Anybody knows that you can run a shell inside Emacs with M-x shell. Few people know you can have Emacs run some additional configuration for the shell after it’s started.

Emacs sends the new shell the contents of the file ~/.emacs_shellname as input, if it exists, where shellname is the name of the name if your shell - bash, zsh, etc. For example, if you use bash, the file sent to it is ~/.emacs_bash. If this file is not found, Emacs tries with ~/.emacs.d/

One popular application of such custom configuration is to have a simpler shell prompt in Emacs, compared to the one you usually employ, since shell-mode doesn’t deal well with fancy prompts.

You can find out more about running interactive shells in Emacs here or by typing C-h r m Interactive Shell RET in your favorite text editor.