ht - headless terminal

ht (short for headless terminal) is a command line program that wraps an arbitrary other binary (e.g. bash, vim, etc.) with a VT100 style terminal interface--i.e. a pseudoterminal client (PTY) plus terminal server--and allows easy programmatic access to the input and output of that terminal (via JSON over STDIN/STDOUT). ht is built in rust and works on MacOS and Linux.

<img src="https://andykonwinski.com/assets/img/headless-terminal.png" alt="screenshot of raw terminal output vs ht output" align="right" style="width:450px">

Use Cases & Motivation

ht is useful for programmatically interacting with terminals, which is important for programs that depend heavily on the Terminal as UI. It is useful for testing and for getting AI agents to interact with terminals the way humans do.

The original motiving use case was making terminals easy for LLMs to use. I was trying to use LLM agents for coding, and needed something like a headless browser but for terminals.

Terminals are one of the oldest and most prolific UI frameworks in all of computing. And they are stateful so, for example, when you use an editor in your terminal, the terminal has to manage state about the cursor location. Without ht, an agent struggles to manage this state directly; with ht, an agent can just observe the terminal like a human does.

Installing

Download and use the latest binary for your architecture.

Building

Building from source requires the Rust compiler (1.84 or later), and the Cargo package manager. If they are not available via your system package manager then use rustup.

To download the source code, build the binary, and install it in $HOME/.cargo/bin run:

cargo install --git https://github.com/andyk/ht

Then, ensure $HOME/.cargo/bin is in your shell's $PATH.

Alternatively, you can manually download the source code and build the binary with:

git clone https://github.com/andyk/ht
cd ht
cargo build --release

This produces the binary in release mode (--release) at target/release/ht. There are no other build artifacts so you can just copy the binary to a directory in your $PATH.

Usage

Run ht to start interactive bash shell running in a PTY (pseudo-terminal).

To launch a different program (a different shell, another program) run ht <command> <args...>. For example:

• ht fish - starts fish shell
• ht nano - starts nano editor
• ht nano /etc/fstab - starts nano editor with /etc/fstab opened

Another way to run a specific program, e.g. nano, is to launch ht without a command, i.e. use bash by default, and start nano from bash by sending nano\r ("nano" followed by "return" control character) to the process input. See input command below.

Default size of the virtual terminal window is 120x40 (cols by rows), which can be changed with --size argument. For example: ht --size 80x24. The window size can also be dynamically changed - see resize command below.

Run ht -h or ht --help to see all available options.

Live terminal preview

ht comes with a built-in HTTP server which provides a handy live terminal preview page.

To enable it, start ht with -l / --listen option. This will print the URL of the live preview.

By default it listens on 127.0.0.1 and a system assigned, dynamic port. If you need it to bind to another interface, or a specific port, pass the address to the -l option, e.g. -l 0.0.0.0:9999.

API

ht provides 2 types of API: STDIO and WebSocket.

The STDIO API allows control and introspection of the terminal using STDIN, STDOUT and STDERR.

WebSocket API provides several endpoints for getting terminal updates in real-time. Websocket API is not enabled by default, and requires starting the built-in HTTP server with -l / --listen option.

STDIO API

ht uses simple JSON-based protocol for sending commands to its STDIN. Each command must be sent on a separate line and be a JSON object having "type" field set to one of the supported commands (below).

Some of the commands trigger events. ht may also internally trigger various events on its own. To subscribe to desired events use --subscribe [<event-name>,<event-name>,...] option when starting ht. This will print the events as they occur to ht's STDOUT, as JSON-encoded objects. For example, to subscribe to view snapshots (triggered by sending takeSnapshot command) use --subscribe snapshot option. See events below for a list of available event types and their payloads.

Diagnostic messages (notices, errors) are printed to STDERR.

sendKeys

sendKeys command allows sending keys to a process running in the virtual terminal as if the keys were pressed on a keyboard.

{ "type": "sendKeys", "keys": ["nano", "Enter"] }
{ "type": "sendKeys", "keys": ["hello", "Enter", "world"] }
{ "type": "sendKeys", "keys": ["^x", "n"] }

Each element of the keys array can be either a key name or an arbitrary text. If a key is not matched by any supported key name then the text is sent to the process as is, i.e. like when using the input command.

