ProjectsWebsiteBlog

Shell Structure

Explaining the inner workings of all the kernel shells

Shells can be built by implementing two different interfaces and base classes. Why two? Because the shell handler relies on:

  • BaseShell and IShell: To hold shell type and initialization code

  • BaseShellInfo<ShellType> and IShellInfo: To hold shell commands and the base shell

Shell Handler

The shell handler, ShellManager, uses the available shell list, which holds the BaseShellInfo abstract class, to manipulate with that shell. That class can be get, depending on the needed type, with the ShellManager.GetShellInfo() function in the ︎Terminaux.Shell.Shells namespace.

The shell handler also contains two properties: CurrentShellType and LastShellType. The former property holds the current shell type, which can be used with the shell management functions. The latter property holds the last shell type, which is usually the shell that you exited. However, there are three cases:

  • If there are no shells in the shell stack, it returns the primary Shell

  • If there is only one shell in the stack, it returns the current shell as the last one

  • If there are two or more shells in the stack, it returns the last shell type

Additionally, when GetLine() is called, it sets Terminaux's reader history to point to the shell's history list. After it's done getting the input, it reverts back to the General history buffer. They are loaded on boot and saved on shutdown or reboot.

You can force a reload on the history by using the loadhistories command across all the shells.

You can manually save the history list for all the shells using the savehistories command.

To add an alternate thread in your command, you can execute AddAlternateThread(), then call GetLine(). Additionally, you can get how many shells that are currently running using the ShellCount() property.

Base Shell

The BaseShell abstract class, which your shell must override, contains the shell type name (ShellType), the flag to bail from the shell (Bail), and the shell initialization code with the shell arguments (InitializeShell()).

public class YourShell : BaseShell, IShell

The shell initialization code usually waits for the Bail value to become true (the shell requested bailing, usually done by exiting the shell using the exit universal command), as in the below example code.

public override void InitializeShell(params object[] ShellArgs)
{
    while (!Bail)
    {
        // Shell code
    }
}

While it's waiting for this to happen, the shell does what it's programmed to do, but in two conditions:

  • All shells must call the ShellManager.GetLine() function, which usually is adaptive to your shell type. This is the below example code inside the shell initialization code to illustrate this:

while (!Bail)
{
    ShellManager.GetLine();
}

  • All shells must also handle both the ThreadInterruptedException, which must set Bail to true, and the general exceptions, which must also set Bail to true after dumping the exception to the debugger or to the console. For example, the below example code, inside the InitializeShell() function:

while (!Bail)
{
    try
    {
        ShellManager.GetLine();
    }
    catch (ThreadInterruptedException)
    {
        CancellationHandlers.DismissRequest();
        Bail = true;
    }
    catch (Exception ex)
    {
        TextWriterColor.Write("There was an error in the shell. {0}", true, ThemeColorType.Error, ex.Message);
        DebugWriter.WriteDebug(DebugLevel.E, "Shell will have to exit: {0}", vars: [ex.Message]);
        DebugWriter.WriteDebugStackTrace(ex);
        InputTools.DetectKeypress();
        Bail = true;
    }
}

The shell registration is required once you're done implementing the shell and all its required values, which will show you how to implement them in the next three pages. The function responsible for this action is ShellTypeManager.RegisterShell() in the Terminaux.Shell.Shells namespace.

Be sure to unregister your shell using the UnregisterShell() function, or the shell registry function will not update your BaseShellInfo class in the available shell lists!

Never call continue on the general catch clause shown above; the shell will enter an infinite loop, depending on how the exception occurs. If you really have to do this, you can use exception filtering to determine whether continue should be called on a specific exception type.

Shell Information

Every BaseShell class you create must accompany it with a separate class that implements the BaseShellInfo and IShellInfo classes, specifying your shell class name when inheriting the generic version of the BaseShellInfo class, as in below:

internal class YourShellInfo : BaseShellInfo<YourShell>, IShellInfo

This is where your commands get together by overriding the Commands variable with the new dictionary containing all your commands, like below:

public override List<CommandInfo> Commands => new()
{
    new CommandInfo("adduser", "Adds users",
        new[] {
            new CommandArgumentInfo(new[]
            {
                new CommandArgumentPart(true, "username"),
                new CommandArgumentPart(false, "password"),
                new CommandArgumentPart(false, "confirm"),
            }, Array.Empty<SwitchInfo>())
        }, new AddUserCommand(), CommandFlags.Strict),
    (...)
};

You can omit the array definition of CommandArgumentInfo instances to create parameterless commands more easily.

