Engineering Craft › Developer Tooling

SSH

Securely logging into and running commands on remote machines.

Also known as: Secure Shell, ssh client, remote login

SSH (Secure Shell) lets you log in to another machine, and run commands there, over an encrypted connection. It’s how you reach most servers.

ssh deploy@203.0.113.10          # log in as user "deploy"
ssh deploy@server.example.com
ssh -p 2222 deploy@server        # server uses a non-default port (default is 22)
ssh deploy@server "df -h"        # run one command, then disconnect

After connecting, your terminal acts as if you were sitting at that machine. Type exit to leave.

Authentication

• Password: simple but weaker, and often disabled on servers.
• Key pair (the normal way): you have a private key (stays on your machine, never share) and a public key (put on the server). The server verifies you hold the private key without it ever being sent. See SSH keys.

First connection

The first time, SSH shows the server’s fingerprint and asks whether to trust it. The answer is saved in ~/.ssh/known_hosts. If it later changes unexpectedly, SSH warns loudly: that can mean the server was rebuilt, or that someone is intercepting the connection. Don’t just delete the line; find out which.

Handy things

scp report.csv deploy@server:/tmp/        # copy a file to the server
scp deploy@server:/var/log/app.log .      # ...and from it
ssh -L 5432:localhost:5432 deploy@server  # forward local port 5432 to the server's database

A ~/.ssh/config file lets you give hosts short names and set the user and key:

Host prod
  HostName 203.0.113.10
  User deploy

Then ssh prod is enough. Git over SSH uses the same keys. Be careful on production machines: you’re running real commands on a live system.