SC2202

[ 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 expand to a sequence of words, one per matching filename. Meanwhile, operators work on single words.

The problematic code is equivalent to [ current.log -nt backup/file1.log backup/file2.log backup/file3.log ], which is invalid syntax. A typical error message is bash: [: too many arguments or dash: somefile: unexpected operator.

Instead, use a for loop to iterate over matching filenames, and apply your condition to each.

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.