# Control mode vs one-shot

Source: https://libtmux.org/en/cxx/latest/concepts/transports/

> Choose subprocesses, command batches, or persistent connections to send commands and receive events.

libtmux sends commands to tmux through subprocesses or persistent control-mode
connections. tmux also accepts several commands in one invocation:

1. **One-shot subprocess.** Each command spawns a fresh [`tmux`](<https://libtmux.org/en/cxx/latest/reference/libtmux-serverimplementation-tmux/>) process,
   which sends the request to the server, prints the result, and exits.
2. **A persistent control-mode client.** [`tmux -C attach-session`](https://libtmux.org/en/tmux/latest/manual/attach-session/) starts one
   long-lived tmux process that stays attached and speaks a line-oriented
   protocol over its stdout: commands go in, replies and asynchronous
   notifications (`%window-add`, `%output`, ...) come out, without starting a
   process per call.
3. **One invocation, several commands.** tmux accepts more than one command
   per invocation (`;`-joined, or one `-F`-tagged `list-*` per line). Grouping
   operations this way reduces process starts without opening a control-mode
   connection.

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## Available transports

The default transport uses bounded subprocesses. [`Chain`](<https://libtmux.org/en/cxx/latest/reference/libtmux-chain/>) groups commands
into an invocation. [`Server::control()`](<https://libtmux.org/en/cxx/latest/reference/libtmux-server-control/>) opens a persistent [`Connection`](<https://libtmux.org/en/cxx/latest/reference/libtmux-connection/>).

Choose based on whether you need command results, notifications, or a batch of
changes.

## A control client is a real client

A persistent control connection attaches a tmux client. It appears in
[`list-clients`](https://libtmux.org/en/tmux/latest/manual/list-clients/), increments [`session_attached`](https://libtmux.org/en/tmux/latest/catalog/formats/session_attached/), and affects [`destroy-unattached`](https://libtmux.org/en/tmux/latest/catalog/options/destroy-unattached/),
client hooks, and idle-client accounting. Each connection counts separately.

## Why fold several commands into one invocation

Creating an object can require a second command to read its resulting state.
Batching can reduce repeated reads and process starts.

A [`Chain`](<https://libtmux.org/en/cxx/latest/reference/libtmux-chain/>) groups operations without attaching a control client.

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## What this costs in practice

A persistent connection avoids starting a client for each command. A chain
groups a known sequence into one invocation. For occasional commands, use the
default subprocess transport; measure your workload before changing transports
for performance. Use a notification stream when your program needs tmux events.

## Sending a command

The example uses the subprocess API. See the reference for batching and
control-mode setup.

```cpp
#include <libtmux/libtmux.hpp>

// One-shot: every call answers with a value; no tmux failure is thrown.
const auto server = libtmux::Server::at_default();
if (!server.has_value()) {
  return 1;
}

const auto session = server->new_session("work");
if (!session.has_value()) {
  return 1;
}

const auto pane = session->active_pane();
if (pane.has_value()) {
  (void)pane->send_text("echo hello");
  (void)pane->send_key("Enter");
}
```

To inspect tmux's control protocol, attach a control client:

```console
$ tmux -C attach-session -t work
```