function capture(iface)
    local hashes = {}, bytes, m, xx, essid_length, essid
    while 1 do
        bytes = {}
        for line in io.popen("exec tcpdump -s0 -lnxi " .. iface .. " subtype probe-req"):lines() do
            m = string.match(line, "^%s*0x%w+:%s+(.+)")
            if m then
                --print(m)
                for xx in string.gmatch(m, "%w%w") do
                    table.insert(bytes, xx)
                end
            elseif bytes[1] ~= nil then
                table.remove(bytes, 1)
                if bytes[1] ~= nil then
                    essid_length = tonumber(table.remove(bytes, 1), 16)
                    essid = ""
                    for _, b in ipairs(bytes) do
                        essid_length = essid_length - 1
                        if essid_length < 0 then
                            break
                        end
                        essid = essid .. string.char("0x" .. b)
                    end
                end
                if hashes[essid] == nil then
                    hashes[essid] = 1
                    if string.len(essid) > 0 then
                        print(essid)
                    end
                end
                bytes = {}
            end
        end
        os.execute("sleep 2")
    end
end

iface = "wlan0"
if arg[1] ~= nil then iface = arg[1] end
capture(iface)