Originally published on MistLab . This is Part 1 of the series Building an SSH Terminal .

Part 1 of "Building an SSH Terminal". Code excerpts are from MistTerm v1.2.26, an open-source (AGPL) SSH terminal client by MistLab, written in Rust; excerpts are trimmed.

TL;DR

A terminal emulator turns a byte stream from a program into pixels in three stages. A parser (a resumable state machine) splits the stream into printable characters and control sequences defined by ECMA-48 and xterm. A grid of character cells applies their meaning: cursor, scrolling regions, modes, wide characters, the alternate screen. A renderer then draws the grid. MistTerm uses alacritty_terminal for the parser and grid, and draws the grid itself with egui.

Type ls --color and colored file names appear. It feels immediate, but inside the terminal there are three distinct stages between the network and the screen. The remote program writes a stream of bytes with control sequences mixed in. A parser turns that stream into actions.…