----- Legend
5 | Critical      | Able to use with options from memory
4 | Very Imprtant | Able to use the bare command
3 | Useful        | Can use after referencing manpage
2 | Nice to know  | Can use after Googling
1 | Be aware of   | Know generally what it does

----- Filesystem
5 | cd
5 | chmod
5 | chown
5 | cp
3 | file
4 | find
3 | ln -s
5 | ls -alph
5 | mkdir
5 | mv
4 | ncdu
5 | pwd
5 | rm -rf
4 | rmdir
3 | stat
4 | touch

----- File manipulation
4 | cat
2 | cut
5 | grep, egrep, zgrep
5 | head
5 | less, zless
5 | nano
2 | sed
5 | tail
2 | tr
2 | vim

----- System status
5 | df -ih
5 | du -sh
5 | free
5 | htop
4 | iftop
4 | iotop
5 | ps aux
5 | top
5 | uptime

----- System management
3 | addgroup, groupmod
3 | adduser, usermod
5 | apt (update,upgrade)
2 | arp -a
4 | crontab -e
5 | dmesg
2 | dpkg (-S, -l)
2 | ifup, ifdown
4 | ip (a, r)
5 | iptables-restore, iptables-save, ip6tables-restore, ip6tables-save
5 | journalctl -uf
5 | kill (-3, -9, SIGUSR1, SIGTERM, SIGKILL)
2 | lsof, fuser
4 | mount, umount
5 | passwd
5 | ss -plntu
5 | sudo
5 | systemctl (status,stop,start,restart,enable,disable)
4 | systemd

----- Important files/directories
2 | ~/.bash_aliases
5 | ~/.bashrc, ~/.profile, ~/.bash_profile
5 | ~/.ssh/authorized_keys, ~/.ssh/id_rsa, ~/.ssh/id_rsa.pub
2 | /etc/
2 | /etc/fstab
3 | /etc/hosts
3 | /etc/init.d
2 | /etc/network/interfaces
1 | /etc/rc.local
3 | /etc/systemd
4 | /etc/ssh/sshd_config
5 | /home/
1 | /root/
5 | /var/log/
4 | /var/www/, /srv/

----- Common programs
1 | awk
4 | certbot
4 | curl
5 | date
2 | diff
5 | dig
1 | git
1 | gpg
2 | hping
2 | iperf
5 | man
2 | md5sum
5 | mtr
4 | nc
2 | netstat
4 | nmap
2 | openssl
5 | ping
1 | python, pip
3 | screen
2 | telnet
5 | traceroute
4 | watch
5 | whois
3 | zip, unzip, gzip, gunzip, tar

----- Recovery
2 | chroot
4 | e2fsck -f
2 | grub
5 | Linode Rescue Mode

----- Remote access
5 | ssh
4 | scp
1 | sftp
3 | rsync
2 | sshfs
5 | FileZilla

----- Common daemons
4 | OpenSSH
4 | Apache, Nginx
3 | MySQL
2 | PHP, CGI
3 | Fail2ban
4 | UFW
1 | dovecot, postfix

----- Ports
5 |   22 SSH
4 |   25 SMTP
3 |   53 DNS
5 |   80 HTTP
1 |  110 POP3
2 |  143 IMAP
4 |  443 HTTPS
2 |  465 SMTP+SSL
2 |  587 SMTP+STARTTLS
1 |  993 IMAP+SSL
1 |  995 POP3+SSL
2 | 3306 MySQL

----- Bash
5 | Quotes (", ')
5 | Semicolons (;)
3 | Variables ($)
4 | Environment variables (export, $PATH)
5 | Piping (|)
5 | Redirection (<, >, >>)
3 | Scripts
3 | Executing from the current directory (./)
2 | Special variables ($?, $#, $@, $1, $2, $3, ...)
1 | alias
4 | history
5 | reset
5 | which

----- Terms
5 | TCP, UDP, PING
5 | IPv4, IPv6
5 | DNS, Domain, Nameserver, A record, CNAME record, NS record
5 | VM, VPS, Host
5 | EXT4, NTFS, Swap
5 | HTTP, HTTPS, SSL, TLS, Let's Encrypt

----- Concepts and Guides
3 | iptables      | https://www.linode.com/docs/security/firewalls/control-network-traffic-with-iptables/
4 | let's encrypt | https://www.linode.com/docs/quick-answers/websites/secure-http-traffic-certbot/