How to connect to a server with the ssh command

1. Connecting with ssh
2. Custom port
3. Using a specific key file
4. The first-connection prompt
5. Host aliases with ~/.ssh/config
6. Debugging with -v
7. Where to go next

#Connecting with ssh

SSH is how I reach almost every server I work with. The basic form is the command, the user, and the host:

ssh jane@203.0.113.10

You can use a hostname instead of an IP, and SSH will resolve it:

ssh jane@app.example.com

If you don't pass a key explicitly, SSH tries the default keys in ~/.ssh. If you don't have a key yet, generate one first: create SSH keys with ssh-keygen.

#Custom port

SSH listens on port 22 by default. Many servers move it elsewhere for security. Use -p (lowercase) to specify the port:

ssh -p 2222 jane@app.example.com

#Using a specific key file

Cloud providers often hand you a .pem file. Point SSH at it with -i (identity file):

ssh -i ~/.ssh/aws-key.pem ubuntu@203.0.113.10

If the key has loose permissions, SSH refuses to use it. Fix it with chmod 400 ~/.ssh/aws-key.pem so only you can read it.

#The first-connection prompt

The first time you connect to a host, SSH shows the server's fingerprint and asks:

The authenticity of host 'app.example.com' can't be established.
ED25519 key fingerprint is SHA256:abc123...
Are you sure you want to continue connecting (yes/no/[fingerprint])?

Type yes and SSH stores the fingerprint in ~/.ssh/known_hosts. On later connections it checks silently. This is expected; it only warns you again if the fingerprint changes.

#Host aliases with ~/.ssh/config

Typing long commands gets old fast. I keep a ~/.ssh/config so I can connect with a short alias:

Host prod
    HostName app.example.com
    User jane
    Port 2222
    IdentityFile ~/.ssh/aws-key.pem

Now the whole thing collapses to:

ssh prod

Every flag above can live in the config, which keeps it out of your muscle memory and your shell history.

#Debugging with -v

When a connection misbehaves, -v (verbose) shows exactly what SSH is doing: which keys it offers, what the server accepts, and where it stops. Add more vs for more detail:

ssh -v prod
ssh -vvv prod

If you hit Permission denied (publickey), I've written up the causes and fixes here: SSH permission denied (publickey).

#Where to go next

Once you can connect, you can do more than open a shell. Run a single command without a full session with run a command over SSH, and move files with scp over SSH.