The Linux Terminal

Getting Help in Linux

MAN Pages

• Use man command to view the manual for a command.
  • Example: man ls

The man pages are navigated using the less command with shortcuts:

• h: Get help within less.
• q: Quit less.
• enter: Show next line.
• space: Show next screen.
• /string: Search forward for a string.
• ?string: Search backward for a string.
• n / N: Next/previous appearance of the search term.

Checking Command Type

• type rm: Check if rm is a shell built-in or an executable file.
  • Example: rm is /usr/bin/rm
• type cd: Check if cd is a shell built-in.
  • Example: cd is a shell builtin

Getting Help for Shell Built-in Commands

• help command: Get help for shell built-in commands.
  • Example: help cd
• command --help: Get help for executable commands.
  • Example: rm --help

Searching Man Pages

• man -k uname: Search for uname in all man pages.
• man -k "copy files": Search for "copy files" in man pages.
• apropos passwd: Search for passwd related man pages.

Keyboard Shortcuts

TAB TAB: Display all commands or filenames starting with written letters.
CTRL + L: Clear the current line.
CTRL + D: Close the shell.
CTRL + U: Cut the current line.
CTRL + A: Move cursor to start of the line.
Ctrl + E: Move cursor to the end of the line.
CTRL + C: Stop the current command.
CTRL + Z: Sleep the running program.
CTRL + ALT + T: Open a terminal.

Bash History

• history: Display the history.
• history -d 100: Remove a specific line from history.
• history -c: Clear the entire history.
• echo $HISTFILESIZE: Print the number of commands saved in the history file.
• echo $HISTSIZE: Print the number of history commands saved in memory.
• !!: Rerun the last command.
• !20: Run a specific command from history.
• !-10: Run the last nth command.
• !abc: Run the last command starting with abc.
• !abc:p: Print the last command starting with abc.
• CTRL + R: Reverse search through history.
• HISTTIMEFORMAT="%d/%m/%y %T ": Record date/time of each command (add to ~/.bashrc for persistence).

Getting Root Access (sudo, su)

• sudo command: Run a command as root (for users in sudo or wheel group).
• sudo su: Become root temporarily.
• sudo passwd root: Set the root password.
• passwd username: Change a user's password.
• su: Become root (if root has a password).

Linux Paths

Paths

• Absolute: Starts with /
• Relative: Relative to the current location
• .: Current working directory
• ..: Parent directory
• ~: User's home directory

Changing Directories

• cd: To user's home directory
• cd ~: To user's home directory
• cd -: To the last directory
• cd /path_to_dir: To path_to_dir
• pwd: Print the current working directory

Installing Tools

• sudo apt install tree: Installs the tree command

Using Tree

• tree directory/: Example: tree .
• tree -d .: Print only directories
• tree -f .: Print absolute paths

The ls Command

Usage: ls [OPTIONS] [FILES]

Listing Directories

• ls, ls .: Current directory
• ls ~ /var /: Multiple directories

Options

• -l: Long listing
• -a: All files (including hidden)
• -1: Single column
• -d: Directory information
• -h: Human-readable sizes
• -S: Sort by size
• -X: Sort by extension
• --hide: Hide specific files
• -R: Recursive listing
• -i: Inode number

Disk Usage

• du -sh ~: Size of home directory

File Timestamps and Date

• ls -lu: Access time (atime)
• ls -l, ls -lt: Modification time (mtime)
• ls -lc: Change time (ctime)
• stat file.txt: All timestamps
• ls -l --full-time /etc/: Full timestamps

Modifying Timestamps with Touch

• touch file.txt: Create or update timestamps
• touch -a file, touch -m file: Modify atime or mtime
• touch -m -t 201812301530.45 a.txt: Specific date/time
• touch -d "2010-10-31 15:45:30" a.txt: Both atime and mtime
• touch a.txt -r b.txt: Copy timestamps

Date and Calendar

• date: Current date/time
• cal, cal 2021, cal 7 2021: Calendars
• cal -3: Previous, current, next month
• date --set="2 OCT 2020 18:00:00": Set date/time

Sorting with ls

• ls -l: Sorted by name
• ls -lt: Sorted by mtime, newest first
• ls -ltu: Sorted by atime
• ls -ltu --reverse: Reverse order

Viewing Files (cat, less, more, head, tail, watch)

