title: tree command on osx bash
body: <p>I'm following a <a href="http://vimeo.com/26391171" rel="noreferrer">screen cast</a> on a ruby gem called <a href="http://pry.github.com/" rel="noreferrer">pry</a>. At 8:10, the .tree command is used, which I believe is a Unix command. </p>
  
  <p>It does not appear to be working on my system:</p>
  
  <pre><code>[24] pry(main)&gt; .tree
  \Error: there was a problem executing system command: tree
  </code></pre>
  
  <p>and I have traced the issue to <a href="http://rubydoc.info/github/pry/pry/master/Pry/DefaultCommands" rel="noreferrer">here</a>, in which pry references a shell command:</p>
  
  <pre><code>Pry::CommandSet.new do
  
    command(/\.(.*)/, "All text following a '.' is forwarded to the shell.", :listing =&gt; ".&lt;shell command&gt;") do |cmd|
      if cmd =~ /^cd\s+(.+)/i
        dest = $1
        begin
          Dir.chdir File.expand_path(dest)
        rescue Errno::ENOENT
          output.puts "No such directory: #{dest}"
        end
  
      else
        if !system(cmd)
          output.puts "Error: there was a problem executing system command: #{cmd}"
        end
      end
    end
  </code></pre>
  
  <p>from the context of bash I tried using the command tree with no luck:</p>
  
  <pre><code>projects/sms(apps2)$ tree
  -bash: tree: command not found
  ~/projects/sms(apps2)$ .tree
  -bash: .tree: command not found
  </code></pre>
  
  <p>This looks incredibly useful, how can I get this command?</p>