Basic Linux Privilege Escalation - https://blog.g0tmi1k.com/2011/08/basic-linux-privilege-escalation/
Linux Privilege Escalation - https://github.com/swisskyrepo/PayloadsAllTheThings/blob/master/Methodology%20and%20Resources/Linux%20-%20Privilege%20Escalation.md
Checklist - Linux Privilege Escalation - https://book.hacktricks.xyz/linux-unix/linux-privilege-escalation-checklist
Sushant 747's Guide (Country dependant - may need VPN) - https://sushant747.gitbooks.io/total-oscp-guide/content/privilege_escalation_-_linux.html