Recursively rename files based on the upper folder names

Question

There is a folder structure like this

./A/B/C/D/file.txt

and I want to start from . and recursively find file.txt and rename it to the two upper level folder names. So the example above should be ./A/B/C/D/C.D.txt.

I can use find to recursively rename the files, but I don't know how combine that with basename and dirname to extract C and D upon a match. How can I do that?

Answer 1

Using bash arrays:

find . -name 'file.txt' -exec bash -c '
    IFS=/
    for p; do
        a=($p)
        n=${#a[@]}
        ((n>3)) && echo mv "$p" "${a[*]:0:n-1}/${a[n-3]}.${a[n-2]}.txt"
    done
' -- {} +