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package tools

// The shell a command is handed to, which differs by platform: /bin/sh on
// Unix, cmd.exe on Windows. Going through a shell is what makes a command in
// the file a *command line*: pipes, globs and && all work, so one entry can be
// a whole sequence. Splitting an argv here would mean inventing quoting rules
// for a file somebody wrote by hand.

// Shell returns the program every command is handed to: /bin/sh on Unix, and
// on Windows the shell %COMSPEC% names, which is cmd.exe.
//
//	exec.Command(tools.Shell(), tools.ShellArgs("go test ./...")...)
func Shell() string { return shellProgram() }

// ShellArgs returns the arguments that make the shell run one command line
// and exit: ["-c", command] for a Unix shell, ["/S", "/C", command] for
// cmd.exe.
//
// The result is meant for the terminal package, which composes the command
// line itself. For an exec.Cmd on Windows the quoting cmd.exe expects is put
// in SysProcAttr.CmdLine instead — see childAttributes.
func ShellArgs(command string) []string { return shellArgs(command) }

// windowsFallbackShell is what runs when COMSPEC is unset, which it never is
// on a working Windows: cmd.exe is found through PATH.
const windowsFallbackShell = "cmd.exe"

// windowsShell returns the shell Windows says is the user's — the value of
// COMSPEC — or the fallback when the variable is empty. It is the same rule
// the terminal package applies, written twice because tools must not import
// terminal.
func windowsShell(comspec string) string {
	if comspec != "" {
		return comspec
	}
	return windowsFallbackShell
}

// cmdExeCommandLine writes the command line cmd.exe parses by its own rules:
// `"cmd.exe" /S /C "command"`. Given /S, cmd.exe strips the first and last
// quote of what follows /C and hands the rest to its parser verbatim, so a
// quote inside the command reaches it as a quote — the C-runtime spelling Go's
// exec would produce, `\"`, would reach it as a backslash and a quote.
func cmdExeCommandLine(shell, command string) string {
	return `"` + shell + `" /S /C "` + command + `"`
}