Why not tmux?

cy shares some basic similarities with tmux. For example, it runs as a daemon, so its state is preserved across sessions. But for cy to be a compelling alternative, it has to do more than just mimic tmux's functionality and be written in a fashionable systems programming language.

cy improves on tmux in three main ways:

1. Session playback: cy records your terminal sessions and lets you play back and search through them.
2. Interface: cy has a simple layout designed for use on large screens.
3. Configuration: cy uses a real programming language, Janet, for configuration.

Session playback

If you use tmux, you might be familiar with copy-mode, which allows you to view lines from the scrollback buffer. This is nice when you're using a program like bash, where you issue commands that produce static output.

But by definition, copy-mode ceases to be useful when you use a program with any interactivity, such as vim: you only see the last lines it left on the screen, and certainly cannot see what the screen used to look like.

cy solves this problem by recording all of your terminal sessions. In replay mode you can seek, play back, and search through the history of a pane--regardless of the application that was running in it. cy saves these recordings to disk and allows you to open them later at your leisure.

Without trying it for yourself, it's hard to appreciate just how useful it is to be able to go back in time to replay everything you've ever seen or done in the terminal. cy aims to augment your memory in a way that other programs cannot.

Interface

cy does not (yet) have windows and panes like tmux does. It displays one terminal session at a time, and by default that session is centered on your screen with a fixed number of columns. This lets you concentrate on one "pane" at a time.

To some, this may seem like a surprising decision. The predominant abstraction for terminal multiplexers for at least a couple of decades has been the familiar pattern of vertical and horizontal splits.

Yet I found that I have spent more time fighting with this (or writing plugins to remove it) than benefiting from it. Rarely do I feel like I need to see more than one pane at a time.

This is practical because cy makes it easy to switch between panes. It emphasizes using fuzzy finding (with previews!) so you can quickly find what you're looking for. It also contains a minimal, filesystem-like abstraction for grouping panes together.

As of writing, cy also lacks tmux's status line. Because there is no notion of windows, there is nothing akin to the tab-like behavior that tmux encourages (and thus little immediate need to display it.)

Neither of these are principled omissions. My intent with cy is to, sooner or later, address these use cases in a flexible way.

Configuration

Anyone who has tried to do anything sophisticated with tmux runs into a familiar set of problems:

1. tmux's configuration language is hacky and primitive, which makes it hard to do anything interesting without running an external command.
2. Its key binding system is limited.

cy allows you to bind arbitrary sequences of keys to Janet functions. It even supports binding regexes, matches for which will be passed to the function you bound.

You can also create bindings that apply only in a specific pane or group of panes.