Skip to content

Latest commit

 

History

History
310 lines (275 loc) · 21.9 KB

NC Linux.md

File metadata and controls

310 lines (275 loc) · 21.9 KB
Raw

#Linux #Networkchuck #HTB

[[Linux_Fundamentals_Module_Cheat_Sheet.pdf|Cheat sheet]] for all linux commands.

Linux is not an Operating System but it is a Kernel on top of which we built our OS.

Everything in Linux is a file.

Kernel's function is to virtualize and control common computer hardware resources like CPU, allocated memory, accessed data, and others. The kernel gives each process its own virtual resources and prevents/mitigates conflicts between different processes.

Basic Linux Commands

  • pwd - Print Working Directory
  • ls <Directory> or ls -la - List
  • cd <Directory> - Change Directory
  • cd - Changes to user directory
  • cd .. - Goes back to previous parent directory
  • cd ../.. - Goes back to previous previous parent directory
  • cd - - Goes back to directory which we were last in
  • whoami - Displays the name of the logged-in user
  • cp <Source> <Destination> - Copies file from source to destination
  • mv <Source> <Destination> - Moves file from source to destination
  • rm <file> - Removes file
  • cat <file> - Concatenates contents of the file
  • mkdir <folder> - Creates a new folder
  • mkdir -p <folder>/<Subfolders> - Creates a parent folder with child folders
  • tree . - Displays the contents of present directory in a tree format
  • which <command> - Displays the location of that linux command
  • sudo updatedb && locate *.txt - Search for files across system
  • find <Directory> <options> - Searches a file in directory with the given options
  • ps - Lists all the running processes
  • echo "Hello World" - Prints the statement in terminal

📝Note - . refers to the present directory and .. refers to the parent directory.

Creating files

touch example.txt
cat > example.txt # Ctrl+D to save file
cat << EOF > example.txt # Type EOF to save file
echo "Contents to be added to the file" > example.txt

Pagers to read file

cat <file>
less <file>
more <file>
head <file>
tail <file>

⚠ Dangerous Command

# Removes french language package
sudo rm -fr / --no-preserve-root

Help in Linux

Solve issues to understand a long command by visiting https://explainshell.com/.

  • man <command> - Displays the manual page of that linux command
  • <command> --help - Lists all functions of the command
  • apropos <keyword> - Searching for a command with the given keyword

Managing Linux users

  • sudo adduser <username> - Adds a user, sets a password and a user directory
  • sudo useradd <username> - Just adds a user and does not set a password nor a directory
  • sudo passwd <username> - Set or change password for a user
  • sudo usermod <username> <argument> - Modifies user account
  • cat /etc/passwd - Displays a file containing all usernames
  • sudo userdel <username> - Deletes a user account

Groups

  • sudo groupadd <groupname> - Adds a new group
  • sudo groupdel <groupname> - Deletes a group
  • cat /etc/group - Displays a file containing all groups

📝Note - Whenever a user account is created, a group is also created for the user.

sudo command stands for Super User do.

Shortcuts

Ctrl+D - Logout Ctrl+L - Clear screen Ctrl+C - Kill a Foreground process Ctrl+Z - Puts a foreground process into sleep Ctrl+R - Search through the command history Ctrl+A - Takes cursor to the beginning of the command Ctrl+E - Takes cursor to the end of the command Ctrl+U - Deletes everything before the cursor Ctrl+K - Deletes everything after the cursor Ctrl+Y - Pastes the deleted content Tab - Autocomplete file names Tab Tab - Lists file names Ctrl+X+E - Opens the command in a text editor to be edited sudo !! - Executes the previous command with root privileges

Set alias for long commands

.bashrc is a hidden file located in your home directory.

nano .bashrc
# Add the following to the file
alias la='ls -la'

Linux Package Manager

dpkg

It is a low-level package manager. Download the package of the application you want to install and run sudo dpkg -i <.deb file>. But dpkg is unable to automatically install any dependency package required to run the program. We have to that manually by searching for packages on web.

apt

It stands for Advanced Package Tool and is a high-level package manager. Unlike dpkg, it can automatically install dependency packages from the repository.

sudo apt update
sudo apt upgrade -y
sudo apt update && sudo apt full-upgrade -y
sudo apt install <Packages>
sudo apt list --installed
sudo apt list --installed | wc -l
sudo apt list --installed | grep ^<Package>
sudo apt show <Package>
sudo apt search <Package>
sudo apt remove <Package>
sudo apt purge <Package>

remove only deletes the package whereas purge also deletes the user data of that package. You can also try aptitude package manager which is apt on steroids.

grep stands for Global Regular Expression Print and it is used for filtering in terminal. Likewise, wc -l displays the total count of the command.

snap

Unlike apt, it does not use repositories to install packages. Rather it is like an app store.

sudo apt install snapd -y
sudo snap install <Package>

git

Learn more at [[Git and Github#^6980b6|clone]].

sudo apt install git
git clone <repo URL>

Permissions ^1927f0

  • (r) - Read
  • (w) - Write
  • (x) - Execute
- rwx rw- r--   1 root root 1641 May  4 23:42 /etc/passwd
- --- --- ---   |  |    |    |   |__________|
|  |   |   |    |  |    |    |        |_ Date
|  |   |   |    |  |    |    |__________ File Size
|  |   |   |    |  |    |_______________ Group
|  |   |   |    |  |____________________ User
|  |   |   |    |_______________________ Number of hard links
|  |   |   |_ Permission of others (read)
|  |   |_____ Permissions of the group (read, write)
|  |_________ Permissions of the owner (read, write, execute)
|____________ File type (- = File, d = Directory, l = Link, ... )

