What causes "invalid duration for --bench: %s" in junegunn/fzf?

Use a valid Go duration literal: --bench 3s, --bench 500ms, --bench 1m

invalid duration for --bench: %s

Error message

invalid duration for --bench: %s

What it means

fzf failed to parse the value passed to --bench as a Go duration. The value is passed straight to time.ParseDuration, so it must use Go duration syntax (e.g. 3s, 500ms, 1m10s). Note the error message wraps the parse failure with the offending string, replacing the underlying Go error text.

Source

Thrown at src/options.go:3459

			if err != nil {
				return err
			}
			opts.WalkerSkip = filterNonEmpty(strings.Split(str, ","))
		case "--threads":
			if opts.Threads, err = nextInt("number of threads required"); err != nil {
				return err
			}
			if opts.Threads < 0 {
				return errors.New("--threads must be a positive integer")
			}
		case "--bench":
			str, err := nextString("duration required (e.g. 3s, 500ms)")
			if err != nil {
				return err
			}
			dur, err := time.ParseDuration(str)
			if err != nil {
				return errors.New("invalid duration for --bench: " + str)
			}
			opts.Bench = dur
		case "--profile-cpu":
			if opts.CPUProfile, err = nextString("file path required: cpu"); err != nil {
				return err
			}
		case "--profile-mem":
			if opts.MEMProfile, err = nextString("file path required: mem"); err != nil {
				return err
			}
		case "--profile-block":
			if opts.BlockProfile, err = nextString("file path required: block"); err != nil {
				return err
			}
		case "--profile-mutex":
			if opts.MutexProfile, err = nextString("file path required: mutex"); err != nil {
				return err
			}

View on GitHub (pinned to bd4efa277b)

Solutions

Use a valid Go duration literal: --bench 3s, --bench 500ms, --bench 1m
If scripting, validate with time.ParseDuration before invoking fzf
Check that no stray characters or unit suffixes like 'sec' are appended

Example fix

# before
fzf --bench 100
# after
fzf --bench 100ms

Defensive patterns

Strategy: validation

Validate before calling

// Go: validate before exec
if _, err := time.ParseDuration(benchFlag); err != nil {
    log.Fatalf("--bench must be a Go duration like 3s or 500ms: %v", err)
}
_ = exec.Command("fzf", "--bench", benchFlag).Run()

Type guard

func isValidGoDuration(s string) bool { _, err := time.ParseDuration(s); return err == nil }

Try / catch

In Go, check err from fzf's exit (exec.ExitError stderr) and if it mentions 'invalid duration for --bench', print the accepted syntax and re-prompt for the value.

Prevention

Always use Go duration units (s, ms, m)
Validate duration strings with time.ParseDuration before building the fzf command line
Centralize fzf flags in one config builder to review units once

When it happens

Trigger: Running fzf with --bench set to a bare number (e.g. --bench 3 instead of --bench 3s), or a non-duration string (e.g. --bench fast). The case block in options.go:3459 calls nextString then time.ParseDuration(str) and returns this error on any parse failure.

Common situations: Users used to millisecond counts (--bench 100) or second-only integers; copying benchmark flags from other tools that accept bare numbers; typos like '3sec' instead of '3s'.

Related errors

AI-assisted analysis of junegunn/fzf@bd4efa277b (2026-08-15). Data as JSON: /api/errors/7121e4af2e6cfc89. Report an issue: GitHub.