As a software developer, I have long-since ingrained the behavior of looking up the documentation before I assume something works in a particular way. Perhaps doubly so when shell scripting, where I have to verify if I can rely on a particular package being present, or the minimum version of something I can expect, or what the output is guaranteed to be.

When I was considering using fgrep, I looked it up; lo and behold, it's not part of the POSIX standard. So I used the option instead, and have since.

I'm a little puzzled why people have problems with these little things when it feels like it's pretty common for tools to change given enough time (for example, ifconfig and netstat to ip and ss). It's just essentially an API that gradually refines over time.