I notice there are (at least) 3 different SE sites that a Linux-related question could get asked: here, StackOverflow or Ubuntu. There could be others too.
But where is best to ask my questions regarding Linux? I don't really know what the differences between Ubuntu, Unix and Linux are (I've only recently started using Linux), nor why they have dedicated SE sites...
There is a slight overlap between the sites, most so between U&L and AskUbuntu.
If your question is primarily about programming (not shell scripting) or if it requires specialist programming knowledge to solve, then use StackOverflow.
If your question is primarily about Ubuntu and if the issue is exclusive to Ubuntu (i.e., it would not apply to any other Linux distribution) or if it requires an Ubuntu specialist to solve, then use AskUbuntu.
Otherwise (or if you're unsure whether it's a specific Ubuntu issue), please feel free to ask here on U&L.
Questions about text-processing with sed and awk, how to use join or any other standard or common Unix command line tool, Unix file permissions, shell scripting in general, etc., are best asked here on U&L. We also take questions regarding the POSIX standard, which includes the system interfaces, which are C library functions. It depends on the framing of the question whether these are best asked here on U&L or on StackOverflow.
Questions about C code, C++ programming, using libraries in your own programs, implementation of programming algorithms in general, etc. are best asked on StackOverflow.
Questions about how to set up your Ubuntu system, services provided for Ubuntu by the company Canonical, or anything not related to the use or administration of any other Linux but Ubuntu, you may take to AskUbuntu.