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>

options.go

@@ -0,0 +1,284 @@
+package main
+
+// Shell options (set -e, set -x, set -o pipefail) and the builtins eval and
+// exec.
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"os/exec"
+	"slices"
+	"sort"
+	"strings"
+	"syscall"
+)
+
+var (
+	// errexit (set -e): exit when a command fails
+	errexit bool
+	// xtrace (set -x): print commands before they run
+	xtrace bool
+	// pipefail (set -o pipefail): the status of a pipeline is the one of
+	// the last command that failed
+	pipefail bool
+
+	// errexitOff is > 0 while commands run whose failure does not end the
+	// shell with set -e: conditions of if, while and until, commands
+	// negated with ! and all but the last command of && and || lists.
+	errexitOff int
+)
+
+// shellOptions are the long names of the options for set -o.
+var shellOptions = []struct {
+	name   string
+	letter byte
+	value  *bool
+}{
+	{"errexit", 'e', &errexit},
+	{"nounset", 'u', nil}, // always on in gosh
+	{"pipefail", 0, &pipefail},
+	{"xtrace", 'x', &xtrace},
+}
+
+// setOption sets an option by letter or long name.
+func setOption(letter byte, name string, on bool) error {
+	for _, o := range shellOptions {
+		if letter != 0 && o.letter == letter || name != "" && o.name == name {
+			if o.value == nil {
+				if !on {
+					return errors.New("set: +u: unset variables are always an error in gosh")
+				}
+				return nil
+			}
+			*o.value = on
+			return nil
+		}
+	}
+	if name != "" {
+		return fmt.Errorf("set: %s: invalid option name", sanitize(name))
+	}
+	return fmt.Errorf("set: -%c: invalid option", letter)
+}
+
+// optionFlags returns the command line flags of gosh that pass the options
+// on to a subshell.
+func optionFlags() []string {
+	var flags []string
+	if errexit {
+		flags = append(flags, "-e")
+	}
+	if xtrace {
+		flags = append(flags, "-x")
+	}
+	if pipefail {
+		flags = append(flags, "-o", "pipefail")
+	}
+	return flags
+}
+
+// builtinSet implements set: set [-+ex] [-+o name] [--] [args], set -o and
+// set +o (show the options), set without arguments (show the variables) and
+// gosh's set VAR=value.
+func builtinSet(args []string, stdout io.Writer) error {
+	args = args[1:]
+	if len(args) == 0 {
+		env := os.Environ()
+		sort.Strings(env)
+		for _, kv := range env {
+			name, value, _ := strings.Cut(kv, "=")
+			if isVarName(name) {
+				fmt.Fprintf(stdout, "%s=%s\n", name, shellQuote(value))
+			}
+		}
+		return nil
+	}
+	if len(args) == 1 && isAssignment(args[0]) {
+		// set VAR=value
+		name, value, _ := strings.Cut(args[0], "=")
+		os.Setenv(name, value)
+		return nil
+	}
+
+	for len(args) > 0 {
+		arg := args[0]
+		if arg == "--" {
+			positional = slices.Clone(args[1:])
+			return nil
+		}
+		if len(arg) < 2 || arg[0] != '-' && arg[0] != '+' {
+			break
+		}
+		on := arg[0] == '-'
+		args = args[1:]
+		if arg[1:] == "o" {
+			if len(args) == 0 {
+				printOptions(stdout, on)
+				return nil
+			}
+			if err := setOption(0, args[0], on); err != nil {
+				return err
+			}
+			args = args[1:]
+			continue
+		}
+		for k := 1; k < len(arg); k++ {
+			if arg[k] == 'o' {
+				// set -euo pipefail: the name is the next argument
+				if len(args) == 0 {
+					return fmt.Errorf("set: %co: option requires an argument", arg[0])
+				}
+				if err := setOption(0, args[0], on); err != nil {
+					return err
+				}
+				args = args[1:]
+				continue
+			}
+			if err := setOption(arg[k], "", on); err != nil {
+				return err
+			}
+		}
+	}
+	if len(args) > 0 {
+		positional = slices.Clone(args)
+	}
+	return nil
+}
+
+// onlyShows reports whether a builtin that changes the shell is called in a
+// way that only shows something (trap -p, set -o), so that it can be used in
+// a pipeline.
