Shell (ala bash) written in Javascript for the browser and Node.js (incomplete prototype!)
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.. toc::

I was thinking about shells, like good ol' bash, and while they fit a certain purpose they aren't necessarily the best way to interact with files and command-line programs. They aren't very modern, and they are very heavily burdened with legacy (heck, there's a Posix standard).

I'd already been thinking about a terminal in the browser (part of a general thought I've had: why do I use any app except a browser?) -- but a terminal emulator always seemed kind of wonky and unexciting. What if the shell itself was in the client?

The browser can't actually run programs (well, without an extension), so I have a small Node.js program ( that serves that purpose, as well as serving up the static files needed by the client. But the server is not itself a shell -- it's more like a JSON/JSONP wrapper for execvpe. It has no context or state, it just runs things. All the context is held in the client.

Moving state to the client itself also interests me. In fact very little is left over when you run a program. Its environmental variables are created then thrown away. It has a PID, but that too is meaningless when the program completes. It might write files. It might output text, but the text doesn't live on the console, it's just text (ignoring for now all interactive terminal programs). So you don't need something like screen to restart a session -- you can store the whole thing in localStorage to the same effect! (Well, I'm not doing that yet, but the point of all of this is that could, not that I am.)

The Name

I should probably change it, JSShell is (over) used already, generally for things entirely unrelated to this project. Clever ideas welcome (though I also kind of like boring/clear ideas too).

The Shell Itself

The shell is currently simple, but does wildcard expansion, ~ expansion, variable expansion, and $() interpolation. While I want to keep the basic flavor of bash/sh, I don't feel a need to replicate its quirks and pitfalls... but still I don't want to mess up everyone's finger memory (I have enough of it myself).

The shell doesn't evaluate simply to a list of strings, instead it creates an AST (parser.js:Node). Various interpolations are done through substitutions in the AST. Only at the last moment when running a command is it turned into a list of strings (which is the only way to run commands -- the one fixed OS-level API we must conform to). I hope to use this to avoid problems with embedded spaces in filenames (i.e., treat filenames as unsplittable strings), and maybe it'll have other handy side-effects? Also it means you can work with real data structures like arrays, and they won't get implicitly turned into strings at any moment. I haven't made much use of this (sometimes it's just wrong right now), but I am hoping it will be useful.

Also I plan on allowing Javascript-side filtering of both input and output, so that things like ls can be prettied up (e.g., making filenames links). And just generally allowing for nice UI around the shell experience. Maybe the idea could be thought of as progressive enhancement.

Pipes, Background Jobs, etc

I am honestly not sure how to handle these. At this exact moment everything is a background job. Pipes aren't supported at all. Probably pipes should be executed on the server, to avoid round-tripping to the client.

The whole thing isn't (yet) a programming language. But with an emphasis on UI (not shell scripting) the target isn't really feature parity with existing shell languages.


This is actually all written in CoffeeScript, not Javascript. There's a script that both starts Node and starts coffeescript so it will recompile everything as it is edited; this is nice for development, except for figuring out line offsets in errors it makes it almost seem like you are editing the native browser language.

I've checked in the .js files so you can still run this without CoffeeScript installed. But since it's more prototype than functional product, you should really just install CoffeeScript (once you have Node installed, which you have to do anyway, it's easy to install CoffeeScript).

If you are curious, I've been quite enjoying CoffeeScript. I'm trying to avoid some of the more clever syntax, like leaving out () on function calls, and sometimes newlines can be... quirky. But being able to use halfway-decent loops more than makes up for that, and then all the other nice CoffeeScript features are just icing. I'm even feeling okay about OO programming.

Interactive/Terminal programs

Some programs interact with the screen more fully than to just dump text. Those programs aren't supported at all. I want them all to be replicated in the browser anyway (e.g., through an in-browser editor).

Basic ANSI escape sequences for color output should be possible and desirable (though it requires interpreting that stuff in Javascript -- messy but doable). What exact cleverness is necessary to fake a program into thinking the output supports color is also a question.


I dunno. Maybe find me on #labs on I'm just messing around with this right now, I don't know where it'll go.