Linux Command CMDsheet

Hardware Information

lscpu	Processor
free	Memory
df / du	Storage

Processes:

ps -A or ps -e	List all processes running on the system
ps ef	Detailed overview in full format
ps a	Displays the running processes from all users
ps u	Displays the running processes along with user or owner
ps x	Displays the processes those have not been executed from or attached to a terminal

Operating System

uname -a	Display all system information, including machine name, kernel name, version, and other details
lsb_release -a	Display all LSB (Linux Standard Base) information, including the distribution name, version, codename, and ID
cat /etc/os-release	Display information about the operating system release
hostnamectl	Display system hostname settings, including the hostname, machine ID, boot ID, virtualization, kernel, architecture, and more

Memory Utilization

cat /proc/meminfo	Displays system’s memory usage
free -m	Displays total amount of free and used physical and swap memory, buffers and caches used by kernel in MB
free -g	Same as above in GB
top	Displays dynamic real-time information about running tasks
vmstat	Report virtual memory statistics, including memory, swap, I/O, system activity, and CPU activity

Disks

lsblk	To list the block devices
iostat	Report Central Processing Unit (CPU) statistics and input/output statistics for devices and partitions
hdparm -I /dev/sda	Display information about disk hardware information
smartctl -a /dev/sda	Display disk health
ncdu	Disk usage analyzer with an ncurses interface
fdisk -l	To list the different partitions
du -h --max-depth=1 \| sort -hr	To get the size of each folder in the current directory
du -ah --max-depth=1	To include hidden files in the size calculation

Disk Space Utilization

df -h	Display disk usage in human-readable format
du -sh <dir>	Estimate file and directory space usage
du -s * | sort -n	Display the size of each file and directory sorted by size

Directories

mkdir <dir>	To create a directory
rmdir <dir>	Remove an empty directory named <dir>
rm -r <dir>	Remove a directory named <dir> and its contents
pwd	Print the current working directory
cd <dir>	Change the current directory to dir
ls	List the contents of the current directory
ls -l	List the contents of the current directory in long format
ls -a	List all files, including hidden files
ls -a --recursive	List all files, including hidden files by looping through all the sub folders
cp -r <dir1> <dir2>	Copy directory <dir1> and its contents to <dir2>
mv <dir1> <dir2>	Move or rename directory <dir1> to <dir2>

User Management

whoami	Display the username of the current user
id <username>	Display the user and group IDs of the specified user
useradd <username>	Create a new user
userdel <username>	Delete a user
passwd <username>	Change a user’s password
su - <username>	Switch to another user account
usermod -aG <groupname> <username>	Add a user to a group
groups <username>	Display the groups a user is a part of
cat /etc/passwd	List all users on the system
cat /etc/group	List all groups on the system

Firewall

sudo ufw enable	Enable the firewall
sudo ufw disable	Disable the firewall
sudo ufw status	Check the status of the firewall
sudo ufw allow <port/service>	Allow traffic on specific port or service
sudo ufw deny <port/service>	Deny traffic on specific port or service
sudo ufw delete allow <port/service>	Delete an allow rule
sudo ufw delete deny <port/service>	Delete a deny rule

Network

nc -l <port>	Listen for incoming connections on a specified port
nc <hostname> <port>	Connect to a specified hostname on a specified port
nc -u <hostname> <port>	Use UDP instead of the default TCP
nc -v <hostname> <port>	Operate in verbose mode, useful for debugging
nc -z <hostname> <port>	Scan for listening daemons, without sending any data
nc -w <seconds> <hostname> <port>	Timeout for connections after the specified number of seconds

DNS

dig <domain>	Query DNS name servers for your DNS lookup
dig <domain> +short	Get just the IP address for a domain
dig -x <ip>	Perform a reverse DNS lookup
nslookup <domain>	Query internet name servers interactively
host <domain>	Perform a simple DNS lookup
whois <domain>	Get whois information for a domain

Logging

dmesg	Display messages from the kernel ring buffer
cat /var/log/syslog	Display system logs
tail -f /var/log/syslog	Follow the system logs in real time
journalctl	Query the systemd journal
journalctl -f	Follow the systemd journal in real time
journalctl -u <service>	Show logs for a specific service
less /var/log/syslog	View system logs with paging
grep 'error' /var/log/syslog	Search for ‘error’ in system logs

Common Commands

echo -n 'string-to-convert' \| openssl base64	Encode a string to base64
echo -n 'string-to-decode' \| openssl base64 -d	Decode a base64 string
echo -n 'string-to-convert' \| md5sum	Generate an MD5 hash of a string
echo -n 'string-to-convert' \| sha256sum	Generate a SHA-256 hash of a string

OpenSSL

openssl version	Display OpenSSL version
openssl genrsa -out private.key 2048	Generate a new private key
openssl req -new -key private.key -out request.csr	Generate a Certificate Signing Request
openssl x509 -req -days 365 -in request.csr -signkey private.key -out certificate.crt	Generate a self-signed certificate
openssl rsa -in private.key -pubout -out public.key	Extract the public key from a private key
openssl s_client -connect <hostname>:<port>	Connect to a server to obtain the SSL/TLS details
openssl enc -aes-256-cbc -salt -in file.txt -out file.txt.enc	Encrypt a file
openssl enc -aes-256-cbc -d -in file.txt.enc -out file.txt	Decrypt a file