Jan. 19, 2017
To understand when .bash_profile or .bashrc are used and differences between them, we introduce several terms. I shall focus on Mac OS, which I use.
In an interactive shell, the user may type commands into it, execute them, and see their output on the screen. A non-interactive shell, in contrast, does not allow the user so. The moment one enters Mac’s native Terminal.app, he or she is using an interactive shell. However, when he runs a shell script, a non-interactive shell is started that runs the script, and then exits when it finishes doing that.
A login shell requires the user to type username and password to log in, but a non-login shell does not. (With definition above, they are interactive.) But Terminal.app automatically logs in when starting up, and thus is a login shell. Of course, the shell that is created by typing the command login, is too a login shell. And the examples of non-login shell include the new Bash instance the user invokes by typing /bin/bash, or another kind of shell, in a terminal.
In Mac, an interactive login shell reads .bash_profile, while an interactive non-login shell reads .bashrc. Default paths of .bash_profile and .bashrc are both the user’s home. On the other hand, a non-interactive shell reads neither.
Actually there are /etc/profile (which Mac has created by default), /etc/bash.bashrc, ~/.bash_login, ~/.profile, BASH_ENV, and ~/.bash_logout, all with subtlety due to historical reasons. But I am not going to figure them out. Instead, the most reliable solution is that the user shall put everything he sets into ~/.bashrc, and make ~/.bash_profile source ~/.bashrc, as I did.
Sketchbook 