# Capture pane output

Source: https://libtmux.org/en/csharp/latest/examples/capture-pane-output/

> Run a complete program that captures a pane and waits for a complete output line.

A pane runs asynchronously: sending a command does not mean its output is
already on screen. Capture repeatedly until the expected line appears, with a
deadline so a failed command cannot leave the program waiting forever.

This complete program creates a private tmux server, captures its output, and
cleans up. Follow the [setup and run instructions](https://libtmux.org/en/csharp/latest/examples/capture-pane-output/#setup-and-run) below. You need
tmux and a Unix environment; no existing tmux session is required.

## Read what's on screen

The program sends `printf` with a leading newline, then waits for the complete
line `libtmux capture ready`. The newline keeps a late shell prompt off that
line. Matching the whole line avoids mistaking the echoed command for its output.

```csharp title="Program.cs"
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using LibTmux;

if (OperatingSystem.IsWindows())
    throw new PlatformNotSupportedException("Run this example on Linux or macOS");

string directory = Path.Combine(
    "/tmp/libtmux-dotnet-dev", Guid.NewGuid().ToString("N"));
Directory.CreateDirectory(directory);
try
{
    using var timeout = new CancellationTokenSource(TimeSpan.FromSeconds(10));
    CancellationToken token = timeout.Token;
    await using OwnedServerScope owned = await Server.CreateOwnedAsync(
        new ServerConnectionOptions
        {
            SocketPath = Path.Combine(directory, "tmux.sock"),
            ConfigurationFile = "/dev/null",
            ChildEnvironment = new Dictionary<string, string?>
            {
                ["TMUX"] = null,
                ["TMUX_PANE"] = null,
                ["ENV"] = null,
                ["BASH_ENV"] = null,
            },
        }, token);
    Session session = await owned.Value.CreateSessionAsync(
        new NewSessionRequest { Name = "capture", Command = "/bin/sh" }, token);
    Pane pane = (await session.GetPanesAsync(token))[0];
    await pane.SendTextAsync(
        "printf '\\nlibtmux capture ready\\n'", cancellationToken: token);

    var elapsed = Stopwatch.StartNew();
    bool captured = false;
    while (elapsed.Elapsed < TimeSpan.FromSeconds(5))
    {
        var lines = await pane.CaptureAsync(cancellationToken: token);
        if (lines.Contains("libtmux capture ready"))
        {
            Console.WriteLine("libtmux capture ready");
            captured = true;
            break;
        }
        await Task.Delay(25, token);
    }
    if (!captured)
        throw new TimeoutException("Output did not arrive within five seconds");
}
finally
{
    Directory.Delete(directory, recursive: true);
}
```

<a id="wait-for-text-instead-of-guessing-a-delay"></a>

## Wait for output or completion

The program above checks the captured screen for up to five seconds. The short
pause between checks limits polling; the observed output determines when the loop
finishes. A tmux capture is a view of the screen and scrollback, so it can miss
output that has already scrolled away. Use a stream or a completion signal for
long-running commands when that distinction matters.

[Capturing output](https://libtmux.org/en/csharp/latest/guides/capturing-output/) covers capture options, while
[Sending keys](https://libtmux.org/en/csharp/latest/guides/sending-keys/#the-race-you-cant-see-from-the-call-site)
explains why sending and waiting are separate operations.

## Setup and run

Use an empty directory. The commands pin the library
revision used to verify the program.

Save the program and project file using the displayed names. Use the .NET 10
SDK. The project disables implicit imports, so every required import
appears in the program.

```xml title="Capture.csproj"
<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net10.0</TargetFramework>
    <ImplicitUsings>disable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <EnableDefaultCompileItems>false</EnableDefaultCompileItems>
  </PropertyGroup>
  <ItemGroup>
    <Compile Include="Program.cs" />
    <ProjectReference Include="libtmux-source/src/LibTmux/LibTmux.csproj" />
  </ItemGroup>
</Project>
```

```console
$ git clone https://github.com/libtmux/libtmux-dotnet libtmux-source &&
  git -C libtmux-source checkout 320dc64f4b8b7815842471327a5e6b84a1499bf8 &&
  dotnet build Capture.csproj --maxcpucount:1 &&
  dotnet run --project Capture.csproj --no-build
```

<a id="source-inclusion"></a>

## Where this comes from

This complete program was run against the library revision pinned above.
The displayed code is checked against the bytes from that run.