The following will find all regular files in or under ./to_keep and will call an in-line sh -c script with these in batches. For each batch of pathnames, the in-line script will call find once to find the regular files under ./to_purge that have the same names. The pathnames of these files under ./to_purge will be printed (to delete them, add -delete after -print).
find to_keep -type f -exec sh -c '
for pathname do
set -- "$@" -o -name "${pathname##*/}"
shift
done
shift
find to_purge \( "$@" \) -type f -print' sh {} +
The in-line script constructs an OR-list of -name tests for the find command that it uses on its last line. The loop constructs this list in the positional parameters from the filename component of each pathname that the outer find has passed to it.
This deals with all allowed filenames, including filenames containing spaces, tabs and newlines. Again, to delete files, add -delete (or -exec rm {} +) after -print in the code.
As a short script that takes the "keep directory" and "purge directory" as command line arguments:
#!/bin/sh
keepdir=$1
purgedir=$2
find "$keepdir" -type f -exec sh -c '
dir=$1; shift
for pathname do
set -- "$@" -o -name "${pathname##*/}"
shift
done
shift
find "$dir" \( "$@" \) -type f -print' sh "$purgedir" {} +
The only issue with this code is that it will use the names in one directory as patterns for finding the names of files in the other directory. This means that if a file in the first directory is called *, all files in the second directory are removed. You can fix that protecting the filenames in the inner find:
for pathname do
sane=$( printf "%s\n" "${pathname##*/}" | sed "s/[[*?]/\\&/g" )
set -- "$@" -o -name "$sane"
shift
done
This modification to the loop in the in-line sh -c script escapes the [, * and ? characters (otherwise used as filename globbing patterns). The script would now not deal with filenames that end in a newline, but that might arguably be something that one could live with.
