Moving files between machines

Sooner or later you need a file that is on another machine. Or you need to send one there. scp copies over an encrypted connection. rsync does the same job, but sends only what has changed. In this lesson you will copy a file both ways, and copy a directory. You will use rsync to preview a transfer before you run it.

What you'll be able to do

• Copy a file to another host, and copy one back.
• Copy a whole directory with scp -r.
• Use rsync to send only the parts that differ.
• Preview a transfer with a dry run before you commit.
• Understand the trailing-slash rule and the risk of --delete.

Step 1: Copy a file to a host

scp copies files over SSH, so both ends must be reachable. You also need an account on the far side. The pattern is scp source destination. A destination that contains a colon is a remote path. This sends a local file to your home directory on the host:

scp notes.txt ada@example.com:~/

notes.txt                                100% 2048     2.0KB/s   00:01

The part before the colon is user@host. The part after it is the path on that host. The tilde means the home directory.

Step 2: Copy a file back

The same command works in reverse. Put the remote path first and the local path second. A single dot means the current directory:

scp ada@example.com:~/report.txt .

report.txt                               100% 8192     4.0KB/s   00:02

The file lands in the directory you are in. Nothing is removed from the host, because scp only reads it.

Step 3: Copy a directory

To copy a directory and everything inside it, add -r for recursive:

scp -r notes ada@example.com:~/

notes.txt                                100% 2048     2.0KB/s   00:01
notes/draft.md                           100% 1024     1.0KB/s   00:01
notes/final.md                           100% 3072     3.0KB/s   00:01

Each file is listed as it moves. On a slow link, a big directory starts to hurt here. scp sends every file again each time you run it.

Step 4: Send only the difference with rsync

rsync does the same job, but it compares the two sides first. Then it sends only what differs. That makes a second run far cheaper than the first. It is not always installed, so add it if needed:

pkg install rsync

Checking availability of current mirror: ok
Reading package lists... Done
Setting up rsync (3.3.0) ...

The -a flag keeps permissions and times, and recurses. The -v flag prints what it does:

rsync -av notes/ ada@example.com:~/notes/

sending incremental file list
./
draft.md
final.md
notes.txt

sent 4,120 bytes  received 92 bytes  2,808.00 bytes/sec
total size is 6,144  speedup is 2.19

The line reading sending incremental file list is the key idea. Only the names below it were sent. Run it a second time and that list is empty, because nothing changed.

For rsync the slash at the end of the source matters. notes/ means the contents of the directory, and notes means the directory itself. Watch what the second form produces:

rsync -av --dry-run notes ada@example.com:~/backup/

sending incremental file list
notes/
notes/draft.md
notes/final.md
notes/notes.txt

With no trailing slash the directory name is carried across. The files then land one level deeper under backup. With the slash, the contents go straight into the destination.

Step 5: Preview before you copy

rsync can show you exactly what it would do without doing it. Add --dry-run, often written -n:

rsync -av --dry-run notes/ ada@example.com:~/notes/

sending incremental file list
./

sent 120 bytes  received 12 bytes  264.00 bytes/sec
total size is 6,144  speedup is 51.20 (DRY RUN)

Nothing moved. The list is empty, so the destination already matches. This is the safe way to check a command before you run it for real.

Quick reference

Command	What it does
scp file user@host:path	Copies a local file to a host
scp user@host:path .	Copies a file from a host to here
scp -r dir user@host:path	Copies a directory and its contents
rsync -av src/ user@host:dest/	Copies only what differs, keeping details
rsync -av --dry-run src/ user@host:dest/	Shows what would move, without moving it
rsync src/ dest/	Copies the contents of src into dest
rsync src dest/	Copies the src folder itself into dest

Next steps

• SSH and remote work covers logging in and running commands on a host.
• HTTP and APIs from the shell shows the other way data moves over the network.
• DNS and addresses explains how the host name is resolved.