I want to copy or backup a large folder A to B without rsync (just because I can): https://stackoverflow.com/a/65485164/1707015.

It works but from time to time I want to clean up B and delete old files in B (when they are deleted in A).

What I have to get the deleted files in A (so I can then delete them in B also):

$ cat A_files.txt  # for example: think the small letters as paths like ./some/path/file.yaml

$ cat B_files.txt

$ \grep -f A_files.txt -F -v B_files.txt

(The backslash \ is just for not using any grep aliases with colors or something.)

This works, but only for small files. For files with more than 100 MB file names each, I need > 100 GB RAM :O

Does anyone have a more resource-efficient variant for me? Of course rsync, but that is not to be used here just for fun and for practice purposes.

  • I am confused: this question is not clear. Mainly with respect to file-names, file-content, lines. I can't work out what is what (without reading code) Jan 2 '21 at 18:12
  • Is it now a bit better?
    – uav
    Jan 2 '21 at 18:20
  • 1
    Yes I see it now. It has nothing to do with file-names: it could be any text. Jan 2 '21 at 19:45

For list conjunctions / subtraction, the standard command is comm. It works on lines of sorted files.

So for the lines of B_files.txt that are not in A_files.txt:

export LC_ALL=C # for a simple and deterministic order and allow any byte
                # in file names.
comm -23 <(sort A_files.txt) <(sort B_files.txt)

If the files are already sorted:

comm -23 A_files.txt B_files.txt

That approach (or yours) doesn't work for arbitrary file names though as file names can contain newline characters, so can't be represented by lines.

If you're on a GNU system, you can use NUL delimited records instead of lines and use the -z option to sort and comm.

Another approach is to use zsh's array conjunction/subtraction operators:

cd /path/to/A || exit
cd /path/to/B || exit


Also note that unless passed the -x option, grep does substring match. grep -F foo/bar matches on blah/foo/barrage for instance.

  • Thanks: comm -23 B_files.txt A_files.txt returns b. Woop-di-doo! Notice that I needed to change the order of the arguments. Otherwise it would output e and f.
    – uav
    Jan 2 '21 at 18:37
  • Dude. 41 seconds for all of my files! Without blowing the RAM! CPU load was first high - but who cares! Thanks!
    – uav
    Jan 2 '21 at 18:56
  • With -z: xargs: argument line too long :(
    – uav
    Jan 2 '21 at 19:46
  • @uav You need xargs -0 to work on nul-delimited records. Jan 2 '21 at 21:36
  • Of course, I did. Since I don't expect any line breaks in the paths, it must also work without -z or -0 - and it does.
    – uav
    Jan 2 '21 at 22:01

I came up with:

readarray -t MY_TARGET_ARRAY < B_files.txt
for LINE in "${MY_TARGET_ARRAY[@]}"; do
    if ! grep -q "${LINE}" "${MY_SOURCE}"; then
        echo "${LINE}";

Not tested yet. Propably problems with missing -x and/or missing -F in grep.

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