command_substitution.lua

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-- PROTOTYPE SCRIPT -- Disabled by default; see below for how to enable it.
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-- Usage:
--
-- This simulates very simplistic command substitutions similar to bash.  Any
-- "$(command)" in a command line is replaced by the output from running the
-- specified command.
--
-- For example:
--
--      echo $(date /t & time /t)
--
-- First runs "date /t & time /t" and replaces the "$(...)" with the output from
-- the command.  Since the output is the current date and the current time,
-- after command substitution the command line becomes something like:
--
--      echo Sat 07/09/2022  11:08 PM
--
-- And then finally the resulting command is executed.
--
-- The following global configuration variables in Lua control how this script
-- functions:
--
--      clink_gizmos_command_substitution
--              [true|false]  Set this global variable to true to enable this
--              script.  This script is disabled by default.
--
--
-- IMPORTANT WARNING:  This is a very simple and stupid implementation, and it
-- does not (and cannot) work the same as bash.  It will not work quite as
-- expected in many cases.  But if the limitations are understood and respected,
-- then it can still be useful and powerful.
--
--
-- Here are some of the limitations:
--
--  - WHETHER a command substitution runs is different than in bash!
--  - The ORDER in which command substitutions run is different than in bash!
--  - Only a small subset of the bash syntax is supported.
--  - Nested substitutions are not supported; neither nested via typing nor
--    nested via substitution.
--  - This spawns new cmd shells to invoke commands.  This means commands cannot
--    affect the current shell's state:  changing env vars or cwd or etc do not
--    affect the current shell.
--  - Newlines and tab characters in the output are replaced with spaces before
--    substitution into the command line.
--  - CMD does not support command lines longer than a total length of about
--    8,000 characters.
--
-- Bash intelligently skips command substitutions that don't need to be
-- performed, for example in an `else` clause that is not reached.  But this
-- script stupidly ALWAYS performs ALL command substitutions no matter whether
-- CMD will actually reach processing that part of the command line.
--
-- Bash intelligently performs command substitutions in the correct order with
-- respect to other parts of the command line that precede or follow the command
-- substitutions.  But this script stupidly performs ALL command substitutions
-- BEFORE any other processing happens.  That means command substitutions can't
-- successfully refer to or use outputs from earlier parts of the command line;
-- because this script does not understand the rest of the command line and
-- doesn't evaluate things in the right order.

if not clink_gizmos_command_substitution then -- luacheck: no global
    return
end

if not clink.onfilterinput then
    print('command_substitution.lua requires a newer version of Clink; please upgrade.')
    return
end

settings.add('color.command_substitution', 'sgr 7', 'Color for command substitutions')

local function find_command_end(line, s)
    local quote
    local level = 1
    local i = s + 2
    while i <= #line do
        local c = line:sub(i, i)
        if c == '"' then
            quote = not quote
        elseif quote then -- luacheck: ignore 542
            -- Accept characters between quotes verbatim.
        elseif c == '^' then
            i = i + 1
        elseif c == '(' then
            level = level + 1
        elseif c == ')' then
            level = level - 1
            if level == 0 then
                return i
            end
        end
        i = i + 1
    end
end

local function substitution(line)
    local i = 1
    local result = ''
    local continue
    while true do
        -- Find a $(command).
        local s = line:find('%$%(', i)
        local e = s and find_command_end(line, s)
        if not s or not e then
            -- Concat the rest of the input line.
            result = result..line:sub(i)
            break
        end

        -- Concat what precedes the $(command).
        result = result..line:sub(i, s - 1)
        i = e + 1

        -- Substitution was found, so halt further onfilterinput processing.
        continue = false

        -- Spawn a new shell to invoke the $(command).
        local c = line:sub(s + 2, e - 1)
        local f = io.popen(c)
        if f then
            -- Read the command's output.
            local o = f:read('*a') or ''

            -- Trim trailing line endings.
            while true do
                local t = o:sub(#o)
                if t ~= '\r' or t ~= '\n' then
                    break
                end
                o = o:sub(1, #o - 1)
            end
            f:close()

            -- Replace problem characters with spaces.
            o = o:gsub('[\r\n\t]', ' ')

            -- Append the output to the input line.
            result = result..o
        end
    end
    return result, continue
end

clink.onfilterinput(substitution)

local cl = clink.classifier(1)

function cl:classify(commands) -- luacheck: no unused
    if not commands[1] then
        return
    end

    local color = settings.get('color.command_substitution')
    if not color or color == '' then
        return
    end

    local line_state = commands[1].line_state
    local classifications = commands[1].classifications
    local line = line_state:getline()

    local i = 1
    while true do
        -- Find a $(command).
        local s = line:find('%$%(', i)
        local e = s and find_command_end(line, s)
        if not s or not e then
            break
        end

        -- Apply color.
        classifications:applycolor(s, e + 1 - s, color, true--[[overwrite]])

        i = e + 1
    end
end