#set, #setenv, #unset, #unsetenv

With #set, and its variant #setenv, you can set environment variables. You can then reference these environment variables in subsequent preprocessor directives, such as #exec, #include, #verbatim, and #echo. (Note that legal environment variable names begin with a letter, followed by any sequence of letters, digits, and/or _ (underscore).)

For example, at pikt.org, we use PIKT as a content management system (CMS) for our web pages. In the file doc_ref_files.cfg, for each web page, we set various environment variables, such as:

#set cat = "Reference"
#set title = "PIKT Reference"
#set descr = "Overview; Config Files; Preprocessing; Programs; Pikt Script Language; Miscellany"
#set keywords = "pikt reference"

...

#include <files/pikt/doc_page_top_files.cfg>

...

In the #include file, doc_page_top_files.cfg, we reference those environment variables by means of the #echo directive:

=doctype

<html>

<head>
#echo "<title>$title</title>"
=meta_content_language
=meta_content_type
#echo "=meta_description($descr)"
#echo "=meta_keywords($keywords)"
=meta_author(=pikt_author)
=meta_copyright(=pikt_license((c)))
//=meta_copyright(=pikt_copyright)
#ifdef homepage
=meta_noodp
#endifdef
=css_style
</head>

...

In any of the config files, if you use

#set foo = "bar"

or the variant

#setenv foo "bar"

you could access the value of the $foo environment variable in a subsequent preprocessor directive.

If you don't specify a value, as in

#set foo

the environment variable is implicitly set to 1. So the previous #set directive is equivalent to

#set foo = "1"

and

#setenv foo "1"

#unset and #unsetenv undo the effects of the #set and #setenv directives. So

#unset foo

#unsetenv foo

leaves the environment variable $foo undefined.

You may set (or unset) environment variables in this way to #include (or not) files (i.e., processes cat'ing the file(s)), or to access environment variable values using the #echo command, to pass parameters to processes embedded within preprocessor directives (e.g., #include [<proc>], or #exec [<proc], or to do some other clever thing.

For example, if you do this in one of your config files

#set foo
#include <files/this_files.cfg>
#unset foo

you could have in this_files.cfg

#include [test $foo && /bin/cat /pikt/lib/configs/files/that_files.cfg]

or perhaps #include the output of some process

#include [/usr/local/bin/bar.pl]

where bar.pl references the $foo environment variable, for example with

if ($ENV{foo}) {
        ...
}

Together with the piktc -|+ E (un)set environment variable option, #set, #setenv, #unset, #unsetenv and the other preprocessor directives make a very powerful combination.

For some examples, refer to the Samples and Developer's Notes pages.

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