Rsync moving window of last k subdirectories

Question

When rsyncing directory src to dest, is there any way I can make dest contain the last k sub-directories, in lexicographical order, of directory src, as in a moving window where sub-directories having fallen outside of the window are automatically removed from dest when re-running the same command at a later time?

For example, if k = 2, src contains

dir_1
dir_2
dir_3
dir_4
dir_5

and dest is empty, after running rsync, dest should contain dir_4 and dir_5. When, at a later time src, contains

dir_1
dir_2
dir_3
dir_4
dir_5
dir_6

and rsync is run again (with the very same command line), I want dir_4 to be deleted from dest, replaced by dir_6.

Answer 1

Yes, this is possible.

A /bin/sh solution:

#!/bin/sh

# number of entries to sync
k=2

# generate file list (sets $1, $2, $3 etc., collectively known as $@)
set -- src/*

# shift off all but the last $k of these entries
shift "$(( $# - k ))"

# create include patterns ($entry is not actually used,
# we work on the 1st element and then add it to the end and shift)
for entry do
    set -- "$@" --include="${1##*/}/***"
    shift
done

# run rsync
rsync --verbose --archive --delete --delete-excluded "$@" --exclude='*' src/ dest/

This creates a list of files in $@ with set first (this list is sorted lexicographically, because globs do that), then removes all but the last $k of these. The loop converts each src/element into --include=element/***. This inclusion pattern will make rsync consider the named element and anything below it. These inclusion patterns are used on the rsync command line together with --exclude='*' which will exclude everything not explicitly included (first match matters).

The rsync run uses --delete which will delete anything in the included subdirectories on the destination that is not available in the source directory, and --delete-excluded will additionally delete everything that was excluded.

Run the script with sh -x to see what happens.

You could do this in bash with arrays as well, but the syntax is a bit messy.