SeaShell.Host 0.1.5

{~} SeaShell

A C# and VB.NET scripting engine with a persistent daemon, NuGet support, hot-swap, REPL, and an embeddable host library. Built on Roslyn.

Install

dotnet tool install -g SeaShell

Quick Start

sea                             Interactive REPL
sea script.cs                   Run a C# script
sea script.vb                   Run a VB.NET script
sea -i Humanizer.Core           REPL with NuGet packages preloaded
sea --associate .cs             Associate .cs files with SeaShell

NuGet packages are resolved automatically — including transitive dependencies and runtime-specific DLLs. Missing packages are downloaded on first use.

Acknowledgements

SeaShell is inspired by and grateful to CS-Script by Oleg Shilo. CS-Script pioneered C# scripting and has been a daily workhorse for years. SeaShell builds on the ideas CS-Script established — directive-based includes, NuGet references, and the principle that C# should be as easy to run as a shell script — while taking a different architectural approach with the persistent daemon and .deps.json generation.

Features

• Persistent daemon — Roslyn stays warm in memory. Second run is instant.
• NuGet just works — Transitive dependency resolution via .nuspec walking. Auto-download. Runtime-specific and native DLLs handled via .deps.json generation.
• Hot-swap — Edit a //sea_watch script while it runs. SeaShell recompiles and swaps the process, with state handoff between instances.
• REPL — sea with no arguments drops into an interactive session. Variables and state carry across evaluations.
• Elevation — //sea_elevate scripts run elevated via a pre-registered Task Scheduler worker. No UAC prompts. Falls back to gsudo sea script.cs if the elevator isn't running. Ignored on Linux.
• Embeddable — SeaShell.Host lets you compile and run scripts from your own application. No daemon required.
• VB.NET support — .vb files compile through the same pipeline with VB-native directives ('sea_nuget).
• CS-Script compatible — //css_inc, //css_nuget directives are recognized. Existing .cscs scripts work with zero changes.
• Cross-platform — Named pipes on Windows, Unix domain sockets on Linux. Shebangs supported (#!/usr/bin/env sea).
• Mixed declarations — Classes, records, and top-level statements can appear in any order. No more "move your class to the bottom."
• Background NuGet updates — The daemon checks all cached packages every 8 hours, across all configured sources including private feeds from nuget.config.

Directives

VB.NET uses the 'sea_ prefix: 'sea_nuget Serilog

CS-Script compatibility: //css_inc, //css_nuget, //css_ref, //css_webapp are all recognized.

Include Search Order

1. Script's own directory
2. //sea_incdir paths (in order declared, across all sources)
3. SEASHELL_INC environment variable
4. Platform include directory (%ProgramData%\seashell\inc\ or /usr/local/share/seashell/inc/)
5. CS-Script fallback: %ProgramData%\cs-script\inc\

Environment Variable Expansion

//sea_incdir and //sea_ref support cross-platform environment variable expansion:

//sea_incdir %ProgramData%/mylibs       Windows-style
//sea_incdir $HOME/scripts              Unix-style
//sea_incdir ${XDG_DATA_HOME}/inc       Unix-style braced
//sea_incdir ~/shared                   Home shorthand

All styles work on all platforms.

Sea Runtime API

Every script has access to the Sea static class:

// Script identity
Sea.ScriptPath          // Absolute path to the main script
Sea.ScriptDir           // Directory containing the script
Sea.ScriptName          // Filename without extension
Sea.ScriptFile          // Filename with extension
Sea.StartDir            // CWD where 'sea' was invoked (never changed)

// Environment
Sea.IsElevated          // Running with admin/root privileges?
Sea.IsConsoleEphemeral  // Console will close on exit? (double-clicked)
Sea.ExitDelay           // Seconds to delay before exit (default: 7, set to 0 to skip)
Sea.Args                // Script arguments

// Build metadata
Sea.Sources             // All compiled source files (main + includes)
Sea.Packages            // NuGet packages resolved (name -> version)
Sea.Assemblies          // All managed DLL paths

// Hot-swap lifecycle
Sea.IsReload            // True if this is a hot-swap restart
Sea.ReloadCount         // Number of reloads so far
Sea.IsShuttingDown      // True after Reloading/Stopping fires
Sea.ShutdownToken       // CancellationToken — cancelled on reload/stop
Sea.Reloading           // Event: script is about to be replaced
Sea.Stopping            // Event: clean stop requested (Ctrl+C)

