Simple Background Mail

local localShellThread = nil
local mailDirectory = "/Mail"
local screenWidth, screenHeight = term.getSize()


function openModem()
        for index, side in pairs(rs.getSides()) do
                if peripheral.getType(side) == "modem" then
                        rednet.open(side)
                        return true
                end
        end

        return false
end

function createNewShell()
        return coroutine.create(function() shell.run("rom/programs/shell") end)
end

function getNumberOfTimesAppearedInMailDirectory(fileName)
        local timesAppeared = 0
        for index, file in ipairs(fs.list(mailDirectory)) do
                if file:find(fileName) then
                        timesAppeared = timesAppeared + 1
                end
        end

        return timesAppeared
end

function handleMail(mail)
        local fileName = "From_" .. mail.sender .. '_'
        fileName = fileName .. getNumberOfTimesAppearedInMailDirectory(fileName)

        local file = fs.open(mailDirectory .. '/' .. fileName, 'w')
        file.write("From " .. mail.sender .. "\n\n" .. mail.message)
        file.close()
end

function listenForAndHandleMail()
        localShellThread = createNewShell()
        coroutine.resume(localShellThread)

        while true do
                local event = {os.pullEvent()}

                if event[1] == "rednet_message" then
                        local mail = textutils.unserialize(event[3])
                        if mail.sender and mail.message then
                                handleMail(mail)
                                local xPos, yPos = term.getCursorPos()

                                term.setCursorPos(screenWidth - ("Mail from " .. mail.sender):len(), 1)
                                term.write("Mail from " .. mail.sender)

                                term.setCursorPos(xPos, yPos)
                                event = {}
                        end
                end

                coroutine.resume(localShellThread, unpack(event))
        end
end

if not openModem() then
        print("Modem not found.")
        return
end

if not fs.isDir(mailDirectory) then
        fs.makeDir(mailDirectory)
end

listenForAndHandleMail()