Bash:
CISCO:
Interconnecting Cisco Network Devices ICND
Crontab:
Crontab Generator
Crontab Guru
Google
History:
Kali:
Linux Hardening:
https://www.tecmint.com/configure-pam-in-centos-ubuntu-linux/
https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/4/html/security_guide/s2-wstation-privileges-noroot
IPtables
UFW
APPARMOR
PAM 7
Use PAM to configure AUTH on an Ubuntu
PAM PWhistory
Mastering Linux Security Intruders