Linux

Basic commands

Specifications & Performance

uptime

Shows details about all block devices

lsblk -f

Display RAM info

free -h

Stats 5s for 10 itinerations

vmstat 5 10

Check RAM remaining to Memory Locking

Check

ulimit -l

Set a new limit

ulimit -l 10240

List USB

lsusb

List Peripherals

lspci

Store Management

List all files recursively

ls -lAR

Show Sizes Current Directory

du -sh *

Largest 10 directories

sudo du -hsx /* | sort -rh | head -n 10

File Enumeration

Print full path

realpath file.txt

Count Lines

wc -l myfile.txt

locate

Look for a file

locate file.txt

Update the db

sudo updatedb

find

Find all .txt files

find /home/user -name "*.txt"

Find files larger than 10MB

find / -type f -size +10M

Search with regex

sudo find / -regex ".*alacritty.*" 2>/dev/null

Process Management

List process

ps -ef | grep process

Processes by Trees

ps -auxwf

User process tree + PID

pstree -p user

Current process ID

echo $$

Background process

bg

List background process

jobs

Foreground process

fg

Users/Groups

Edit sudoers file

sudo visudo

Users

Creates a new user

sudo useradd newuser

Creates a new user with home directory and bash shell

sudo useradd -m -s /bin/bash newuser

Add user to the sudo group

sudo usermod -aG sudo newuser

Set or change password

sudo passwd newuser

Set zsh as default shell

sudo usermod --shell /usr/bin/zsh alice

Groups

Create group

sudo groupadd newgroup

Delete Group

sudo groupdel group

Add user to group

sudo gpasswd -a newuser newgroup

File Ownership

Set file permissions to rwxr-xr-x

sudo chmod 755 file.txt

Read permissions for all users

sudo chmod a+r shell.sh

Change file owner and group

sudo chown user:group file.txt

Add execution permission

sudo chmod +x script.sh

Change group ownership

sudo chgrp group file.txt

Logs

Current login user

w

Display login records

last

Display bad login attempts

lastb

Kernel and boot messages

dmesg

journalctl

Shows logs live

journalctl -f 

Logs from the current boot

journalctl -b

Logs from the previous boot

journalctl -b -1

Logs from a specific service

journalctl -u <service>

Logs for a specific process

journalctl _PID=<pid>

Shows logs from the kernel

journalctl -k

Logs by time-frame

journalctl --since "2024-11-01" --until "2024-11-10"

Priority Levels

journalctl -p <level>

• 0 --> emergency

• 1 --> alert

• 2 --> critical

• 3 --> error

• 4 --> warning

• 5--> notice

• 6 --> informational

• 7 --> debug

Access Control Lists

Grant read/write permissions to 'user' on file.txt

setfacl -m u:user:rw file.txt

Show the ACL

getfacl file.txt

Prevent world-writable files

umask 022

Change a route's metric

sudo route change -net 192.168.2.0 netmask 255.255.255.0 gw 192.168.1.254 metric 200

SELinux

Display mode

getenforce

Disable SELinux

sudo setenforce 0

Chroot

Decrypt the system

Decrypt partition

sudo cryptsetup luksOpen /dev/sda3 cryptdisk

Mounts the decrypted device

sudo mount /dev/mapper/cryptdisk /mnt

Mounts the boot partion

sudo mount /dev/sda1 /mnt/boot

chroot into a mounted partition

sudo arch-chroot /mnt

Connect using WPA

Shows connection status

wpa_cli status

Creates the passphrase

sudo wpa_passphrase "MyWiFi" "mypassword123" | sudo tee /etc/wpa_supplicant/wpa_supplicant.conf

Connect using passphrase

sudo wpa_supplicant -B -i wlan0 -c /etc/wpa_supplicant/wpa_supplicant.conf

Boot/UEFI Management

Displays the current boot entries in the UEFI firmware

efibootmgr

deletes the boot entry "0"

sudo efibootmgr --delete-bootnum --bootnum 0

base64

Generate MD5 hash

md5sum <file>

Generates a SHA256 hash

sha256sum file.txt

Encoding

Encode File + Redirect Output

base64 file.txt > hash.txt

Encode String + Redirect Output

echo "your_string_here" | base64 > encoded_file.txt

• In Base64 encoding, by default, some implementations add line breaks every 76 characters (following the RFC 2045 standard).

Encode File Without line break

base64 -w 0 <file>

Encode String without line breaks

echo -n "your_string_here" | base64 -w 0

---

Safe URL encoding

base64 -w 0 <file> | tr '+/' '-_' > url_safe_encoded.txt

Decoding

Decode String + Output to a file

echo <string> | base64 -d > <file>

Decode File + Output to a file

base64 -d -i <file> > <decoded_file>

Decode File without line breaks

base64 -d -w 0 file.txt > decoded_output.bin

Qemu

Installation

sudo pacman -S qemu libvirt dnsmasq virt-manager bridge-utils ebtables
sudo systemctl enable --now libvirtd

Check/Backup the XML

sudo virsh net-dumpxml c2-lab

Manual XML Virtual Network Configuration

Locked-down Version

<network>
  <name>c2-lab</name>
  <bridge name="virbr2" stp="off" delay="0"/>  <!-- Disable STP (not needed) -->
  <forward mode="none"/>                       <!-- NO NAT, NO ROUTING -->
  <interface type="network">
  <mac address="52:54:00:XX:XX:XX"/>  <!-- Set a static MAC -->
  <source network="c2-lab"/>
  <model type="virtio"/>
</interface>
  <ip address="192.168.100.1" netmask="255.255.255.0">
  <ip family="ipv6" address="fe80::1" prefix="64"/>
    <!-- No DHCP (assign IPs manually) -->
  </ip>
</network>

Disable ICMP

sudo iptables -I FORWARD -i virbr2 -p icmp -j DROP

Start the Virtual-Network

sudo virsh net-define c2-lab.xml
sudo virsh net-start c2-lab

Start on boot

sudo virsh net-autostart c2-lab

Check Network Info

sudo virsh net-info c2-lab

Check for leaks

sudo iptables -L -v -n | grep virbr2