add startup files and the source builtin

- login shells (-l or argv[0] starting with -) read /etc/gosh/profile
  and ~/.gosh_profile, interactive shells /etc/gosh/goshrc and ~/.goshrc;
  --noprofile and --norc skip them
- source file / . file runs a file in the current shell, nesting is
  limited to stop files that source themselves
- errors in startup files and scripts are reported with file and line
- ~ and the startup files use $HOME (like sh), the user database is
  only the fallback

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

main.go

@@ -74,12 +74,15 @@ func versionInfo() string {
 // previousDir tracks the previous working directory for 'cd -' support
 var previousDir string
 
-// getHomeDir returns the user's home directory
+// getHomeDir returns the user's home directory: $HOME like in sh, or the
+// home directory from the user database if $HOME is not set
 func getHomeDir() string {
+	if home := os.Getenv("HOME"); home != "" {
+		return home
+	}
 	usr, err := user.Current()
 	if err != nil {
-		// Fallback to HOME environment variable
-		return os.Getenv("HOME")
+		return ""
 	}
 	return usr.HomeDir
 }
@@ -699,7 +702,7 @@ type shellCompleter struct {
 
 func newShellCompleter() *shellCompleter {
 	return &shellCompleter{
-		commands: []string{"cd", "pwd", "echo", "exit", "quit", "set", "export", "help"},
+		commands: []string{"cd", "pwd", "echo", "exit", "quit", "set", "export", "source", "help"},
 	}
 }
 
@@ -866,33 +869,48 @@ func completeFilePath(line, prefix string) []string {
 	return matches
 }
 
-const usage = `usage: gosh [-l] [-i] [script]
-       gosh [-l] -c command
+const usage = `usage: gosh [-l] [-i] [--norc] [--noprofile] [script]
+       gosh [-l] [--noprofile] -c command
        gosh -version`
 
+// sysConfDir holds the system-wide startup files.
+var sysConfDir = "/etc/gosh"
+
 func main() {
-	os.Exit(run(os.Args[1:]))
+	args := os.Args[1:]
+	// login(1), sshd and "su -" start a login shell with a leading - in
+	// argv[0], e.g. "-gosh"
+	if strings.HasPrefix(os.Args[0], "-") {
+		args = append([]string{"-l"}, args...)
+	}
+	os.Exit(run(args))
 }
 
 // run parses the command line and runs gosh in the matching mode. It returns
 // the exit status of the shell.
 func run(args []string) int {
-	// -l/--login is accepted for compatibility (e.g. "su -l", login(1)), but
-	// gosh does not read a startup file yet. -i forces interactive mode even
-	// if stdin is not a terminal.
-	forceInteractive := false
+	var login, forceInteractive, noRC, noProfile bool
 flags:
 	for len(args) > 0 {
 		switch args[0] {
 		case "-l", "--login":
+			login = true
 		case "-i":
+			// Interactive mode even if stdin is not a terminal
 			forceInteractive = true
+		case "--norc":
+			noRC = true
+		case "--noprofile":
+			noProfile = true
 		default:
 			break flags
 		}
 		args = args[1:]
 	}
 
+	// Determine what to run, before the startup files are read
+	var script func() int
+	interactiveMode := false
 	if len(args) > 0 {
 		switch arg := args[0]; {
 		case arg == "-version" || arg == "--version" || arg == "-V":
@@ -909,13 +927,13 @@ flags:
 				fmt.Fprintln(os.Stderr, usage)
 				return 2
 			}
-			return runScript(strings.NewReader(args[1]))
+			script = func() int { return runScript(strings.NewReader(args[1]), "") }
 		case strings.HasPrefix(arg, "-") && arg != "-":
 			fmt.Fprintf(os.Stderr, "gosh: %s: invalid option\n", arg)
 			fmt.Fprintln(os.Stderr, usage)
 			return 2
 		case arg == "-":
-			return runScript(os.Stdin)
+			script = func() int { return runScript(os.Stdin, "") }
 		default:
 			f, err := os.Open(arg)
 			if err != nil {
@@ -923,30 +941,107 @@ flags:
 				return 127
 			}
 			defer f.Close()
-			return runScript(bufio.NewReader(f))
+			script = func() int { return runScript(bufio.NewReader(f), arg) }
 		}
+	} else if forceInteractive || readline.IsTerminal(int(os.Stdin.Fd())) {
+		interactiveMode = true
+	} else {
+		// Without a terminal (e.g. "echo ls | gosh") read the commands from
+		// stdin
+		script = func() int { return runScript(os.Stdin, "") }
 	}
 
-	// Without a terminal (e.g. "echo ls | gosh") read the commands from stdin.
-	if !forceInteractive && !readline.IsTerminal(int(os.Stdin.Fd())) {
-		return runScript(os.Stdin)
+	for _, file := range startupFiles(login && !noProfile, interactiveMode && !noRC) {
+		sourceStartupFile(file)
 	}
 
