of A since these will be broken in the moved / copied directory.

Answer 1

You want a program called realpath, used in conjunction with find.

E.g.:

find . -type l -exec realpath {} \; | grep -v "^$(pwd)"

Answer 2

Use bindfs to create another view of that directory tree.

mkdir /tmp/view
bindfs /some/directory /tmp/view

Then use the symlinks utility (shipped by many distributions, or compile it from source) to detect cross-filesystem links.

symlinks -r /tmp/view | sed -n 's/^\(absolute\|other_fs\): //p'

(Note that parsing the output assumes that your symbolic links and their targets do not contain newlines, nor do paths to symbolic links contain the substring  -> .) That same utility can also convert absolute symlinks to relative (but you'd want to do that from the original location).

Answer 3

With zsh:

cd -P -- "$dir"
for i (**/*(ND@)) [[ $i:A = $PWD/* ]] || [[ $i:A = $PWD ]] || print -r -- "$i => $i:A"

Now, if the directory is /foo and you have /foo/bar that's a symlink to /foo/baz, that's a link whose target is in /foo, but once moved, the link will still be broken, so you may want also to match symlinks to absolute paths.

But even then, a bar => ../foo/baz in /foo would be an issue (false negative), so would a a => b where b is a symlink outside the tree (false positive, depending on how you want to look at it)

Answer 4

GNU coreutils provedes realpath, which resolves symlinks. With this, you could compare each symlink's target to the current working directory with something like:

#!/bin/bash

find . | while read filename
do
  if realpath $filename | grep -E "^$PWD" > /dev/null
  then
    echo 'this file is safe'
  else
    echo 'this file links externally'
  fi
done