[tomlm]

Adds Background(program, args...) (on CShell and Globals) for servers and daemons that should outlive the script, and bumps the version to 3.3.0.

int pid = Background("dotnet", "MyServer.dll", "--port", "8080");
Background(opt => opt.WorkingDirectory(dir).EnvironmentVariable("X", "1"), "dotnet", "run");

Why

Start() and Run() can't promise that the new process stays out of the caller's way:

• On Windows, Process.Start passes every handle the caller can share to the child. That includes the stdout pipe of whoever is capturing the script's output, so $(script), pipes and test harnesses wait until the child exits.
• On Unix, the child shares the caller's stdin, stdout, stderr and session.

Start() keeps its current behaviour, which is right for apps you open and leave.

What it does

• Returns the program's own process id straight away.
• No window, and none of the caller's handles or streams. The program's output goes nowhere.
• Detached from the terminal:
  • Windows: CreateProcess with bInheritHandles=FALSE, CREATE_NO_WINDOW and CREATE_NEW_PROCESS_GROUP. The process gets a console no one can see, and programs it starts don't each open a window.
  • Linux: /bin/sh points the program's input and output at /dev/null, then replaces itself with setsid so the program leads its own session.
  • macOS: there's no setsid command, so it runs under nohup. It survives the terminal closing, but not a Ctrl+C sent while the script is still running. This is documented.
• Like the other process methods: it uses CurrentFolder (or WorkingDirectory), takes environment variables, follows Echo, and throws Win32Exception for a program that can't be found.
• Docs: the README gets a fourth row in the "ways to run something" table and a Background() section.

Tests

Tests/BackgroundHost is a small helper program that records what it was started with. BackgroundTests checks:

• the returned id is the program's own;
• it runs in the shell's folder, and the WorkingDirectory option overrides that;
• environment variables arrive;
• awkward arguments arrive unchanged: quotes, trailing backslashes, empty strings, tabs, non-ASCII text;
• a caller whose output is captured can start a 60-second background process and exit, and reading its output finishes promptly. Flipping bInheritHandles to TRUE makes this test fail, so it does catch a leak;
• on Linux, the process leads its own session.

Results:

• Windows: the full suite passes, 321 tests with 1 skipped.
• Linux (WSL, Ubuntu 24.04): the 7 Background tests pass on two runs. I didn't run the full suite there.
• macOS: not tested.

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