Displaying File Contents

• cat filename: Display content
• cat -n filename: Line numbers
• cat filename1 filename2 > filename3: Concatenate

Less Shortcuts

• h: Help
• q: Quit
• enter: Next line
• space: Next screen
• /string: Search forward
• ?string: Search backward
• n / N: Next/previous search result

Tail and Head

• tail filename: Last 10 lines

• tail -n 15 filename: Last 15 lines

• tail -n +5 filename: Starting with line 5

• tail -f filename: Real-time updates

• head filename: First 10 lines

• head -n 15 filename: First 15 lines

Monitoring Commands

• watch -n 3 ls -l: Refresh every 3 seconds

Working with Files and Directories

Creating and Updating Files

• touch filename: Create a new file or update timestamps.

Creating Directories

• mkdir dir1: Create a new directory.
• mkdir -p mydir1/mydir2/mydir3: Create nested directories.

The cp Command

Copy files and directories:

• cp file1 file2: Copy file1 to file2.
• cp file1 dir1/file2: Copy to another directory with a different name.
• cp -i file1 file2: Prompt before overwrite.
• cp -p file1 file2: Preserve permissions.
• cp -v file1 file2: Verbose output.
• cp -r dir1 dir2/: Recursively copy directories.
• cp -r file1 file2 dir1 dir2 dest_dir/: Copy multiple items to a destination.

The mv Command

Move or rename files and directories:

• mv file1 file2: Rename a file.
• mv file1 dir1/: Move to a directory.
• mv -i file1 dir1/: Prompt before overwrite.
• mv -n file1 dir1/: Prevent overwriting.
• mv -u file1 dir1/: Update based on modification time.
• mv file1 dir1/file2: Move and rename.
• mv file1 file2 dir1/ dir2/ dest_dir/: Move multiple items.

The rm Command

Remove files and directories:

• rm file1: Remove a file.
• rm -v file1: Verbose removal.
• rm -r dir1/: Remove a directory.
• rm -rf dir1/: Force removal without prompt.
• rm -ri file1 dir1/: Prompt for each removal.

Secure File Deletion

• shred -vu -n 100 file1: Securely overwrite and remove a file.

Piping and Command Redirection

Piping Examples

• ls -lSh /etc/ | head: View the top 10 largest files.
• ps -ef | grep sshd: Check if sshd is running.
• ps aux --sort=-%mem | head -n 3: Top 3 processes by memory.

Command Redirection

Redirect output and errors:

• ps aux > processes.txt: Output to a file.
• id >> users.txt: Append output.
• tail -n 10 /var/log/*.log > output.txt 2> errors.txt: Separate output and errors.
• tail -n 2 /etc/passwd /etc/shadow > all.txt 2>&1: Redirect all to one file.
• cat /var/log/auth.log | grep "fail" | wc -l: Count occurrences.

Finding Files with locate and find

locate

• sudo apt install plocate: Install plocate.
• sudo updatedb: Update the database.
• locate filename: Find a file by name.
• locate -i filename: Case insensitive search.
• locate -b '\filename': Exact name search.
• locate -r 'regex': Regular expression search.
• locate -e filename: Check file existence.
• which command: Show command path.

find

Search with various options:

• find ~ -type f -size +1M: Files over 1MB.
• Options include -type, -name, -iname, -size, -perm, -links, -atime, -mtime, -ctime, -user, and -group.

Searching for Text Patterns with grep

Usage: grep [OPTIONS] PATTERN FILE

Options

• -n: Print line number.
• -i: Case insensitive.
• -v: Invert match.
• -w: Match whole words.
• -a: Include binary files.
• -R: Recursive search.
• -c: Count matches.
• -C n: Context display (n lines around the match).

Extracting ASCII Characters from Binary Files

• strings binary_file: Example with /bin/ls.

VIM - Text Editor

Modes

• Command Mode: Default on entry.
• Insert Mode: Editing text.
• Last Line Mode: Save/exit commands.

Config File

• VIM settings: ~/.vimrc.

Commands

• i, I, a, A, o: Enter Insert Mode.
• :w!, :q!, :wq!, :e!: Save/quit commands in Last Line Mode.
• x, dd, ZZ, u, G, $, 0, ^: Editing commands in Command Mode.
• /string, ?string, n, N: Search commands in Command Mode.
• vim -o file1 file2: Open files in stacked windows.
• vim -d file1 file2: Highlight differences.

