dirGIDFile := filepath.Join(setgiddir, "setgid")
	if err := sh.moveOrCopyFile(dirGIDFile, pkgfile.Name(), 0666, true); err != nil {
		t.Fatalf("moveOrCopyFile: %v", err)
	}

	got := strings.TrimSpace(cmdBuf.String())
	want := sh.fmtCmd("", "cp %s %s", pkgfile.Name(), dirGIDFile)
	if got != want {
		t.Fatalf("moveOrCopyFile(%q, %q): want %q, got %q", dirGIDFile, pkgfile.Name(), want, got)
	}

// Try not to use this unless there's really no sensible Action available.
func (b *Builder) BackgroundShell() *Shell {
	return b.backgroundSh
}

// moveOrCopyFile is like 'mv src dst' or 'cp src dst'.
func (sh *Shell) moveOrCopyFile(dst, src string, perm fs.FileMode, force bool) error {
	if cfg.BuildN {
		sh.ShowCmd("", "mv %s %s", src, dst)
		return nil
	}

	dir, _ := filepath.Split(a.Target)
	if dir != "" {
		if err := sh.Mkdir(dir); err != nil {
			return err
		}
	}

	if !a.buggyInstall {
		defer b.cleanup(a1)
	}

	return sh.moveOrCopyFile(a.Target, a1.built, perm, false)
}

// AllowInstall returns a non-nil error if this invocation of the go command is
// allowed to install a.Target.
//