Linux Overview

# Copying files
cp source.txt destination.txt # Copies 'source.txt' to 'destination.txt'
cp -r source_directory/ destination_directory/ # Recursively copies a directory

# Moving/Renaming files
mv oldfile.txt newfile.txt # Renames or moves 'oldfile.txt' to 'newfile.txt'
mv files/* /backup/ # Moves all files from 'files' directory to '/backup/'

# Deleting files
rm unwanted.txt # Removes 'unwanted.txt' permanently
rm -r old_directory/ # Recursively deletes 'old_directory' and its contents

# Creating directories
mkdir new_directory # Creates a new directory named 'new_directory'
mkdir -p path/to/new_directory # Creates all intermediate directories as needed

# Listing files
ls -lah # Lists all files in a detailed, human-readable format
ls -ld */ # Lists only directories within the current directory

# Find files larger than 100MB
find / -type f -size +100M # Finds files larger than 100MB throughout the filesystem

# Find files modified in the last 24 hours
find / -type f -mtime -1 # Locates files modified within the last day across the entire filesystem

# Find files by name ignoring case
find / -type f -iname "*pattern*" # Searches for files with 'pattern' in their names, case-insensitively

# Find directories named 'config' anywhere in the filesystem
find / -type d -name "config" # Locates all directories named 'config'

# Find empty files and directories
find / -type d -empty -o -type f -empty # Finds all empty directories and files

# Find and remove files larger than 500MB
find / -type f -size +500M -exec rm -i {} \; # Interactively deletes files over 500MB

# Rapid filesystem searches with updated database
locate pattern # Searches for files matching 'pattern' using a database updated by 'updatedb'

# Search for specific text within files
grep 'pattern' filename # Finds occurrences of 'pattern' within 'filename'
grep -ri 'pattern' /path/to/directory # Recursively/case-insensitive searches for 'pattern' in all files under the specified directory

# Search for 'pattern' in .log files, excluding lines starting with '#'
grep -v '^#' /path/to/*.log | grep 'pattern' # Filters out comments, then searches for 'pattern'

# Search files using regex and print lines before and after the match
grep -B 1 -A 1 'pattern' /path/to/file.log # Shows the line with 'pattern' and one line before and after each match

# Display files containing 'config' in their name
ls -la | grep 'config' # Uses 'grep' to filter the output of 'ls' to show only files containing 'config'

# Use the results of 'find' to search for 'error' in log files
find /var/log -name '*.log' | xargs grep 'error' # Searches for 'error' in all log files located in '/var/log'

# Count the number of files in a directory
ls -1 /path/to/directory | wc -l # Uses 'wc -l' to count the number of lines output by 'ls', which represents the number of files

# Find and delete files older than 30 days
find /path/to/directory -type f -mtime +30 -exec rm {} \; # Finds files older than 30 days and deletes them using 'rm'

# Replace 'old' with 'new' in 'filename'
sed 's/old/new/g' filename # Uses global replacement to change all occurrences of 'old' to 'new'

# Delete lines containing 'pattern' in 'filename'
sed '/pattern/d' filename # Deletes all lines that match 'pattern'

# Print lines 5 to 10 from 'filename'
sed -n '5,10p' filename # Uses '-n' to suppress automatic printing and 'p' to print specific lines

# Append 'text' after lines containing 'pattern'
sed '/pattern/a text' filename # Appends 'text' after lines that contain 'pattern'

# Substitute 'foo' with 'bar' only on lines containing 'baz'
sed '/baz/s/foo/bar/' filename # Performs substitution only on lines that include 'baz'

# Print lines matching 'pattern' in 'filename'
awk '/pattern/ {print $0}' filename # Prints whole lines ($0) that match 'pattern'

# Sum the values in the first column
awk '{sum += $1} END {print sum}' filename # Adds up all values in the first column ($1)

# Print lines where the second column equals 'value'
awk '$2 == "value" {print $0}' filename # Checks if the second column ($2) is 'value' and prints the line

