I have written a Bash script to backup a folder. At the core of the script is an rsync instruction
rsync -abh --checksum /path/to/source /path/to/target
I am using --checksum because I neither want to rely on file size nor modification time to determine if the file in the source path needs to be backed up. However, most -- if not all -- of the time I run this script locally, i.e., with an external USB drive attached which contains the backup destination folder; no backup over network. Thus, there is no need for a delta transfer since both files will be read and processed entirely by the same machine. Calculating the checksums even introduces a speed down in this case. It would be better if rsync would just diff the files if they are both on stored locally.
After reading the manpage I stumbled upon the --whole-file option which seems to avoid the costly checksum calculation. The manpage also states that this is the default if source and destination are local paths.
So I am thinking to change my rsync statement to
rsync -abh /path/to/source /path/to/target
Will rsync now check local source and target files byte by byte or will it use modification time and/or size to determine if the source file needs to be backed up? I definitely do not want to rely on file size or modification times to decide if a backup should take place.
UPDATE
Notice the -b option in the rsync instruction. It means that destination files will be backed up before they are replaced. So blindly rsync'ing all files in the source folder, e.g., by supplying --ignore-times as suggested in the comments, is not an option. It would create too many duplicate files and waste storage space. Keep also in mind that I am trying to reduce backup time and workload on a local machine. Just backing up everything would defeat that purpose.
So my question could be rephrased as, is rsync capable of doing a file comparison on a byte by byte basis?