Execute command on Terminal launch (macOS)

The most common way is to add the command to .bash_profile file in your home directory.

Quoting man bash which lists other options as well:

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. After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and reads and executes commands from the first one that exists and is readable.

If you want to run a command when Terminal.app opens a shell, not just for any interactive shell:

1. Open Terminal → Preferences → Profiles.
2. Select the profile you use, then switch to the Shell tab.

3. Under Startup, enter the command you wish to run as the Run command.