Shell script to delete all files of certain type in different folders, except 10 latest

First of all, please excuse my poor english.

I am trying to write a shell script to clean up some files of certain type in my backup folders.

I need to remove files of certain type, except 10 newest in each folder.

Folder structure looks like this:

Root folder
│
├── folder_1
│   ├── file_1.txt
│   └── file_2.txt
│   └── file_n.txt
│
├── folder_2
│   ├── file_1.txt
│   └── file_2.txt
│   └── file_n.txt
│
├── folder_n
│   ├── file_1.txt
│   └── file_2.txt
│   └── file_n.txt
│

I've used script from David Foerster as a basis, but can't figure out on how to make it work in different folders separately without manually writing each folder name in script.

Currently, script looks like this:

find /volume1/rootfolder/ -type f -name *.txt -printf '%T@ %p\0' |
sort --zero-terminated --reverse --numeric-sort --field-separator=' ' --key 1,1 |
gawk -F ' ' -v RS='\0' -v ORS='\0' -v retain_count=10 \
  'BEGIN{ maxage = systime() - retain_younger_days * 24 * 3600; }
  (NR > retain_count) && (int($1) < maxage) { print(substr($0, length($1) + 2)); }' |
xargs -r0 -- rm --

But the issue is that it removes all files in all folders, except 10 latest. So, overall i have just 10 files in all folders, not 10 in each folder.

Please, help me figure out, how to make script to process each folder separately.

You divide it into two parts. First, you build a directory structure, and in this case, we only need directories containing files. In the second part, we sort the newest file first and skip those with tail.

#!/bin/bash

while IFS= read -rd ''; do
    find "$REPLY" \
    -maxdepth 1 -type f -name '*.txt' -printf '%T@\t%p\0' | sort -zk1rn | cut -zf2- | tail -zn +$((10+1)) | xargs -r -0 echo rm
done < <( \
    find $PWD/data \
    -mindepth 2 -type f -printf %h\\0 | sort -zu \
)

There is also the possibility to use globstar with a for-loop:

#!/bin/bash
shopt -s globstar nullglob

for a in data/*/**/; do
    find "$a" \
    -maxdepth 1 -type f -name '*.txt' -printf '%T@\t%p\0' | sort -zk1rn | cut -zf2- | tail -zn +$((10+1)) | xargs -r -0 echo rm
done

Lastly, we make use of an array of counters to keep track of the file count:

#!/bin/bash

find $PWD/data -mindepth 1 -type f -name '*.txt' -printf '%T@\t%p\0' \
    | sort -zk1rn | cut -zf2- \
    | awk -F / -vRS='\0' -vORS='\0' '{a=$0; NF=NF-1} b[$0]++ >= 10 {print a}' | xargs -r -0 echo rm