Capture pane output

This complete Kotlin program starts a private tmux server, sends a command, and captures the line it prints. It includes imports, setup, and cleanup. You need tmux and a Unix environment; no existing session is required.

Read what’s on screen¶

The leading newline puts the output on a fresh row. Matching the whole line avoids mistaking the echoed command for its output.

import io.github.libtmux.ServerConfig
import io.github.libtmux.ServerEndpoint
import io.github.libtmux.kotlin.capture
import io.github.libtmux.kotlin.killServer
import io.github.libtmux.kotlin.newSession
import io.github.libtmux.kotlin.sendLine
import io.github.libtmux.kotlin.withServer
import java.nio.file.Files
import java.nio.file.Path
import java.time.Duration
import kotlinx.coroutines.NonCancellable
import kotlinx.coroutines.delay
import kotlinx.coroutines.runBlocking
import kotlinx.coroutines.withContext
import kotlinx.coroutines.withTimeout

fun main() = runBlocking {
    val root = Files.createDirectories(Path.of("/tmp/libtmux-java-dev"))
    val directory = Files.createTempDirectory(root, "capture-")
    val socket = directory.resolve("tmux.sock")
    val config = ServerConfig.builder()
        .endpoint(ServerEndpoint.socketPath(socket))
        .configFile(Path.of("/dev/null"))
        .defaultTimeout(Duration.ofSeconds(1))
        .build()
    var failure: Throwable? = null
    try {
        withServer(config) { server ->
            try {
                val session = server.newSession {
                    name = "capture"
                    running("/bin/sh")
                    env("ENV", "/dev/null")
                }
                val pane = session.windows.first().panes.first()
                pane.sendLine("printf '\\nlibtmux capture ready\\n'")
                withTimeout(5_000) {
                    while (!pane.capture().contains("libtmux capture ready")) {
                        delay(25)
                    }
                }
                println("libtmux capture ready")
            } catch (error: Throwable) {
                failure = error
                throw error
            } finally {
                withContext(NonCancellable) {
                    try {
                        if (Files.exists(socket)) server.killServer()
                        Files.deleteIfExists(socket)
                        Files.delete(directory)
                    } catch (cleanup: Throwable) {
                        val original = failure
                        if (original == null) throw cleanup
                        original.addSuppressed(cleanup)
                    }
                }
            }
        }
    } finally {
        // Opening a client can fail before the cleanup block is entered.
        if (!Files.exists(socket)) Files.deleteIfExists(directory)
    }
}

Wait for output or completion¶

Capture and input calls suspend. withTimeout bounds the polling loop, and delay yields between captures. withServer closes the client; the finally block also stops the tmux server this program created. Cleanup errors stay visible, and a server that cannot be stopped keeps its socket.

Capture reads screen state and scrollback, so output that has scrolled away may be absent. The program prints libtmux capture ready when its check passes and exits unsuccessfully if an operation fails.

Setup and run¶

Use an empty directory and save the files using the displayed names. You need JDK 25. The pinned repository supplies Gradle; the project files select Kotlin 2.4.10 and include the library build.

Save settings.gradle.kts beside Capture.kt.

rootProject.name = "capture"
includeBuild("libtmux-source")

Save build.gradle.kts beside Capture.kt.

plugins {
    application
    kotlin("jvm") version "2.4.10"
}

repositories { mavenCentral() }
dependencies {
    implementation("io.github.libtmux:libtmux-kotlin:0.0.1-alpha.17-SNAPSHOT")
}
kotlin {
    jvmToolchain(25)
    sourceSets.main { kotlin.srcDir("."); kotlin.include("Capture.kt") }
}
application { mainClass.set("CaptureKt") }

Save gradle.properties beside Capture.kt.

org.gradle.jvmargs=-Xmx2g -XX:MaxMetaspaceSize=768m -Dfile.encoding=UTF-8
kotlin.daemon.jvmargs=-Xmx2g
org.gradle.workers.max=2

The commands pin the library revision used to run this program.

$ git clone https://github.com/libtmux/libtmux-java libtmux-source &&
  git -C libtmux-source checkout 85ebf6955e56703c5be74e2afd34a18309044741 &&
  ./libtmux-source/gradlew --project-dir . run --console=plain --max-workers=2

Where this comes from¶

The displayed files were compiled or loaded with their native tools and run on Linux with tmux 3.2a and 3.7c. The rendering checks preserve those file bytes.

Scripting & web

Compiled & native

JVM

.NET

tmux

Capture pane output

Read what’s on screen¶

Wait for output or completion¶

Setup and run¶

Where this comes from¶