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Remote Machines

One of Linux's superpowers: the terminal works the same whether the computer is under your desk or across the planet. SSH (Secure Shell) gives you an encrypted terminal on another machine.

Logging in

ssh paul@192.168.1.50

This connects as user paul to the machine at 192.168.1.50. You'll be asked for paul's password on that machine, then your prompt changes — you're typing on the remote computer now. exit (or Ctrl+D) logs out and returns you home.

Check: After logging in, the prompt shows the remote machine's name, and pwd/whoami answer for that machine.

The first connection asks you to confirm the host's fingerprint:

The authenticity of host '192.168.1.50' can't be established.
Are you sure you want to continue connecting (yes/no)?

Type yes. SSH remembers the fingerprint; if it ever changes, SSH warns you loudly — that can mean an attack, or just a reinstalled machine. Don't ignore the warning; do verify which it is before proceeding.

Copying files

scp notes.txt paul@192.168.1.50:/home/paul/   # copy TO the remote
scp paul@192.168.1.50:/home/paul/notes.txt .   # copy FROM the remote
scp -r projects/ paul@192.168.1.50:~/          # -r for directories

scp reads like cp, but either side can be user@host:path. Think of it as cp with a network.

Passwordless login with keys

Typing passwords gets old, and passwords can be guessed. The better way: a key pair. You keep the private key; the server gets the public key.

ssh-keygen -t ed25519        # create a key pair (accept the defaults)
ssh-copy-id paul@192.168.1.50  # install your public key on the server

Check: ssh paul@192.168.1.50 now logs you in without asking for a password.

Careful: Your private key (~/.ssh/id_ed25519) is your identity. Anyone holding it can log in as you everywhere it's installed. Never email it, never paste it into a chat, never copy it to a machine you don't trust. The public key (id_ed25519.pub) is safe to share — that's the whole point of the pair.

Users and groups (the 2-minute version)

whoami            # who am I here?
who               # who's logged in right now?
id                # my user id, group id, and all my groups
sudo usermod -aG sudo paul   # add paul to the sudo group (-aG = append!)

That last one deserves respect: -aG appends paul to the sudo group. Without -a, usermod -G sudo paul replaces his groups — locking him out of everything else. The -a is the difference between "also" and "only."

Next: Disks and Services