There are so many things I wish I had known way before I ended up learning about them. I based these on conversations I have with first and second year computer science students, and the things they don't know about.
I might come back to this and add more items at a later point.
There are some honourable mentions, which in themselves don't have anything to do with Linux.
Python's argparse and the Namespace object These are only in the same bullet point because they are described in the same documentation file. I, however, included them for completely different reasons:
argparses programming interface for parsing command line input is, hands down, the best CLI parser I have used. It's simple but powerful, you can define subparsers easily, using it is shorter than hacky DIY solutions and safer. The output of
-his top of the class too. A close second is clap-rs/clap for Rust. I appreciate that clap allows you to define the CLI through many different means.
musl vs. glibc: libc is a complicated piece of software. You interact with it more often than you think when you code anything. It also is the cause of the weirdest errors and frustration you can imagine. You think you can cross-compile to a lot of systems? You better hope they have the same glibc version you do! Or you use musl, a non glibc libc, and deal with the disadvantages that come with not compiling against the standard that glibc has become, ... but what if your dependencies do? If you have never heard about this problem, it a nice rabbit-hole to go down, that I'll leave up to you.
git: honestly, if you write any code, or even anything plaintext, you should use a version control system! Using git gives you the most choice in platforms to host on.
The juicy bits, that I think every computer-science-inclined person should have heard about:
I you need to use LaTeX, you'll know, but for every other type of typesetting or notetaking in general, you should use a markdown flavour and maybe a markdown notetaking app. I still write a lot of pure Markdown in vscode but have started using notable a few years ago, and now have my entire work and study notes organised in it, easily accessible as text, convertible into almost any known format, easy to read and pretty. I don't know if I would still recommend notable to everyone as there's been some internal controversities as it has moved from open-source to closed-source, but the app itself is still very good (esp. the inclusion of AsciiMath is great).
But Markdown can help with more than just notetaking, I've held multiple presentations written in Markdown using reveal.js and got compliments on the slidesets. Turns out using Markdown forces you to be more structured and brings a certain simplicity to the slides. As you can allways fall back to HTML (even though that is rarely necessary) you can recreate any slideset using Markdown. Another advantage is that you can more easily scan adjacent slides in the plaintext Markdown file as compared to something like PowerPoint or LibreOffice Impress.
man pages' numbers are systematic. If you don't know what manpages are, you have to fix that! But once you know they exist there's still a lot to learn. It scares me how much space information from or about man pages take up in my brain. Just like a knee-jerk reaction I have to say:
man <number> <page> when a particular topic comes up in conversation. But one part that I only learned about embarassingly recently: The number of the page is based on the type of information on the page. I should've read
man man earlier:
The table below shows the section numbers of the manual followed by the types of pages they contain. 1 Executable programs or shell commands 2 System calls (functions provided by the kernel) 3 Library calls (functions within program libraries) 4 Special files (usually found in /dev) 5 File formats and conventions, e.g. /etc/passwd 6 Games 7 Miscellaneous (including macro packages and conventions), e.g. man(7), groff(7) 8 System administration commands (usually only for root) 9 Kernel routines [Non standard]