Linux

I'm surprised to encounter someone who uses a D compiler but doesn't know much about environment variables or unix shells.

Due to the nonexistent documentation for the issue,

Are you familiar with the man command, for reading the system's manual pages? When you get some time, you might want to run man bash and read whatever sections are relevant to things you want to do, such as where the Invocation section talks about startup files.

I’ll give my paths a more generic names in the future, so I can just rename a folder, then use the compiler right out of the box without having to mess around with the path yet again.

Creating a symlink (symbolic link) with a generic name to point at the version-specific directory name is a common way to do this. (For example, ln -s dmd-2.107.1 dmd) You could then add the symlink's path to your PATH environment variable, and replace the symlink whenever you want to point it at a different compiler version. Run man ln for more info.

I also suggest finding a beginner's guide to linux (or unix) when you get a chance. This stuff gets a lot easier once you learn the basics.

You can set the initial value directly in /etc/environment, did you check that? It could also be set only for your user, so it might be in ~/.profile, ~/.bashrc or ~/.bash_profile` (or the rc file for your shell if you're not using the default bash).

Edit: I suppose you could also have added a startup script in /etc/init/ or /etc/init.d/, or in /etc/rc.local

Found it in .bashrc, editing it makes my pi no longer wanting to log in to GUI. Ctrl+Alt+F1 still works. Commenting out the export PATH part does not fix it. Some forums recommended deleting some files, so I'll try doing that.