Navigation

• Ctrl+w: Switch between files.

Account Management

## Account Management
/etc/passwd # users and info: 
/etc/shadow # users' passwords
/etc/group # groups

## User Commands
useradd [OPTIONS] username # Create user.
usermod [OPTIONS] username # Modify user.
userdel -r username        # Delete user.

## Group Commands
groupadd group_name # Create group.
groupdel group_name # Delete group.

## Examples
useradd -m -d /home/john -c "C++ Developer" -s /bin/bash -G sudo,adm,mail john # Example of creating a user.
usermod -aG developers,managers john # Example of modifying a user.

Monitoring Users

## Commands
who -H    # User info.
id        # User info.
whoami    # User info.
w         # System usage.
uptime    # System usage.
last      # Login history.
last -u username # Login history for a specific user.

File Permissions

Understanding Permissions

• Legend: u (user), g (group), o (others), a (all), r (read), w (write), x (execute), - (no access).

Displaying Permissions

• ls -l /etc/passwd: View file permissions.
• stat /etc/shadow: Detailed permission stats.

Changing Permissions

• chmod u+r filename: Add read to user.
• chmod u+r,g-wx,o-rwx filename: Adjust multiple permissions.
• chmod ug+rwx,o-wx filename: Set multiple permissions.
• chmod ugo+x filename: Add execute to all.
• chmod a+r,a-wx filename: Modify all permissions.

Absolute Mode

• chmod 777 filename: Set all permissions for all.
• chmod 755 filename: Read & execute for group and others.
• chmod 644 filename: Read-only for group and others.

Special Permissions

• SUID: chmod u+s executable_file.
• SGID: chmod g+s projects/.
• Sticky Bit: chmod o+t temp/.

UMASK

• Display: umask.
• Set new value: umask new_value.

Ownership

• Owner: chown new_owner file.
• Group: chgrp new_group file.
• Both: chown new_owner:new_group file.
• Recursive: chown -R new_owner file.

File Attributes

• Display: lsattr filename.
• Change: chattr +-attribute filename.

Processes

Process Viewing

• type rm: Check if rm is built-in or executable.
• ps: Processes in current terminal.
• ps -ef, ps aux, ps aux | less: System processes.
• ps aux --sort=%mem | less: Sort by memory usage.
• ps -ef --forest: ASCII process tree.
• ps -f -u username: Processes by user.
• pgrep -l sshd, pgrep -f sshd, ps -ef | grep sshd: Check for sshd.
• pstree, pstree -c: Hierarchical process tree.

Dynamic Real-Time View

• top: Start system monitor.
• top shortcuts: h for help, space for refresh, d for delay, etc.
• top -d 1 -n 3 -b > top_processes.txt: Top in batch mode.
• Install htop for an interactive view.

Killing Processes

• kill -l: List signals.
• kill pid, kill -SIGNAL pid1 pid2 ...: Send signals.
• kill -2 pid, kill -HUP pid: Send specific signal.
• pkill process_name, killall process_name: Kill by name.
• kill $(pidof process_name): Kill using pidof.

Background and Foreground Management

• command &: Run in background.
• jobs: List jobs.
• Ctrl + Z: Stop process.
• fg %job_id: Resume in foreground.
• bg %job_id: Resume in background.
• nohup command &: Immune to hangups.

Networking

Getting Network Interface Information

• ifconfig: Enabled interfaces.
• ifconfig -a, ip address show: All interfaces.
• ifconfig enp0s3, ip addr show dev enp0s3: Specific interface.
• ip -4 address: Only IPv4 info.
• ip -6 address: Only IPv6 info.
• ip link show, ip link show dev enp0s3: L2 info, including MAC.
• route, route -n, ip route show: Default gateway.
• systemd-resolve --status: DNS servers.

Setting Network Interfaces

• ifconfig enp0s3 down, ip link set enp0s3 down: Disable interface.
• ifconfig enp0s3 up, ip link set enp0s3 up: Enable interface.
• ifconfig -a, ip link show dev enp0s3: Check status.
• ifconfig enp0s3 192.168.0.222/24 up, ip address add 192.168.0.112/24 dev enp0s3: Set IP.
• ifconfig enp0s3:1 10.0.0.1/24: Secondary IP.
• route del default gw, ip route del default, ip route add default via: Default gateway.
• ifconfig enp0s3 hw ether, ip link set dev enp0s3 address: Change MAC.

