Rename the most recent file in each group - linux

Question

i try to create a script that should detect the latest file of each group, and add prefix to its original name.
ll $DIR
asset_10.0.0.1_2017.11.19 #latest
asset_10.0.0.1_2017.10.28
asset_10.0.0.2_2017.10.02 #latest
asset_10.0.0.2_2017.08.15
asset_10.1.0.1_2017.11.10 #latest
...
2 questions:
1) how to find the latest file of each group?
2) how to rename adding only a prefix
I tried the following procedure, but it looks for the latest file in the entire directory, and doesn't keep the original name to add a prefix to it:
find $DIR -type f ! -name 'asset*' -print | sort -n | tail -n 1 | xargs -I '{}' cp -p '{}' $DIR...
What would be the best approach to achieve this? (keeping xargs if possible)

Answer 1

Selecting the latest entry in each group
You can use sort to select only the latest entry in each group:
find . -print0 | sort -r -z | sort -t_ -k2,2 -u -z | xargs ...
First, sort all files in reversed lexicographical order (so that the latest entry appears first for each group). Then, by sorting on group name only (that's second field -k2,2 when split on underscores via -t_) and printing unique groups we get only the first entry per each group, which is also the latest.
Note that this works because sort uses a stable sorting algorithm - meaning the order or already sorted items will not be altered by sorting them again. Also note we can't use uniq here because we can't specify a custom field delimiter for uniq (it's always whitespace).
Copying with prefix
To add prefix to each filename found, we need to split each path find produces to a directory and a filename (basename), because we need to add prefix to filename only. The xargs part above could look like:
... | xargs -0 -I '{}' sh -c 'd="${1%/*}"; f="${1##*/}"; cp -p "$d/$f" "$d/prefix_$f"' _ '{}'
Path splitting is done with shell parameter expansion, namely prefix (${1##*/}) and suffix (${1%/*}) substring removal.
Note the use of NUL-terminated output (paths) in find (-print0 instead of -print), and the accompanying use of -z in sort and -0 in xargs. That way the complete pipeline will properly handle filenames (paths) with "special" characters like newlines and similar.

Answer 2

If you want to do this in bash alone, rather than using external tools like find and sort, you'll need to parse the "fields" in each filename.
Something like this might work:
declare -A o=() # declare an associative array (req bash 4)
for f in asset_*; do # step through the list of files,
IFS=_ read -a a <<<"$f" # assign filename elements to an array
b="${a[0]}_${a[1]}" # define a "base" of the first two elements
if [[ "${a[2]}" > "${o[$b]}" ]]; then # compare the date with the last value
o[$b]="${a[2]}" # for this base and reassign if needed
fi
done
for i in "${!o[#]}"; do # now that we're done, step through results
printf "%s_%s\n" "$i" "${o[$i]}" # and print them.
done
This doesn't exactly sort, it just goes through the list of files and grabs the highest sorting value for each filename base.