Find all files with the extension ".txt" in the current directory and all subdirectories, print out the second field of each line, remove any blank lines, convert all characters to lowercase, sort the output, and count the number of occurrences of each unique line, then sort the output by the number of occurrences in descending order.
find ./ -mindepth 3 -name "*.txt" -exec cat {} + | cut -d':' -f2 | grep -vE "\s*$" | tr '[A-Z]' '[a-z]' | sort | uniq -c | sort -nr