keep N files in subdirectories

Question

I'm trying to do something that looks pretty simple but I haven't been able to solve.

I have a directory with a bunch of subdirectories all of them with multiple files (jpg files). I want to execute a command which keep only 4 or N files inside of these directories. The order of the files isn't important as I have seen related questions depending on the time they were created

I have played with ls + head and trying to put find in a loop with the -delete option but still no luck.

Answer 1

Maybe something like:

for dir in /target/dir/*/; do
  (cd -- "$dir" && set -- *.jpg && [ "$#" -gt 4 ] && shift 4 && rm -f -- "$@")
done

Which with zsh, you could shorten to:

for dir (/target/dir/*(/)) rm -f $dir/*.jpg(N[5,-1])

Answer 2

Assuming those are regular files with no blanks, newlines, quotes, backslashes or invalid characters in their paths:

find /target/dir/ -type f \
  | awk '{t=$0; gsub(/\/[^/]*$/, "", t); if(a[t]++ < 4) print}' \
  | xargs rm --

See the comments below for how you could possibly adapt it for more funny file names if your utilities support certain extensions.