add parameter expansion, arithmetic, set -e/-x/pipefail, trap, eval, exec

- ${#v}, ${v#p} ${v##p} ${v%p} ${v%%p}, ${v/p/r} ${v//p/r} ${v/#p/r}
  ${v/%p/r}, ${v:off:len}, ${v-w} ${v:=w} ${v:+w} ${v:?msg}; nested ${}
- arithmetic expansion $((...)) with the C operators, ** and assignments
- set -e, set -x, set -o pipefail (also as gosh options, passed on to
  subshells); set shows variables, set -o the options
- trap for EXIT and signals; scripts with an EXIT trap run it on
  SIGINT/SIGTERM/SIGHUP too
- eval, exec (also exec > file without a command), $!, wait pid

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

trap.go

@@ -0,0 +1,286 @@
+package main
+
+// The trap builtin. A trapped signal is only noted when it arrives; its
+// command runs in the shell at the next safe point: after the running
+// command has finished (like in sh, a shell waiting for a foreground command
+// handles the signal afterwards).
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"os/signal"
+	"sort"
+	"strconv"
+	"strings"
+	"sync"
+	"syscall"
+)
+
+// trapSignals are the signals that can be trapped, besides EXIT.
+var trapSignals = map[string]syscall.Signal{
+	"HUP":   syscall.SIGHUP,
+	"INT":   syscall.SIGINT,
+	"QUIT":  syscall.SIGQUIT,
+	"TERM":  syscall.SIGTERM,
+	"USR1":  syscall.SIGUSR1,
+	"USR2":  syscall.SIGUSR2,
+	"ALRM":  syscall.SIGALRM,
+	"WINCH": syscall.SIGWINCH,
+}
+
+// fatalSignals end a non-interactive shell; with an EXIT trap the shell
+// catches them to run it first.
+var fatalSignals = []string{"HUP", "INT", "TERM"}
+
+var (
+	// traps maps EXIT and signal names to their commands ("" = ignore).
+	traps = map[string]string{}
+	// trapChans are the channels of the signals the shell catches.
+	trapChans = map[string]chan os.Signal{}
+
+	pendingMu      sync.Mutex
+	pendingSignals []string
+
+	inTrap bool
+
+	// interruptChan swallows SIGINT in an interactive shell, so that Ctrl+C
+	// only stops the running command.
+	interruptChan = make(chan os.Signal, 1)
+)
+
+// signalName returns the canonical name (EXIT, INT, ...) of a signal given
+// by name (with or without SIG) or number, or "".
+func signalName(spec string) string {
+	s := strings.ToUpper(strings.TrimPrefix(strings.ToUpper(spec), "SIG"))
+	if s == "EXIT" || s == "0" {
+		return "EXIT"
+	}
+	if _, ok := trapSignals[s]; ok {
+		return s
+	}
+	if n, err := strconv.Atoi(spec); err == nil {
+		for name, sig := range trapSignals {
+			if int(sig) == n {
+				return name
+			}
+		}
+	}
+	return ""
+}
+
+// builtinTrap implements trap [-lp] [[action] signal ...].
+func builtinTrap(args []string, stdout io.Writer) error {
+	args = args[1:]
+	switch {
+	case len(args) == 0 || args[0] == "-p":
+		names := args
+		if len(names) > 0 {
+			names = names[1:]
+		}
+		return printTraps(stdout, names)
+	case args[0] == "-l":
+		names := []string{"EXIT"}
+		for name := range trapSignals {
+			names = append(names, name)
+		}
+		sort.Slice(names, func(i, j int) bool { return signalNumber(names[i]) < signalNumber(names[j]) })
+		for _, name := range names {
+			fmt.Fprintf(stdout, "%2d) %s\n", signalNumber(name), name)
+		}
+		return nil
+	case args[0] == "--":
+		args = args[1:]
+	}
+
+	action := "-"
+	if len(args) > 1 || len(args) == 1 && signalName(args[0]) == "" {
+		action, args = args[0], args[1:]
+	}
+	if len(args) == 0 {
+		return fmt.Errorf("trap: usage: trap [-lp] [[action] signal ...]")
+	}
+	var err error
+	for _, spec := range args {
+		name := signalName(spec)
+		if name == "" {
+			err = fmt.Errorf("trap: %s: invalid signal specification", sanitize(spec))
+			continue
+		}
+		setTrap(name, action)
+	}
+	return err
+}
+
+func signalNumber(name string) int {
+	if name == "EXIT" {
+		return 0
+	}
+	return int(trapSignals[name])
+}
+
+func printTraps(w io.Writer, only []string) error {
