Why `--exclude-dir "_book"` works but `--exclude-dir={_book}` failed when using BSD grep?

Unix & Linux: Why `--exclude-dir "_book"` works but `--exclude-dir={_book}` failed when using BSD grep? (2 Solutions!) Helpful? Please support me on Patreon: https://www.patreon.com/roelvandepaar With thanks & praise to God, and with thanks to the many people who have made this project possible! | Content (except music & images) licensed under cc by-sa 3.0 | Music: https://www.bensound.com/royalty-free-music | Images: https://stocksnap.io/license & others | With thanks to user Zen (https://unix.stackexchange.com/users/74226), user Wildcard (https://unix.stackexchange.com/users/135943), user muru (https://unix.stackexchange.com/users/70524), and the Stack Exchange Network (http://unix.stackexchange.com/questions/433729). Trademarks are property of their respective owners. Disclaimer: All information is provided "AS IS" without warranty of any kind. You are responsible for your own actions. Please contact me if anything is amiss at Roel D.OT VandePaar A.T gmail.com.

Видео Why `--exclude-dir "_book"` works but `--exclude-dir={_book}` failed when using BSD grep? автора Linux Lovers