# Replace spaces with commas in 'filename'
awk '{gsub(" ", ",", $0); print}' filename # Uses gsub function to globally replace spaces with commas

# Sort lines by the numerical value of the second column
awk '{print $2, $0}' filename | sort -n | cut -d' ' -f2- # Prints and sorts lines by the second column numerically

# Search recursively for 'pattern' in directory
grep -r 'pattern' /path # '-r' allows recursive search through all directories and subdirectories

# Exclude log files while searching recursively for 'pattern'
grep --exclude='*.log' -r 'pattern' /path # '--exclude' prevents grep from searching files ending in .log

# Count the number of occurrences of 'pattern'
grep -c 'pattern' filename # '-c' counts the occurrences of 'pattern' in 'filename'

# Display line numbers of 'pattern' in 'filename'
grep -n 'pattern' filename # '-n' shows line numbers along with the matching lines

# Highlight matches for 'pattern' in 'filename'
grep --color=always 'pattern' filename # '--color=always' highlights all occurrences of 'pattern'

# Search for lines that do not contain 'pattern'
grep -v 'pattern' filename # '-v' inverts the search, showing lines without 'pattern'

# Find patterns that are whole words only
grep -w 'pattern' filename # '-w' matches only whole words, ensuring 'pattern' is not a substring

# Display lines before and after 'pattern' matches
grep -B 1 -A 1 'pattern' filename # '-B 1' and '-A 1' show one line before and one line after each match

# Create aliases for commonly used commands
alias ll='ls -la' # 'll' now lists all files with detailed information
alias rm='rm -i' # Prompts for confirmation before removing files

# Alias to quickly navigate to frequently accessed directories
alias docs='cd ~/Documents'

# Write functions to perform complex tasks
backup() {
  tar -czf "$1.tar.gz" "$1" # Compresses the directory
  echo "Directory $1 backed up as $1.tar.gz"
}

# Function to update and clean system with a single command
sys_update() {
  sudo apt update && sudo apt upgrade -y
  sudo apt autoremove -y
  echo "System updated and cleaned."
}

# Add these aliases and functions to your .bashrc for permanent availability
# Edit .bashrc with `nano ~/.bashrc` and paste the alias or function at the end

# Repeat the last command executed
!! # Executes the last command entered in the terminal

# Search command history for a specific command
history | grep 'install' # Finds all past instances of 'install'

# Search history using reverse-i-search
Ctrl + r 'part_of_command' # Interactively search and complete commands from history

# Persist command history across sessions
shopt -s histappend # Append rather than overwrite the history on disk
history -a # Writes the current session's history to the disk (.bash_history)

# Protect .bash_history from being overwritten
chmod 600 ~/.bash_history # Sets read and write permissions for the user only

# Clear command history
history -c # Clears the terminal history

# Exclude certain commands from history
HISTIGNORE='ls:cd:cd -:pwd:exit:date:* --help' # Ignore basic commands and any command with '--help'

# Basic transfer from local to remote
scp localfile.txt user@remote:/path/ # Copies 'localfile.txt' to the remote server at the specified path

# Transfer from remote to local
scp user@remote:/path/remotefile.txt /local/path # Downloads 'remotefile.txt' from a remote server to a local directory

# Transfer entire directory recursively from local to remote
scp -r localdir/ user@remote:/path/ # Recursively copies the entire 'localdir' directory to remote location

# Access Windows share from Linux
sudo mount -t cifs -o username=user_name,password=password //Windows_Server/share /mnt/windows_share # Mounts a Windows share on a Linux system

# Share Linux directory with Windows using Samba
# Step 1: Install Samba
sudo apt install samba

# Step 2: Configure the Samba share
# Open the Samba configuration file in a text editor, such as vim
sudo vim /etc/samba/smb.conf

# Step 3: Add the following entry at the end of the smb.conf file
# This configures a share that Windows users can access

