Copying files and saving connection settings

Once you can log in, you will want to move files between machines and to stop typing the same long ssh -i ... user@host over and over. SSH carries file transfers inside the same secure connection, and a small configuration file turns every server into a short name.

Copy files and directories in both directions with scp, sftp and rsync.
Explain when rsync is the better choice, and use -a, -n and the trailing slash correctly.
Define host aliases in ~/.ssh/config.

Host aliases: ~/.ssh/config

Instead of repeating options, give the server a nickname in ~/.ssh/config (mode 600). Each Host block applies to connections to that name:

[student@workstation ~]$ cat ~/.ssh/config
Host opsa
    HostName servera
    User ops
    IdentityFile ~/.ssh/id_ops
    IdentitiesOnly yes
[student@workstation ~]$ ssh opsa 'echo hello from $(hostname) as $USER'
hello from servera.lab.example.com as ops

Keyword	Means
`Host`	The nickname you type
`HostName`	The real name or address
`User`	The account to log in as
`IdentityFile`	Which private key to use
`IdentitiesOnly yes`	Offer only that key (not every key in the agent)
`Port`	A non-standard port

The same alias works for every tool below, because they all use SSH.

scp: simple copies

scp copies like cp but between hosts. A remote path is written HOST:PATH; an empty path after the colon means the remote home directory. The first argument is the source, the last is the target:

[student@workstation ~]$ scp inventory.txt opsa:                 # upload to ~ops
[student@workstation ~]$ scp -r reports opsa:/tmp/               # a directory (-r)
[student@workstation ~]$ scp opsa:/etc/hostname ./servera-hostname.txt   # download
[student@workstation ~]$ cat servera-hostname.txt
servera.lab.example.com

scp is silent on success when not run on a terminal that shows progress. It overwrites without asking, so mind the target.

sftp: browse, then copy

sftp opens an interactive session with commands like ls, cd, get, put and bye, which is handy when you are not sure of the exact file name:

[student@workstation ~]$ sftp opsa
Connected to opsa.
sftp> cd /tmp
sftp> ls reports
reports/a.txt   reports/b.txt
sftp> get reports/a.txt
Fetching /tmp/reports/a.txt to a.txt
sftp> bye

rsync: copy only what changed

rsync compares source and target and sends only the differences, so it is the right tool for repeated copies and for backups.

[student@workstation ~]$ rsync -av reports/ opsa:rsyncdir/
sending incremental file list
created directory rsyncdir
./
a.txt
b.txt

sent 191 bytes  received 88 bytes  558.00 bytes/sec
total size is 6  speedup is 0.02
[student@workstation ~]$ rsync -av reports/ opsa:rsyncdir/
sending incremental file list

sent 117 bytes  received 19 bytes  272.00 bytes/sec
total size is 6  speedup is 0.04

The second run transferred nothing. Key options:

Option	Does
`-a`	Archive: recurse and preserve permissions, owners, times and links
`-v`	Verbose: list the files
`-n`	Dry run: show what would happen, change nothing
`--delete`	Remove files at the target that are gone at the source (mirror)
`-z`	Compress during transfer
`--progress`	Show progress for large files

The trailing slash matters. rsync -av reports opsa:dir/ creates dir/reports/... (the directory itself), while rsync -av reports/ opsa:dir/ puts the contents directly in dir/. Always rehearse a destructive mirror with -n first:

[student@workstation ~]$ rsync -avn --delete reports/ opsa:rsyncdir/

Choosing a transfer tool: Select a tool to see when it fits.

Try it

Check your understanding

Which command copies the file notes.txt to ops's home directory on servera?
What does rsync do that scp does not?
In rsync, what is the difference between reports and reports/ as the source?
What is ~/.ssh/config used for?
Which command starts an interactive file transfer session?
rsync -avn --delete a/ host:b/ does what?