# Capture and filter an existing server

Capture once, then query the immutable result without more tmux commands.
This example works with the published package. Install it in
an Elixir project as shown in the [README](../README.md#installation).

Save this program as `capture_and_filter.exs` in your application:

<!-- example: examples/capture_and_filter.exs -->

```elixir
alias LibTmux.{Selection, Server, Snapshot}

[executable, socket | _] = System.argv()
{:ok, _} = Application.ensure_all_started(:libtmux)

server =
  Server.new(
    executable: executable,
    socket: socket,
    env: Map.drop(System.get_env(), ["TMUX", "TMUX_PANE"])
  )

{:ok, snapshot} = Server.snapshot(server)

active_ids =
  snapshot
  |> Snapshot.panes()
  |> Selection.where(active: true)
  |> Enum.map(& &1.id)

IO.puts(JSON.encode!(%{active_panes: active_ids}))
```

<!-- /example -->

Set `LIBTMUX_TMUX` to the absolute tmux executable and `LIBTMUX_SOCKET` to
the absolute socket of the existing server you intend to observe. Run:

```console
$ mix run capture_and_filter.exs "$LIBTMUX_TMUX" "$LIBTMUX_SOCKET"
```

The JSON result lists active physical pane IDs in numeric order. A pane is
active within its physical window; this is not a choice of one session's
active window. Linked windows do not duplicate their panes.

`Server.new/1` builds a descriptor without I/O. `Server.snapshot/2` runs
bounded one-shot reads without attaching a control client or creating a
missing server. `Selection.where/2` and `Enum.map/2` use the captured graph.
Capturing covers an interval, not an atomic tmux transaction. Capture again
explicitly when fresh state is needed.

The pattern match makes a capture error visible; production callers can
handle `{:error, %LibTmux.Error{}}`. The default capture budgets are 5000ms,
512 commands, 1024 contextual topology rows, 64 KiB per response and 8 MiB total.
A limit failure returns an error rather than a partial graph.

For an isolated demonstration, use the [owned-server recipe](owned-server.md).
Next: [filter and select](filtering.md), [one-shot actions](one-shot-actions.md),
or [managed control](managed-control.md).