The first thing you'll want to do is consult rsync's man page and --help readme, which will explain a lot of options.
The basic operation is:
rsync [options] source destination
This sync is, by default, not destructive for files on the destination that do not exist in the source. So, if you have some files on your backup destination that you have since deleted from the source, they will still exist by default. There are a number of options for when to delete these (if you want them deleted), which all start with --delete. For my purposes, --delete-during is fine.
In general, you will want to use verbose mode (-v) and archive mode (-a), which turn on information about files being synced or deleted and also manage creation timestamps, permissions, etc properly. If you want to run a trial run, use -n.
I also generally run rsync through tee, so I can see what is going on but also have a log afterwards. If you are concerned about deletion, you can run with -n --delete to see a log of what will be deleted. For instance:
rsync -av -n --delete /src /dest |tee full-log.txt grep '^deleted' full-log.txt
Will give you a log of all transfers and allow you to see info about what was deleted (or more), but not obscure the progress from your terminal.
Rsync treats slashes specially. It's easiest to explain this with examples, which is what rsync --help actually does.
These two are equivalent:
rsync /src/foo /dest rsync /src/foo/ /dest/foo
The trailing slash on the second example will sync the contents of foo into the destination, whereas the first example looks for the directory foo in the destination.
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