In addition, you can override the ShellPresets class with a new dictionary containing all the presets for your shell, like below:

public override Dictionary<string, PromptPresetBase> ShellPresets => new()
{
    { "Default", new DefaultPreset() }
};

The ShellType variable found within the BaseShellInfo class is a wrapper for the ShellBase.ShellType variable for easier access. It's not overridable and is defined like this:

public string ShellType => ShellBase.ShellType;

By default, your shells don't accept network connections. To make them accept network connections, you must override the AcceptsNetworkConnection so that it holds the value of true instead of false. This causes the network connection selector, especially OpenConnectionForShell() which can be invoked in your networked shell launch code in your command class, to be able to acknowledge your shell.

public override bool AcceptsNetworkConnection => true;

By default, all the shells provide you a multi-line prompt, but if you want your input to be in one line wrapped mode, you can override the below property:

public override bool OneLineWrap => false;

If your shell meets the following conditions:

  • You need to handle written text in a way, and

  • You need to use your commands with a slash character,

Then, you need to override the two properties in order for your special non-slash handler to execute:

public override bool SlashCommand => true;
public override CommandInfo NonSlashCommandInfo => new CommandInfo(...);

If you need to know how to define a command information class, consult here.

You'll have to adapt your shell to take the first argument, ShellArgs[0], as the network connection instance in your Shell instance. For example, we've done this to the FTP shell and shell info instances:

FTPShell.cs
public override void InitializeShell(params object[] ShellArgs)
{
    // Parse shell arguments
    NetworkConnection ftpConnection = (NetworkConnection)ShellArgs[0];
    FtpClient clientFTP = (FtpClient)ftpConnection.ConnectionInstance;

    // Finalize current connection
    FTPShellCommon.clientConnection = ftpConnection;
FTPShellInfo.cs
internal class FTPShellInfo : BaseShellInfo<FTPShell>, IShellInfo
{
    (...)
    public override bool AcceptsNetworkConnection => true;
}

Base Command

The base command is required to be implemented, since it contains overridable command execution code. Your command must implement the command base class below:

class YourCommand : BaseCommand, ICommand

The only function that you need to override is Execute(), which you can override like below:

public override void Execute(CommandParameters parameters, ref string variableValue)

To support dumb consoles that don't support positioning or complex console functions, you can override ExecuteDumb():

public override void ExecuteDumb(CommandParameters parameters, ref string variableValue)

Additionally, you can override the extra help function, HelpHelper(), like this:

public override void HelpHelper()

If you want to support redirection or wrapping, you must either take dumb console support into account on the Execute() function by not calling any of the below console wrappers, or you must override the ExecuteDumb() function shown above to be compatible with the dumb consoles.

The following wrappers should not be called (explicitly and implicitly) on that function:

  • CursorLeft (set)

  • CursorTop (set)

  • ForegroundColor

  • BackgroundColor

  • CursorVisible

  • OutputEncoding

  • InputEncoding

  • KeyAvailable

  • Beep()

  • Clear()

  • OpenStandardError()

  • OpenStandardInput()

  • OpenStandardOutput()

  • ReadKey()

  • ResetColor()

  • SetCursorPosition()

  • SetOut()

Registering your command

In order for your command to be usable, applications are now required to register the commands manually using a function that helps doing this. That function is defined in the CommandManager class.

CommandManager.cs
public static void RegisterCustomCommand(string ShellType, CommandInfo commandBase)
public static void RegisterCustomCommands(string ShellType, CommandInfo[] commandBases)

Similarly, if your application is going to stop, or if a command is to be unregistered, you can unregister all your commands. You can use the following functions:

CommandManager.cs
public static void UnregisterCustomCommand(string ShellType, string commandName)
public static void UnregisterCustomCommands(string ShellType, string[] commandNames)

If you've registered your commands correctly, the help command list should list your commands that you've registered using one of the RegisterCustomCommand functions.

Setting command values

There is a special switch called set that allows your command to set the final variable value to any value. For example, if you run calc with the -set switch to a variable called result, that variable will be set to an output value (in this case an arithmetic result) using the variableValue argument.

To take advantage of the feature, just write the following code at the end of Execute():

variableValue = myValue;

...where myValue is a string representation of the resulting value that the command produces. A real-world example of this is provided (from the echo command code):

string result = PlaceParse.ProbePlaces(StringArgs);
TextWriterColor.Write(result);
variableValue = result;

Return codes

Your commands all feature return codes. The return code is zero by default, which means that the command has executed successfully. In case of a failure, some commands may return numbers other than zero, which indicate that there is something wrong when executing a command, possibly due to either a failed operation, a general error, or some other error.

You can change the return code of a command by writing return 1 or any other number other than zero to indicate failure, or return 0 to indicate success.

Command flags

Finally, the command flags (CommandFlags) can be defined. One or more of the command flags can be defined using the OR (|) operator when defining the command flags. These flags are available:

  • Obsolete: The command is obsolete.

    • The flag value is 1

  • RedirectionSupported: Redirection is supported, meaning that all the output to the commands can be redirected to a file.

    • The flag value is 2

  • Wrappable: This command is wrappable to pages.

    • The flag value is 4

More?

For guidance on how to define your command, click the below button:

Command Information

For guidance on how to define and manage your command's switches, click the below button:

Command Switches

For information about the help system and how it works, consult the below page:

Help System

For command parsing, click the below button:

Command Parsing

For shell scripting, click the below button:

Shell Scripting

For shell presets, click the below button:

Shell Presets

For extra shell features, click the below button:

Extra Shell Features

Last updated