add non-interactive mode: -c, script files and commands from stdin

- gosh -c 'command' runs the command and exits, so gosh works as login
  shell for ssh host command, scp and rsync
- gosh script runs a script file, lines starting with # are skipped
- without a terminal, commands are read from stdin (byte by byte, so
  started commands can read the rest of stdin)
- the exit status is the one of the last command (127 not found,
  126 not executable, 128+n killed by signal n)
- -i forces interactive mode, -l/--login is accepted

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

main_test.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"errors"
 	"bytes"
 	"io"
 	"os"
@@ -1173,7 +1174,7 @@ func TestShell_History(t *testing.T) {
 	histFile := filepath.Join(t.TempDir(), "gosh", "history")
 	workDir := t.TempDir()
 
-	cmd := exec.Command(bin)
+	cmd := exec.Command(bin, "-i")
 	cmd.Dir = workDir
 	cmd.Env = append(os.Environ(), "GOSH_HISTFILE="+histFile)
 	cmd.Stdin = strings.NewReader("echo first\n set SECRET=hunter2\nexit\n")
@@ -1205,7 +1206,7 @@ func TestShell_History(t *testing.T) {
 func TestShell_SIGINTKillsOnlyChild(t *testing.T) {
 	bin := buildShell(t)
 
-	cmd := exec.Command(bin)
+	cmd := exec.Command(bin, "-i")
 	// Own process group, so that we can signal it like the terminal does
 	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
 	stdin, err := cmd.StdinPipe()
@@ -1267,3 +1268,83 @@ func TestExecInput_PassesStdin(t *testing.T) {
 		t.Errorf("command read %q from stdin, want %q", got, "hello")
 	}
 }
+
+func TestExitStatus(t *testing.T) {
+	tests := []struct {
+		input string
+		want  int
+	}{
+		{"true", 0},
+		{"false", 1},
+		{`sh -c "exit 42"`, 42},
+		{`sh -c "kill -TERM \$\$"`, 128 + 15},
+		{"gosh-no-such-command", 127},
+		{"cd /gosh-no-such-dir", 127},
+	}
+	for _, tc := range tests {
+		if got := exitStatus(execInput(tc.input)); got != tc.want {
+			t.Errorf("exitStatus(execInput(%q)) = %d, want %d", tc.input, got, tc.want)
+		}
+	}
+	if got := exitStatus(errors.New("other")); got != 1 {
+		t.Errorf("exitStatus(other error) = %d, want 1", got)
+	}
+}
+
+func TestRunScript(t *testing.T) {
+	origDir, _ := os.Getwd()
+	defer os.Chdir(origDir)
+
+	dir := t.TempDir()
+	script := "#!/usr/bin/gosh\n" +
+		"# comment\n" +
+		"\n" +
+		"cd " + dir + "\n" +
+		"  # indented comment\n" +
+		"touch first\n" +
+		"false\n" +
+		"touch second" // no trailing newline
+	if got := runScript(strings.NewReader(script)); got != 0 {
+		t.Errorf("runScript() = %d, want 0", got)
+	}
+	for _, name := range []string{"first", "second"} {
+		if _, err := os.Stat(filepath.Join(dir, name)); err != nil {
+			t.Errorf("expected %s to be created: %v", name, err)
+		}
+	}
+
+	// The status of the last command is returned
+	if got := runScript(strings.NewReader("true\nsh -c 'exit 3'\n")); got != 3 {
+		t.Errorf("runScript() = %d, want 3", got)
+	}
+}
+
+func TestRun_Options(t *testing.T) {
+	tests := []struct {
+		args []string
+		want int
+	}{
+		{[]string{"-c", "true"}, 0},
+		{[]string{"-c", "false"}, 1},
+		{[]string{"-l", "-c", "sh -c 'exit 5'"}, 5},
+		{[]string{"-c"}, 2},
+		{[]string{"-x"}, 2},
+		{[]string{"/gosh/no/such/script"}, 127},
+	}
+	for _, tc := range tests {
+		if got := run(tc.args); got != tc.want {
+			t.Errorf("run(%q) = %d, want %d", tc.args, got, tc.want)
+		}
+	}
+}
+
+func TestReadLine_DoesNotReadAhead(t *testing.T) {
+	r := strings.NewReader("first\nsecond\n")
+	line, err := readLine(r)
+	if line != "first" || err != nil {
+		t.Fatalf("readLine() = %q, %v", line, err)
+	}
+	if r.Len() != len("second\n") {
+		t.Errorf("readLine() consumed more than one line, %d bytes left", r.Len())
+	}
+}