the length of stdout should be greater than 0 when using ren

Error message

the length of stdout should be greater than 0 when using render post hook with change

What it means

RunPostHooks in skaffold's render hook pipeline runs a user-defined post-render hook that may modify the rendered manifests. After such a 'change' hook runs, skaffold reloads the manifest from the hook's captured stdout buffer; if the buffer is empty (the hook printed nothing), skaffold cannot reload anything and fails fast with this explicit message rather than silently treating the manifests as deleted/empty.

Source

Thrown at pkg/skaffold/hooks/render.go:99

	}
	env := r.getEnv()
	for _, h := range r.PostHooks {
		if h.HostHook != nil {
			hook := hostHook{latest.HostHook{
				Command: h.HostHook.Command,
				OS:      h.HostHook.OS,
				Dir:     h.HostHook.Dir,
			}, env}
			var b bytes.Buffer
			if h.HostHook.WithChange {
				if err := hook.run(ctx, updated.Reader(), &b); err != nil {
					if errors.Is(err, &Skip{}) {
						continue
					}
					return manifest.ManifestList{}, err
				}
				if b.Len() == 0 {
					return manifest.ManifestList{}, fmt.Errorf("the length of stdout should be greater than 0 when using render post hook with change")
				}
				updated, err = manifest.Load(&b)
				if err != nil {
					return manifest.ManifestList{}, fmt.Errorf("failed to load manifest")
				}
			} else {
				if err := hook.run(ctx, updated.Reader(), logWriter); err != nil && !errors.Is(err, &Skip{}) {
					return manifest.ManifestList{}, err
				}
			}
		}
	}
	if len(r.PostHooks) > 0 {
		output.Default.Fprintln(logWriter, fmt.Sprintf("Completed %s hooks", phases.PostRender))
	}
	return updated, nil
}

View on GitHub (pinned to a1189de023)

Solutions

Make the hook command print the full modified manifest JSON/YAML to stdout (e.g. `yq eval '...'` without -i; use `yq eval -i` only for file mode, not stdout mode)
Fix redirection mistakes: remove `> file`, `2>&1 >/dev/null`, or tee-to-file patterns that consume stdout
Verify the hook actually processes the input manifests rather than exiting early on a no-match condition

Example fix

// before (skaffold.yaml)
postHooks:
  host:
    command: ["sh", "-c", "cat manifests.yaml | yq e '.metadata.namespace = \"prod\"' -i"]
// after
postHooks:
  host:
    command: ["sh", "-c", "cat manifests.yaml | yq e '.metadata.namespace = \"prod\"' -"]

Defensive patterns

Strategy: validation

Validate before calling

// before relying on a change-type render post hook, verify it emits the manifest on stdout:
out=$(sh -c "$hook_command" <<< "$manifests")
if [ -z "$out" ]; then echo 'hook stdout is empty; fix the hook to print manifests' >&2; exit 1; fi

Prevention

Never redirect hook stdout to a file; print the transformed manifest to stdout
Send logs/diagnostics to stderr only
Test change hooks locally by piping a sample manifest and checking output is non-empty and parseable

When it happens

Trigger: A render post-hook configured with OS command + `when: change` runs successfully (exit 0, no Skip) but writes nothing to stdout, so b.Len() == 0 when manifest.Load(&b) is attempted.

Common situations: Hook command redirects output to a file instead of stdout; hook echoes to stderr; hook silently no-ops because its condition didn't match (e.g. no matching files); a YAML-processing tool (yq, kustomize) invoked with wrong flags that produce empty output; writing a script that logs progress to stdout and pipes the manifest elsewhere.

Related errors

AI-assisted analysis of GoogleContainerTools/skaffold@a1189de023 (2026-09-05). Data as JSON: /api/errors/12e60f6723eff9e1. Report an issue: GitHub.