Boot process

Filesystem hierarchy

Storage & LVM

# Disk tools
lsblk                           # list block devices (tree view)
fdisk /dev/sda                  # partition disk (MBR)
gdisk /dev/sda                  # partition disk (GPT)
mkfs.ext4 /dev/sda1             # format as ext4
mkfs.xfs /dev/sda1              # format as XFS
mount /dev/sda1 /mnt            # mount device
df -h                           # disk usage by filesystem

# LVM — create: PV → VG → LV
pvcreate /dev/sdb               # physical volume
vgcreate vg01 /dev/sdb          # volume group
lvcreate -L 10G -n lv01 vg01   # logical volume
lvextend -L +5G /dev/vg01/lv01  # extend LV
resize2fs /dev/vg01/lv01        # resize ext4 to fill LV

Shell operations

# Navigation & file ops
pwd; cd /path; ls -la; cp -r src/ dst/; mv file new; rm -rf dir/
find / -name "*.conf" -type f   # find by name

# Text processing
grep -r "pattern" /dir          # recursive search
awk '{print $1}' file           # field extraction
sed 's/old/new/g' file          # find and replace

# Redirection
cmd > file                      # stdout to file (overwrite)
cmd >> file                     # stdout to file (append)
cmd 2>&1 | tee out.log          # merge stderr, tee to file

# Archiving
tar -czf archive.tar.gz /dir    # create gzip archive
tar -xzf archive.tar.gz         # extract
rsync -avz src/ dst/            # sync (preserves permissions)

Permissions

# chmod — r=4, w=2, x=1
chmod 755 file                  # rwxr-xr-x
chmod 644 file                  # rw-r--r--
chmod -R 755 /dir               # recursive
chmod u+s file                  # SUID — runs as file owner
chmod g+s dir                   # SGID — new files inherit group
chmod +t dir                    # sticky bit — only owner can delete
chown user:group file           # change owner and group

User management

useradd -m -s /bin/bash username    # add user with home dir
passwd username                     # set password
usermod -aG sudo username           # add to sudo group
userdel -r username                 # delete user and home dir
groupadd groupname; groupdel name
id username                         # show UID, GID, groups

# Key files
/etc/passwd    # user accounts (UID, shell, home)
/etc/shadow    # encrypted passwords
/etc/group     # group memberships
/etc/sudoers   # sudo permissions (edit with visudo!)

systemd & process management

systemctl start|stop|restart|status service
systemctl enable|disable service    # boot persistence
journalctl -u service -f            # follow service logs
journalctl -xe                      # show errors with context

ps aux; top; htop                   # process viewers
kill -9 PID; killall processname    # terminate processes
nice -n 10 cmd; renice 5 -p PID     # process priority (-20 high, 19 low)

Package management

# Debian/Ubuntu (APT)
apt update
apt install packagename
apt remove packagename
apt autoremove
dpkg -i package.deb

# RHEL/CentOS/Fedora (DNF)
dnf update
dnf install packagename
dnf remove packagename
rpm -ivh package.rpm
rpm -qa  # list all installed

Containers (Docker)

docker pull image:tag               # pull image
docker run -d -p 80:80 nginx        # run detached, map port
docker ps; docker ps -a             # running / all containers
docker stop|rm container            # stop / remove
docker build -t myapp .             # build from Dockerfile
docker exec -it container bash      # interactive shell
docker logs container               # view logs
docker network ls                   # list networks

Firewalls

# UFW (Ubuntu)
ufw enable; ufw status verbose
ufw allow 22/tcp; ufw allow 80/tcp
ufw deny 23/tcp

# firewalld (RHEL)
firewall-cmd --list-all
firewall-cmd --add-service=ssh --permanent
firewall-cmd --add-port=80/tcp --permanent
firewall-cmd --reload

