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Latest commit 3cda957 Jun 27, 2018
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app Rename Shell to Action Jun 26, 2018
src Rename Shell to Action Jun 26, 2018
test Add Located type Jun 24, 2018
.gitignore Proper project scaffold Jun 19, 2018 Proper project scaffold Jun 19, 2018
bad.hell Setup exe Jun 23, 2018
demo.hell Setup exe Jun 23, 2018
hell.cabal Start of command interpreting Jun 24, 2018
stack.yaml Implement combinators Jun 24, 2018



I've been working on a more practical version based somewhat on my work with the jl tool which is simply typed lambda calculus for querying JSON data structures. I believe the same mini-language can work well for a shell.

I think that the language can be laid out as a simple command language that looks very much like regular sh, but embedded within it the ability to write pure functional code (similar to jl or haskell).

$ let xs = [1,3,5]; ls | grep -o ^[0-9]+ | ${filter (elem xs) | take 5} > nums.txt

Script files may look like this. ${ } splices in the pure language, whereas do { .. } or simply do x; y (a la Haskell) quotes shell commands.

main = do

files = [ "types.sql", "table_schema.sql", "functions.sql" ]

out = "../dumps/out.sql"

n = "demo"

reset = do
  dropdb $n
  createdb $n -O $n
  echo "" > $out
  for $files \f -> do cat ${"../schema/pg/" <> f} >> $out

build = do
 stack build
 rm -f
 time stack exec -- tsql ../schema/sqlserver/schema.sql > $out

extensions =
 [ 'create extension if not exists "uuid-ossp"'
 , 'create extension if not exists "citext"' ]

import = do
  unlines extensions | pg_import
  $out < pg_import > /dev/null

pg_import = do time psql -d pg --quiet -X

test = do stack test

I'm using this document as a brainstorming area and to write up observations.

Shell is a templating language

The good thing about Sh and its descendents is that by default it is "quoted". I.e. it produces lists of strings:

ls -al foo

This is the list of strings:


The special syntactical character is (space). Other special characters are listed below. Aside from these special characters, all other text is quoted. This makes shells a templating language.

I believe it's neccessary to preserve this property of shells.

Shell is a free monad

This idea of a shell is fairly well described as a rough ADT:

data Shell
  = Command [String] -- ls -al foo
  | Pipe [Shell] -- |
  | Sequence [Shell] -- ;
  | Redirect Shell FilePath -- ls > foo.txt
  | Background Shell -- ls &
  | Substitution Shell (String -> Shell) -- $(...) or `...`

Side note Actually, there are some questions here. Pipe can't really pipe ls > x.txt with cat because the output has been redirected to x.txt. Should we disallow that in the ADT? Or perhaps all Shell can be piped and if it's a redirected then the output is simply empty, because stdout is closed. The same applies to background ls& which doesn't output to stdout but rather a new pipe output.

An example type might be:

data In
data Out
data None

data Shell i o a where
  Command :: [String] -> Shell In Out a
  Pipe :: Shell i Out a -> Shell In o a -> Shell i o a
  Sequence :: Shell i _o a -> Shell _i o a -> Shell i o a
  Redirect :: Shell i Out a -> FilePath -> Shell i None a
  Background :: Shell i _o a -> Shell i None a
  Substitution :: Shell None Out a -> (String -> Shell i o a) -> Shell i o a

Substitution is where the shell gets its join operator, or >>=, in which it can make decisions. Before that, it's more of an arrow.

nc $(docker-machine ip server)

is similar to

Command ["docker-machine","ip","server"]
  >>= \sub1 ->
    Command ["nc", sub1]

The syntax in sh ls x.* is a runtime expansion depending on the contents of the directory. It might expand to ls x.txt etc. This logic can be handled by an additional MatchSubstitution construtor:

MatchSubstitution [Pattern] ([String] -> Shell)

data Pattern = Plain String | Wild | AnyChar

Special commands like cd, pwd, time would be additional constructors.

Syntactical analysis of sh/bash

Special reserved words in bash: ! case do done elif else esac fi for function if in select then until while { } time [[ ]]

Special commands in bash: cd, pwd, eval, time

Not mentioned in lists is: = which defines variables

Special sh characters:

  • Lexical helpers: # comment, " quote, ' quote, \ char quote
  • Variables: $ variable, ${...} variable, $<foo> various globals, % job number, , ~ home directory (and ~foo)
  • Process control: | pipe, & background job, ` command substitute, ; command separator, $(..) command substitute, ( .. ; ..) subshell, { .. ; ..} sequence, >, <, <<, >> redirect IO
  • Matching: * match 0+ characters, ? match character
  • Arithmetic: ((...))
  • Misc: !

Short version: