Shell Commands

Finding Commands and Files:

• which: Locates the full path to an executable file on your system. It's helpful for finding the exact location of a command you want to run.

  • Example: which ls (might output something like /bin/ls)

• locate: Quickly finds files based on names or patterns using a pre-built database. This can be faster than find for simple searches.

  • Example: locate demo* (finds all files starting with "demo")

  • Important: The locate database needs to be updated periodically using updatedb to ensure it reflects the latest file system changes.

• updatedb: Updates the database used by the locate command. This should be run regularly to keep the file location information accurate.

• find: Searches for files and directories based on various criteria, offering more flexibility than locate.

  • Example: find / -name demo* (finds all files starting with "demo" anywhere on the system)

User Management:

• passwd: Changes a user's password. It prompts you for the current password and then the new password you want to set.

System Service Management (Systemd):

• systemctl start ssh: Starts the SSH (Secure Shell) service, allowing remote logins to the system.

• systemctl enable ssh: Enables the SSH service to start automatically when the system boots.

Network Tools:

• netstat -antp | grep ssh: Shows network connections, including those related to SSH (port 22 by default). The options used (-antp) provide detailed information about connections, including listening ports and processes using the connections.

Downloading Files:

• wget: Downloads a file from the internet.

  • Example: wget https://example.com/file.txt (downloads the file "file.txt" from the specified URL)

Extracting Script Names from HTML (Not recommended for security reasons):

• The provided command pipe (grep -o '<script[^>]src="[^"].js"' index.html | grep -o '"[^"]*.js"') attempts to extract script filenames from an HTML file (index.html) using regular expressions (grepping). However, it's generally not recommended to parse HTML with shell commands due to security risks and potential for errors. Consider using a safer approach like a programming language library designed for HTML parsing.

Here's a breakdown of the commands you requested:

File Permissions:

• Chown: Changes the ownership of a file or directory.

  • Syntax: chown [owner][:group] file/directory

  • Example: chown john:users myfile.txt (changes owner to "john" and group to "users")

• Chmod: Modifies file or directory permissions.

  • Syntax: chmod [ugo][+-=][rwxX] file/directory

    • u (user), g (group), o (others)

    • + (add permission), - (remove permission), = (set exact permissions)

    • r (read), w (write), x (execute), X (execute for directories)

  • Example: chmod 754 myfile.txt (sets full access for owner, read/execute for group and others)

• Setuid: Sets the owner's user ID for the process started by the file, regardless of the user who executes it (often used for security purposes).

  • Syntax: chmod u+s file (gives setuid permission to the owner)

• Setgid: Sets the group ID for the process started by the file, regardless of the user who executes it (used for group-specific actions).

  • Syntax: chmod g+s file (gives setgid permission to the group)

Basic Commands:

• Echo: Displays a line of text on the terminal.

  • Syntax: echo "This is some text"

Network Tools:

• Nmap: A powerful network scanner used for discovering hosts and services on a network.

  • Example: nmap scanme.example.com (scans the host "scanme.example.com")

• Arping: Sends ARP (Address Resolution Protocol) requests to identify MAC addresses associated with IP addresses on a network.

  • Example: arping -a 192.168.1.0/24 (sends ARP requests to all devices on the 192.168.1.0 subnet)

• Dig: Queries DNS (Domain Name System) servers to look up domain name information.

  • dig microsoft.com mx: Retrieves mail exchange (MX) records for microsoft.com

  • dig microsoft +short: Performs a quick lookup and displays only the answer

• Whois: Looks up information about a domain name registrant.

  • Example: whois google.com

• W: Displays logged-in users and their terminals.

• Lastlog: Shows the last login time for all users.

File Viewing and Management:

• Cat: Displays the contents of a file.

  • Example: cat /etc/passwd (shows the system password file)

Caution: Be careful when using cat with sensitive files like /etc/passwd as it might display usernames and password hashes.

• Less/More: Similar to cat but allow page-by-page navigation for large files.

Permissions and Processes:

• /etc/group: This system file stores information about groups on the system.

• /etc/sudoers: This sensitive file defines which users have administrative privileges (sudo) and what commands they can run.

Caution: Never modify /etc/sudoers without a clear understanding of its implications, as it can compromise system security.

• Lsof: Lists open files and the processes that have them open.

  • lsof -u <username>: Shows files opened by a specific user.

Package Management (Debian/Ubuntu):

• apt-get install net-tools: Installs the net-tools package, which provides basic networking utilities. Note that some newer systems may use apt instead of apt-get.

Finding Files:

• which: Locates the full path to an executable file.

  • Example: which ls (shows the location of the ls command)

• locate: Quickly finds files based on names or patterns in a pre-built database (update the database with updatedb).

  • Example: locate demo* (finds all files starting with "demo")

• find: Searches for files and directories based on various criteria (more powerful than locate).

  • Example: find / -name demo* (finds all files starting with "demo" anywhere on the system)

User Management:

• passwd: Changes a user's password (prompts for the current and new password