SSH glossary: the terms you'll meet in the terminal

Connection basics

• SSHSecure Shell — an encrypted protocol for running commands on a remote machine, and the foundation most of this glossary builds on
• SFTPthe file-transfer subsystem of SSH; same connection, same authentication, but for browsing and moving files
• SCPa simpler, older file-copy command that also rides on SSH
• Telnetan unencrypted remote-shell protocol from before SSH; still common on older network gear
• Seriala direct cable connection to a device, used for switches, routers and embedded boards; settings include baud, data bits, stop bits, parity and flow control
• Moshthe mobile shell — stays connected over flaky links and roaming networks by using UDP alongside SSH
• PTYpseudo-terminal; the kernel device that lets a remote program behave as if it were attached to a real keyboard and screen
• Shellthe program, such as bash or zsh, that reads your commands once you are logged in

Authentication and identity

• Password authenticationlogging in with a password; simple, but weaker than keys and often disabled on servers
• Public-key authenticationthe server holds your public key and you prove ownership of the matching private key
• Ed25519a modern, fast, small elliptic-curve key type; the usual default for new keys today
• RSAthe older, widely supported key type; still common, but long RSA keys are slower and being phased out
• ECDSAanother elliptic-curve key type, supported broadly but with a more complicated history than Ed25519
• Passphrasea password that protects a private key file on disk, so a stolen key is not instantly usable
• SSH agenta background helper that holds unlocked keys in memory and answers authentication requests for you
• Agent forwardingletting a remote server reach back through your local agent, so you can hop onward without copying keys; powerful, and worth using only on hosts you trust
• authorized_keysthe file on a server listing the public keys allowed to log in as a user
• Certificate authenticationa signed certificate that proves your key's identity, often used with short-lived credentials in larger fleets

Host verification

• Host keythe server's own public key; your client checks it to make sure you are talking to the right machine
• Fingerprinta short hash of a key that is easy to compare out of band
• Known hoststhe local record of server keys you have already accepted
• TOFUtrust on first use — accept a server's key the first time, then warn loudly if it ever changes
• Man-in-the-middlean attacker sitting between you and the server; host key checks exist to catch exactly this

TerminalX prompts on first use and again if a known host's key changes, which is TOFU working as intended.

Networking and forwarding

• Jump hostan intermediate server you connect through to reach a machine that is not directly reachable
• Bastionanother name for a hardened jump host, usually the only door into a private network
• ProxyJumpthe OpenSSH option that expresses a jump chain in your config
• Port forwardingtunnelling other traffic over an SSH connection
• Local forwardingsending a local port's traffic through the server, for reaching a remote service on your own machine
• Remote forwardingthe reverse: exposing a service on the far side back toward you
• Dynamic forwardingturning SSH into a SOCKS proxy for arbitrary destinations
• SOCKS5the proxy protocol dynamic forwarding speaks; point a browser or app at the local port to route through your server
• Port knockinghitting a sequence of closed ports to signal a firewall to open SSH for you

Keys, files and permissions

• Public vs private keythe public key is shared and goes to servers; the private key stays on your machine and is never sent
• Keychaina client's store of keys, passwords and certificates, reused across many hosts
• chmodthe command that sets file permissions; a client with an SFTP pane can change them without a shell
• Octal notationpermissions written as digits: 600 is owner read-write only, 644 adds group and public read, 755 adds execute for all
• Recursiveapplying an operation to a folder and everything inside it, such as a recursive delete or chmod
• ~/.ssh/configthe plain-text file where OpenSSH users keep host aliases, keys and jump settings; most GUI clients can import it

Terminal behaviour

• Scrollbackthe buffer of past output above the current screen; search-in-scrollback lets you find a line you scrolled past
• xterm-256colora widely supported terminal type that advertises 256 colours to remote programs
• True color24-bit colour, which some modern tools use for richer output
• Bracketed pastea mode that tells the shell when input was pasted rather than typed, so it does not misread it as commands
• Split panesdividing one window into several terminals, side by side or stacked
• Multiplexera tool like tmux that keeps sessions alive on the server and lets you reattach later
• Command palettea searchable list of app actions, faster than hunting through menus

Security and storage

• Vaultan encrypted store for hosts, keys and credentials; local in TerminalX, cloud-synced in some subscription clients
• Argon2ida modern key-derivation function that turns a master password into an encryption key; slow by design to resist guessing
• XChaCha20-Poly1305the authenticated cipher TerminalX uses to protect the vault, providing both confidentiality and tamper detection
• Zeroizationwiping key material from memory once it is no longer needed
• Auto-locklocking the vault after a period of inactivity so a left-open app does not expose your secrets
• Zero-knowledgea design where a server only ever holds encrypted data it cannot read; TerminalX goes further and has no server at all
• Telemetryusage data sent back to a vendor; TerminalX sends none

File transfer operations

• Transfer queuean ordered list of uploads and downloads, each with progress you can watch
• Resumecontinuing an interrupted transfer instead of starting over
• Dual-paneshowing local and remote folders together, which makes drag and drop obvious
• Previewopening a remote file to inspect it before deciding to download