profile bash_profile bashrc on Ubuntu Linux, Macs, and Windows

I'm trying to get the same script to run and also set the $PATH for the login and non-login terminal shell on all 3 OSs: Windows, Mac, and Linux.

There's 3 parts to making it work:

Does the shell start in login or non-login mode?

1. Mac's Terminal.app by default starts bash as a login shell.
   
2. Ubuntu graphically logs in as a GUI shell (i.e. your desktop environment, gnome-session, etc.), so it's just not bash at all.  But if you then open a terminal in the desktop environment, your manually opened bash shell on Ubuntu may well start as a non-login (i.e. interactive) shell.
3. Debian LXDE graphically logs in with bash and runs as a login shell by default.
   
4. Windows Git Bash (MINGW64) seems to start as a login shell by default as well.

Which script runs on shell startup: .profile, .bash_profile, .bashrc?

1. Gnome-session or whatever is your desktop environment (DE) is not bash, but the script that starts your DE is supposed to source ~/.profile but NOT ~/.bash_profile.  However, it's possible with Debian LXDE that it'll prefer ~/.bash_profile if it exists over the fallback that is ~/.profile.
   
2. Bash as a login mode shell sources ~/.bash_profile and if that's missing then ~/.profile as a fallback.
   
3. Bash as a non-login (interactive) shell sources ~/.bashrc and NOT those other two.

Where should scripts go to run on shell startup?

1. So scripts that run only when your desktop environment or graphical shell starts goes into ~/.profile.
   
2. Scripts that run only when you start bash in login mode --- e.g. started with bash -l, via the text console mode in Debian or Ubuntu e.g. via Ctrl + Alt + F2, when Mac's Terminal.app opens, or when Git's MINGW Bash opens on Windows --- goes into ~/.bash_profile.
   
3. Scripts that run only when you start bash interactively (non-login mode) --- e.g. started by clicking the terminal icon having graphically logged in to Ubuntu's DE --- goes into ~/.bashrc.  This assumes you didn't configure your terminal emulator to run as a login shell!
   

Want the script to always run however shell starts up?

Suppose you want a bit of code to run when your shell starts.  It doesn't matter if it's a login, non-login (i.e. interactively started), graphical login from your desktop environment, on Macs, on Windows MINGW, etc.

In principle, I think ~/.bash_profile and ~/.profile both should source in ~/.bashrc as well, so scripts in ~/.bashrc should run whichever way you get your hands into a terminal UI in Ubuntu.  But Macs and Windows Git Bash doesn't have a proper .bash_profile to start.

Thus for Macs and Windows, favor ~/.bash_profile.

For Ubuntu Linux, favor ~/.bashrc.

A single method that works on all 3 platforms would be to put the code in ~/.bashrc, then make sure ~/.bash_profile sources in bashrc.

Bibliography

Why ~/.bash_profile is not getting sourced when opening a terminal?

DotFiles