An experimental Unix shell

This is a work in progress. Things may change and/or break without notice. You have been warned...

Obligatory screenshots

All software websites should have screenshots. -- Someone on the Internet

Syntax highlighting (also showcasing right-hand-side prompt):

Tab completion:

The Editor

Those marked with ✔ are implemented (but could be broken from time to time).

Like fish:

• Syntax highlighting ✔
• Auto-suggestion

Like zsh:

• Right-hand-side prompt ✔
• Dropdown menu completion ✔
• Programmable line editor

And:

• A vi keybinding that makes sense
• More intuitive multiline editing
• Some method to save typed snippets into a script
• A "navigation mode" integrating functionalities of ranger

The Language

(Like the previous section, only those marked with ✔ have been implemented.)

• Running external programs and pipelines, of course (> represents the prompt): ✔

  > vim README.md
  ...
  > cat -v /dev/random
  ...
  > dmesg | grep bar
  ...
  

• Some constructs look like lisp without the outermost pair of parentheses: ✔

  > + 1 2
  3
  > * (+ 1 2) 3
  9
  

• Use backquote for literal string (so that you can write both single and double quotes inside), double backquotes for a literal backquote: ✔

  > echo `"He's dead, Jim."`
  "He's dead, Jim."
  > echo ```He's dead, Jim."`
  ``He's dead, Jim."
  

• Barewords are string literals:

  > = a `a`
  true
  

• Tables are a hybrid of array and hash (a la Lua); tables are first-class values: ✔

  > println [a b c &key value]
  [a b c &key value]
  > println [a b c &key value][0]
  a
  > println [a b c &key value][key]
  value
  

• Declare variable with var, set value with set; var also serve as a shorthand of var-set combo: ✔

  > var v
  > set v = [foo bar]
  > var u = [foo bar] # equivalent
  

• First-class closures, lisp-like functional programming:

  > map {|x| * 2 $x} [1 2 3]
  [2 4 6]
  > filter {|x| > $x 2} [1 2 3 4 5]
  [3 4 5]
  > map {|x| * 2 $x} (filter {|x| > $x 2} [1 2 3 4 5])
  

• Get rid of lots of irritating superfluous parentheses with pipelines (put is the builtin for outputting compound data):

  > put [1 2 3 4 5] | filter {|x| > $x 2} | map {|x| * 2 $x}
  [6 8 10]
  

• Use the table $env for environmental variables:

  > put $env[HOME]
  /home/xiaq
  > set env[PATH] = $env[PATH]:/bin
  

There are many parts of the language that is not yet decided. See TODO.md for a list of things I'm currently thinking about.

Motivation

This experiment has a number of motivations. Some of them:

• It attempts to prove that a shell language can be a handy interface to the operating system and a decent programming language at the same time; Many existing shells recognize the former but blatantly ignore the latter.

• It attempts to build a better interface to the operating system, trying to strike the right balance between the tool philosophy of Unix and the tremendous usefulness of a more integrated system.

• It also attempts to build a better language, learning from the success and failure of programming language designs.

• It attempts to exploit a facility Shell programmers are very familiar with, but virtually unknown to other programmers - the pipeline. That leads us to the topic of the next few sections.

Pipeline, the Good

A Concatenative Programming Facility

Pipelines make for a natural notation of concatenative programming.

So what's concatenative programming? In some of its most common use cases, we can say it's just functional programming without lots of irritating superfluous parentheses. Consider this fictional piece of lisp to find in strs, a list of strings, all members containing "LOL", transform them into upper case, sort them, and store them in another list lols:

(def lols (sort (map upper-case
                     (filter (lambda (x) (contains? x "LOL")) strs))))

(See Appendix A for this piece of code in real lisps.)

It looks OK until you try to read the code aloud:

Put in lols what results from sorting what results from turning into upper case what results from filtering the strings that contain "LOL" in strs.

An deep hierarchy of parentheses map into a deep hierarchy of clauses. Worse, this reads backwards.

What would you do it in shell, with pipelines? Assuming that the strings are stored in the file strs, it is just:

lols=`cat strs | grep LOL | tr a-z A-Z | sort`

The historically weird names aside, it reads perfectly natural: assign to lols the result of the following: take the lines in strs, find those having "LOL", change them to upper case, and sort them. This matches our description of the procedure except for the assignment. There is an obvious restriction with this shell pipeline approach, but that will be the topic of the next section.

Concatenative programming is the notion of building programs by connecting data-transforming constructs together. In our case, the constructs are cat strs, grep LOL, tr a-z A-Z and sort; the pipe symbol is the connector. The interesting thing is that each construct itself is actually a valid program; thus it could be said that a more complex program is formed by concatenating simpler programs, hence the term "concatenative programming". Compare this to the functional approach, where constructs are nested instead of connected one after another.

A Concurrency Construct

(TO BE WRITTEN)

Pipeline, the Bad and the Ugly

(TO BE WRITTEN)

Building

You need go >= 1.1 to build this. Just run make. The resulting binary is called das.

Name

Indeed, das is not a very good name for a Unix shell. The name is actually a corrupted form of dash, which also happens to be the German definite neuter article.

I have some other ideas in mind. One of them is elv, since I found "elvish" to be a great adjective - I can't use "elf" though, since it's already taken and may be confused with the well known file format.

Another possible source of names is the names of actual seashells; but my English vocabulary is too small for me to recall any beyond "nautilus", which is both too long and already taken.

I'm not avoiding names ending in "sh" though; but I do find "bash" to be a terrible name. "fish" is clever, but it has a quite unpleasant adjective. I find "dash" really good though, which is why it came to my mind :).

License

BSD 2-clause license. See LICENSE for a copy.

Appendix A

This fictional lisp code:

(def lols (sort (map upper-case
                     (filter (lambda (x) (contains? x "LOL")) strs))))

written in Clojure:

(require ['clojure.string :refer '(upper-case)])
(def strs '("aha" "LOLaha" "hahaLOL" "hum?"))
(def lols (sort (map upper-case
                     (filter #(re-find #"LOL" %) strs))))

written in Racket:

(define strs '("aha" "LOLaha" "hahaLOL" "hum?"))
(define lols (sort (map string-upcase
                        (filter (lambda (x) (regexp-match? #rx"LOL" x)) strs))
                   string<?))

I'm by no means a Lisp hacker, so feel free to fire an issue if my code is not idiomatic.