Linux

Basic commands

System Commands

File & Directory Management

List all files + details

ls -al

List all files recursively

ls -lAR

Read raw binary data

cat example | hexdump -C

Look for a given word inside a file

cat file.txt | grep word

Count number of lines in a file

wc -l myfile.txt

Print Working Directory

pwd

Print full path of a file/directory

realpath file.txt

Copy recursively

cp -r dir/ /home/user/

Find all .txt files

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

Find files larger than 10MB

find / -type f -size +10M

Search the entire filesystem for any match with the regex pattern

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

Updates the database used by locate

sudo updatedb

Look file from the database

locate file.txt

Full path of a binary

which binary

Creates a new file/Update timestamps

touch newfile

Delete directory

rm -r directory/

Process Management

List details for process

ps -ef | grep process

Show a detailed process tree with wide output

ps -auxwf

Show process tree for a specific user

pstree user

Show process IDs (PIDs) with the tree view

pstree -p

Show real-time process stats

htop

Get the current process ID

echo $$

Kill a process

sudo kill -9 <PID>

System Management

Print Info about the system

uname -a 

Print Environment

env

Shows details about all block devices

lsblk -f

List PCI devices

lspci

Display Disk Usage

df -h

Show the size of files and directories in the current directory

du -sh *

Shows the 10 largest directories on the system

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

Display RAM info

free -h

Displays the maximum allowed size of a process's lockable memory

ulimit -l

Put a process in the background

bg

List all process in the background

jobs

Puts a process into the foreground

fg

Display overall system performance stats

vmstat

Print stats every 5 seconds

vmstat 5

Print stats every 5 seconds for 10 iterations

vmstat 5 10

Show the system uptime

uptime

Show info about current login user

w

Logs

Display login records

last

Display bad login attempts

lastb

Shows log as they are written

journalctl -f 

Shows logs from the current boot

journalctl -b

Shows logs from the previous boot

journalctl -b -1

Shows logs from a specific service

journalctl -u <service>

Shows logs from a time range

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

Show logs by priority level

journalctl -p <level>

Priority Levels:

• 0 --> emergency

• 1 --> alert

• 2 --> critical

• 3 --> error

• 4 --> warning

• 5 --> notice

• 6 --> informational

• 7 --> debug

Shows logs for a specific process

journalctl _PID=<pid>

Shows logs from the kernel

journalctl -k

Show kernel and boot messages

dmesg

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User/Group Management

Users

Display user and group ID of the current user

id 

id from a specific user

id username

Edit sudoers file

sudo visudo

Switch to the root user

su

Switch to another user

su - username

User Management

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

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Permission & Security

File Ownership

Set file permissions to rwxr-xr-x

sudo chmod 755 file.txt

Adds read permissions for all users

sudo chmod a+r shell.sh

Add execution permission

sudo chmod +x script.sh

Change file owner and group

sudo chown user:group file.txt

Change group ownership

sudo chgrp group file.txt

Prevent world-writable files

umask 022 

Access Control Lists

Show the ACL

getfacl file.txt

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

setfacl -m u:user:rw file.txt

Security Policies

Display the current SELinux mode

getenforce

Disable SELinux

sudo setenforce 0

File Integrity

Generates a SHA256 file

sha256sum file.txt

Generates a MD5 hash

md5sum file.txt

Initialize the Tripwire database

sudo tripwire --init

Check the integrity of the system

sudo tripwire --check

Generate a report

sudo tripwire --update

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Networking Command

Display network interfaces and their IP addresses

ifconfig

Display IP addresses of all network interfaces

ip a 

Bring interface up

sudo ip link set eth0 up

Show networks accessible via the VPN

netstat -rn

Show listening TCP/UDP ports

netstat -tuln

Show listening ports and services

ss -tuln

Show listening ports for a process

ss -tulnp | grep PID

Display the Routing Table

ip route show

Add a route

sudo ip route add 192.168.2.0/24 via 192.168.1.254

Delete a Route

sudo ip route del 192.168.2.0/24

Add a Default Gateway

sudo ip route add default via 192.168.1.1

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

Trace the network path to example.com

traceroute example.com

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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 usingWPA

Shows the current connection status for the wpa

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

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base64

• Using base64 help to keep file integrity

• Check hashes to avoid file corruption

Generate hash

md5sum <file>

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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

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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