1. https://www.wikiwand.com/en/Linux.
2. Fig1 Shells bash, csh, tcsh, zsh hells, Available at: https://code.snipcademy.com/tutorials/linux-command- line/environment/shells.
3. https://www.thegeekdiary.com/unix-linux-what-is-a-shell-what-are- different-shells/.
4. https://www.geeksforgeeks.org/introduction-linux-shell-shell-scripting/.
5. https://developer.ibm.com/tutorials/l-linux-shells/.