DB Subst is a general-purpose program for expanding user-defined text templates into an output file using data extracted from a database. It is similar in some respects to "report generator" software, but its command line usage and wider application differentiates it.

A configuration file specifies what template and what data to combine to generate the output file. A single operation may use single or multiple template files to generate a set of multiple output files.

DB Subst only operates on 8 bit text files. This means that both the templates and output files must be normal 8 bit text. The database source may be Unicode provided that the output transform selected in DB Subst is compatible, e.g. html, utf-8.