osx bash上的ruby – tree命令

它似乎在我的系统上工作：

[24] pry(main)> .tree
Error: there was a problem executing system command: tree

我已经将问题追溯到here，其中pry引用了一个shell命令：

Pry::CommandSet.new do

  command(/.(.*)/,"All text following a '.' is forwarded to the shell.",:listing => ".<shell command>") do |cmd|
    if cmd =~ /^cds+(.+)/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

从bash的上下文我尝试使用没有运气的命令树：

projects/sms(apps2)$ tree
-bash: tree: command not found
~/projects/sms(apps2)$ .tree
-bash: .tree: command not found

这看起来非常有用，我该如何得到这个命令？