# LibTmux::Selection#one

- **Module:** LibTmux
- **Package:** libtmux
- **Language:** Ruby
- **Kind:** method
- **Source:** https://github.com/libtmux/libtmux-ruby/blob/dcc9f8b5a6970144a9e7d80cd5f381d8f6198372/gems/libtmux/lib/libtmux/selection.rb#L65
- **Page:** https://libtmux.org/en/ruby/v0.1.0.alpha.2/reference/libtmux-selection-one/

```
def one: (?untyped, **untyped) -> Elem
```

Behavior: Selections.

Behavior contract: Selections.

## Examples

Filter a captured snapshot, distinguish a missing session, and acquire a fresh pane selection. one raises unless exactly one record matches; one_or_nil returns nil only for zero matches. Use Ruby 4.0.7, Bundler, Git and tmux 3.2a or newer on Unix, with /bin/cat and /bin/sh available. In an empty directory, fetch the documented library revision:

```console
$ git clone https://github.com/libtmux/libtmux-ruby.git libtmux-source && \
  git -C libtmux-source checkout dcc9f8b5a6970144a9e7d80cd5f381d8f6198372
```

Save this dependency file as Gemfile:

```ruby
source "https://rubygems.org"

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

Save this complete program as query.rb. Its Server.start block owns a private tmux server and stops it on exit, including after an error.

[Source example](https://github.com/libtmux/libtmux-ruby/blob/dcc9f8b5a6970144a9e7d80cd5f381d8f6198372/examples/api/query.rb).

```ruby
# frozen_string_literal: true

require "libtmux"

begin
  LibTmux::Server.start do |server|
    work = server.new_session(name: "work", command: ["/bin/cat"], receipt: true)
    server.new_session(name: "logs", command: ["/bin/cat"])
    snapshot = server.snapshot
    session = snapshot.sessions.where(name: "work").one
    puts "matching session: #{session.name}"
    missing = snapshot.sessions.one_or_nil(name: "missing")
    puts "missing session: #{missing.nil?}"
    panes = server.search_panes(where: {id: work.pane.id})
    puts "matching pane: #{panes.one.id}"
  end
rescue StandardError => error
  warn "#{error.class}: #{error.message}"
  exit 1
end
```

Install the runtime dependencies and run the saved program. An operation or cleanup error makes it exit unsuccessfully:

```console
$ bundle config set --local path vendor/bundle && \
  bundle install --jobs 2 && \
  bundle exec ruby query.rb
```

Expected output:

```text
matching session: work
missing session: true
matching pane: %0
```

## Raises

- [`NoMatchError`](/en/ruby/v0.1.0.alpha.2/reference/libtmux-nomatcherror/)