# Step 4: Restart the Samba service to apply changes
sudo systemctl restart smbd

# Step 5: Set a Samba password for the user
sudo smbpasswd -a user_name

# Ensure the directory permissions allow the appropriate access
sudo chmod -R 775 /path/to/share
sudo chown -R user_name:group_name /path/to/share

# Zip multiple files into a single archive
zip compressed.zip file1.txt file2.txt # Compresses 'file1.txt' and 'file2.txt' into 'compressed.zip'

# Zip an entire directory recursively
zip -r archive.zip /path/to/directory # Recursively compresses the directory and its contents into 'archive.zip'

# Unzip files into the current directory
unzip compressed.zip # Extracts all files from 'compressed.zip' into the current directory

# List contents of a zip archive without extracting
unzip -l compressed.zip # Lists the contents of 'compressed.zip'

# Create a tar.gz archive of a directory
tar -czf archive.tar.gz folder/ # Compresses 'folder/' into 'archive.tar.gz' using gzip

# Extract a tar.gz archive
tar -xzf archive.tar.gz # Extracts 'archive.tar.gz' into the current directory

# View contents of a tar.gz archive without extracting
tar -tzf archive.tar.gz # Lists the contents of 'archive.tar.gz'

# Mount a filesystem
mount /dev/sdx1 /mnt/data # Attaches the filesystem found on device '/dev/sdx1' to the directory '/mnt/data'

# Mount a USB drive with specific filesystem type
mount -t vfat /dev/sdx1 /mnt/usb # Mounts a USB drive formatted with FAT32 to '/mnt/usb'

# Unmount a device safely
umount /mnt/data # Detaches the filesystem from '/mnt/data'

# Force unmount when the device is busy
umount -l /mnt/data # Performs a lazy unmount, detaching the filesystem as soon as it is not busy

# Copy files with verification
cp -v source.txt destination.txt # Copies 'source.txt' to 'destination.txt' and displays what is being copied

# Move and rename files
mv -v old_location.txt new_location.txt # Moves/Renames 'old_location.txt' to 'new_location.txt', showing the process

# Remove a directory and its contents
rm -r directory_name # Recursively removes 'directory_name' and all its contents

# Check disk space usage
df -h # Displays free disk space on all mounted filesystems in human-readable form

# Summarize disk usage of the directory
du -sh /path/to/directory # Shows the total space used by the specified directory in human-readable form

# Display block devices connected to the system
lsblk # Lists all block devices connected to the system along with their mount points

# Create a new user with a home directory and a specific shell
useradd -m -s /bin/bash newuser # '-m' creates a home directory, '-s' specifies the shell

# Add a user to the sudo group to grant administrative privileges
usermod -aG sudo newuser # '-aG' appends the user to the group without removing them from other groups

# Delete a user and their home directory
userdel -r olduser # '-r' removes the user along with their home directory

# Create a new group
groupadd newgroup # Adds a new group to the system

# Modify a user's primary group
usermod -g newgroup username # '-g' sets the primary group for the user

# Add a user to multiple groups
usermod -aG group1,group2 username # Appends the user to both 'group1' and 'group2'

# Change file permissions
chmod 755 filename # Sets read, write, and execute permissions for the owner, and read and execute for others

# Change ownership of a file
chown user:group filename # Changes both the owner and group of 'filename'

# Set permissions recursively for a directory and its contents
chmod -R 755 /path/to/directory # '-R' applies the permissions recursively to all files and subdirectories

# Set the SetUID permission on a file
chmod u+s /path/to/file # Enables users to execute the file with the permissions of its owner

# Set the SetGID on a directory
chmod g+s /path/to/directory # Files created in the directory inherit the group ID of the directory, not of the user who created the file

# Set sticky bit on a directory to restrict file deletion
chmod +t /path/to/directory # Only the file's owner, the directory's owner, or root can delete files within the directory