// Reload state handoff (max 8 KB)
Sea.SetReloadState(bytes)       // Pass state to next instance
Sea.SetReloadState(string)      // String convenience overload
Sea.GetReloadState()            // Retrieve state from previous instance
Sea.GetReloadStateString()      // String convenience overload

// Host↔Script messaging (when running via ScriptHost)
Sea.MessageReceived             // Event: Host sent a message (byte[] payload, string? topic)
Sea.SendMessage(bytes, topic)   // Send binary message to Host
Sea.SendMessage(string, topic)  // Send string message to Host (UTF-8)
Sea.SendMessageAsync(...)       // Async variants

Hot-Swap Example

//sea_watch

var counter = int.TryParse(Sea.GetReloadStateString(), out var c) ? c : 0;

Sea.Reloading += () => Sea.SetReloadState(counter.ToString());

while (!Sea.IsShuttingDown)
{
    counter++;
    Console.WriteLine($"counter={counter}");
    await Task.Delay(2000, Sea.ShutdownToken);
}

Edit and save the file — the counter picks up where it left off.

Embedding (SeaShell.Host)

var host = new ScriptHost();

// Run a script file
var result = await host.RunAsync("path/to/script.cs");
Console.WriteLine(result.StandardOutput);

// Run a code snippet
var result = await host.RunSnippetAsync("""
    Console.WriteLine("Hello!");
    return 0;
    """);

// Compile once, run many times
var compiled = host.Compile("script.cs");
await host.ExecuteAsync(compiled);
await host.ExecuteAsync(compiled);

// Background NuGet updates on your schedule
var updater = host.CreateUpdater();
updater.Log += Console.WriteLine;
await updater.CheckForUpdatesAsync();

Sea.IsConsoleEphemeral is always false when running via Host. Sea.ExitDelay has no effect — the Host does not perform exit delays.

Host↔Script Messaging

The Host and script can exchange binary messages during execution over the existing pipe:

var conn = new ScriptHost.ScriptConnection();
conn.MessageReceived += (payload, topic) =>
    Console.WriteLine($"Script [{topic}]: {Encoding.UTF8.GetString(payload)}");

var result = await host.RunAsync("script.cs", connection: conn);

Script side:

Sea.MessageReceived += (payload, topic) =>
    Sea.SendMessage("acknowledged", "reply");

Sea.SendMessage("{\"status\":\"ready\"}", "status");

Messages are binary (byte[]) with an optional string topic for routing. String convenience overloads encode as UTF-8.

Task Scheduler (Windows)

Register the daemon and optionally the elevator for automatic startup. Each is a separate, explicit registration:

sea --install-daemon              Register daemon (limited privileges)
sea --install-elevator            Register elevator (requires elevation once)
sea --uninstall-daemon            Remove daemon task
sea --uninstall-elevator          Remove elevator task
sea --start                       Start registered tasks
sea --stop                        Stop registered tasks
sea --status                      Show daemon and elevator status

The daemon starts automatically on first sea invocation even without Task Scheduler. The elevator is optional — without it, elevated scripts fall back to gsudo sea script.cs.

Project Structure

SeaShell.Cli       CLI (sea.exe) — argument parsing, script execution, exit delay
SeaShell.Daemon    Persistent compilation server, REPL host, file watcher
SeaShell.Elevator  Pre-elevated worker (connects to daemon, no public pipe)
SeaShell.Engine    Roslyn compiler, NuGet resolver, include system, .deps.json writer
SeaShell.Script    Sea runtime context (loaded into every script process)
SeaShell.Ipc       Binary IPC: MessageChannel (MessagePack over System.IO.Pipelines)
SeaShell.Protocol  Daemon/Elevator protocol messages + transport (named pipes / UDS)
SeaShell.Host      Embeddable library for other applications

See doc/ARCHITECTURE.md for the full architecture, doc/SECURITY.md for the security model, and doc/HISTORY.md for the changelog.

File Association (Windows)

Register .cs files to open with SeaShell when double-clicked:

sea --associate              Associate .cs (default)
sea --associate .vb          Associate .vb
sea --unassociate .cs        Remove association

Per-user registration via HKCU — no elevation needed. The SeaShell icon appears on associated files in Explorer.

