The Text::Template module allows arbitary
Perl code to be embedded in a template file by surrounding it in
braces ("{" and "}").
The code inside the braces is interpreted and its return
value replaces the section between, and including, the braces.
For instance:
The answer is { 2 + 2 }becomes
The answer is 4
Variables can be passed in from the program which is expanding the template, hence:
Shopping list:
{
$OUT = '';
for my $item ( qw(bread milk bananas) )
{
$OUT .= "* $item\n";
}
}would expand to:
Shopping list: * bread * milk * bananas
The SME Server template system uses this mechanism to automatically pass in global configuration variables from the configuration database which can then be used to fill out the configuration files.
For example, the /etc/hosts template is fairly simple and composed of two fragments:
[gordonr@smebuild hosts]$ pwd /etc/e-smith/templates/etc/hosts [gordonr@smebuild hosts]$ ls 10localhost 20hostname
Let's look at those fragments. The first is a piece of static text, which
Text::Template will include verbatim:
127.0.0.1 localhost
The second is more complex and relies on values from the configuration database:
{
$OUT .= "$LocalIP\t";
$OUT .= " ${SystemName}.${DomainName}";
$OUT .= " ${SystemName}";
}Note that the whole fragment is enclosed in braces. Within those braces is a section of Perl code. When this template is expanded, it results in the following configuration file:
#------------------------------------------------------------ # !!DO NOT MODIFY THIS FILE!! # # Manual changes will be lost when this file is regenerated. # # Please read the developer's guide, which is available # at http://www.contribs.org/development/ # # Copyright (C) 1999-2006 Mitel Networks Corporation #------------------------------------------------------------ 127.0.0.1 localhost 192.168.10.1 smebuild.gormand.com.au smebuild
The header block comes "for free" as part of the template system, courtesy of an optional file template-begin, which is always processed as the first fragment. If it isn't provided, the text shown with # comments is included.
The other lines are provided by the two fragments shown above. Note the use of the configuration database variables: $LocalIP, $SystemName and $DomainName. All simple entries in the configuration database are provided as global variables to the templates.
Note that all of the template fragments are concatenated together before evaluation, so it is possible to set values in fragments which are used in later fragments. This is a very useful model for reducing the code in individual template fragments.
The complex entries in the configuration database are also provided as global variables to the templates. However, they are provided as Perl hashes instead of simple scalars. For example, here is how you might configure the Network Time Protocol (NTP) server /etc/ntp.conf file:
server { $ntpd{NTPServer} }
driftfile /etc/ntp/drift
authenticate noThe NTPServer setting is stored in the ntpd configuration database record, and so can be accessed via the hash accessor $ntpd{NTPServer}.
Each template directory can contain two optional files template-begin and template-end . The template-begin file is always processed as the first file of the template, and the template-end file is always processed as the last file.
If the directory does not contain a template-begin file, the contents of /etc/e-smith/templates-default/template-begin is used automatically.
If the directory does not contain a template-end , nothing is appended to the template output. It is mostly used to provide the closing block for configuration files written in languages such as HTML and PHP, through a link to an entry in the templates-default/ directory.
The /etc/e-smith/templates-default directory contains a set of template-begin and template-end files for various languages. For example, if your template generates a perl script, you would link template-begin to /etc/e-smith/templates-default/template-begin-perl and automatically get the #!/usr/bin/perl -w line and a comment containing the contents of the default template-begin file.
[gordonr@sevendev1 devguide]$ ls /etc/e-smith/templates-default/ template-begin template-begin-perl template-end-php template-begin-html template-begin-php template-begin-pam template-begin-shell
Note: You may also need a templates.metadata configuration file if your generated file needs to be executable.
Template fragments are assembled in ASCII-betical order, with two exceptions: template-begin always comes first, and template-end always comes last. Template fragments are often named to start with a two digit number to make the ordering obvious, but this is not required.
Note: The number of fragments and the order of those fragments within a template directory is subject to change between releases.
Most of the templates on the system map to single, fixed output files, such as /etc/hosts. However, templates are also used to generate configuration files such as mail delivery instructions for users. These templates are stored in the /etc/e-smith/template-user/ tree.
For example, the template for the .qmail file in user home directories (which details how mail is to be handled), is stored under /etc/e-smith/template-user/.qmail/. As these templates have a variable output filename, they are expanded using small pieces of Perl code in action scripts.
It is possible that the standard templates are not correct for a particular installation, and so the local system administrator can override the extsing templates by placing files in the templates-custom tree. This is a parallel tree to the normal templates hierarchy, and is normally empty. There is also a template-user-custom tree for overriding entries in the templates-user tree.
Note: Never edit the standard templates. Your changes will be overwritten when packages are upgraded.
Note: The template-custom trees should be reserved for local system overrides. Software should not install files in this tree.
If a templates-custom entry exists for a template, it is merged with the standard templates directory during template expansion, using the following rules:
If a fragment of the same name exists in both templates and templates-custom, the one from templates-custom is used, and the one from the standard templates tree is ignored.
If the fragments in templates-custom have different names from those in templates, they are merged into the template as if they were in the templates directory.
If the templates-custom entry is a file, rather than a directory, it completely overrides the standard template.
To make this concrete, let's assume we have the following template structure:
/etc/e-smith/templates/etc/book.conf: 10intro 30chapter3 40chapter4 80synopsis
and
/etc/e-smith/templates-custom/etc/book.conf: 30chapter3 50chapter5
The resulting template would be processed in this order:
template-begin from /etc/e-smith/templates-default
10intro from /etc/e-smith/templates/etc/book.conf
30chapter3 from /etc/e-smith/templates-custom/etc/book.conf
40chapter4 from /etc/e-smith/templates/etc/book.conf
50chapter5 from /etc/e-smith/templates-custom/etc/book.conf
80synopsis from /etc/e-smith/templates/etc/book.conf
template-end (empty), nominally from /etc/e-smith/templates-default
It is possible that the standard templates may specify behaviour which is not appropriate for your application. In many cases the templates will be driven by configuration database settings which allow their behaviour to be customized, which should be the first thing to check.
In many cases, your application only needs to extend the behaviour of the template by adding one or more fragments. This should be your second option and can be achieved by simply adding your fragment in the correct place in the list of fragments.
In rare cases the standard template specifies a behaviour which conflicts with your application. In these cases, you should do all of the following:
Create a templates-custom directory to match the existing one in the templates hierachy.
Copy the conflicting fragment, and only that fragment, to the templates-custom directory. The fragment should have the same name in both directories. At this point you have not changed the behaviour of the system as the templates-custom entry will be preferred, but will behave identically.
Modify the copy in templates-custom to suit your required behaviour.
Raise a New Feature Request here: http://www.contribs.org/bugzilla/. Please attach your modified template (or even better, a patch file) and provide details of why you think that the standard template should be changed.
Note: You should not release RPMs which install templates in the templates-custom directories. If the behaviour of a base template needs to be changed, please raise a bug to discuss the change.
It is also possible to split templates into further subdirectories. This can be very useful for evaluating the same fragments in a loop, for example for each virtual domain in httpd.conf or each ibay in smb.conf.
Two examples of this can be found in /etc/e-smith/templates/etc/httpd/conf/httpd.conf/80VirtualHosts which loops over the /etc/e-smith/templates/etc/httpd/conf/httpd.conf/VirtualHosts/ directory, and /etc/e-smith/templates/etc/smb.conf/90ibays which performs a similar loop over the /etc/e-smith/templates/etc/smb.conf/ibays/ directory.