I knew basic CLI commands (such as cd and ls) for a while, but did not do learn much more. Some things have helped me grow my skills:

• Necessity: Some times I need to do something on a VM or container that does not have a graphical interface installed. Some utilities only have a command line interface and not a graphical client. My only option is to Google how to do it. The more I do it, the less I have to Google and the more focused my searches become (instead of searching for "How to do x", I search for "How to do x in utility").
• Learning from others: For many tasks, I follow internal or external guides, which typically use CLI commands. Often I look at how my coworkers accomplish tasks and pay attention to what commands they use. Then, when I have time, I look up any new commands I saw and decide if they will be useful for me too. Lately, I have been doing code reviews that involve shell scripts. Those are especially nice, because I can take my time, going line by line, and understand what each command does.
• Keep notes: Every time I find a command that I think I will need again, I copy it into a text file (and I have many such text files). It also makes it easier when I need to run the command with slightly different arguments (a different commit id or something), I can just edit the command in my editor (with searching and undo) and paste it in to my terminal with all the flags and arguments correct.