How to create a custom magic file database

In this article: https://linux.die.net/man/1/file (or man file)

search for magic.mgc for explanation

The information identifying these files is read from the compiled magic file /usr/share/file/magic.mgc, or /usr/share/file/magic if the compile file does not exist. In addition file will look in $HOME/.magic.mgc, or $HOME/.magic for magic entries.

You can man magic for a description for how to create your own magic file. Then use file -C -m <your magic file> to compile it, and file -m <your magic file> to use it.

I'm using the following magic file now for my unit tests:

0       search/1/c      =<?php                  PHP script text
!:mime  text/x-unittest

With it, every php file (beginning with <?php) will get the MIME type text/x-unittest.

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Also see http://cweiske.de/tagebuch/custom-magic-db.htm