7 ERB templates #

ERB stands for Embedded Ruby. ERB uses the power of the Ruby programming language to generate different kinds of content. With ERB, you can include some Ruby code in your profiles to adapt them at runtime, depending on the installation system.

When using ERB, the Ruby code is enclosed between <% and %> signs. Use an equals sign, =, to include command output in the resulting profile.

<bootloader>
  <% require "open-uri" %>
  <%= URI.open("http://192.168.1.1/profiles/bootloader-common.xml").read %>
</bootloader> <!-- this line gets replaced with the content of bootloader-common.xml -->

You can use Ruby facilities to run arbitrary commands. If you want to get the output of a command, then enclose it between backticks. If you want to know whether a command was successful or not, run the command with the system function.

Also, you can use more advanced Ruby code structures such as conditions and loops.

AutoYaST offers a small set of helper functions to retrieve information from the underlying system, like disks or network_cards. You can check the list of helpers and their values in the Section 7.2, “Template helpers” section.

Template helpers are sets of Ruby methods that can be used in the profiles to retrieve information about the installation system.

<partitioning t="list">
  <drive>
    <% disk = disks.sort_by { |d| d[:size] }.last %> <!-- find the largest disk -->
    <device><%= disk[:device] %></device> <!-- print the disk device name -->
    <initialize t="boolean">true</initialize>
    <use>all</use>
  </drive>
</partitioning>

<interfaces t="list">
  <% with_link = network_cards.sort_by { |n| n[:name] }.find { |n| n[:link] } %>
  <% if with_link %>
    <interface>
      <device><%= with_link[:device] %></device>
      <startmode>auto</startmode>
      <bootproto>dhcp</bootproto>
      </interface>
  <% end %>
</interfaces>

You can use the Ruby console to run AutoYaST ERB helpers and find out what they offer. All ERB helpers are accessed through an instance of the Y2Autoinstallation::Y2ERB::TemplateEnvironment class. Start the Ruby interactive interpreter by running, as root: irb -ryast -rautoinstall/y2erb.

The AutoYaST command line provides a check-profile command that can be used to generate a profile from an ERB file. This command asks AutoYaST to parse, run the ERB code, and generate the resulting profile. You can inspect the rendered profile to check that everything worked as expected. See the command help for all the options it supports: autoyast check-profile --help. In the following example, check-profile asks AutoYaST to download and parse the profile, interpret the ERB code and run the pre-scripts. The result will be dumped to the result.xml file.

Warning: check-profile permissions

In most cases, check-profile requires root permissions, so be careful when running pre-installation scripts and ERB profiles as root. Use only profiles that you trust.

For those cases in which you want to stop the ERB evaluation and check what is happening, YaST offers integration with the byebug debugger. Install the rubygem(byebug) package and set the Y2DEBUGGER environment variable to 1.

Adding breakpoints is as easy as adding <% byebug %> where you want to stop. For more information about byebug, see https://github.com/deivid-rodriguez/byebug.

Although both ERB and rules/classes enable generating profiles dynamically, in general ERB profiles are easier to read and understand. One important difference is that rules and classes can merge profiles, and ERB cannot. See more about merging profiles in Chapter 6, Rules and classes. On the other hand, ERB brings all the power of a high-level language, Ruby. Let's see an example using both. In the following example, we want to place /home directory in /dev/sdb if it exists.