Netplan for Static Network Configuration on Ubuntu

1. Stop/Disable NetworkManager.
2. Create/Modify YAML in /etc/netplan.
3. Apply config: sudo netplan apply.
4. Verify: ifconfig, route -n.

OpenSSH Configuration and Management

Installation

# Ubuntu
sudo apt update && sudo apt install openssh-server openssh-client

# CentOS
sudo dnf install openssh-server openssh-clients

Server Connection

ssh -p 22 username@server_ip  # Connect using default SSH port
ssh -p 22 -l username server_ip  # Connect with a specific username
ssh -v -p 22 username@server_ip  # Connect in verbose mode for detailed information

# Ubuntu
sudo systemctl status ssh  # Check SSH status
sudo systemctl stop ssh    # Stop SSH service
sudo systemctl restart ssh # Restart SSH service
sudo systemctl enable ssh  # Enable SSH to start on boot
sudo systemctl is-enabled ssh  # Check if SSH is enabled on boot

# CentOS
sudo systemctl status sshd  # Check SSH status
sudo systemctl stop sshd    # Stop SSH service
sudo systemctl restart sshd # Restart SSH service
sudo systemctl enable sshd  # Enable SSH to start on boot
sudo systemctl is-enabled sshd  # Check if SSH is enabled on boot

Security Configuration

Edit /etc/ssh/sshd_config and then apply changes by restarting SSH:

• Change port: Port 2278
• Disable root login: PermitRootLogin no
• Restrict user access: AllowUsers user1 user2
• Configure firewall to filter SSH access
• Enable Public Key Authentication, disable password-based login
• Use SSH Protocol 2 only
• Set client session intervals and max attempts for security

Remember to consult the man page (man sshd_config) for detailed configuration options.

File Transfer Techniques with SCP and RSYNC

SCP Usage

# Copy local file to remote host
scp a.txt john@80.0.0.1:~
scp -P 2288 a.txt john@80.0.0.1:~ # Custom port

# Copy from remote to local
scp -P 2290 john@80.0.0.1:~/a.txt .

# Copy entire directory to remote
scp -P 2290 -r projects/ john@80.0.0.1:~

RSYNC Commands

# Sync local directory to local backup
sudo rsync -av /etc/ ~/etc-backup/

# Mirror directory, deleting extraneous files from dest
sudo rsync -av --delete /etc/ ~/etc-backup/

# Exclude files during sync
rsync -av --exclude-from='~/exclude.txt' /source/ /dest/

# Sync over SSH with custom port
sudo rsync -av -e 'ssh -p 2267' /etc/ student@192.168.0.108:~/etc-backup/

Exclude Patterns Example

# exclude.txt could include patterns like:
*.avi
music/
abc.mkv

# Exclude specific file types during transfer
rsync -av --exclude='*.mkv' /source/ /dest/

WGET for File Download

# Install wget
sudo apt install wget # Ubuntu
sudo dnf install wget # CentOS

# Basic file download
wget https://example.com/file.iso

# Resume incomplete download
wget -c https://example.com/file.iso

# Download with bandwidth limit
wget --limit-rate=100k https://example.com/file.iso

# Download multiple files
wget -i urls.txt # urls.txt contains list of URLs

# Recursive download for offline viewing of a website
wget -mkEpnp http://example.org

Use these commands to efficiently copy files and directories across systems and for downloading content from the internet, ensuring data synchronization and maintaining web accessibility.

NETSTAT and SS Usage

# Display all ports and connections
sudo netstat -tupan
sudo ss -tupan

# Check if port 80 is open
netstat -tupan | grep :80

LSOF Commands

# List open files
lsof

# Files opened by a specific user
lsof -u username

# Files opened by a specific command/process
lsof -c sshd

# Open files for TCP ports in LISTEN state
lsof -iTCP -sTCP:LISTEN
lsof -iTCP -sTCP:LISTEN -nP

Use these commands to monitor network connections, check for open ports, and view files opened by users or processes, especially for security and troubleshooting.

