I live in my terminal. Neovim, tmux, git, SSH — that's my whole day. I've used every terminal emulator out there. iTerm2, Alacritty, Kitty, Ghostty. All great.
But I never understood what actually happens inside one. Bytes come out of a shell, escape sequences get parsed, characters appear on screen. What does ESC[38;2;255;100;0m do to the internal state? How does a key press travel through a pseudoterminal and come back as text? I had no clue.
Only way I know how to learn something is to build it.
So I built Attyx — a GPU-accelerated terminal emulator, from scratch, in Zig. Started on a Saturday. Five days later I was daily-driving it. I'm still daily-driving it.
Why Though
"We don't need another terminal emulator." Sure. But I didn't build it because the world needed one. I built it for two selfish reasons.
I wanted to learn Zig. Not from docs and tutorials — from a real project that would punch me in the face with systems programming problems. Terminal emulators hit everything: parsing, GPU rendering, font rasterization, Unicode, PTY management, platform APIs. Perfect.
I needed a testable terminal core. This one's less obvious, so let me explain.
I build TUI apps. I made Glyph — a React renderer for the terminal. Flexbox, components, hooks, the whole deal, but rendering to your terminal instead of a browser. On top of it I built Aion (Calendar TUI) and Epist (an email client with vim keybindings). Real apps I use every single day.
Here's my problem: how do you test what a TUI app actually looks like? You can test component state. You can test logic. But the final output — the grid of characters and styles after the terminal interprets your escape sequences — that's a black hole. Screenshot diffing? Fragile. Asciinema recordings? Not automated. Unit testing raw escape codes? Doesn't catch interpretation bugs.
What I wanted was dead simple — feed bytes into a terminal engine, get a deterministic grid back, assert against it. No terminal app existed that let me do that. So I built one where the core is pure from the ground up.
var engine = try Engine.init(allocator, 24, 80);
engine.feed(my_app_output);
const cell = engine.state.grid.getCell(0, 0);
try expectEqual('A', cell.char);
try expect(cell.style.bold);
No GPU. No window. No PTY. Just bytes in, state out. That's the testing primitive I've wanted for years.
What You Get
GPU-accelerated rendering — Metal on macOS, OpenGL 3.3 on Linux. Full VT100/xterm compatibility. Truecolor, 256-color, the works. Mouse tracking, Kitty graphics protocol for inline images, hyperlinks, alternate screen buffer, scroll regions, cursor shapes. 20,000-line scrollback with reflow on resize. Popup terminals that float over your session. Search. TOML config with hot reload.
Under 5MB.
The whole thing — GPU rendering, VT parser, font handling, platform code — in a tiny binary. That's what happens when you write Zig with minimal deps, use native platform frameworks instead of bundling a GUI toolkit, and let the compiler strip dead code. No runtime. No GC. No Electron. Just a Zig binary talking to the OS.
Under the Hood (Without Boring You to Death)
I wrote a deep technical dive on the Semos blog if you want all the details. But here's the gist.
Everything flows through a pipeline:
Raw bytes → Parser → Actions → State → Grid
The parser is an incremental state machine. One byte at a time, spits out actions — "print H", "move cursor to row 3 col 5", "set foreground to red." Fixed-size buffers, zero heap allocations, handles partial sequences across read() boundaries. The whole parser struct is stack-sized.
The grid is a flat array of cells. Each cell: a Unicode codepoint, two combining mark slots (for diacriticals and such), a style, and a hyperlink ID. One alloc on init. Scroll regions are memcpy on contiguous memory. No linked lists, no indirection.
Damage tracking keeps rendering fast. 256-bit dirty bitset — four u64s. State machine flips a bit when it touches a row. Renderer only redraws dirty rows. Most frames, that's one or two rows.
Two threads, no locks. PTY thread reads bytes and fills a shared cell buffer. Main thread renders at vsync. A seqlock — just an atomic generation counter — prevents torn reads. If the renderer catches the PTY mid-write, it skips a frame. You'll never notice a single dropped frame. You will notice mutex contention.
GPU rendering turns each character into a textured quad. Glyphs get rasterized on demand into an atlas that lives in GPU memory. Drawing 10,000 cells costs about the same as drawing 100 — that's the whole point of offloading to the GPU. The CPU does parsing and state. The GPU does pixels. Each does what it's good at.
The Semos Stack
Attyx is part of Semos — the collection of dev tools I've been building. It fits into a stack that's been growing over the past few months:
Glyph is the React terminal renderer. Write terminal apps with JSX, flexbox, hooks — the same DX you know from the web, but painting to a terminal. Aion and Epist are the apps built on top of it — calendar and email, both living in my terminal where they belong.
Attyx closes the loop. Glyph apps needed a testable terminal. Attyx needed real-world apps to stress-test against. They push each other forward.
Give It a Shot
brew install semos-labs/tap/attyx
Or build from source:
git clone https://github.com/semos-labs/attyx
cd attyx
zig build -Doptimize=ReleaseFast
Or download it from the website if you happened to be on a mac. You will also get auto-updates in this case.
Config goes in ~/.config/attyx/attyx.toml — fonts, colors, keybindings, opacity, blur. Change it, hit Ctrl+Shift+R, done. No restart.
Open source, MIT licensed. github.com/semos-labs/attyx
Is it as mature as Ghostty or Kitty? Not yet. But I understand every line in it, I use it every day, and the whole thing fits in under 5MB. That counts for something.
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