Templator

Templator is a command line tool allowing to generate text documents from templates written in the ERB template language.

It also provides a Domain Specific Language, the Parameter DSL, to define a set of parameters that can be referenced from template files in order to generate the target document with expected values.

Templator is developped in Ruby. It requires Ruby 1.8.7 and higher, or any version of the 1.9 branch.

To quickly install Templator, use the following command:

gem install templator

The following command allows to display the online help:

$ templator help

Two tasks are available from the command line:

• gen

This task is responsible for the transformation of a given template to a target document, taking into account any provided parameter files.

Here is the most simple command line invokation.

$ templator gen path/to/template path/to/target

Files that define parameters can be passed to Templator with the -p switch:

$ templator gen path/to/template path/to/target -p path/to/paramaters1 path/to/parameters2

When parameter files are passed, Templator firstly parses these files with respect to the Parameter DSL (see below). Files are parsed in the same order that they are provided by the -p switch. All parameters exported from these files are then visible by the template.

The -c switch allows to define a default context from which Templator will try to resolve parameter names that are not fully qualified in the template. More details are provided at the end of this document.

• get_param

This task allows to get the value of a parameter from the provided parameter files.

$ templator get_param 'my_parameter' -p path/to/parameters 

The set of parameters is expressed in a Ruby DSL that provides following methods:

• export

This method allows to define new parameters and make them visible from a template during the generation process. Following example shows how to define the parameter 'my_parameter' with the value 'my_value'

export "my_parameter" => "my_value"

It is also possible to define several parameters in a single export line:

export "my_parameter" => "my_value", "my_other_parameter" => "my_other_value"

It is worth noting that parameter names can be a Ruby Symbol:

export :my_parameter => "my_value"

More over, the parameter value can be any Ruby valid expression, for example:

export :integer => 3
export :now => Time.now
export :upper_parameter => "my_value".upcase

Last but not least, you can use the value of previously defined parameters to build a more complex parameter:

export :parameter1 => 1
export :parameter2 => 2
export :sum => parameter1 + parameter2

Each time the Parameter DSL parser encounters an exported parameter, it defines a method with the same name. In the previous example, the value of :parameter1 and :parameter2 is gotten by invoking the corresponding methods, parameter1 et parameter2.

• group

The group method allows to define a subset of parameters.

group :my_group do
    export :my_parameter => "my_value"
end

Nested group is also possible:

group :top do
    group :inner do
        ...
    end
end

Value of parameters defined in other groups must be retrieved with the fully qualified name of the parameter in dot notation.

group :foo_group do
    export :foo => "foo"
end

group :bar_group do
    export :bar => "bar"
end

group :foobar_group do
    export :foobar => foo_group.foo + bar_group.bar
end

A group can de defined multiple times. The resulting group is a merge of all definitions taking into account the order of the parsing:

#file1
group :my_group do
    export :parameter1 => 1
    export :parameter2 => 2
end

#file2
group :my_group do
    export :parameter1 => 0.99999
    export :parameter3 => 3
end

Assuming that file1 and file2 are parsed in this order, the resulting group is semantically equivalent to this one:

group :my_group do
    export :parameter1 => 0.99999
    export :parameter2 => 2
    export :parameter3 => 3
end

• include_group

The include_group method is an interesting way to share some common parameters between different groups. It allows to mix the parameters of a group in another one. It is conceptually equivalent to the well known Ruby include method.

Consider the following example:

group :mixin do
    export :mixme => "some value"
end

group :my_group do
    include_group :mixin
    export :another_parameter => "another_value"
end

Thus, the resulting group is equivalent to :

group :my_group do
    export :mixme => "some value"
    export :another_parameter => "another_value"
end

As said before, the template language used by Templator is ERB.

In addition to the features provided by ERB, the following extra methods can be invoked from a template:

• param

This method allows to retrieve the value of parameters passed to Templator by the -p swicth.

Here is a concrete example:

File parameters.txt:

group :my_group do
    export :my_parameter => "my_value"
end
...

File template.txt:

The value of the parameter "my_parameter" defined in the group "my_group" is <%= param "my_group.my_parameter" %>

Command line invokation from the shell:

$ templator gen template.txt output -p parameters.txt

The resulting output file should have the following content:

The value of the parameter "my_parameter" defined in the group "my_group" is my_value

• param_exists?

This method tests if a parameter is defined. Consider the following template example:

<% if param_exists? "my_group.my_parameter" %>
The parameter "my_parameter" is well defined in group "my_group".
<% else %>
There is no parameter "my_parameter" defined in group "my_group".
<% end %>

• include_file

This method parses the content of the given file as an ERB template, and appends the resulting text into the output stream of the source template.

This is a convenient method to spread a template on multiple files.

Here is an example that dynamically generates the name of the template to include according to the value of a parameter:

blah blah blah
<%= include_file "#{param :my_parameter}.txt" %>

The path of the template to include is interpreted relatively from the path of the source template.

Contextual resolution of parameter names

A context is defined with the -c switch. It is eventually used by the param and param_exists? methods to resolve the provided parameter names. Whenever the resolution of a parameter name fails, if a context is defined, it is prepended to the parameter name and a new resolution is tried with the resulting name. In this case, you must ensure that the context matches a valid fully qualified group name.

Context is a convenient way to generate different documents from the same template, assuming that a group of parameters is defined for each expected documents.

Here is a concrete example where the objective is to generate a Debian /etc/network/interfaces file for three different hosts.

File hosts.txt:

group :common_parameters do
    export :gateway => "192.168.121.254"
end

group :host_a do
    export :address => "192.168.121.1"
    export :netmask => "255.255.255.0"
    include_group :common_parameters
end

group :host_b do
    export :address => "192.168.121.2"
    export :netmask => "255.255.255.0"
    include_group :common_parameters
end

group :host_c do
    export :address => "192.168.121.3"
    export :netmask => "255.255.255.0"
    include_group :common_parameters
end

File interfaces.txt:

iface eth0 inet static
    address <%= param :address %>
    netmask <%= param :netmask %>
    gateway <%= param :gateway %>

Command line execution from shell:

for host in host_a host_b host_c
do
    templator gen interfaces.txt interfaces.$host -p hosts.txt -c $host
done

Copyright © 2011 Christophe Arguel. See LICENSE for details.