What causes "not a valid file descriptor number: %#v" in kovidgoyal/kitty?

Use a plain decimal integer: `--password fd:3`

not a valid file descriptor number: %#v

Error message

not a valid file descriptor number: %#v

What it means

For --password fd:VALUE, the part after 'fd:' must parse as an integer via strconv.Atoi. When it does not (Atoi returns an error), the raw value is reported as not a valid file descriptor number.

Source

Thrown at kittens/clipboard/main.go:53

		}
		switch ptype {
		case "text":
			opts.Password = val
		case "fd":
			if fd, err := strconv.Atoi(val); err == nil {
				if f := os.NewFile(uintptr(fd), "password-fd"); f == nil {
					return 1, fmt.Errorf("invalid file descriptor: %d", fd)
				} else {
					data, err := io.ReadAll(f)
					f.Close()
					if err != nil {
						return 1, fmt.Errorf("failed to read from file descriptor: %d with error: %w", fd, err)
					}
					opts.Password = strings.TrimRightFunc(string(data), unicode.IsSpace)
				}

			} else {
				return 1, fmt.Errorf("not a valid file descriptor number: %#v", val)
			}
		case "file":
			if data, err := os.ReadFile(val); err == nil {
				opts.Password = strings.TrimRightFunc(string(data), unicode.IsSpace)
			} else {
				return 1, fmt.Errorf("failed to read from file: %#v with error: %w", val, err)
			}
		}
	}
	if len(args) > 0 {
		return 0, run_mime_loop(opts, args)
	}
	if opts.Password != "" || opts.HumanName != "" {
		return 1, fmt.Errorf("cannot use --human-name or --password in filter mode")
	}
	return 0, run_plain_text_loop(opts)
}

View on GitHub (pinned to 6d5d0c4406)

Solutions

Use a plain decimal integer: `--password fd:3`
If you meant a path, switch the type: `--password file:/path/to/pw`
Trim any accidental whitespace or shell-expanded characters from the value

Example fix

# before
kitten clipboard get --human-name me --password fd:/tmp/pw
# after
kitten clipboard get --human-name me --password file:/tmp/pw

Defensive patterns

Strategy: validation

Validate before calling

val=${PW#fd:}
[[ "$val" =~ ^[0-9]+$ ]] || { echo 'fd: value must be an integer'; exit 2; }

Prevention

Validate the fd suffix is numeric in wrappers
Use file: when tempted to pass a path after fd:

When it happens

Trigger: Passing `--password fd:abc`, `--password fd:3a`, `--password fd:` (empty), or any non-numeric value after the fd: prefix.

Common situations: Typos such as `fd:0x3` or `fd: 3` (leading space); passing a filename instead of a number after fd: because the user meant the file: type; copy-paste from examples that used a different syntax.

Related errors

AI-assisted analysis of kovidgoyal/kitty@6d5d0c4406 (2026-08-27). Data as JSON: /api/errors/24a2e17523683f4e. Report an issue: GitHub.