While searching for things to strengthen my Linux skills, I stumbled upon wargames, which are like CTFs but they are neither time-bound nor competitive. You can practise them whenever you want. https://overthewire.org/wargames/. suited this wargame and boy am I happy to play this game in my free time. OverTheWire challenges introduced me to the basics and more advanced skills of using Linux by a game called Bandit.
For connecting to OverTheWire levels, I had to SSH(which is basically a secure method to connect to a server) every time, copy the password of the level and then paste it. Not to worry about how to connect to the server. Several hints are dropped for you to do what you need to get to the next level.
Start reading manuals of all the commands by using man. During my journey in Cyber Security, I realised that once you come to this field, you need to be open to learning and exposing yourself to new tools and techniques. You don’t have to focus on learning the commands, it will come to you gradually as you start using those commands. You will have an idea about the command by reading the manual so that the next time you need that command for any reason, you just have that in your mind.
One more thing which you should be aware of while coming to this field is that when you come to Linux(or the well known KALI you might have heard of), you should learn to use terminal very well like the text editor of Linux terminal is way too powerful. I personally prefer Vim for this. Just install Vim in the system, start vimtutor and do as it says. It will be difficult at first but once you start getting comfortable and start using it, it is going to help you a lot.
Start reading about Linux commands, regular expressions and Python. Python will come to your rescue in further games of OverTheWire challenges. The game levels increase gradually so go in order. Some of the levels will look repetitive but some levels are mind-blowing which will make you think in more creative ways. Always make a note of how you solve those levels. That will help you later as it happened in my case that I know I have seen this kind of challenge before but don’t remember how it was done. These cheat sheets you make are going to come in handy when you see similar challenges but forget how you approached them.
If you are a beginner in this field I would highly recommend OverTheWire challenges.
Good luck and best of all, enjoy and have fun!!