2019/09/12: Updated with some new links and information based on my later usage of Elvish. See https://zzamboni.org/tags/elvish for other things I have written about it.

I’m always on the lookout for new toys, particularly if they make my work more productive or enjoyable. For a couple of months now I have been using a new Unix shell called Elvish as my default login shell on macOS, and I love it. It’s a young project but very usable and with some very nice features.

Here are a few of the things I like about it:

• The Elvish language is clean and powerful, with clear syntax. It supports name spaces, exception handling and proper data structures, including lists and maps.
• It has a rich built-in library of functions, including string manipulation, regex matching, JSON parsing, etc.
• Commands and functions can output two types of streams: bytes (what we usually see as standard output/error) and data (data values, potentially containing structured data), which makes it very flexible.
• It has very nice interactive features, including as-you-type syntax checking and highlighting, asynchronous prompt themes, support for custom completions, a built-in command-line file browser, prompt hooks, configurable keybindings, and many others.
• Exceptions! In Elvish, if a command exits with a non-zero code, it generates an exception that stops the execution of the current script or function. This breaks completely with the Unix-shell tradition of silently setting return codes, and it takes a while to get used to it, but once you do it’s a very powerful idea, as it forces you to think about failures and to plan for them in your code.
• It is extensively documented. Take a look at https://elv.sh and you will see.

Elvish is very powerful, but it’s different enough from other shells that it’s worth your time to read through some of the documentation when you start. I would recommend the Some Unique Semantics page, which assumes you know other shells already. From there you can move to some of the other tutorials and reference documentation.

Of course, Elvish is not without its quirks and drawbacks. None of these has been a deal-breaker for me, but just for completeness:

• It’s very young. Occasionally I still encounter crashes, but they are few and far between, and the developer is always very responsive. Also, the language is still subject to change and there are still backwards-incompatible changes with relative frequency. If you are looking for absolute stability, it’s not yet ready. (2019/06/12 update: it has been months (probably at least a year) without encountering a single crash)
• Its language syntax is still a bit quirky. Spacing is sometimes important. For example, in the if=/=else construct, the “} else {” has to be just like that—with spaces, and in a single line, for it to be recognized by the parser. (2019/06/12 update: the reason for this is explained in Effective Elvish)
• The data/byte separation is not fully clear (at least to me) yet. Sometimes the data stream can be interpreted as bytes as well, sometimes it does not. The language is still evolving, so I am sure this will become clearer in the future. (2019/06/12 update: this is just a matter of getting used to it)
• There is not yet a large body of code/scripts you can use. This is very noticeable in completions—while Elvish supports completions, there are very few implementations. Coming from Fish, which has an impressive library of custom completions out-of-the-box, this lack is very noticeable, particularly with complex commands like git. (2019/06/12 update: there is still not as much as for Fish or other shells, but steadily increasing. See Awesome Elvish for a list)

I am very happy with Elvish, and if you are interested in this sort of thing, I encourage you to take a look. If you need a starting point, you can use my configuration files at as an example of the kind of things you can do with it.

I will post more Elvish tips and tricks over time.