Moving files between machines

scp copies a single file by default, so a directory needs the -r option for recursive, which walks the tree and sends every file inside it. Without it scp refuses the directory outright. The direction is set by which argument carries the colon, so because the colon is on the destination side this command sends rather than receives.

With the trailing slash the contents go straight into the destination. Without it the directory name is carried across, so the files land one level deeper under the destination. That is why the same command can quietly put your files in the wrong place if the slash is not what you meant, and it is worth checking before a run with --delete.

A dry run performs the whole comparison and prints the file list it would transfer, then stops before moving anything. That is the safe way to check a command, and it matters most alongside --delete, because the dry run shows you the deletions before they happen. Sending only what changed is what rsync does on every run, not something --dry-run adds.

rsync compares the source and the destination before it moves any data, so on a second run the file list is empty or short and almost nothing crosses the link. scp, by contrast, sends every file again each time you run it, which is the difference the lesson draws out. That comparison step is also what makes the dry run meaningful, since it is the same work without the transfer.