If you want to master the shell (it will save years of your life), follow these guides. I highly recommend reading the entire BASH manual, it will answer every question you'll ever have, or at least give you a hint about it, as well as expose you to all the hidden knowledge (some call it "gotchas") you'll wish you knew later.

  101
  - Bash for Beginners (https://tldp.org/LDP/Bash-Beginners-Guide/html/)
  - Bash Tutorial (https://www.w3schools.com/bash/)

  201
  - Bash Programming (https://tldp.org/HOWTO/Bash-Prog-Intro-HOWTO.html)
  - Advanced Bash Scripting Guide (https://tldp.org/LDP/abs/html/)

  301
  - The Bash Manual (https://www.gnu.org/software/bash/manual/bash.html) (https://man7.org/linux/man-pages/man1/bash.1.html)
  - ShellCheck (https://www.shellcheck.net/)

To find every Unix-y program and get a 1-line description of it (and referenced programs/functions), run:

  for i in $(ls /bin/* /usr/bin/* /sbin/* /usr/sbin/* | sed -E 's?.*/??g' | sort -u) ; do
    echo "command: $i"
    whatis "$(basename "$i")" | cat
    echo ""
  done | tee command-descriptions.log

View 'command-descriptions.log' with less command-descriptions.log, use arrow-keys and page up/down to navigate, and type 'q' to exit. To find out more about a program like df(1), run man 1 df.