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6.2 SSH in, and live in tmux

Working at a VM's console window is fine for installing an operating system and miserable for everything else: no copy and paste worth having, a fixed window size, and you have to keep the hypervisor in front of you. Real server work happens over SSH, from your own machine.

This is also where lesson 1.5 finally pays off. I promised you'd meet tmux for real once your lab had a server in it. It does now.

Connect​

SSH is built into Windows, macOS, and Linux, so open a terminal on your own computer. On Windows that's PowerShell or Terminal, and no extra software is needed.

# ssh <username>@<address>, using the account you created during the
# Ubuntu install.
ssh sam@10.10.10.20

The first time, it warns that the host's authenticity can't be established and shows a fingerprint. That's not a fault: SSH is telling you it has never seen this server before and can't vouch for it. Type yes. It records the fingerprint, and from now on it will warn you loudly if the answer ever changes, which is how you'd notice something impersonating your server.

You're in. Same shell, better window.

Start living in tmux​

Before you do anything substantial, start a tmux session. This is the habit lesson 1.5 asked you to file away:

# Ubuntu Server usually ships with tmux. This installs it if yours
# didn't, and does nothing if it's already there.
sudo apt install -y tmux

# Start a named session. The name matters when you have more than one.
tmux new -s lab

Now do something long-running, and interrupt yourself deliberately:

# A slow command, so there's something to lose.
ping -c 300 1.1.1.1

While it runs, close your terminal window entirely. Not Ctrl+C, not exit. Close the window, the way a dropped wifi connection would.

Then reconnect and look:

ssh sam@10.10.10.20

# What sessions are alive on this server?
tmux ls

# Reattach to yours.
tmux attach -t lab

Your ping is still counting. It never stopped, because it was never attached to your terminal in the first place; it was attached to a session living on the server. Press Ctrl+C to stop it.

That's the whole value, and it's why every long operation in the rest of this course should start with tmux attach -t lab or tmux new -s lab. Kernel upgrades, database restores, anything that takes twenty minutes: inside tmux, a dropped connection costs you nothing.

The rest of tmux, when you want it:

# Detach on purpose, leaving everything running: Ctrl+b, then d
# Reattach:        tmux attach -t lab
# List sessions:   tmux ls
# Split the window into two panes: Ctrl+b, then %
# Move between panes:              Ctrl+b, then arrow keys

Ctrl+b is tmux's "listen to me" key. Everything else is one keystroke after it.

Make the connection yours​

Two small things that make daily use pleasant enough that you'll actually do it.

Give the server a short name on your own machine. On your computer, not the server, create or edit ~/.ssh/config (on Windows that's C:\Users\you\.ssh\config):

Host ubnt01
    HostName 10.10.10.20
    User sam

Now ssh ubnt01 is the whole command. Add entries as your lab grows and you'll never type an address again.

Know how to get out. exit closes the SSH session. If a connection freezes and won't respond to anything, press Enter, then ~, then . which is SSH's own escape sequence, and it kills the connection when Ctrl+C can't.

From here on, assume every lesson means "over SSH, inside tmux" unless it says otherwise.