charmbracelet/glow · error

unable to write to writer: %w

Error message

unable to write to writer: %w

What it means

The default display path writes the rendered output with fmt.Fprint(w, out). Failure means writing to the destination writer failed - most commonly a broken pipe (EPIPE) when the downstream consumer exits early, e.g. glow README.md | head. It can also mean the output target is unwritable: disk full (ENOSPC) or a redirected file on a dead mount.

Source

Thrown at main.go:341

		if err != nil || len(fields) == 0 {
			return fmt.Errorf("unable to parse PAGER command: %s", pagerCmd)
		}
		c := exec.Command(fields[0], fields[1:]...) //nolint:gosec
		c.Stdin = strings.NewReader(out)
		c.Stdout = os.Stdout
		if err := c.Run(); err != nil {
			return fmt.Errorf("unable to run command: %w", err)
		}
		return nil
	case tui || cmd.Flags().Changed("tui"):
		path := ""
		if !isURL(src.URL) {
			path = src.URL
		}
		return runTUI(path, content)
	default:
		if _, err = fmt.Fprint(w, out); err != nil {
			return fmt.Errorf("unable to write to writer: %w", err)
		}
		return nil
	}
}

func runTUI(path string, content string) error {
	// Read environment to get debugging stuff
	cfg, err := env.ParseAs[ui.Config]()
	if err != nil {
		return fmt.Errorf("error parsing config: %v", err)
	}

	// use style set in env, or auto if unset
	if err := validateStyle(cfg.GlamourStyle); err != nil {
		cfg.GlamourStyle = style
	}

	cfg.Path = path

View on GitHub (pinned to e3970c813d)

Solutions

1. If piping into head-like tools, treat EPIPE as benign - it is expected, not a glow bug
2. Check disk space when redirecting to a file: df -h .
3. Write to a file first and inspect the tail: glow README.md > out.txt
4. Use less as the consumer (glow -p) instead of piping to head

func isEPIPE(err error) bool { return errors.Is(err, syscall.EPIPE) }

if _, err := fmt.Fprint(w, out); err != nil {
	if errors.Is(err, syscall.EPIPE) {
		return nil // downstream reader closed early: not a real failure
	}
	return fmt.Errorf("unable to write to writer: %w", err)
}

When it happens

Trigger: Piping glow into head/less/grep -m1 that exits before glow finishes writing (EPIPE); stdout redirected to a file on a full filesystem; stdout closed or revoked mid-write.

Common situations: Piping rendered output into short-lived readers, CI log capture with size caps, disk-full conditions when redirecting to files.

Related errors

• unable to write config file: %w
• unable to read http response body: %w
• unable to read http response body: %w
• unable to open file: %w
• unable to read from reader: %w

AI-assisted analysis of charmbracelet/glow@e3970c813d (2026-08-15). Data as JSON: /api/errors/689b317e42856b14. Report an issue: GitHub.