+func onlyShows(args []string) bool {
+	switch args[0] {
+	case "trap":
+		return len(args) == 1 || args[1] == "-p" || args[1] == "-l"
+	case "set":
+		return len(args) == 1 || len(args) == 2 && (args[1] == "-o" || args[1] == "+o")
+	}
+	return false
+}
+
+// printOptions shows the options like set -o (human readable) or set +o
+// (as commands).
+func printOptions(w io.Writer, human bool) {
+	for _, o := range shellOptions {
+		on := o.value == nil || *o.value
+		switch {
+		case human && on:
+			fmt.Fprintf(w, "%-15s on\n", o.name)
+		case human:
+			fmt.Fprintf(w, "%-15s off\n", o.name)
+		case on:
+			fmt.Fprintf(w, "set -o %s\n", o.name)
+		default:
+			fmt.Fprintf(w, "set +o %s\n", o.name)
+		}
+	}
+}
+
+// shellQuote quotes s for the shell if it contains special characters.
+func shellQuote(s string) string {
+	if s != "" && strings.Trim(s, "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_-+=.,:/@%") == "" {
+		return s
+	}
+	return "'" + strings.ReplaceAll(s, "'", `'\''`) + "'"
+}
+
+// traceCommand prints a simple command for set -x.
+func traceCommand(st stage) {
+	var words []string
+	for _, a := range st.assigns {
+		name, value, _ := strings.Cut(a, "=")
+		words = append(words, name+"="+shellQuote(value))
+	}
+	for _, arg := range st.args {
+		words = append(words, shellQuote(arg))
+	}
+	if len(words) > 0 {
+		fmt.Fprintln(stdio(2), "+", sanitize(strings.Join(words, " ")))
+	}
+}
+
+// checkErrexit ends the shell with set -e if the pipeline pl, the last one
+// of its and-or list, failed. Compound commands are not checked themselves
+// (the failing command in them is), but subshells and pipelines are.
+func checkErrexit(pl *pipelineNode, err error) {
+	status := exitStatus(err)
+	if !errexit || errexitOff > 0 || pl.negate || status == 0 {
+		return
+	}
+	var (
+		lc  loopControl
+		ret returnSignal
+	)
+	if errors.As(err, &lc) || errors.As(err, &ret) {
+		return
+	}
+	cmd := pl.cmd
+	if r, ok := cmd.(*redirNode); ok {
+		cmd = r.cmd
+	}
+	switch cmd.(type) {
+	case *simpleNode, *pipeNode, *subshellNode:
+		printError(err)
+		exitShell(status)
+	}
+}
+
+// builtinEval implements eval: the arguments, joined with blanks, run as a
+// command in the shell.
+func builtinEval(args []string) error {
+	text := strings.Join(args[1:], " ")
+	if strings.TrimSpace(text) == "" {
+		return nil
+	}
+	tree, err := parse(text)
+	if errors.Is(err, errIncomplete) {
+		return errors.New("eval: syntax error: unexpected end of input")
+	}
+	if err != nil {
+		return err
+	}
+	return tree.exec()
+}
+
+// builtinExec implements exec: without a command its redirections apply to
+// the shell itself (exec > log), otherwise the command replaces the shell.
+func builtinExec(args []string, fds [3]*os.File, assigns []string) error {
+	for fd, f := range fds {
+		if f != stdio(fd) {
+			if err := dupTo(f, fd); err != nil {
+				return fmt.Errorf("exec: %w", err)
+			}
+		}
+	}
+	if len(args) == 1 {
+		for _, a := range assigns {
+			name, value, _ := strings.Cut(a, "=")
+			os.Setenv(name, value)
+		}
+		return nil
+	}
+
+	path, err := exec.LookPath(args[1])
+	if err != nil {
+		err = fmt.Errorf("%s: %w", sanitize(args[1]), errCommandNotFound)
+		return execFailed(err)
+	}
+	releaseTerminal()
+	err = syscall.Exec(path, args[1:], append(os.Environ(), assigns...))
+	// Exec only returns if it failed
+	return execFailed(fmt.Errorf("exec: %s: %w", sanitize(args[1]), err))
+}
+
+// execFailed reports a failed exec. A non-interactive shell exits then, like
+// sh.
+func execFailed(err error) error {
+	if interactiveShell {
+		return err
+	}
+	printError(err)
+	exitShell(exitStatus(err))
+	return err
+}