How to load ~/.bash_profile when entering bash from within zsh?

An interactive bash reads your ~/.bash_profile if it's a login shell, or your ~/.bashrc if it's not a login shell.

A typical .bash_profile will contain something like:

if [ -f ~/.bashrc ]; then . ~/.bashrc; fi

so .bashrc can contain commands to be executed by either login or non-login shells.

If you run bash -l rather than just bash, it should read your .bash_profile.

Reference: https://www.gnu.org/software/bash/manual/html_node/Bash-Startup-Files.html

Open ~/.zshrc, and at the very bottom of the file, add the following:

if [ -f ~/.bash_profile ]; then 
    . ~/.bash_profile;
fi

Every time you open the terminal, it will load whatever is defined in ~/.bash_profile (if the file exist). With that, you can keep your custom settings for zsh (colors, and etc). And you get to keep your custom shell settings in .bash_profile file.

This is much cleaner than using bash -l IMO.

If you prefer putting your settings in .bashrc, or .bash_login, or .profile , you can do the same for them.

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Similarly, you could also move the common profile settings to separate file, i.e. .my_common_profile, and add the following to both .bash_profile and .zshrc:

if [ -f ~/.my_common_profile ]; then 
    . ~/.my_common_profile;
fi