SSH Cheatsheet

The Config File

Use this SSH reference while you build software engineering projects, review code for technical interview prep, or polish examples for a software engineer resume.

Overview

~/.ssh/config lets you alias hosts and set per-host options so you type ssh prod instead of ssh -p 2222 -i ~/.ssh/deploy_key pablo@203.0.113.5.

chmod 600 ~/.ssh/config    # required — SSH ignores world-readable configs

Structure

# Global defaults — apply to every host
Host *
    ServerAliveInterval 60
    ServerAliveCountMax 3
    AddKeysToAgent yes
    IdentityFile ~/.ssh/id_ed25519

# A named alias
Host prod
    HostName 203.0.113.5
    User deploy
    Port 2222
    IdentityFile ~/.ssh/deploy_ed25519

# Wildcard alias
Host *.internal
    User ubuntu
    ProxyJump bastion

Rules are read top-to-bottom; the first matching value wins. Put specific hosts before Host *.

Common Directives

Directive	Default	What it does
`HostName`	(alias)	Real hostname or IP
`User`	`$USER`	Remote login name
`Port`	`22`	Remote port
`IdentityFile`	`~/.ssh/id_*`	Private key to offer
`IdentitiesOnly`	`no`	Only use listed identity files (don't try agent keys)
`ProxyJump`	—	Comma-separated jump hosts
`ProxyCommand`	—	Arbitrary command to open the transport
`ServerAliveInterval`	`0`	Keepalive interval in seconds
`ServerAliveCountMax`	`3`	Max unanswered keepalives before disconnect
`ForwardAgent`	`no`	Forward local agent to remote
`ForwardX11`	`no`	Forward X11 display
`StrictHostKeyChecking`	`ask`	`yes`/`no`/`ask`/`accept-new`
`UserKnownHostsFile`	`~/.ssh/known_hosts`	Where to store/check host keys
`LogLevel`	`INFO`	`QUIET` / `INFO` / `VERBOSE` / `DEBUG`
`Compression`	`no`	Enable zlib compression
`ControlMaster`	`no`	`auto` to enable multiplexing
`ControlPath`	—	Socket path for mux (`~/.ssh/cm-%r@%h:%p`)
`ControlPersist`	`no`	Keep master open (`10m`, `yes`)
`AddKeysToAgent`	`no`	Auto-add key to ssh-agent on use
`BatchMode`	`no`	Fail instead of prompting (scripts)
`ConnectTimeout`	(OS)	Seconds before giving up

Jump Host Example

Host bastion
    HostName bastion.example.com
    User ubuntu
    IdentityFile ~/.ssh/bastion_key

Host db
    HostName 10.0.1.50
    User postgres
    ProxyJump bastion
    IdentityFile ~/.ssh/db_key

Host *.internal
    ProxyJump bastion
    User ubuntu

Now ssh db automatically tunnels through bastion.

Multiplexing (Reuse Connections)

Host *
    ControlMaster auto
    ControlPath ~/.ssh/cm-%r@%h:%p
    ControlPersist 10m

After the first connection, subsequent ssh/scp/rsync to the same host reuse the socket.

Separate Known Hosts Per Host

Host staging
    HostName staging.example.com
    UserKnownHostsFile ~/.ssh/known_hosts_staging
    StrictHostKeyChecking accept-new

CI / Scripting Boilerplate

Host deploy-*
    StrictHostKeyChecking accept-new
    BatchMode yes
    ConnectTimeout 10
    IdentitiesOnly yes
    IdentityFile ~/.ssh/ci_deploy

Token Substitutions in Config Values

Token	Expands to
`%h`	Remote hostname
`%p`	Remote port
`%r`	Remote username
`%n`	Original hostname (before alias)
`%u`	Local username
`%d`	Local home directory
`%i`	Local user ID

ControlPath ~/.ssh/cm-%r@%h:%p     # typical use of tokens