- 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
- 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
- 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 strings /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
- Stop/Disable NetworkManager.
- Create/Modify YAML in /etc/netplan .
- Apply config: sudo netplan apply .
- 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 bootSecurity 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.orgUse 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 :80LSOF 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 -nPUse 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.txtOnly 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
Task Scheduling using Cron
crontab -e # Edit crontab crontab -l # List tasks crontab -r # Remove tasks # Schedule Format: * * * * * command # Every minute 15 * * * * command # Hourly 30 18 * * * command # Daily 3 22 * * 1 command # Weekly 10 6 1 * * command # Monthly @yearly command # Yearly @reboot command # At rebootGetting System Hardware Information
General Hardware
lshw # Full hardware info lshw -short # Short format lshw -json # JSON format lshw -html # HTML formatCPU Information
lscpu # CPU details lshw -C cpu # Hardware-specific CPU details lscpu -J # JSON formatMemory Information
dmidecode -t memory # RAM specs dmidecode -t memory | grep -i size dmidecode -t memory | grep -i max free -m # Memory usagePCI 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 outputStorage Devices
lshw -short -C disk fdisk -l # List disks fdisk -l /dev/sda lsblk # Block devices listNetwork Devices
lshw -C network iw list # Wi-Fi cards iwconfig # Wi-Fi configuration iwlist scan # Wi-Fi networks scanSystem 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 infoBattery Power
acpi -bi # Battery info acpi -V # All ACPI infoWorking 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=progressUse these commands to check hardware specifications and perform operations with device files safely.
Service Management
# 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 nginxUbuntu
sudo systemctl status ssh # Check SSH service 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 bootCentOS
sudo systemctl status sshd # Check SSHD service status sudo systemctl stop sshd # Stop SSHD service sudo systemctl restart sshd # Restart SSHD service sudo systemctl enable sshd # Enable SSHD to start on boot sudo systemctl is-enabled sshd # Check if SSHD is enabled on bootSecurity Configuration
To configure security settings, edit /etc/ssh/sshd_config . Apply changes by restarting SSH. Key configurations include:
- Change SSH port:
- Port 2278
- PermitRootLogin no
- AllowUsers user1 user2
Note: Consult the man page ( man sshd_config ) for detailed configuration options.
Bash Programming
Bash Aliases
alias # List all aliases alias name='command' # Create an alias unalias name # Remove an aliasUseful 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 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 variableSpecial Variables
$0, $1, $2, ..., $ # Script name & positional arguments $# # Number of positional arguments "$*" # All positional arguments as a single string $? # Exit status of the last commandProgram 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 statementTest 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 functionCommand Examples
crontab -e # Edit crontab file crontab -l # List crontab entries crontab -r # Remove crontab entriesCombine these constructs to write effective bash scripts for task automation and system management.
