Program.cs

internal class Program
{
    /// <param name="args">The files to read</param>
    /// <param name="number">Print Numbers?</param>
    /// <param name="showEnds">Put an "$" at the End?</param>
    private static void Main(string[] args, bool number = false, bool showEnds = false)
    {
        if (args.Length == 0) PrintFromSTDIN(number, showEnds);

        string text = "";

        foreach (string file in args)
        {
            try
            {
                text += File.ReadAllText(file);
            }
            catch (System.IO.FileNotFoundException)
            {
                Console.WriteLine($"{file} file not found!");
                return;
            }
        }

        Console.Write(ParseString(text, number, showEnds));
    }

    public static string ParseString(string text, bool number, bool showEnds)
    {
        if (number) text = AddNumbers(text);
        if (showEnds) text = AddEnds(text);
        return text;
    }


    public static void PrintFromSTDIN(bool number, bool showEnds)
    {
        while (true) Console.WriteLine(Console.ReadLine());
    }


    public static string AddEnds(string text)
    {
        var splitText = text.Split(new[] { Environment.NewLine }, StringSplitOptions.None);

        text = "";

        for (var i = 0; i < splitText.Length - 1; i++) text += $"{splitText[i]}$ \n";

        return text;
    }

    public static string AddNumbers(string text)
    {
        var splitText = text.Split(new[] { Environment.NewLine }, StringSplitOptions.None);

        text = "";

        for (var i = 0; i < splitText.Length - 1; i++) text += $"  {i}   {splitText[i]}\n";

        return text;
    }
}