Why do I need to source bash_profile every time

Question

Now that we've narrowed down the problem:

1. Run ps -p $$ at the command line to determine that you are, in fact, using a bash shell.
2. Realize that you are in zsh, which means you should be editing your profile in .zshrc.
3. Copy the offending lines from .bash_profile to .zsh, OR
4. Modify your .zshrc to directly source your .bash_profile.

UPDATE: Do what @TC1 mentions in the comments and keep the shell-specific code in each shell's own profile, and from those profiles, only source shell-agnostic code.

On Mac Catalina, I just had to open "preferences" on terminal and change the "shells open with" from "default" to "Command(complete path)", which the default path was "/bin/zsh". touch ~/.zshrc, if that file doesn't exist already, and copy/paste your stuff from ".bash_profile" into the ".zshrc" file.

bash_profile.sh is applicable for bash shell. if your default shell is not bash and if your default shell is someother shell for example zsh then you have to manually load the .bash_profile using source ~/.bash_profile.

You can always change the default shell to bash shell so that the .bash_profile file will be automatically loaded.

Inorder to automatically load .bash_profile, you can update your default shell to bash using the command chsh -s /bin/bash

cat /etc/shells will list the default shells available in the machine

echo $SHELL will display the currently active shell in your machine

To change active shell to a different shell, use chsh -s /bin/bash. Then echo $SHELL to verify if the shell has changed.

Terminal -> Preference -> profile -> Shell -> Run command : source ~/.bash_profile

Tick on run inside shell.

After doing all those , just logout and check weather everything works fine or not

I tried the approved answer. Changing the .zshrc file works for one of my machines. But for the other one, when I run ps -p $$, it is -sh under the command. And I changed both bash and zsh files, neither of them works for me this time.

So I found this https://www.gnu.org/software/bash/manual/html_node/Bash-Startup-Files.html

it mentioned "When Bash is invoked as an interactive login shell, or as a non-interactive shell with the --login option, it first reads and executes commands from the file /etc/profile, if that file exists. "

so I went to that file /etc/profile and add "source ~/.bashrc" in that file. Then it works since every time a terminal is opened, it runs the command in that /etc/profile file.