I have a Nextcloud instance that various things automatically upload photos to. These automatic folders sync to a directory on my desktop. I wanted to pull things out of that directory without deleting them, and only once. (My wife might move them out of the directory on her computer, and I might arrange them into targets on my end.)

In other words, I wanted to copy a file from a source to a destination, but remember what had been copied before so it only ever copies once.

rsync doesn’t quite do this. But it turns out that tar’s listed-incremental feature can do exactly that. Ordinarily, it would delete files that were deleted on the source. But if we make the tar file with the incremental option, but extract it without, it doesn’t try to delete anything at extract time.

Here’s my synconce script:

#!/bin/bash

set -e

if [ -z "$3" ]; then
    echo "Syntax: $0 snapshotfile sourcedir destdir"
    exit 5
fi

SNAPFILE="$(realpath "$1")"
SRCDIR="$2"
DESTDIR="$(realpath "$3")"

cd "$SRCDIR"
if [ -e "$SNAPFILE" ]; then
    cp "$SNAPFILE" "${SNAPFILE}.new"
fi
tar "--listed-incremental=${SNAPFILE}.new" -cpf - . | \
    tar -xf - -C "$DESTDIR"
mv "${SNAPFILE}.new" "${SNAPFILE}"

Just have the snapshotfile be outside both the sourcedir and destdir and you’re good to go!