The quick and dirty way is to us the mv (move) command with the -n or --no-clobber option.
From the mv man page:
-n, --no-clobber
do not overwrite an existing file
Example:
You two directories:
/files/
/backup/
if /files is the original directory that is missing the files and the /backup directory is your most current backup, the command would be:
mv -n /backup/* /files
If your version of Linux/Unix does not support the -n option you can do the same thing with the -i option, except it will ask if you want to overwrite the existing file. You will need to answer no each file. It will not ask you if the file does not exist in the destination directory.
From the mv man page:
-i, --interactive
prompt before overwrite
i.e.
mv -i /backup/* /files
Be careful.
JimT