Filter mode

--filter QUERY (or -f QUERY) makes fzf scan its input, apply the query, print every match in score order, and exit — no UI involved. This is the canonical way to use fzf inside scripts.

Behavior

$ printf 'foo\nbar\nbaz\nfox\n' | fzf -f 'fo'
foo
fox

• Reads from stdin (or FZF_DEFAULT_COMMAND if stdin is a TTY) just like the interactive mode.
• Applies the same matching pipeline as the interactive mode: extended-search syntax, normalization, scheme, scoring.
• Prints matches to stdout in descending score order, with tiebreakers from --tiebreak.
• Exits with code 0 if at least one item matched, 1 (ExitNoMatch) otherwise.

Streaming filter

When fzf can stream — which means: --no-sort is set, --no-tac is set, --sync is not, and --bench is not — Run in src/core.go skips chunk-list snapshotting entirely and runs the matcher inline as items arrive:

// Filtering mode (in Run)
if streamingFilter {
    reader := NewReader(func(runes []byte) bool {
        item := Item{}
        if chunkList.trans(&item, runes) {
            ...
            if result, _, _ := pattern.MatchItem(&item, false, slab); result.item != nil {
                opts.Printer(transformer(&item))
                found = true
            }
        }
        return false
    }, eventBox, executor, opts.ReadZero, false)
    reader.ReadSource(...)
}

Streaming mode is what makes cat huge-log | fzf -f foo --no-sort start producing output immediately.

When sorting is required, fzf falls back to the standard scan-and-sort path: read everything into the chunk list, snapshot it, run a parallel scan via Matcher.scan, print the merger.

Useful flags

| Flag | Effect | | --------------------- | ------------------------------------------------------------------ | ----------------------------- | | --no-sort | Stream output as matches are found. | | --print-query | Print the query as the first output line. Useful when chaining. | | --exit-0 | Exit immediately with ExitNoMatch if there are no matches. | | --select-1 | Auto-accept if exactly one item matches (interactive mode mostly). | | -d, --delimiter=STR | Field delimiter (regex) for --nth / --with-nth. | | -n, --nth=... | Restrict matching to specific fields. | | --with-nth=... | Transform what is matched against. | | --accept-nth=... | Choose which fields to print. | | --tiebreak=... | Comma-separated tiebreak criteria. | | --scheme=path | history | Apply a tuned scoring scheme. | | --ansi | Honor ANSI escape sequences in input. |

--bench

--bench DURATION (e.g. --bench=2s) runs the matcher in a tight loop on the same input for the given duration and prints timing statistics. It's a microbenchmark for the matcher, not a real filter; the rest of the pipeline is bypassed.

Where it lives

Concern	File
Filter branch	src/core.go (after if opts.Filter != nil)
Streaming filter	src/core.go (streamingFilter block)
Tests	test/test_filter.rb