The oathtool is the standard tool on Linux. It comes as a command-line tool or a dynamic and static link lib to be used in your own applications.
You can statically link the code into your application and generate TOTP codes and also validate them.
The homepage contains information about the command line tool "oathtool" and also the lib "liboath".
https://www.nongnu.org/oath-toolkit/
Example how to use it on command-line.
The example used the base32 encoded secret for "test".
oathtool --totp -b ORSXG5AK
This is a companion discussion topic for the original entry at https://blog.nashcom.de/nashcomblog.nsf/dx/adding-totp-to-your-own-application.htm