How can I work with the real file behind a symlink?

Question

I have a file and a symlink:

file: ~/${USER_HOME}/blabla/.cheatsheet
symlink: ~/.cheatsheet (is linked to file above)

Now in my script I do some operations with the file, e.g. add a line and sort the file alphabetically and move it:

addOneCommand() {
  file"=~/.cheatsheet"
  # add it to the file
  echo "${cmd}" >> "${file}"
  # sort file instantly
  cat "${file}" | sort > "${file}".tmp
  mv "${file}".tmp "${file}"
}

But the real file behind the symlink doesn't get affected by the script (e.g. not sorted alphabetically). What can I do to work with "symlinked files" in bash scripts?

Thanks.

Answer 1

The point is, that you create a new file ${file}.tmp, that is now not a symlink, then rename.

You might try

cat "${file}".tmp >"${file}"
rm "${file}".tmp

instead of

mv "${file}".tmp "${file}"

If you don't mind the race condition.

P.S.: If I understand your intention correctly, you might want sort -u instead of plain sort

Answer 2

Your issue is that you're using mv, which will replace the symlink.

You can use a number of other approaches (see Eugen Rieck's answer for one), but fundamentally, you need to reuse the symlink, not replace it.

sponge is a good tool that will also allow you to remove handling of the *.tmp file as well:

With a file a, that is symlinked from b:

$ echo -e "3\n1\n2" > a
$ ln -s a b
$ ls -l
total 1
-rw-r--r-- 1 attie attie 6 Mar  5 15:44 a
lrwxrwxrwx 1 attie attie 1 Mar  5 15:44 b -> a

You can sort and rewrite a via b, without affecting the symlink:

$ cat b | sort | sponge b
$ ls -l
total 1
-rw-r--r-- 1 attie attie 6 Mar  5 15:44 a
lrwxrwxrwx 1 attie attie 1 Mar  5 15:44 b -> a
$ cat b
1
2
3

Answer 3

You can find the real file behind the symlink using the readlink or realpath commands.