Execute Fish Shell from Bash at end of .bashrc -- while being able to invoke Bash shell from fish without re-running Fish -- and without having to use --norc

.bashrc

# Many systems, such as Arch, MSys2, Cygwin, etc., need to have Bash interpret 
# their startup scripts, set Environment variables, etc. Therefore, setting your 
# default shell to anything other than Bash might not be a good idea. In these 
# cases, the conventional advice is to append 'exec fish' at the end of your 
# .bashrc. However, this approach creates another problem; should you need to 
# drop into Bash to run a bash script, you need to be careful to use 
# 'bash --norc' every time. Otherwise, Fish will start up again when you don't 
# want it to. Obviously, this is cumbersome and inefficient. Thus, instead of 
# doing a naked 'exec fish' at the end of your .bashrc, use the following:

## Edited on 4/17/24 with [Ilan's suggestion|https://gist.github.com/voidptr/d77a5a527ed2cc06cd277df9ec366f32?permalink_comment_id=5026742#gistcomment-5026742]
if [[ $(ps --no-header --pid=$PPID --format=comm) != "fish" && -z ${BASH_EXECUTION_STRING} ]]
then
  shopt -q login_shell && LOGIN_OPTION='--login' || LOGIN_OPTION=''
  exec fish $LOGIN_OPTION
fi

Ouch! Painful! Sorry!