+	var names []string
+	for name := range traps {
+		names = append(names, name)
+	}
+	if len(only) > 0 {
+		names = nil
+		for _, spec := range only {
+			name := signalName(spec)
+			if name == "" {
+				return fmt.Errorf("trap: %s: invalid signal specification", sanitize(spec))
+			}
+			if _, ok := traps[name]; ok {
+				names = append(names, name)
+			}
+		}
+	}
+	sort.Slice(names, func(i, j int) bool { return signalNumber(names[i]) < signalNumber(names[j]) })
+	for _, name := range names {
+		fmt.Fprintf(w, "trap -- %s %s\n", quoteAlias(traps[name]), name)
+	}
+	return nil
+}
+
+// setTrap sets the action of a trap: "-" restores the default, "" ignores
+// the signal, anything else is a command.
+func setTrap(name, action string) {
+	if action == "-" {
+		delete(traps, name)
+	} else {
+		traps[name] = action
+	}
+	if name == "EXIT" {
+		updateFatalCatch()
+		return
+	}
+	sig := trapSignals[name]
+	stopCatching(name)
+	switch action {
+	case "-":
+		signal.Reset(sig)
+		restoreShellSignal(sig)
+		if _, exit := traps["EXIT"]; exit {
+			updateFatalCatch()
+		}
+	case "":
+		// Ignored signals stay ignored in the programs the shell starts
+		signal.Ignore(sig)
+	default:
+		catch(name)
+	}
+}
+
+// catch makes the shell note the signal, to run its trap later.
+func catch(name string) {
+	if _, ok := trapChans[name]; ok {
+		return
+	}
+	ch := make(chan os.Signal, 4)
+	trapChans[name] = ch
+	signal.Notify(ch, trapSignals[name])
+	go func() {
+		for range ch {
+			pendingMu.Lock()
+			pendingSignals = append(pendingSignals, name)
+			pendingMu.Unlock()
+		}
+	}()
+}
+
+func stopCatching(name string) {
+	if ch, ok := trapChans[name]; ok {
+		signal.Stop(ch)
+		close(ch)
+		delete(trapChans, name)
+	}
+}
+
+// restoreShellSignal installs the handling the shell itself needs for sig
+// again after signal.Reset.
+func restoreShellSignal(sig syscall.Signal) {
+	if sig == syscall.SIGINT && interactiveShell {
+		signal.Notify(interruptChan, os.Interrupt)
+	}
+}
+
+// updateFatalCatch catches the fatal signals in a non-interactive shell with
+// an EXIT trap (unless they have traps of their own), so that the EXIT trap
+// runs before the shell ends.
+func updateFatalCatch() {
+	if interactiveShell {
+		return
+	}
+	_, exitTrap := traps["EXIT"]
+	for _, name := range fatalSignals {
+		if _, own := traps[name]; own {
+			continue
+		}
+		if exitTrap {
+			catch(name)
+		} else {
+			stopCatching(name)
+		}
+	}
+}
+
+// hasPendingSignal reports whether a caught signal waits for its trap.
+func hasPendingSignal() bool {
+	pendingMu.Lock()
+	defer pendingMu.Unlock()
+	return len(pendingSignals) > 0
+}
+
+// runPendingTraps runs the traps of the signals that arrived. A fatal signal
+// without a trap of its own (caught for the EXIT trap) ends the shell.
+func runPendingTraps() {
+	if inTrap {
+		return
+	}
+	pendingMu.Lock()
+	names := pendingSignals
+	pendingSignals = nil
+	pendingMu.Unlock()
+
+	for _, name := range names {
+		if action, ok := traps[name]; ok {
+			if action != "" {
+				runTrap(action)
+			}
+			continue
+		}
+		// Caught only for the EXIT trap: run it and die from the signal
+		runExitTrap()
+		sig := trapSignals[name]
+		signal.Reset(sig)
+		releaseTerminal()
+		syscall.Kill(os.Getpid(), sig)
+		os.Exit(128 + int(sig))
+	}
+}
+
+// runTrap runs the command of a trap; $? is kept.
+func runTrap(action string) {
+	saved := lastStatus
+	inTrap = true
+	defer func() { inTrap = false }()
+	tree, err := parse(action)
+	if err == nil {
+		err = tree.exec()
+	}
+	printError(err)
+	lastStatus = saved
+}
+
+// runExitTrap runs the EXIT trap (once) before the shell exits.
+func runExitTrap() {
+	action, ok := traps["EXIT"]
+	if !ok {
+		return
+	}
+	delete(traps, "EXIT")
+	if action != "" {
+		runTrap(action)
+	}
+}