Shell

$SHELL tells you the current shell you are using
To find if the default location is just a symlink, go the directory containing the shell and run ls -lah to see if your shell is just a symlink to another shell, very common for sh → bash
To see the working directory, you can use the environment variable $PWD which stands for print working directory
To create a symlink, use ln -s FILE DESTINATION
If you want to see all registered shells (important for using another default shell), you can cat the content of /etc/shells
To get a list of all environment variables, use export -p
The $ sign in the terminal stands for a variable, as example if you want to write the following in the terminal: echo that is the path for brew which brew will not work because which brew is part of the command now but with the use of a variable: echo that is the path for $(which brew) it will work
To change the default shell to zsh as example. Use chsh -s $(which zsh)
To check the directories that are registered in your path, use echo $PATH
If a directory containing a binary file is registered in your path, you can directly access it from everywhere
An alternative to see the location of a command: command -v brew is which brew
TODO
The .profile file is loaded once you login in the system
The .rc file is loaded on every new terminal
There are a lot of different shells. The Bourne Shell (sh) created by Stephen Bourne was the default shell in UNIX 7.
The Bourne Shell does not load the .rc file
The Bourne shell is mostly dead by now and was replaced with bash (Bourne Again Shell) or on newer linux versions, with dash
Another very popular shell is zsh (Z Shell)
If you want to add a path to your PATH variable you need to do it in either the .profile or .rc file, otherwise it will get lost once you restart the terminal
To add a new alisas, go into your .rc file and add it with alias aliasname=command