find / -type f -name "filename" 2>/dev/null
find / -type d -name "dirname" 2>/dev/null

find . -type f -exec grep -l 'version' {} \;

journalctl | grep logrotate

dmidecode -s system-manufacturer

du -hs * | sort -h

(echo > /dev/tcp/10.254.4.54/22) >/dev/null 2>&1 && echo "It's up" || echo "It's down"

sudo lsof -i -P -n | grep LISTEN
sudo netstat -tulpn | grep LISTEN
sudo ss -tulpn | grep LISTEN
sudo lsof -i:22 ## see a specific port such as 22 ##
sudo nmap -sTU -O IP-address-Here

openssl x509 -enddate -noout -in /path/to/certificate.pem

sudo yum check-update

needs-restarting -r

rm -rf /var/cache/yum

sysctl -w net.ipv4.icmp_echo_ignore_all=1

sed -i '/^$/d' <filename>

sudo -i

rm -f /tmp/names; for user in $(getent passwd | cut -d: -f1); do count=$((count+1)); if sudo -l -U "$user" | grep -q "ALL"; then echo "$user" >> /tmp/names; echo "Checked $count of $(getent passwd | cut -d: -f1 | wc -l) users."; fi; done; clear; cat /tmp/names; rm -f /tmp/names

/var/log/example.log {
    daily
    rotate 7
    compress
    missingok
    notifempty
    create 0640 root adm
}

/var/log/myapp/*.log {
    daily
    rotate 10
    size 100M
    compress
    delaycompress
    missingok
    notifempty
    create 0640 appuser adm
    sharedscripts
    postrotate
        systemctl reload myapp > /dev/null 2>&1 || true
    endscript
}

lsblk

df -h

pvck        # Check PV metadata
pvdisplay   # Display PV attributes
pvs         # Report PV information
pvscan      # Scan for PVs

pvcreate /dev/sda /dev/sdb /dev/sdc /dev/sdd

pvremove /dev/sdX

pvchange     # Change PV attributes
pvmove       # Move physical extents
pvresize     # Resize PV

vgck        # Check VG metadata
vgdisplay   # VG attributes
vgs         # Report VG info
vgscan      # Scan for VGs

vgcreate vg0 /dev/sda

vgremove vg_name

vgcfgbackup   # Backs up volume group (VG) metadata
vgcfgrestore  # Restores VG metadata from backup
vgchange      # Change VG attributes
vgconvert     # Metadata format change
vgexport      # Exports a VG to make it unknown to the system
vgimport      # Imports a VG previously exported
vgimportclone # Imports and renames a VG to avoid conflicts
vgmerge       # Merges two VGs into one
vgsplit       # Splits a VG into two separate VGs
vgextend vg0 /dev/sdc /dev/sdd  # Adds physical volumes to a VG
vgreduce      # Removes physical volumes from a VG
vgrename      # Renames a VG
vgmknodes     # Recreates device nodes for LVM devices

lvdisplay    # Show LV attributes
lvmdiskscan  # Scan for devices
lvs          # Report info
lvscan       # Scan for LVs

lvcreate -n lvbackup -L 50G vgbackup -r

lvremove /dev/vg_name/lv_name

lvchange     # Change LV attributes
lvconvert    # Mirror/snapshot conversion
lvextend /dev/vg_name/lv_name
lvreduce     # Reduce size
lvresize     # Resize
lvrename     # Rename
resize2fs /dev/vg0/lv_root   # Resize filesystem
xfs_growfs -d /dev/vg-group-name/lv-name

mkfs.ext4 /dev/vgbackup/lvbackup

blkid /dev/vgbackup/lvbackup    # Get UUID
mkdir /path/to/folder           # Mount point
vim /etc/fstab                  # Add entry with UUID
mount -a                        # Mount all from fstab

pvs     # List physical volumes
vgs     # List volume groups
lvs     # List logical volumes

pvdisplay
vgdisplay
lvdisplay

lvcreate --size 1G --snapshot --name snap_name /dev/vg0/lvdata
lvconvert --merge /dev/vg0/snap_name

lvextend -L +10G /dev/vgbackup/lvbackup
resize2fs /dev/vgbackup/lvbackup
xfs_growfs /path/to/your/mount/point

fdisk [options] /dev/sdX

sudo fdisk -l

sudo fdisk /dev/sdX

Command (m for help): n
Select default or choose primary (p) or extended (e)
Partition number: 1
First sector: [Press Enter to accept default]
Last sector: +1G  # or specify size like +20G

Command (m for help): t
Partition number: 1
Hex code (type L for list): 83  # Linux filesystem

Command (m for help): a
Partition number: 1

Command (m for help): w

Command (m for help): q

-f       # Use a script file
-l       # List partition tables for all devices
-u       # Display sectors in cylinders or sectors

mkfs.ext4 /dev/sdX1
mkfs.vfat /dev/sdX2

mount /dev/sdX1 /mnt/mydisk

#!/bin/bash

# Ensure script is run as root
if [[ $EUID -ne 0 ]]; then
   echo "❌ This script must be run as root"
   exit 1
fi

# Prompt for new username
read -p "Enter new username: " username

# Check if user already exists
if id "$username" &>/dev/null; then
    echo "⚠️ User '$username' already exists."
    exit 1
fi

# Prompt for password (silent input)
read -s -p "Enter password for $username: " password
echo
read -s -p "Confirm password: " password_confirm
echo

# Check passwords match
if [[ "$password" != "$password_confirm" ]]; then
    echo "❌ Passwords do not match."
    exit 1
fi

# Create user with home directory and bash shell
useradd -m -s /bin/bash "$username"

# Set user password
echo "${username}:${password}" | chpasswd

# Add user to sudo group
usermod -aG sudo "$username"

# Create a sudoers file to allow passwordless sudo
echo "$username ALL=(ALL) NOPASSWD:ALL" > "/etc/sudoers.d/$username"
chmod 440 "/etc/sudoers.d/$username"

echo "✅ User '$username' created with bash shell and passwordless sudo access."