Other resources

• awesome-bash is a curated list of Bash scripts and resources
• awesome-shell is another curated list of shell resources
• bash-it provides a solid framework for using, developing and maintaining shell scripts and custom commands for your daily work.
• Bash Guide for Beginners a good resource between the HOWTO and the Bash Scripting guide.
• dotfiles.github.io is a good source of pointers to the various dotfiles collections and shell frameworks available for bash and other shells.
• learnyoubash helps you write your first bash script
• shellcheck is a static analysis tool for shell scripts. You can either use it from a web page at www.shellcheck.net or run it from the command line. Installation instructions are on the koalaman/shellcheck github repository page.

Finally, Stack Overflow has many questions that are tagged as bash that you can learn from and is a good place to ask if you're stuck.