I'm using my yubikey 5 NFC with U2F as well as for OTP codes. I get OTP codes using Yubico Authenticator app which seems to be a little too complicated and I was wondering if there is a way to assign it to short/long touch on my key so I don't need to open that app every time for codes I use often enough?
It seems that the authenticator uses something else than slots to store credentials, is it possible to read them with ykman or some other official command line utility/sdk?
There are two types of 6-digit OTP codes that are part of OATH: HMAC-based (HOTP), which are generated in a fixed sequence, and time-based (TOTP), which update every 30 seconds or so. TOTP are more commonly used.
The Yubikey can generate HOTP codes on touch, in either slot 1 (short touch) or 2 (long touch). You can set this up with ykman otp hotp 1 or ... 2 as the case may be. It expects the secret key in base 32 format.
This can't be done for TOTP, for the simple reason that in order to generate a time-based code, you have to know what time it is, and the Yubikey doesn't have a real-time clock on board (because it doesn't have any power source to keep it running). So it can't generate TOTP codes without assistance from the software application, which feeds it the current time from the system clock.
If you don't like the graphical authenticator app, you can generate HOTP/TOTP codes from the command line by running ykman oath code.