Simple Mail Sender

tArgs = {...} -- Capture arguments that are passed when this program is run.
sMailIndicator = "MAIL_MESSAGE" -- The indicator in rednet messages that identifies them as mail messages.

-- Returns a file hanlde if the desired path is valid.
function getFileHandle(sPath, cMode)
    if not fs.isDir(sPath) then
        local fileHandle = fs.open(sPath, cMode)
        if fileHandle then
            return fileHandle
        else
            return nil
        end
    else
        return nil
    end
end

-- Takes a file handle in read mode, then loops through it using fs.readLine(),
-- finally returning the length of the file in number of lines.
function getFieLength(sPath)
    -- Make sure that the given file path is valid.
    if not fs.isDir(sPath) and fs.exists(sPath) then
        -- Get a handle on the file to read from.
        local fileHandle = io.open(sPath, 'r')
        local nLines = 0
        -- Validate the file handle.
        if fileHandle then
            -- Loop through every line of the file and return the number of lines in the file.
            for nLine in fileHandle:lines() do
                nLines = nLines + 1
            end
            fileHandle:close() -- Close the file handle.

            -- Return the number of lines in the file.
            return nLines
        else
            return 0
        end
    else
        return 0
    end
end

-- Reads a file and sends it as an email to the requested id.
function sendMail(nReceiver, sPath)
    -- Check if the parameters given are valid.
    if nReceiver and sPath then
        -- If the file handle is valid, then go ahead and get the length of the file to send.
        local nFileLength_InLines = getFieLength(sPath)
        -- Get a handle on the file to read and send.
        local mailFileHandle = getFileHandle(sPath, 'r')
        local sMessage = sMailIndicator -- Make sure to start the message with the mail indicator so it is recognized as mail by the receiver.
        -- Validate the file handle.
        if mailFileHandle then
            for nLine = 1, nFileLength_InLines do
                -- Add the line to the current message.
                sMessage = sMessage .. mailFileHandle.readLine() .. '\n' -- Not sure if the \n character already exists on the line.
            end

            -- Send the message over rednet to the desired computer.
            if type(nReceiver) == "number" or tonumber(nReceiver) then
                rednet.send(tonumber(nReceiver), sMessage)
                return true
            else
                return false
            end
        else
            return false
        end
    else
        return false
    end
end

-- Prints the proper syntax to be used when calling this program from the shell.
function printUsage()
    print(shell.getRunningProgram() .. " <receiver_ID> <message_file_name>")
end

-- Opens any modem that exists, throws an error if a modem isn't found.
function openExistantModem()
    local tSides = rs.getSides()

    for nSideIndex, sSide in ipairs(tSides) do
        if peripheral.isPresent(sSide) and peripheral.getType(sSide) == "modem" then
            rednet.open(sSide)
            return true
        end
    end

    -- If the loop executed and did not return, then there is not modem on the computer.
    -- Throw an error.
    error("No modem found!")
end

-- Make sure that the arguments are plenty and valid for this program to work.
function checkArguments()
    -- The program takes two arguments; check if there are exactly that many.
    if #tArgs >= 2 then
        -- Check if the id passed is either already a number, or can be cast to a number.
        if type(tArgs[1]) == "number" or tonumber(tArgs[1]) then
            local nReceiver = tonumber(tArgs[1]) -- Establish a variable for the receiver ID passed as an argument to the program.
            local sPath = tArgs[2] -- Establish a variable for the path of the message file to be read then sent.
            -- Check if the passed file path is valid.
            if fs.exists(sPath) and not fs.isDir(sPath) then
                -- Now that all of the argument checks have been passed, return a success so the program may proceed.
                return true
            -- If the passed message file path is not valid, print the proper syntax of the program and return a failure.
            else
                printUsage()
                return false
            end
        -- If the passed ID is not valid, print the proper syntax of the program and return a failure.
        else
            printUsage()
            return false
        end
    -- If the arguments are not enough or are invalid, print the proper usage of the program and return a failure.
    else
        printUsage()
        return false
    end
end

-- Make sure all of the arguments are valid for the program to execute properly.
if checkArguments() then
    -- Open any modem that exists on the computer.
    openExistantModem()
    -- Attempt to send the mail requested from the passed file.
    -- tArgs[1] is the receiver id, and tArgs[2] is the file path for the message that will need to be read, then sent.
    if sendMail(tonumber(tArgs[1]), tArgs[2]) then
        print("Mail sent!")
    else
        print("Send failure.")
    end
-- If the argument checks are not valid, then DO NOT attempt to run the program.
else
    return -- Escape the program. (Programs are run as functions underneath the shell, so we can return from them using a retunr call
        -- in the global scope.)
end

-- SUPERFLOUS METHOD: MAIL IS READ FROM A PRE-EXISTING FILE; DOES NOT NEED TO HAVE ITS OWN METHOD TO COMPOSE MAIL.
--[[
-- Runs the edit program on a temporary file and then sends the contents of the message to the desired computer.
function composeMail(nReceiver)
    if type(nReceiver) == "number" or tonumber(nReceiver) then
        -- Make sure that the temporary file doesn't exist. If it does, then delete it.
        if fs.exsists(".tmpFile") then
            fs.delete(".tmpFile")
        end
        -- Run the 'edit' program for the user to enter their message.
        shell.run("edit", ".tmpFile")
        -- Send the message.
        if sendMail(nReceiver, ".tmpFile") then
            print("Message sent to " .. nReceiver .. '!')
        else
            print("Message send failure.")
        end

        return
    else
        error("Receipient ID must be a positive intenger.")
    end
end
--]]