# iptables (legacy)
iptables -L -v -n
iptables -A INPUT -p tcp --dport 22 -j ACCEPT
iptables -A INPUT -j DROP

SSH hardening & OS hardening

# /etc/ssh/sshd_config
PermitRootLogin no
PasswordAuthentication no
Port 2222
AllowUsers username

# Find risky permissions
find / -perm -4000      # SUID files
find / -perm -o+w       # world-writable files

SELinux & AppArmor

Ansible basics

# Run playbook
ansible-playbook playbook.yml
ansible all -m ping
ansible-inventory --list

# Basic playbook structure
---
- name: Configure web server
  hosts: webservers
  become: yes
  tasks:
    - name: Install nginx
      package:
        name: nginx
        state: present

Bash scripting essentials

#!/bin/bash
NAME="Linux"
echo "Hello $NAME"

# Conditionals
if [ "$VAR" = "value" ]; then
    echo "matched"
fi

# Loops
for file in *.txt; do echo "$file"; done
while [ $COUNT -lt 5 ]; do COUNT=$((COUNT+1)); done

# Functions
greet() { echo "Hello, $1"; }
greet "World"

# File tests
[ -f file ] # exists and is regular file
[ -d dir ]  # directory exists
[ -x file ] # file is executable

# Exit code
echo $?     # 0=success, non-zero=error

Git version control

git init; git clone url
git add .; git commit -m "message"; git push origin main
git pull; git status; git log --oneline
git branch feature; git checkout feature; git merge feature
git tag v1.0; git stash

System monitoring

top; htop                         # real-time process/CPU view
free -h                           # memory usage
vmstat 1                          # VM stats (swap, CPU, I/O)
iostat -x 1                       # disk I/O statistics
journalctl -f                     # follow system logs
journalctl -p err --since today   # today's errors only
dmesg | grep -i error             # kernel error messages

Network troubleshooting

ping -c 4 8.8.8.8                 # connectivity test
traceroute 8.8.8.8                # hop-by-hop path
ip addr; ip route                 # interfaces and routing
ss -tulnp                         # open ports and sockets
dig google.com; nslookup          # DNS queries
tcpdump -i eth0 port 80           # capture HTTP traffic
nc -zv host 443                   # test TCP port connectivity

Performance troubleshooting

Network components

TCP vs UDP

IPv6 address types

Switching concepts

VLAN & trunk configuration

! Create VLAN and access port
vlan 10
 name SALES
interface Gi0/1
 switchport mode access
 switchport access vlan 10
 switchport voice vlan 20       ! voice VLAN

! Trunk port
interface Gi0/0
 switchport mode trunk
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan 10,20,30
 switchport trunk native vlan 99   ! change native VLAN!

show vlan brief
show interfaces trunk

EtherChannel (LACP)

interface range Gi0/1-2
 channel-group 1 mode active    ! active = initiates LACP
interface port-channel 1
 switchport mode trunk
show etherchannel summary

Rapid PVST+ Spanning Tree

! Root bridge — lower priority wins
spanning-tree vlan 10 priority 4096    ! must be multiple of 4096

! Access port optimizations
interface Gi0/2
 spanning-tree portfast                ! skip listening/learning
 spanning-tree bpduguard enable        ! shutdown if BPDU received

! Upstream protection
 spanning-tree guard root              ! prevent unauthorized root
 spanning-tree guard loop              ! protect against unidirectional link

show spanning-tree
show spanning-tree vlan 10

Wireless architectures

Routing table & route selection

Router# show ip route
Codes: C - connected, S - static, O - OSPF, R - RIP, B - BGP

O    10.1.0.0/24 [110/2] via 192.168.1.1, Gi0/0
!             ^AD  ^metric ^next-hop     ^exit-int

Administrative distance

Static routing

! Network route
ip route 10.0.0.0 255.255.255.0 192.168.1.1

! Default route (gateway of last resort)
ip route 0.0.0.0 0.0.0.0 192.168.1.1

! Host route (/32)
ip route 10.0.0.5 255.255.255.255 192.168.1.1

! Floating static (backup — AD 150 > OSPF 110)
ip route 10.0.0.0 255.255.255.0 192.168.2.1 150

