I am trying to install create a simple .deb dpkg package.
I have seen multiple packages where, during installation, they output custom messages like Reboot after installation! or Clearing caches or (in the case of iOS) Thanks to [developer]!.
I can't seem to find any way to replicate this myself during installation of my package.
How do I output a custom message to the terminal while dpkg is installing my package?
You can do this by providing a postinst script to the package.
See section 7.6 What is a Debian preinst, postinst, prerm, and postrm script?