# Change the owner and group recursively for a directory
chown -R user:group /path/to/directory # '-R' changes owner and group recursively for all files within the directory

grep 'error' /var/log/syslog # Find all error messages in the system activity log
grep -i 'fail' /var/log/auth.log # Find failed login attempts, case-insensitive search
grep 'Failed password' /var/log/auth.log # Search for failed SSH login attempts
grep 'COMMAND=' /var/log/auth.log # Check for sudo command usage
grep 'segfault' /var/log/kern.log # Find segmentation faults in kernel messages
grep '192.168.1.1' /var/log/apache2/access.log # Search for specific IP address access in web logs
grep -i 'error' /var/log/*.log # Find all occurrences of "error" in all logs, ignoring case
grep 'denied' /var/log/audit/audit.log # List denied services from SELinux
grep 'reboot' /var/log/syslog # Check for reboot records in the system activity log
grep -i 'out of memory' /var/log/kern.log # Find out of memory issues in kernel logs

#!/bin/bash
echo "Checking security-related log entries..."
keywords="fail|error|denied|segfault|unauthorized|exception"
log_files="/var/log/auth.log /var/log/syslog /var/log/kern.log /var/log/apache2/error.log"
for file in $log_files; do
    echo "Scanning $file for security keywords..."
    grep -E $keywords $file | while read -r line ; do
        echo "Security alert in $file: $line"
    done
done

# Disk usage checking with 'df'
df -h # Display disk space usage in human-readable format

# Directory size with 'du'
du -sh /path/to/directory # Summarize disk usage of the directory, human-readable

# Top command for process monitoring
top -o %MEM # Sort processes based on memory usage

# Memory and CPU usage with 'htop'
htop # Interactive process viewer, better visualization than 'top'

# Monitoring disk I/O with 'iostat'
iostat -mx 5 # Display extended disk I/O statistics every 5 seconds

# Network interface statistics with 'ifstat'
ifstat 5 5 # Report network interface status every 5 seconds for 5 intervals

# Viewing block device information with 'lsblk'
lsblk -f # Display filesystems in addition to block device attributes

# Checking active connections and listening ports with 'ss'
ss -tulwn # Display TCP, UDP, listening, and non-listening sockets with IPs and ports

# Using 'vmstat' for system performance
vmstat 1 10 # Report virtual memory statistics every 1 second for 10 intervals

# Real-time monitoring with custom alerts script
#!/bin/bash
usage=$(df / | grep / | awk '{ print $5 }' | sed 's/%//g')
if [ $usage -gt 90 ]; then
    echo "Disk usage is above 90%, current usage is $usage%" | mail -s "Disk Usage Alert" admin@example.com
fi

# Configure network interfaces with ifconfig
ifconfig eth0 up # Activates the 'eth0' network interface
ifconfig eth0 down # Deactivates the 'eth0' network interface
ifconfig eth0 192.168.1.100 netmask 255.255.255.0 # Sets IP address and netmask for 'eth0'

# Manage wireless networks with iwconfig
iwconfig wlan0 essid "ExampleNetwork" key s:password # Sets the SSID and WEP key for 'wlan0'

# Check connectivity to a host with ping
ping -c 4 google.com # Sends 4 ICMP packets to 'google.com' to check connectivity

# Trace the path packets take to a host with traceroute
traceroute google.com # Shows the route packets take to reach 'google.com'

# Scan network devices, ports, and services with nmap
nmap -v -A scanme.nmap.org # Performs a verbose scan with OS and service detection

# Connect to remote systems with telnet to test connectivity
telnet example.com 80 # Connects to 'example.com' on port 80 to check web server connectivity

# `curl` for testing HTTP/HTTPS endpoints.
curl http://example.com # Sends a GET request to example.com and displays the response
curl -O http://example.com/file.zip # Downloads the file from the specified URL and saves it with the same filename as on the server
curl -d "param1=value1&param2=value2" -X POST http://example.com/submit # Sends a POST request with data to the server

