interp: add an Interactive option to be used by cmd/gosh

Otherwise aliases don't get expanded, as they are only on by default for
interactive shells in Bash, and we follow POSIX shell and Bash options.

Updates #1100.

cmd/gosh/main.go

@@ -33,7 +33,7 @@ func main() {
 }
 
 func runAll() error {
-	r, err := interp.New(interp.StdIO(os.Stdin, os.Stdout, os.Stderr))
+	r, err := interp.New(interp.Interactive(true), interp.StdIO(os.Stdin, os.Stdout, os.Stderr))
 	if err != nil {
 		return err
 	}

cmd/gosh/main_test.go

@@ -179,11 +179,10 @@ var interactiveTests = []struct {
 		pairs: []string{
 			"gosh_alias arg || true\n",
 			"\"gosh_alias\": executable file not found in $PATH\n$ ",
-			// TODO: aliases should be expanded by default, gosh is an interactive shell
 			"alias gosh_alias=echo\n",
 			"$ ",
 			"gosh_alias arg || true\n",
-			"\"gosh_alias\": executable file not found in $PATH\n$ ",
+			"arg\n$ ",
 			"unalias gosh_alias\n",
 			"$ ",
 			"gosh_alias arg || true\n",
@@ -200,7 +199,7 @@ func TestInteractive(t *testing.T) {
 			qt.Assert(t, qt.IsNil(err))
 			outReader, outWriter, err := os.Pipe()
 			qt.Assert(t, qt.IsNil(err))
-			runner, _ := interp.New(interp.StdIO(inReader, outWriter, outWriter))
+			runner, _ := interp.New(interp.Interactive(true), interp.StdIO(inReader, outWriter, outWriter))
 			errc := make(chan error, 1)
 			go func() {
 				errc <- runInteractive(runner, inReader, outWriter, outWriter)
@@ -255,7 +254,7 @@ func TestInteractiveExit(t *testing.T) {
 		inWriter.Close()
 	}()
 	w := io.Discard
-	runner, _ := interp.New(interp.StdIO(inReader, w, w))
+	runner, _ := interp.New(interp.Interactive(true), interp.StdIO(inReader, w, w))
 	if err := runInteractive(runner, inReader, w, w); err != nil {
 		t.Fatal("expected a nil error")
 	}

interp/api.go

@@ -192,17 +192,17 @@ func New(opts ...RunnerOption) (*Runner, error) {
 		statHandler:    DefaultStatHandler(),
 	}
 	r.dirStack = r.dirBootstrap[:0]
+	// turn "on" the default Bash options
+	for i, opt := range bashOptsTable {
+		r.opts[len(shellOptsTable)+i] = opt.defaultState
+	}
+
 	for _, opt := range opts {
 		if err := opt(r); err != nil {
 			return nil, err
 		}
 	}
 
-	// turn "on" the default Bash options
-	for i, opt := range bashOptsTable {
-		r.opts[len(shellOptsTable)+i] = opt.defaultState
-	}
-
 	// Set the default fallbacks, if necessary.
 	if r.Env == nil {
 		Env(nil)(r)
@@ -266,6 +266,16 @@ func Dir(path string) RunnerOption {
 	}
 }
 
+// Interactive configures the interpreter to behave like an interactive shell,
+// akin to Bash. Currently, this only enables the expansion of aliases,
+// but later on it should also change other behavior.
+func Interactive(enabled bool) RunnerOption {
+	return func(r *Runner) error {
+		r.opts[optExpandAliases] = enabled
+		return nil
+	}
+}
+
 // Params populates the shell options and parameters. For example, Params("-e",
 // "--", "foo") will set the "-e" option and the parameters ["foo"], and
 // Params("+e") will unset the "-e" option and leave the parameters untouched.

interp/example_test.go

@@ -28,6 +28,7 @@ func Example() {
 	`
 	file, _ := syntax.NewParser().Parse(strings.NewReader(src), "")
 	runner, _ := interp.New(
+		// Use [interp.Interactive] to enable interactive shell defaults like expanding aliases.
 		interp.Env(expand.ListEnviron("GLOBAL=global_value")),
 		interp.StdIO(nil, os.Stdout, os.Stdout),
 	)