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.go

@@ -1,9 +1,11 @@
 package main
 
 import (
+	"bufio"
 	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"os"
 	"os/exec"
 	"os/signal"
@@ -11,6 +13,7 @@ import (
 	"path/filepath"
 	"runtime/debug"
 	"strings"
+	"syscall"
 	"unicode"
 	"unicode/utf8"
 
@@ -611,15 +614,151 @@ func completeFilePath(line, prefix string) []string {
 	return matches
 }
 
+const usage = `usage: gosh [-l] [-i] [script]
+       gosh [-l] -c command
+       gosh -version`
+
 func main() {
-	if len(os.Args) > 1 {
-		switch os.Args[1] {
-		case "-version", "--version", "-V":
+	os.Exit(run(os.Args[1:]))
+}
+
+// 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
+flags:
+	for len(args) > 0 {
+		switch args[0] {
+		case "-l", "--login":
+		case "-i":
+			forceInteractive = true
+		default:
+			break flags
+		}
+		args = args[1:]
+	}
+
+	if len(args) > 0 {
+		switch arg := args[0]; {
+		case arg == "-version" || arg == "--version" || arg == "-V":
 			fmt.Println("gosh", versionInfo())
-			return
+			return 0
+		case arg == "-h" || arg == "--help":
+			fmt.Println(usage)
+			return 0
+		case arg == "-c":
+			// Like sh, further arguments are allowed (they would become $0,
+			// $1, ...), but gosh has no positional parameters yet.
+			if len(args) < 2 {
+				fmt.Fprintln(os.Stderr, "gosh: -c: option requires an argument")
+				fmt.Fprintln(os.Stderr, usage)
+				return 2
+			}
+			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)
+		default:
+			f, err := os.Open(arg)
+			if err != nil {
+				fmt.Fprintln(os.Stderr, "gosh:", err)
+				return 127
+			}
+			defer f.Close()
+			return runScript(bufio.NewReader(f))
 		}
 	}
 
+	// 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)
+	}
+
+	interactive()
+	return 0
+}
+
+// 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 {
+	status := 0
+	for {
+		line, err := readLine(r)
+		if trimmed := strings.TrimSpace(line); trimmed != "" && !strings.HasPrefix(trimmed, "#") {
+			cmdErr := execInput(line)
+			status = exitStatus(cmdErr)
+			// A command that ran and failed reports its own errors.
+			var exitErr *exec.ExitError
+			if cmdErr != nil && !errors.As(cmdErr, &exitErr) {
+				fmt.Fprintln(os.Stderr, "gosh:", cmdErr)
+			}
+		}
+		if err != nil {
+			if err != io.EOF {
+				fmt.Fprintln(os.Stderr, "gosh:", err)
+				return 1
+			}
+			return status
+		}
+	}
+}
+
+// readLine reads a single line from r without the trailing newline. It reads
+// byte by byte so that nothing after the line is consumed: when the script
+// comes from stdin, the commands it starts read the rest of stdin (like in
+// sh, "printf 'cat\nhello\n' | gosh" prints hello). Pass a bufio.Reader if
+// that does not matter.
+func readLine(r io.Reader) (string, error) {
+	var b strings.Builder
+	buf := make([]byte, 1)
+	for {
+		n, err := r.Read(buf)
+		if n > 0 {
+			if buf[0] == '\n' {
+				return b.String(), nil
+			}
+			b.WriteByte(buf[0])
+		}
+		if err != nil {
+			return b.String(), err
+		}
+	}
+}
+
+// exitStatus converts the result of execInput into a shell exit status: the
+// exit code of the command, 128+n if it was killed by signal n, 127 if it was
+// not found, 126 if it could not be executed and 1 for other errors.
+func exitStatus(err error) int {
+	if err == nil {
+		return 0
+	}
+	var exitErr *exec.ExitError
+	if errors.As(err, &exitErr) {
+		if ws, ok := exitErr.Sys().(syscall.WaitStatus); ok && ws.Signaled() {
+			return 128 + int(ws.Signal())
+		}
+		return exitErr.ExitCode()
+	}
+	if errors.Is(err, exec.ErrNotFound) || errors.Is(err, fs.ErrNotExist) {
+		return 127
+	}
+	if errors.Is(err, fs.ErrPermission) {
+		return 126
+	}
+	return 1
+}
+
+// interactive runs the interactive shell with line editing, history and
+// completion until the user exits.
+func interactive() {
 	// Ctrl+C sends SIGINT to the whole foreground process group. Catch it so
 	// that it only stops the running command and not the shell. (signal.Ignore
 	// would be inherited by the child processes.)
@@ -800,7 +939,9 @@ Features:
     (keys user, root, path; values are ANSI SGR codes, empty = no color);
     disabled with $NO_COLOR, TERM=dumb or when output is not a terminal
   - Environment variable expansion ($VAR, ${VAR}, ${VAR:-default});
-    using an unset variable is an error, use ${VAR:-} for an empty default`)
+    using an unset variable is an error, use ${VAR:-} for an empty default
+  - Non-interactive use: gosh -c 'command', gosh script, command | gosh;
+    the exit status is the one of the last command`)
 		return nil
 	}