Wow they actually did it! Congratulations and welcome to the age of second-gen systems languages!
this post was submitted on 16 Jan 2024
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Seems one of the main reasons is to use Rust's thread safety to enable "concurrent mode". Anyone with the knowledge able to explain what advantages that would yield for an end fish user?
Here's one issue they hope to solve with this rewrite: https://github.com/fish-shell/fish-shell/issues/238
One big, long-standing issue is that fish can't run builtins, blocks or functions in the background or at the same time.
That means a pipeline like
seq 1 5 | while read -l line
echo line; sleep 0.1;
end | while read -l line
echo line; sleep 0.1
end
will have to wait for the first while loop to complete, which takes 0.5s, and then run the second.
So it takes 0.5s until you get the first output and a full second until you get all of it.
Making this concurrent means you get the first line immediately and all of it in 0.5s.
While this is an egregious example, it makes all builtin | builtin pipelines slower.
Other shells solve this via subshells - they fork off a process for the middle part of the pipeline at least. That has some downsides in that it's annoyingly leaky - you can't set variables or create a background job in those sections and then wait for them outside, because it's a new process and so the outer shell never sees them.
Now we have nu and fish. Great!
Interesting that you mention it. I mainly use fish but always do some stuff in nu to check out its progress. They are in my opinion the two most interesting interactive shells at the moment that I know of, the third shell I keep an eye on is oil but rather as a replacement for bash when used in scripts rather than interactive. The project also has ysh which also doesn't look too bad and seems to go in a similar direction as fish.