Fun with C# and a command line tool for calculating the UNIX file permission number from the text listing

ChmodNum.cs

using System.Text.RegularExpressions;

if (args.Length == 0 || !args[0].IsUnixFileModeAndPermissionsString())
{
    Console.WriteLine(
        "Argument must be in the form of a UNIX file permission string, " +
        "i.e. -rwxrwxrwx or -rwxr-xr-x etc.");
    return;
}
Console.WriteLine(args[0].ToClasses().ToPermissionNumber());



static class ChmodNumEx
{
    public static IEnumerable<string> ToClasses(this string self) =>
        new List<string>
        {
            self[1..4],  // group
            self[4..7],  // other
            self[^3..]   // user
        };

    public static int ToPermissionNumber(this IEnumerable<string> self) =>
        int.Parse(string.Join("",
            self.Select(@class => @class.ToPermissionNumber()).ToList()));

    public static bool IsUnixFileModeAndPermissionsString(this string self) =>
        new Regex(@"^(-|d|l|p|s|b|c|D)((-|r)(-|w)(-|x)){3}$").IsMatch(self);

    private static bool IsPermission(this char self) => self != '-';

    private static int ToInt(this bool self) => self ? 1 : 0;

    private static int ToPermissionNumber(this string self)
    {
        var bits = new int[3];
        for (var b = 2; b >= 0; b--)
        {
            bits[b] = self[b].IsPermission().ToInt();
        }
        return Convert.ToInt16(string.Join("", bits), 2);
    }
}

chmodnum -rwxr--r--