Nmap Scanning Guide

# SYN Scan (root required)
nmap -sS 192.168.0.1

# TCP Connect Scan
nmap -sT 192.168.0.1

# Scan All Ports
nmap -p- 192.168.0.1

# Scan Specific Ports
nmap -p 20,22-100,443,1000-2000 192.168.0.1

# Service Version Detection
nmap -p 22,80 -sV 192.168.0.1

# Ping Scan Network
nmap -sP 192.168.0.0/24

# Skip Host Discovery
nmap -Pn 192.168.0.0/24

# Exclude Specific IP from Scan
nmap -sS 192.168.0.0/24 --exclude 192.168.0.10

# Output Scan to File
nmap -oN output.txt 192.168.0.1

# OS Detection
nmap -O 192.168.0.1

# Aggressive Scan
nmap -A 192.168.0.1

# Read Targets from File & Output to File without DNS Resolution
nmap -n -iL hosts.txt -p 80 -oN output.txt

Only scan your own networks and systems, or those you have explicit permission to test. Unauthorized scanning can be illegal.

Software Management with DPKG and APT

DPKG

• View .deb file info: dpkg --info package.deb
• Install from .deb: sudo dpkg -i package.deb
• List installed programs: dpkg --get-selections or dpkg-query -l
• Find by name: dpkg-query -l | grep ssh
• List package files: dpkg -L openssh-server
• Find owning package: dpkg -S /bin/ls
• Remove package: sudo dpkg -r package
• Purge package: sudo dpkg -P package

APT

• Update index: sudo apt update
• Install/update: sudo apt install apache2
• List upgradable: sudo apt list --upgradable
• Full upgrade: sudo apt full-upgrade
• Remove: sudo apt remove package
• Purge: sudo apt purge package
• Auto remove dependencies: sudo apt autoremove
• Clean cache: sudo apt clean
• List all packages: sudo apt list
• Search: sudo apt list | grep nginx
• Show package info: sudo apt show nginx
• List installed: sudo apt list --installed

Cron Task Scheduling

• Edit crontab: crontab -e
• List tasks: crontab -l
• Remove tasks: crontab -r
• Schedule format:
  • Every minute: * * * * * command
  • Hourly: 15 * * * * command
  • Daily: 30 18 * * * command
  • Weekly: 3 22 * * 1 command
  • Monthly: 10 6 1 * * command
  • Yearly: @yearly command
  • At reboot: @reboot command

Getting System Hardware Information

General Hardware

lshw               # Full hardware info
lshw -short        # Short format
lshw -json         # JSON format
lshw -html         # HTML format

CPU Information

lscpu              # CPU details
lshw -C cpu        # Hardware-specific CPU details
lscpu -J           # JSON format

Memory Information

dmidecode -t memory           # RAM specs
dmidecode -t memory | grep -i size
dmidecode -t memory | grep -i max
free -m                       # Memory usage

PCI and USB Devices

lspci                         # PCI buses and connected devices
lspci | grep -i wireless
lspci | grep -i vga
lsusb                         # USB controllers and devices
lsusb -v                      # Verbose output

Storage Devices

lshw -short -C disk
fdisk -l                      # List disks
fdisk -l /dev/sda
lsblk                         # Block devices list

Network Devices

lshw -C network
iw list                       # Wi-Fi cards
iwconfig                      # Wi-Fi configuration
iwlist scan                   # Wi-Fi networks scan

System Information via /proc

cat /proc/cpuinfo             # CPU info
cat /proc/meminfo             # Memory info
cat /proc/version             # System version
uname -r                      # Kernel version
uname -a                      # All system info

Battery Power

acpi -bi                      # Battery info
acpi -V                       # All ACPI info

Working with Device Files (dd)

# Backup MBR
dd if=/dev/sda of=~/mbr.dat bs=512 count=1

# Restore MBR
dd if=~/mbr.dat of=/dev/sda bs=512 count=1

# Clone partition
dd if=/dev/sda1 of=/dev/sdb2 bs=4M status=progress

Use these commands to check hardware specifications and perform operations with device files safely.

