Shell – Delete directories that contain a certain file

(Your question is not clear: if a directory contains some.foo and some.bar, should it be deleted? I interpreted it as requiring such a directory to be kept.)

The following script should work, provided that no file name contains a newline and no directory matches *.foo. The principle is to traverse the directory from the leaves up (-depth), and as *.foo files are encountered, the containing directory and all parents are marked as protected. Any reached file that is not *.foo and not protected is a directory to be deleted. Because of the -depth traversal order, a directory is always reached after the *.foo files that might protect it. Warning: minimally tested, remove the echo at your own risk.

find . -depth -name '*.foo' -o -type d | awk '{
    if ($0 ~ /\.foo$/) {
        while (sub("/+[^/]+$", "")) {protect[$0]=1;}
    } else {
        if (!protect[$0]) {
            gsub("[\\\001-/]", "\\\\&"); # protect file names for xargs
            print;
        }
    }
}' | xargs echo rm -rf

_{For once, I'm not proposing a zsh solution.}

You don't need to use awk at all. Use the built-in tests that ksh provides, something like this:

#!/bin/ksh

for NAME in *
do
    FOUND=no
    if [[ -d $NAME && $NAME != '.' && $NAME != '..' ]]
    then
        for SUBNAME in $NAME/*
        do
            if [[ -d $SUBNAME ]]
            then
                FOUND=yes
                break
            fi
        done
        if [[ $FOUND == no ]]
        then
            echo Found only files in $NAME
        fi
    fi
done

That little script looks in all the directories in the current directory, and tells you if they only contain files, no sub-directories.