# Use fping to check the reachability of multiple hosts
fping -a -g 192.168.1.0/24 # Lists all alive hosts in the '192.168.1.0/24' subnet

# DNS lookup and network diagnostics with dig
dig google.com # Queries DNS information for 'google.com'
dig +trace +all google.com # Traces the path of the DNS query for 'google.com'

# Use nslookup for DNS querying
nslookup google.com # Queries DNS for records associated with 'google.com'
nslookup -type=mx google.com # Queries for mail exchange records for 'google.com'

# Capture and analyze network packets with tcpdump
tcpdump -i eth0 -w capture_file.pcap # Captures packets on 'eth0' and writes them to 'capture_file.pcap'
tcpdump -r capture_file.pcap # Reads and displays packets from a pcap file

# Basic SSH connection
ssh user@host # Logs into 'host' as 'user', establishing a secure shell session

# Generate SSH keys to use for authentication
ssh-keygen -t rsa -b 4096 # Generates an RSA key pair with 4096-bit encryption

# Copy SSH key to a remote host
ssh-copy-id user@host # Copies your public key to 'user@host' to enable key-based authentication

# Secure SSH configuration tips
# Edit the SSH config file to disable root login and enforce key-based authentication
sudo vim /etc/ssh/sshd_config
# Add or modify the following lines:
PermitRootLogin no
PasswordAuthentication no

# Securely copy a file to a remote server with SCP
scp localfile.txt user@host:/remote/directory/ # Copies 'localfile.txt' to the remote directory

# Start an SFTP session to interactively transfer files
sftp user@host # Initiates a secure FTP session with 'host', allowing file management commands

# Securely download a directory from a remote server with SCP
scp -r user@host:/remote/directory/ local_directory # Recursively copies a remote directory to a local directory

# Using iptables for basic firewall rules
iptables -A INPUT -p tcp --dport 22 -j ACCEPT # Allows incoming SSH traffic on port 22
iptables -A OUTPUT -p tcp --dport 80 -j ACCEPT # Allows outgoing HTTP traffic on port 80
iptables -A INPUT -s 192.168.1.0/24 -j DROP # Blocks all incoming traffic from the 192.168.1.0/24 subnet

# Save iptables rules
iptables-save > /etc/iptables/rules.v4 # Saves the current IPv4 rules to a file

# Using Firewalld to manage firewall rules with zones
firewall-cmd --zone=public --add-port=80/tcp --permanent # Permanently opens HTTP port 80 in the public zone
firewall-cmd --zone=public --add-service=https --permanent # Permanently enables HTTPS service in the public zone
firewall-cmd --reload # Reloads Firewalld to apply changes

# List all rules in a specific zone
firewall-cmd --zone=public --list-all # Displays all settings for the public zone

# Regular system updates to patch vulnerabilities
sudo apt update && sudo apt upgrade # Updates and upgrades all packages on Debian-based systems

# Enforcing SELinux policies to enhance security
setenforce 1 # Puts SELinux in enforcing mode, making the security policies active
semanage port -a -t http_port_t -p tcp 8080 # Assigns TCP port 8080 a specific SELinux type for HTTP services

# Check SELinux status
sestatus # Displays the current status of SELinux, including current mode and policy version

# Using fail2ban to protect against brute force attacks
sudo apt-get install fail2ban # Installs fail2ban on Debian-based systems
sudo systemctl start fail2ban # Starts the fail2ban service
sudo fail2ban-client status # Checks the status of fail2ban to see what services are being protected

# Writing Efficient Scripts
# Use shell built-ins and minimize complexity
[[ $1 -gt 100 ]] && echo "Value is greater than 100" # Efficient condition check using built-in

# Run tasks in the background to free up the terminal
nohup ./long_running_script.sh & # Runs script in the background with no hangup

# Error Handling in Scripts
# Set up traps to catch and handle errors gracefully
trap 'echo "An error occurred." >&2; exit 1' ERR # Sets a trap for errors

