Bash shell script: recursively cat TXT files in folders - bash

Question

I have a directory of files with a structure like below:
./DIR01/2019-01-01/Log.txt
./DIR01/2019-01-01/Log.txt.1
./DIR01/2019-01-02/Log.txt
./DIR01/2019-01-03/Log.txt
./DIR01/2019-01-03/Log.txt.1
...
./DIR02/2019-01-01/Log.txt
./DIR02/2019-01-01/Log.txt.1
...
./DIR03/2019-01-01/Log.txt
...and so on.
Each DIRxx directory has a number of subdirectories named by date, which themselves have a number of log files that need to be concatenated. The number of text files to concatenate varies, but could theoretically could be as many as 5. I would like to see the following command performed for each set of files within the dated directories (note that the files must be concatenated in reverse order):
cd ./DIR01/2019-01-01/
cat Log.txt.4 Log.txt.3 Log.txt.2 Log.txt.1 Log.txt > ../../Log.txt_2019-01-01_DIR01.txt
(I understand the above command will give an error that certain files do not exist, but the cat will do what I need of it anyways)
Aside from cding into each directory and running the above cat command, how can I script this into a Bash shell script?

Answer 1

If you just want to concatenate all files in all subdirectories whose name starts with Log.txt, you could do something like this:
for dir in DIR*/*; do
date=${dir##*/};
dirname=${dir%%/*};
cat $dir/Log.txt* > Log.txt_"${date}"_"${dirname}".txt;
done
If you need the files in reverse numerical order, from 5 to 1 and then Log.txt, you can do this:
for dir in DIR*/*; do
date=${dir##*/};
dirname=${dir%%/*};
cat $dir/Log.txt.{5..1} $dir/Log.txt > Log.txt_"${date}"_"${dirname}".txt;
done
That will, as you mention in your question, complain for files that don't exist, but that's just a warning. If you don't want to see that, you can redirect error output (although that might cause you to miss legitimate error messages as well):
for dir in DIR*/*; do
date=${dir##*/};
dirname=${dir%%/*};
cat $dir/Log.txt.{5..1} $dir/Log.txt > Log.txt_"${date}"_"${dirname}".txt;
done 2>/dev/null

Answer 2

Not as comprehensive as the other, but quick and easy. Use find and sort your output however you like (-zrn is --zero-terminated --reverse --numeric-sort) then iterate over it with read.
find . -type f -print0 |
sort -zrn |
while read -rd ''; do
cat "$REPLY";
done >> log.txt