[CentOS] Recommendation/pointers please - Need to brush up on CentOS/Linux command line tools

If you are going to be working on RH-centric systems eg. RH/Centos/FC, I
recommend RedHat's PDF and online manuals found here:
https://www.redhat.com/docs/manuals/enterprise/

These manuals mostly describe the use of command line utils, and they
are broken down into categories such as administration, security etc.