# Provide clear custom error messages
cp file1 file2 || echo "Failed to copy file1 to file2" >&2 # Custom error message if cp fails

# Basics of Bash Scripting
#!/bin/bash
# Script to backup logs
tar -czf /backup/logs-$(date +%F).tar.gz /var/log # Backs up logs with a timestamp

# Error checking after operations
cp /source /destination
if [ $? -eq 0 ]; then
    echo "Copy successful"
else
    echo "Copy failed" >&2
fi

# Automating with Cron
# Cron job to check disk usage daily at midnight
0 0 * * * /home/user/checkDiskUsage.sh

#!/bin/bash
# Script to check disk usage and send email alerts
usage=$(df / | grep / | awk '{ print $5 }' | sed 's/%//g')
if [ $usage -gt 90 ]; then
    mail -s "Disk usage alert: $usage%" user@example.com <<< "Your root partition remaining free space is critically low."
fi

# Regex in Scripting
# Use regular expressions to validate and manipulate data
#!/bin/bash
# Script to find and replace phone numbers in a text file
sed -r 's/\([0-9]{3}\) [0-9]{3}-[0-9]{4}/+1-\1-\2-\3/g' contacts.txt # Standardizes phone number format

# Character classes
grep '[A-Z]' file.txt # Matches any uppercase letter
grep '[0-9]' file.txt # Matches any digit

# Anchors
grep '^A' file.txt # Finds lines starting with 'A'
grep 'end$' file.txt # Finds lines ending with 'end'

# Quantifiers
grep 'o+' file.txt # Matches one or more 'o's in a line
grep '0*1' file.txt # Matches any number of '0's followed by '1'

# Exact number of characters
grep '^[0-9]{3}' file.txt # Finds lines starting with exactly three digits
grep '\b[0-9]{4}\b' file.txt # Finds four-digit numbers as whole words

# Optional characters
grep 'colou?r' file.txt # Matches both 'color' and 'colour'

# Range of characters
grep '[0-9A-Fa-f]' file.txt # Matches any hexadecimal digit
grep '[1-9][0-9]{2,4}' file.txt # Matches numbers between 100 and 99999

# Groups and Ranges
grep 'gr(a|e)y' file.txt # Matches 'gray' or 'grey'
sed -E 's/([0-9]{4})-([0-9]{2})-([0-9]{2})/\3-\2-\1/' file.txt # Changes date format from YYYY-MM-DD to DD-MM-YYYY

# Special Characters
grep 'Co\.ltd' file.txt # Finds 'Co.ltd' (dot is treated literally)
grep '\$[0-9]+' file.txt # Matches dollar amounts like $100, $20

# Backreferences
grep '\([a-z]\)\1' file.txt # Matches doubled letters like 'ee' in 'see'
sed -n '/\(.*\)\1/p' file.txt # Prints lines where any sequence of characters is repeated

# Non-capturing groups
grep -oP '(?:[0-9]{1,3}\.){3}[0-9]{1,3}' file.txt # Matches IP addresses without capturing the groups

# Lookahead and lookbehind assertions
grep -P '(?<=\bUSD)\d+' file.txt # Matches amounts that follow 'USD'
grep -P '\d+(?=\s*USD)' file.txt # Matches numbers that are immediately followed by 'USD'

# Complex nested patterns
grep -P 'foo(bar|baz)(qux|quux)' file.txt # Matches 'foobarqux', 'foobarquux', 'foobazqux', 'foobazquux'

# Validating email input
read email
if [[ $email =~ ^[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,4}$ ]]; then
  echo "Valid email"
else
  echo "Invalid email"
fi

# Extracting IP addresses from logs
grep -oP '(\d{1,3}\.){3}\d{1,3}' server.log # Extracts IP addresses using Perl-compatible regex

# Parsing structured data
awk '/ERROR [0-9]{4}/ {print $0}' system.log # Finds and prints error messages with a four-digit code