bavbavhaus.net/pandoc/links.lua

-- local l = require("pandoc.logging")
local read = pandoc.read

assert(#arg > 0, "\n" ..
  "[ERROR] usage: pandoc lua links.lua [--follow] <file>...")

local should_follow = arg[1] == "--follow" and true or false

local function file_exists (fp)
  local f = io.open(fp)
  if f ~= nil then f:close() return true else return false end
end

local visited = {}
local function find_links (fp)
  local f = assert(io.open(fp), "\n" ..
    "[ERROR] could not open " .. fp .. " for reading.")
  local data = f:read("a")
  f:close()
  read(data):walk({ Link = function (link)
    if not file_exists(link.target) then return end
    if visited[link.target] then return end
    visited[link.target] = true
    io.write(link.target .. "\n")
    if should_follow then find_links(link.target) end
  end })
end

for i = should_follow and 2 or 1, #arg do
  find_links(arg[i])
end