Google Music sort script (Linux)

#!/usr/bin/env lua
-- Copy your MP3s from src folder to dest folder, where they will be
-- sorted into subdirectory structure: Dest_dir/genre/album/song.mp3
require("lfs") --sudo apt-get install liblua5.1-filesystem0
math.randomseed(123)

local function sanitize(str)
    return (str:gsub("[^a-zA-Z0-9. ]","_"))
end

local function get_tag(tagname, str)
    local val = str:match(" %("..tagname.."%)%: (.-)>")
    if val=="" then
        val = false
    end
    return val
end

local function scan_dir(directory, albums)
    print("Entering directory: "..directory)
    for fname in lfs.dir(directory) do
        if fname ~= "." and fname ~= ".." then
            local attrs = lfs.attributes(directory..fname)
            if attrs.mode == "directory" then
                scan_dir(directory..fname.."/", albums)
            else
                --print (fullname)
                assert(fname:match("mp3$"))
                --print(fullname)
                local escaped = fname:gsub("%$","\\$")
                escaped = escaped:gsub("`","\\`")
--              print (escaped)
                local fd = assert(io.popen("eyeD3 -v \""..directory..escaped.."\""))
                local txt = fd:read("*a")
                fd:close()
                assert(txt:match("ID3"))
                local tags = {}
                for i,tagname in ipairs{"TPE1", "TPE2", "TALB", "TCON"} do
                    tags[tagname] = get_tag(tagname, txt)
                end
                --print(tags.TPE2)
                local album = tags.TALB or "_no_album_"..math.random(999999)
                local genre = tags.TCON or "_no_genre"
--              local dirname = sanitize(genre).."/"..sanitize(album)
                album = genre.."/"..album
                if not albums[album] then
                    albums[album] = {}
                end
                table.insert(albums[album],{dir=directory,fname=escaped, genre=genre})
            end
        end
    end
end

local function main()
    local dir_src = assert(arg[1])
    local dir_dest = assert(arg[2])
    assert(dir_src ~= dir_dest)
    assert(dir_src:match("/$"))
    assert(dir_dest:match("/$"))
    local albums = {}
    scan_dir(dir_src, albums)
    for album, songs in pairs(albums) do
        table.sort(songs,function(a,b) return (a.fname < b.fname) end)
        --print(#songs, album)
        local destdir = album
        assert(#destdir > 0)
--      destdir = dir_dest..destdir:sub(1,1).."/"..destdir.."/"
        destdir = dir_dest..destdir.."/"
        print("===Copying "..#songs.." songs in album: "..destdir)
        local stat, err = os.execute('mkdir -p "'..destdir..'"')
        if not stat then
            print(err)
        else
            for i, song in ipairs(songs) do
                local destfname = destdir..sanitize(song.fname)
                if lfs.attributes(destfname) then
                    print("File exists, skipping: "..destfname)
                else
                    local command = 'cp "'..song.dir..song.fname..'" "'..destfname..'"'
                    print(command)
                    os.execute(command)
                end
            end
        end
    end
end

main()

--[[
-------------------------------------------------------------------------------
ID3 Frames:
<Title/songname/content description (TIT2): The Terrace>
<Lead performer(s)/Soloist(s) (TPE1): John Williams>
!!! Album artist = TPE2
<Composer (TCOM): John Williams>
<Album/Movie/Show title (TALB): Superman>
<Year (TYER): >
<Track number/Position in set (TRCK): 3/0>
<Part of a set (TPOS): 2/0>
<Content type (TCON): Soundtrack>
<Attached picture Frame (APIC)>
]]