Okay so first there was Unix. It was semi Open Source and a bunch of companies were making different versions that were becoming increasingly incompatible. That is why POSIX was created, it standardizes major parts of Unix. Linux is a Unix like operating system, meaning it functions similarly but doesn’t share any code. One thing that POSIX standardizes is the shell meaning there’s a standard how a loop works etc. Most shell on Linux like bash and zsh are POSIX compliant but some (like fish aren’t). This means a command that works one way in bash might work differently in fish. Basic stuff is mostly the same in my experience so if you’re not having any problems you shouldn’t worry about being POSIX compliant. If you want most of the same stuff but POSIX compliant checkout zsh. Fish provides documentation for adjusting your commands so I’d just ignore it until you run into a problem and then take a look at the docks