+	if !interactiveMode {
+		return script()
+	}
 	interactive()
 	return 0
 }
 
+// startupFiles returns the startup files to read, in order: for login
+// shells /etc/gosh/profile and ~/.gosh_profile, for interactive shells
+// /etc/gosh/goshrc and ~/.goshrc. Non-interactive shells (gosh -c, scripts)
+// read no files unless they are login shells.
+func startupFiles(login, interactive bool) []string {
+	home := getHomeDir()
+	var files []string
+	if login {
+		files = append(files, filepath.Join(sysConfDir, "profile"))
+		if home != "" {
+			files = append(files, filepath.Join(home, ".gosh_profile"))
+		}
+	}
+	if interactive {
+		files = append(files, filepath.Join(sysConfDir, "goshrc"))
+		if home != "" {
+			files = append(files, filepath.Join(home, ".goshrc"))
+		}
+	}
+	return files
+}
+
+// sourceStartupFile runs a startup file in the current shell. A missing file
+// is skipped silently, other errors are reported.
+func sourceStartupFile(path string) {
+	f, err := os.Open(path)
+	if errors.Is(err, fs.ErrNotExist) {
+		return
+	}
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "gosh:", err)
+		return
+	}
+	defer f.Close()
+	runScript(bufio.NewReader(f), path)
+}
+
+// sourceDepth counts nested source commands, to stop a file that sources
+// itself.
+var sourceDepth int
+
+// sourceFile runs the commands of a file in the current shell (the source and
+// . builtins) and returns an error carrying the status of the last command.
+func sourceFile(cmd, path string) error {
+	if sourceDepth >= 100 {
+		return fmt.Errorf("%s: %s: maximum nesting depth exceeded", cmd, sanitize(path))
+	}
+	f, err := os.Open(path)
+	if err != nil {
+		var pathErr *fs.PathError
+		if errors.As(err, &pathErr) {
+			err = pathErr.Err
+		}
+		return fmt.Errorf("%s: %s: %v", cmd, sanitize(path), err)
+	}
+	defer f.Close()
+
+	sourceDepth++
+	defer func() { sourceDepth-- }()
+	if status := runScript(bufio.NewReader(f), path); status != 0 {
+		return statusError(status)
+	}
+	return nil
+}
+
 // runScript executes the commands from r line by line. Like sh, it does not
 // stop at a failing command and returns the exit status of the last one.
 // Empty lines and lines starting with # (e.g. a #!/usr/bin/gosh line) are
-// skipped.
-func runScript(r io.Reader) int {
+// skipped. If name is set, errors are reported with file name and line.
+func runScript(r io.Reader, name string) int {
 	status := 0
-	for {
+	for lineNo := 1; ; lineNo++ {
 		line, err := readLine(r)
 		if trimmed := strings.TrimSpace(line); trimmed != "" && !strings.HasPrefix(trimmed, "#") {
-			printError(execInput(line))
+			cmdErr := execInput(line)
 			status = lastStatus
+			if name != "" && cmdErr != nil {
+				cmdErr = fmt.Errorf("%s:%d: %w", sanitize(name), lineNo, cmdErr)
+			}
+			printError(cmdErr)
 		}
 		if err != nil {
 			if err != io.EOF {
@@ -1476,6 +1571,8 @@ var builtins = map[string]struct{ changesState bool }{
 	"set":    {changesState: true},
 	"export": {changesState: true},
 	"help":   {},
+	"source": {changesState: true},
+	".":      {changesState: true},
 }
 
 // runBuiltin executes a built-in command, writing its output to stdout.
@@ -1555,6 +1652,12 @@ func runBuiltin(args []string, stdout io.Writer) error {
 		}
 		os.Setenv(parts[0], parts[1])
 		return nil
+	case "source", ".":
+		// source file - run the commands of file in the current shell
+		if len(args) < 2 {
+			return fmt.Errorf("usage: %s file", args[0])
+		}
+		return sourceFile(args[0], args[1])
 	case "help":
 		fmt.Fprintln(stdout, "gosh - Go Shell\nVersion: " + versionInfo() + `
 
@@ -1566,6 +1669,7 @@ Built-in commands:
   quit [code]   - Quit shell with status code (default: $?)
   set VAR=val   - Set environment variable
   export VAR=val - Export environment variable
+  source file   - Run the commands of file in this shell (also: . file)
   help          - Show this help
 
 Features:
@@ -1591,7 +1695,10 @@ Features:
   - Pipes (a | b) and redirections: < file, > file, >> file, 2> file,
     2>> file, 2>&1, >&2, &> file (stdout and stderr), &>> file
   - Non-interactive use: gosh -c 'command', gosh script, command | gosh;
-    the exit status is the one of the last command`)
+    the exit status is the one of the last command
+  - Startup files: login shells (-l or argv[0] starting with -) read
+    /etc/gosh/profile and ~/.gosh_profile, interactive shells read
+    /etc/gosh/goshrc and ~/.goshrc (skip with --noprofile, --norc)`)
 		return nil
 	}
 	return fmt.Errorf("%s: not a builtin", args[0])