First cut of solution for https://github.com/myitcv/x/issues/83

main.go

package main

import (
	"fmt"
	"os"
	"strings"

	"mvdan.cc/sh/syntax"
)

type slModifier func(*syntax.StmtList) error

func main() {

	sete := func(sl *syntax.StmtList) error {
		if sl.Stmts == nil {
			return nil
		}

		sete := &syntax.Stmt{
			Cmd: &syntax.CallExpr{
				Args: []*syntax.Word{
					&syntax.Word{Parts: []syntax.WordPart{&syntax.Lit{Value: "set"}}},
					&syntax.Word{Parts: []syntax.WordPart{&syntax.Lit{Value: "-euo"}}},
					&syntax.Word{Parts: []syntax.WordPart{&syntax.Lit{Value: "pipefail"}}},
				},
			},
		}

		sl.Stmts = append([]*syntax.Stmt{sete}, sl.Stmts...)

		return nil
	}

	{
		r := strings.NewReader("x=$(ls); (ls)")
		f, err := syntax.NewParser().Parse(r, "")
		if err != nil {
			panic(err)
		}

		fmt.Println("Before:")
		syntax.NewPrinter().Print(os.Stdout, f)

		syntax.Walk(f, func(n syntax.Node) bool {
			var f slModifier
			var sl *syntax.StmtList

			switch n := n.(type) {
			case *syntax.CmdSubst:
				f = sete
				sl = &n.StmtList
			case *syntax.Subshell:
				f = sete
				sl = &n.StmtList
			default:
				return true
			}

			if err := f(sl); err != nil {
				panic(err)
			}

			return true
		})

		fmt.Println()
		fmt.Println("After:")
		syntax.NewPrinter().Print(os.Stdout, f)
	}
}