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Next, we'll add the 
-b
 (backup) option:
sudo rsync -avb --delete /src /target 
This one is particularly useful. Normally, when a file is updated on 
/src
 and then 
copied over to 
/target
, the copy on 
/target
 is overwritten with the new version. 
But what if you don't want any files to be replaced? The 
-b
 option renames files on 
the target that are being overwritten, so you'll still have the original file. If you add 
the 
--backup-dir
 option, things get really interesting:
sudo rsync -avb --delete --backup-dir=/backup/incremental /src /target 
Now, we're copying files from 
/src
 to 
/target
 as we were before, but we're now 
sending replaced files to the 
/backup/incremental
 directory. This means that when 
a file is going to be replaced on the target, the original file will be copied to 
/backup/
incremental
. This works because we used the 
-b
 option (backup) but we also used 
the 
--backup-dir
 option, which means that the replaced files won't be renamed, 
they'll simply be moved to the designated directory. This allows us to effectively 
perform incremental backups.
Building on our previous example, we can use the Bash shell itself to make 
incremental backups work even better. Consider these commands:
CURDATE=$(date +%m-%d-%Y) 
export $CURDATE 
sudo rsync -avb --delete --backup-dir=/backup/incremental/$CURDATE /src 
/target 
With this example, we grab the current date and store it in a variable (
CURDATE
). 
We'll also 
export
 the new variable so that it's fully available. In the 
rsync
 portion 
of the command, we use that variable for the 
--backup-dir
 option. This will copy 
the replaced files to a 
backup
 directory named after the 
date
 the command was run. 
Basically, if today's date was 
08-17-2020
, the resulting command would be the same 
as if we had run the following:
sudo rsync -avb --delete --backup-dir=/backup/incremental/08-17-2020 /
src /target 
Hopefully, you can see how flexible 
rsync
 is and how it can be used to not only copy 
files between directories and/or nodes, but also to serve as a backup solution as well 
(assuming you have a remote destination to copy files to). The best part is that this 
is only the beginning. If you consult the man page for 
rsync
, you'll see that there are 
a lot of options you can use to customize it even further. Give it some practice, and 
you should get the hang of it in no time.

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Chapter 12