! IPv6 static
ipv6 route 2001:DB8::/32 2001:DB8::1

OSPFv2

router ospf 1
 router-id 1.1.1.1
 network 192.168.1.0 0.0.0.255 area 0   ! wildcard mask

! Verify adjacencies
show ip ospf neighbor
show ip ospf
show ip route ospf

FHRP — First Hop Redundancy

NAT configuration

! PAT (overload) — most common
access-list 1 permit 10.0.0.0 0.0.0.255
ip nat inside source list 1 interface Gi0/1 overload
interface Gi0/0
 ip nat inside
interface Gi0/1
 ip nat outside

show ip nat translations
show ip nat statistics

DHCP & NTP

! DHCP server config
ip dhcp excluded-address 192.168.1.1 192.168.1.10
ip dhcp pool LAN
 network 192.168.1.0 255.255.255.0
 default-router 192.168.1.1
 dns-server 8.8.8.8

! DHCP relay (ip helper-address)
interface Gi0/0
 ip helper-address 10.0.0.5

! NTP
ntp server 192.168.1.1
show ntp status; show ntp associations

QoS & syslog

ACLs

! Standard ACL (source IP only) — apply near destination
access-list 10 permit 192.168.1.0 0.0.0.255
access-list 10 deny any
interface Gi0/0
 ip access-group 10 in

! Extended ACL (src+dst+protocol+port) — apply near source
ip access-list extended ALLOW_WEB
 permit tcp 192.168.1.0 0.0.0.255 any eq 80
 permit tcp 192.168.1.0 0.0.0.255 any eq 443
 deny ip any any

show access-lists
show ip interface Gi0/0         ! shows applied ACLs

Layer 2 security

! DHCP Snooping — must enable before DAI
ip dhcp snooping
ip dhcp snooping vlan 10,20
interface Gi0/1                    ! trusted uplink
 ip dhcp snooping trust

! Dynamic ARP Inspection
ip arp inspection vlan 10,20
interface Gi0/1
 ip arp inspection trust

! Port Security
interface Gi0/2
 switchport port-security
 switchport port-security maximum 2
 switchport port-security mac-address sticky
 switchport port-security violation shutdown

show ip dhcp snooping binding
show ip arp inspection
show port-security interface Gi0/2

AAA

SDN architecture

REST APIs & JSON

// JSON example — Cisco device interface
{
  "interface": {
    "name": "GigabitEthernet0/0",
    "ip-address": "192.168.1.1",
    "enabled": true
  }
}

Configuration management tools

Essential Cisco IOS show commands

show version
show running-config
show startup-config
show interfaces
show ip interface brief
show ip route
show arp

show vlan brief
show interfaces trunk
show spanning-tree
show mac address-table
show cdp neighbors
show ip ospf neighbor
show ip nat translations

Core models

Security control categories & types

Zero Trust

Cryptographic solutions

Change management

Threat actors

Threat vectors & attack surfaces

Social engineering

Vulnerability types

Malware types

Mitigation techniques

Architecture models

Security infrastructure concepts

Firewall types

Secure communication & data protection

Resilience & recovery

Hardening & asset management

Vulnerability management

Enterprise security capabilities

Identity & access management

Incident response process

Governance

Risk management

Third-party risk & compliance

Audits, assessments & awareness

Cloud models

Cloud service types

Benefits & pricing

Core architectural components

Compute services

Networking services

Storage services

Migration & transfer tools

Identity, access & security

Cost management

Governance & compliance tools

Deployment & management tools

Monitoring tools

Cloud models

Cloud service types

AWS global infrastructure

Cloud benefits

IAM — Identity & Access Management

Security services

Compliance & governance

Compute services

Storage services

S3 storage classes

Networking services

Database services

Other key services

Pricing models

Cost management tools

Support plans