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>

jobs.go

@@ -286,7 +286,16 @@ func (j *job) result() error {
 	for _, p := range j.procs[:len(j.procs)-1] {
 		printError(p.err)
 	}
-	return j.procs[len(j.procs)-1].err
+	last := j.procs[len(j.procs)-1].err
+	if pipefail && exitStatus(last) == 0 {
+		// The status of the last command that failed
+		for i := len(j.procs) - 2; i >= 0; i-- {
+			if err := j.procs[i].err; exitStatus(err) != 0 {
+				return silent(err)
+			}
+		}
+	}
+	return last
 }
 
 // waitForeground waits for a job in the foreground. If it is stopped, it is
@@ -504,9 +513,12 @@ func builtinBg(args []string, stdout io.Writer) error {
 func builtinWait(args []string) error {
 	jobs := append([]*job(nil), jobTable...)
 	if len(args) > 1 {
-		j, err := findJob("wait", args)
-		if err != nil {
-			return err
+		j := jobByPid(args[1])
+		if j == nil {
+			var err error
+			if j, err = findJob("wait", args); err != nil {
+				return err
+			}
 		}
 		jobs = []*job{j}
 	}
@@ -523,6 +535,23 @@ func builtinWait(args []string) error {
 	return err
 }
 
+// jobByPid returns the job with a process with the ID in spec (wait $!), or
+// nil.
+func jobByPid(spec string) *job {
+	pid, err := strconv.Atoi(spec)
+	if err != nil {
+		return nil
+	}
+	for _, j := range jobTable {
+		for _, p := range j.procs {
+			if p.cmd != nil && p.cmd.Process.Pid == pid {
+				return j
+			}
+		}
+	}
+	return nil
+}
+
 // exitShell exits the shell, unless there are stopped jobs and the user was
 // not warned yet (like bash).
 func exitShell(code int) error {
@@ -530,6 +559,8 @@ func exitShell(code int) error {
 		exitWarned = true
 		return errors.New("there are stopped jobs")
 	}
+	lastStatus = code // $? in the EXIT trap
+	runExitTrap()
 	releaseTerminal()
 	os.Exit(code)
 	return nil