Service Management with systemd and systemctl

# Analyze boot process
systemd-analyze
systemd-analyze blame

# List active units
systemctl list-units
systemctl list-units | grep ssh

# Service status
sudo systemctl status nginx.service

# Stop service
sudo systemctl stop nginx

# Start service
sudo systemctl start nginx

# Restart service
sudo systemctl restart nginx

# Reload service config
sudo systemctl reload nginx
sudo systemctl reload-or-restart nginx

# Enable service at boot
sudo systemctl enable nginx

# Disable service at boot
sudo systemctl disable nginx

# Check if service is enabled at boot
sudo systemctl is-enabled nginx

# Mask service
sudo systemctl mask nginx

# Unmask service
sudo systemctl unmask nginx

```markdown
## Bash Programming

### Bash Aliases
```bash
alias                     # List all aliases
alias name='command'      # Create an alias
unalias name              # Remove an alias

Useful Aliases

alias c='clear'
alias cl='clear; ls; pwd'
alias root='sudo su'
alias ports='netstat -tupan'
alias sshconfig='sudo vim /etc/ssh/sshd_config'
alias update='sudo apt update && sudo apt dist-upgrade -y && sudo apt clean'

Interactive File Manipulation

alias cp='cp -i'
alias mv='mv -i'
alias rm='rm -i'

Bash Programming

Bash Variables

variable="value"          # Define a variable
echo $variable            # Reference a variable
declare -r const=100      # Define a read-only variable
unset variable            # Unset a variable
env | grep PATH           # Find an environment variable
export PATH=$PATH:~/bin   # Modify the PATH variable

Special Variables

$0, $1, $2, ..., ${10}    # Script name & positional arguments
$#                        # Number of positional arguments
"$*"                      # All positional arguments as a single string
$?                        # Exit status of the last command

Program Flow Control

if [ condition ]; then command; fi                          # Basic if statement
if [ condition ]; then command; else other_command; fi       # If-else statement
if [ condition ]; then command; elif [ condition ]; then...  # If-elif-else statement

Test Conditions

# Numeric comparisons: -eq, -ne, -lt, -le, -gt, -ge
# File checks: -s, -f, -d, -x, -w, -r
# String comparisons: =, !=, -n (not zero), -z (is zero)
# Logical operators: && (and), || (or)

Loops and Functions

for i in {1..5}; do echo "Loop $i"; done               # For loop
while [ condition ]; do command; done                  # While loop
case "$variable" in pattern) command;; esac            # Case statement
function name() { command; }                           # Function definition
name() { command; }                                    # Alternative function syntax
name                                                   # Call a function

Command Examples

crontab -e    # Edit crontab file
crontab -l    # List crontab entries
crontab -r    # Remove crontab entries

Combine these constructs to write effective bash scripts for task automation and system management.

OpenSSH Configuration and Management

Installation

Ubuntu

• Update packages and install OpenSSH:

  sudo apt update && sudo apt install openssh-server openssh-client

CentOS

• Install OpenSSH:

  sudo dnf install openssh-server openssh-clients

Server Connection

• Connect using the default SSH port:

  ssh -p 22 username@server_ip

• Connect with a specific username:

  ssh -p 22 -l username server_ip

• Connect in verbose mode for detailed information:

  ssh -v -p 22 username@server_ip

Service Management

Ubuntu

• Check SSH service status:

  sudo systemctl status ssh

• Stop SSH service:

  sudo systemctl stop ssh

• Restart SSH service:

  sudo systemctl restart ssh

• Enable SSH to start on boot:

  sudo systemctl enable ssh

• Check if SSH is enabled on boot:

  sudo systemctl is-enabled ssh

CentOS

• Check SSHD service status:

  sudo systemctl status sshd

• Stop SSHD service:

  sudo systemctl stop sshd

• Restart SSHD service:

  sudo systemctl restart sshd

• Enable SSHD to start on boot:

  sudo systemctl enable sshd

• Check if SSHD is enabled on boot:

  sudo systemctl is-enabled sshd

Security Configuration

To configure security settings, edit /etc/ssh/sshd_config. Apply changes by restarting SSH. Key configurations include:

• Change SSH port:
  • Port 2278
• Disable root login:
  • PermitRootLogin no
• Restrict user access to specified users only:
  • AllowUsers user1 user2
• Configure firewall to filter SSH access
• Enable Public Key Authentication and disable password-based login
• Use SSH Protocol 2 only
• Set client session intervals and maximum attempts for increased security

Note: Consult the man page (man sshd_config) for detailed configuration options.