SC2203

[[ current.log -nt backup/*.log ]] && echo "This is the latest file"

newerThanAll=true
for log in backup/*.log
do
  [[ current.log -nt "$log" ]] || newerThanAll=false
done
[[ "$newerThanAll" = "true" ]] && echo "This is the latest file"

Globs in [[ ]] will not filename expand, and will be treated literally (or as patterns on the right-hand side of =, == and !=).

The problematic code is equivalent to [[ current.log -nt 'backup/*.png' ], and will look for a file with a literal asterisk in the name.

Instead, you can iterate over the filenames you want with a loop, and apply your condition to each filename.

If you know your glob will only ever match one file, you can check this explicitly and use the first file:

set -- backup/*.log
[[ $# -eq 1 ]] || { echo "There are too many matches."; exit 1; }
[[ file.log -nt "$1" ]] && echo "This is the latest file"

Alternatively, ignore this warning.