For me, the hard part is remembering if .bashrc is supposed to source .bash_profile or vice versa.
.profile is only for login sessions, .shrc is sourced on every shell fork, about 1000x more than .profile.
For example, do you want to make an autoloaded function named "foo" that comes into existence the first time you call it? Write a file named ~/.config/fish/functions/foo.fish, and inside it define a function named "foo". There - done.
Follow this pattern to make a function named "fish_greeting", and it will run every time you launch a new fish shell. Want to make a custom prompt? Write a function named "fish_prompt". Decide you want one of those fancy prompts where the timestamp is displayed on the right side of your terminal window? Make a "fish_right_prompt.fish" file and put in it:
function fish_right_prompt --description 'Write out the right prompt'
set_color 666666
date +'%Y-%m-%d %H:%M:%S'
set_color normal
end
* There are two scripts run by a login shell, $HOME/.profile and /etc/profile .
* These scripts set up environment variables and other things (e.g. umask) to be inherited by the whole login session.
* They, specifically, optionally set an ENV variable whose value names a third script that is run by every interactive shell, including the interactive login shell.
That's exactly three scripts: one for the system administrator and another one for the user to set up login-session-wide stuff, and one for the user to set up this-interactive-shell stuff (such as the shell variables for fancy prompts, completion settings, command-line editing stuff, useful interactive shell aliases, and whatnot). They are always in the same order. And they are simply based upon two things: whether the shell is a login shell, and whether the shell is an interactive shell.
The C shell mechanism is similarly simple, on its own.
The complexity mainly comes from the fact that the Bourne Again shell and the Z shell, amongst others, have multiple mechanisms mixed together and extensions. They look for scripts that are specifically in their own shell languages rather than potentially shared with other shells; .zprofile and .bash_profile instead/ahead of .profile , for example. They have extra scripts that mirror the C shell way of doing things; .zlogin, .zlogout, .bash_login, .bash_logout, .bashrc, and .zshrc, for example. They have idiosyncratic and non-strictly-standard ways of deciding what constitutes an interactive shell. They have mechanisms for disabling the extensions, and ways to come up in alternative not-strictly-conformant sh-compatible modes.
info bash -n "Bash Startup Files"