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.

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 (1.74 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.

API

Communication with ht is performed via stdin, stdout and stderr.

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).

ht sends responses (where applicable) to its stdout, as JSON-encoded objects.

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.

This command doesn't produce any output on stdout.

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

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.

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 "\x03" as the payload:

{ "type": "input", "payload": "\x03" }

This command doesn't produce any output on stdout.

getView

getView command allows obtaining a textual view of a terminal window.

{ "type": "getView" }

This command responds with the current view on stdout. The view is a multi-line string, where each line represents a terminal row.

{ "view": "[user@host dir]$                 \n                       \n..." }

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 doesn't produce any output on stdout.

Testing on command line

ht is aimed at programmatic use given its JSON-based API, however one can play with it by just launching it in a normal desktop terminal emulator and typing in JSON-encoded commands from keyboard and observing the output on stdout.

rlwrap can be used to wrap stdin in a readline based editable prompt, which also provides history (up/down arrows).

To use rlwrap with ht:

rlwrap ht [ht-args...]

Python and Typescript libs

Here are some experimental versions of a simple Python and Typescript libraries that wrap ht: htlib.py and a htlib.ts.

TODO: either pull those into this repo or fork them into their own htlib repo.

Possible future work

• support higher-level "keyboard like" input in the input command, e.g. parse any string in the form of "<ctrl+d>" and automatically turn it into 0x04 before sending it to the process.
• update the interface to return the view with additional color and style information (text color, background, bold/italic/etc) also in a simple JSON format (so no dealing with color-related escape sequence either), and the frontend could render this using HTML (e.g. with styled pre/span tags, similar to how asciinema-player does it) or with SVG.
• support subscribing to view updates, to avoid needing to poll (see issue #9)
• native integration with asciinema for recording terminal sessions (see issue #8)

Alternatives and related projects

expect is an old related tool that let's you spawn an arbitrary binary and then send input to it and specify what output you expect it to generate next.

Also, note that if there exists an explicit API to achieve your given task (e.g. a library that comes with the tool you're targeting), it will probably be less bug prone/finicky to use the API directly rather than working witht your tool through ht.

See also this hackernews discussion where a bunch of other tools were discussed!

Design doc

Here is the original design doc we used to drive the project development.

License

All code is licensed under the Apache License, Version 2.0. See LICENSE file for details.