Capture pane output

This complete Ruby 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.

require "libtmux"

LibTmux::Server.start do |server|
  created = server.new_session(
    name: "capture", command: ["/bin/sh"],
    environment: {"ENV" => "/dev/null"}, receipt: true
  )
  pane = created.pane
  pane.send_text("printf '\\nlibtmux capture ready\\n'")
  pane.send_keys("Enter")

  deadline = Process.clock_gettime(Process::CLOCK_MONOTONIC) + 5
  loop do
    if pane.capture.stdout.lines(chomp: true).include?("libtmux capture ready")
      puts "libtmux capture ready"
      break
    end
    if Process.clock_gettime(Process::CLOCK_MONOTONIC) >= deadline
      raise "Output did not arrive within five seconds"
    end
    sleep 0.025
  end
end

Wait for output or completion¶

mLibTmux::Server.start owns the private server. Leaving its block stops the server, including when capture or the deadline raises an error.

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 Ruby 4.0.7 and Bundler. Bundler installs the library and its runtime dependencies.

Save this dependency file beside the program.

source "https://rubygems.org"

gem "libtmux", path: "libtmux-source/gems/libtmux"

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

$ git clone https://github.com/libtmux/libtmux-ruby libtmux-source &&
  git -C libtmux-source checkout 2599d45369515aaf2fd5793bfe71cdc20de641b6 &&
  bundle config set --local path vendor/bundle &&
  bundle install &&
  bundle exec ruby capture.rb

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.