For a pretty good example of a readable/fun program written in bash, the KISS package manager stands out: https://github.com/kisslinux/kiss/blob/master/kiss

Neofetch is also written in bash: https://github.com/dylanaraps/neofetch/blob/master/neofetch

For my own work, I once wrote a functional file system based irc client in ~50 lines of bash: https://github.com/retrohacker/irc-sh/blob/master/main.sh

To learn bash:

* https://tldp.org/LDP/abs/html/

* https://github.com/dylanaraps/pure-bash-bible