Find a file or directory

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

find files that contain a pattern

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

Find a file or directory

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

check if machine is a vm or barebone

dmidecode -s system-manufacturer

check folder disk usage

du -hs * | sort -h

check open port

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

check for listening ports

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

Check expiry date on .pem file

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

dry-run an update on OS

sudo yum check-update

Check if a reboot is required

needs-restarting -r

remove cache of updates for old data

rm -rf /var/cache/yum

block icmp

sysctl -w net.ipv4.icmp_echo_ignore_all=1

remove empty lines from file

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

change to root

sudo -i

Grep sudo users

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

This cheat sheet provides an extensive list of Logrotate configuration directives, their descriptions, and examples.
Use this as a quick reference to master log rotation on Unix-like systems.

Each configuration block is tied to a log file or set of log files. Example:

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

Basic Settings:

rotate <count>
Keep number of old log files before deleting them.

daily | weekly | monthly | yearly
Frequency of rotation.

Compression Options:

compress
Compress old versions of log files with gzip.

nocompress
Do not compress old logs.

delaycompress
Postpone compression to the next rotation cycle (used with compress).

File Handling:

missingok
Ignore missing log files and don’t issue an error.

notifempty
Do not rotate the log if it is empty.

ifempty
Rotate the log even if it is empty (default behavior).

create <mode> <owner> <group>
Create a new log file with specified permissions.

copy
Make a copy of the log file and truncate the original.

copytruncate
Truncate the original log file after copying it (useful for active logs).

Date & Naming:

dateext
Append an extension with the current date to rotated log files.

dateformat .%Y-%m-%d
Custom format for dateext (e.g., .2025-06-06).

extension <ext>
Force specific extension for rotated files (e.g., .log).

Size-Based Rotation:

maxage <days>
Remove rotated logs older than .

minsize <size>
Rotate only if log size is above .

size <size>
Rotate if log file size meets threshold, regardless of time.

maxsize <size>
Do not rotate if log is larger than specified size.

Directory & Scripts:

olddir <dir>
Move rotated logs to a specified directory.

sharedscripts
Run postrotate script once for all matching logs.

postrotate/endscript
Script to run after log rotation.

prerotate/endscript
Script to run before log rotation.

firstaction/endscript
Run only once before rotation begins (before prerotate).

lastaction/endscript
Run once after rotation finishes (after postrotate).

tabooext + <ext>
Treat additional extensions as taboo (not rotated).

• Run logrotate -d <config> to debug your config without applying changes.
• Use logrotate -f <config> to force rotation for testing.
• Logrotate is typically triggered via cron or systemd timers.
• Keep your config DRY by centralizing shared logic in /etc/logrotate.conf and using includes.

Create a sudo user that won’t prompt for password on executing sudo commands.

#!/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."