Building

dotnet build

Run from source:

dotnet run --project src/SeaShell.Cli -- script.cs

Pack NuGet packages:

dotnet pack -c Release -o ./nupkg

Install as a global tool (from local build):

dotnet tool install -g --add-source ./nupkg SeaShell

Notes

Compilation Cache

Compiled scripts are cached in %TEMP%/seashell/cache/. The cache key is a SHA256 hash of:

• All source files (main script + includes, by content)
• All resolved NuGet package versions (name@version)
• The Engine and Script assembly timestamps

A cache hit skips Roslyn entirely — the daemon returns the existing DLL. The cache invalidates automatically when:

• Any source file changes
• A NuGet package is updated to a new version
• The SeaShell engine or script runtime is rebuilt
• The .NET SDK is updated (Engine DLL is recompiled)

Exit Delay (Windows)

When sea.exe detects it was launched from an ephemeral console (e.g., double-clicked from Explorer rather than run from a terminal), it shows an interactive countdown after the script exits. This prevents the console window from vanishing before the user can read the output.

• Default delay: 7 seconds (Sea.ExitDelay)
• Scripts can change it: Sea.ExitDelay = 0 to exit immediately, or Sea.ExitDelay = 30 for more time
• The countdown is interactive: Enter skips, Escape cancels, arrow keys adjust, P pauses
• Non-ephemeral consoles (terminals, IDE, CI) never delay
• The Host library never delays — Sea.IsConsoleEphemeral is always false

Detection uses GetConsoleProcessList: a .NET app in an ephemeral console has exactly 2 processes (apphost + runtime), while a terminal adds its shell process (3+).

Webapp Hot-Swap

//sea_webapp and //sea_watch combine for live-reloading web services. The key requirement: the old Kestrel instance must release the port before the new one starts. The CLI already waits for the old process to exit before spawning the new one, but Kestrel needs a graceful shutdown to release the port cleanly:

//sea_webapp
//sea_watch

var app = WebApplication.CreateBuilder().Build();
app.MapGet("/", () => "Hello!");

Sea.Reloading += () => app.StopAsync().Wait(TimeSpan.FromSeconds(2));

app.Run("http://localhost:5199");

Edit, save — the new version is serving on the same port in seconds. No downtime, no port conflicts. Without the StopAsync call, the killed process may leave the port in TIME_WAIT and the new instance would fail to bind.

Elevation Cascade

When a script has //sea_elevate, the CLI resolves it in order:

1. Already elevated? (e.g., gsudo sea script.cs) — spawn directly
2. Elevator worker connected to daemon? — delegate to it (no UAC prompt)
3. Neither — error with a helpful message suggesting gsudo or sea --install-elevator

On Linux, //sea_elevate is silently ignored.

NuGet Resolution

Packages are resolved from the global NuGet cache (~/.nuget/packages/). Transitive dependencies are walked via .nuspec files. Runtime-specific DLLs (runtimes/{rid}/lib/{tfm}/) and native DLLs (runtimes/{rid}/native/) are included in the generated .deps.json, which the dotnet host reads for correct assembly probing. This is the key improvement over CS-Script, which loads the generic lib/{tfm}/ stub and fails on packages like Microsoft.Data.SqlClient.

All configured NuGet sources from the user's nuget.config are checked. The background updater probes sources before iterating packages (circuit breaker) and distinguishes 404 (package not on this source) from 5xx (service error) to avoid false positives.

Logging

The Engine uses Serilog with Log.ForContext<T>() per class. The daemon configures:

• Windows: EventLog sink (Info+) — visible in Event Viewer under "SeaShell"
• Linux: Syslog sink (Info+) — visible in journalctl
• Dev mode: Console sink (Debug+) — enabled with --console flag or SEASHELL_CONSOLE_LOG=1

The Host inherits Serilog from the Engine. The caller configures Log.Logger before creating ScriptHost. If unconfigured, all log output is silently dropped.

Documentation

• ARCHITECTURE.md — Component diagram, IPC layers, compilation pipeline, execution paths
• SECURITY.md — Threat model, IPC access control, elevation security
• HISTORY.md — Changelog

License

MIT