The key and modifier specifications were inspired by tmux.

The following key specifications are currently supported:

• Enter
• Space
• Escape or ^[ or C-[
• Tab
• Left - left arrow key
• Right - right arrow key
• Up - up arrow key
• Down - down arrow key
• Home
• End
• PageUp
• PageDown
• F1 to F12

Modifier keys are supported by prepending a key with one of the prefixes:

• ^ - control - e.g. ^c means <kbd>Ctrl</kbd> + <kbd>C</kbd>
• C- - control - e.g. C-c means <kbd>Ctrl</kbd> + <kbd>C</kbd>
• S- - shift - e.g. S-F6 means <kbd>Shift</kbd> + <kbd>F6</kbd>
• A- - alt/option - e.g. A-Home means <kbd>Alt</kbd> + <kbd>Home</kbd>

Modifiers can be combined (for arrow keys only at the moment), so combinations such as S-A-Up or C-S-Left are possible.

C- control modifier notation can be used with ASCII letters (both lower and upper case are supported) and most special key names. The caret control notation (^) may only be used with ASCII letters, not with special keys.

Shift modifier can be used with special key names only, such as Left, PageUp etc. For text characters, instead of specifying e.g. S-a just use upper case A.

Alt modifier can be used with any Unicode character and most special key names.

This command doesn't trigger any event.

input

input command allows sending arbitrary raw input to a process running in the virtual terminal.

{ "type": "input", "payload": "ls\r" }

In most cases it's easier and recommended to use the sendKeys command instead.

Use the input command if you don't want any special input processing, i.e. no mapping of key names to their respective control sequences.

For example, to send Ctrl-C shortcut you must use "\u0003" (0x03) as the payload:

{ "type": "input", "payload": "\u0003" }

This command doesn't trigger any event.

takeSnapshot

takeSnapshot command allows taking a textual snapshot of the the terminal view.

{ "type": "takeSnapshot" }

This command triggers snapshot event.

resize

resize command allows resizing the virtual terminal window dynamically by specifying new width (cols) and height (rows).

{ "type": "resize", "cols": 80, "rows": 24 }

This command triggers resize event.

WebSocket API

The WebSocket API currently provides 2 endpoints:

/ws/events

This endpoint allows the client to subscribe to events that happen in ht.

Query param sub should be set to a comma-separated list of desired events. E.g. /ws/events?sub=init,snapshot.

Events are delivered as JSON encoded strings, using WebSocket text message type.

See events section below for the description of all available events.

/ws/alis

This endpoint implements JSON flavor of asciinema live stream protocol, therefore allows pointing asciinema player directly to ht to get a real-time terminal preview. This endpoint is used by the live terminal preview page mentioned above.

Events

The events emitted to STDOUT and via /ws/events WebSocket endpoint are identical, i.e. they are JSON-encoded objects with the same fields and payloads.

Every event contains 2 top-level fields:

• type - type of event,
• data - associated data, specific to each event type.

The following event types are currently available:

init

Same as snapshot event (see below) but sent only once, as the first event after ht's start (when sent to STDOUT) and upon establishing of WebSocket connection.

In addition to the fields from snapshot event this one includes:

• pid - PID of the top-level process started by ht (e.g. PID of bash)

output

Terminal output. Sent when an application (e.g. shell) running under ht prints something to the terminal.

Event data is an object with the following fields:

• seq - a raw sequence of characters written to a terminal, potentially including control sequences (colors, cursor positioning, etc.)

resize

Terminal resize. Send when the terminal is resized with the resize command.

Event data is an object with the following fields:

• cols - current terminal width, number of columns
• rows - current terminal height, number of rows

snapshot

Terminal window snapshot. Sent when the terminal snapshot is taken with the takeSnapshot command.

Event data is an object with the following fields:

• cols - current terminal width, number of columns
• rows - current terminal height, number of rows
• text - plain text snapshot as multi-line string,