We can modify permissions using the chmod command, permission group references (u - owner, g - Group, o - others, a - All users), and either a + or a - to add remove the designated permissions.

chmod a+r <file>
chmod 754 <file>
Binary Notation:                4 2 1  |  4 2 1  |  4 2 1
----------------------------------------------------------
Binary Representation:          1 1 1  |  1 0 1  |  1 0 0
----------------------------------------------------------
Octal Value:                      7    |    5    |    4
----------------------------------------------------------
Permission Representation:      r w x  |  r - x  |  r - -

Daemons in Linux

'Daemons' in Greek mean 'supernatural beings which is not associated neither with good nor bad'. Most Linux distributions have now switched to systemd. This daemon is an Init process started first and thus has the process ID (PID) 1. Daemons are automatically initiated running programs. ps -aux lists all the processes running on your system.

ps -aux
pstree
ps -aux | grep <Process Name>

Names of processes ending with d (like sshd or ntpd) are all daemon processes. All daemons are processes but not all processes are daemons.

Managing Daemons

systemd is the master daemon and it manages all other daemons. It is the first daemon to be initiated and all other daemons are forked from it. We use a tool called systemctl to manage all daemons. 📝Note- Daemons are also called Units.

systemctl start <Process Name>
systemctl status <Process Name>
systemctl stop <Process Name>

systemctl restart <Process Name>
systemctl reload-or-restart <Process Name>

systemctl disable <Process Name>
systemctl enable <Process Name>

systemctl is-enabled <Process Name>
systemctl is-active <Process Name>

systemctl list-units

sudo !!

Managing Processes in Linux

There are two types of processes.

  • Foreground Process
  • Background Process
sleep 600
ping -c 1000 google.com
# Run a command and put it into sleep using Ctrl+Z
jobs
bg <job ID>
fg <job ID>

ps -u <User> | grep <Process Name> - Lists all the processes running in a user account pgrep <Process Name> - Displays the process ID of a process kill <Process ID> - Kills the process ps -aux - Displays all the running process top or htop - Displays the CPU usage of running processes

Execute Multiple Commands

  • Semicolon (;) is a command separator and executes the commands by ignoring previous commands' results and errors.
  • Double ampersand characters (&&) are used to run the commands one after the other. If there is an error in one of the commands, the following ones will not be executed anymore, and the whole process will be stopped.
  • Pipes (|) depend not only on the correct and error-free operation of the previous processes but also on the previous processes results.

Web Services

curl

cURL is a tool that allows us to transfer files from the shell over protocols like HTTP, HTTPS, FTP, SFTP, FTPS, or SCP. This tool gives us the possibility to control and test websites remotely. Besides the remote servers' content, we can also view individual requests to look at the client's and server's communication.

wget

An alternative to curl is the tool wget. With this tool, we can download files from FTP or HTTP servers directly from the terminal, and it serves as a good download manager.

Components of Linux

Component Description
Bootloader A piece of code that runs to guide the booting process to start the operating system.
OS Kernel The kernel is the main component of an operating system. It manages the resources for system's I/O devices at the hardware level.
Daemons Background services are called "daemons" in Linux. Their purpose is to ensure that key functions such as scheduling, printing, and multimedia are working correctly. These small programs load after we booted or log into the computer.
OS Shell The operating system shell or the command language interpreter (also known as the command line) is the interface between the OS and the user. The most commonly used shells are Bash, Tcsh/Csh, Ksh, Zsh, and Fish.
Graphics server This provides a graphical sub-system (server) called "X" or "X-server" that allows graphical programs to run locally or remotely on the X-windowing system.
Window Manager Also known as a graphical user interface (GUI). There are many options, including GNOME, KDE, MATE, Unity, and Cinnamon. A desktop environment usually has several applications, including file and web browsers.
Utilities Applications or utilities are programs that perform particular functions for the user or another program.

Linux File System

The Linux operating system is structured in a tree-like hierarchy and is documented in the Filesystem Hierarchy Standard (FHS). ![[Linux_file_system.png]]

Path Description
/ The top-level directory is the root filesystem and contains all of the files required to boot the operating system before other filesystems are mounted, as well as the files required to boot the other filesystems. After boot, all of the other filesystems are mounted at standard mount points as subdirectories of the root.
/bin Contains essential command binaries like ls, cd and much more.
/boot Consists of the static bootloader, kernel executable, and files required to boot the Linux OS.
/dev Contains device files to facilitate access to every hardware device attached to the system.
/etc Local system configuration files. Configuration files for installed applications may be saved here as well.
/home Each user on the system has a subdirectory here for storage.
/lib Shared library files that are required for system boot.
/media External removable media devices such as USB drives are mounted here.
/mnt Temporary mount point for regular filesystems.
/opt Optional files such as third-party tools can be saved here.
/root The home directory for the root user.
/sbin This directory contains executables used for system administration (binary system files).
/tmp The operating system and many programs use this directory to store temporary files. This directory is generally cleared upon system boot and may be deleted at other times without any warning.
/usr Contains executables, libraries, man files, etc.
/var This directory contains variable data files such as log files, email in-boxes, web application related files, cron files, and more.

Customising the Prompt

The prompt can be customized using special characters and variables in the shell’s configuration file (.bashrc for the Bash shell). For example, we can use: the \u character to represent the current username, \h for the hostname, and \w for the current working directory.

Special Character Description
\d Date (Mon Feb 6)
\D{%Y-%m-%d} Date (YYYY-MM-DD)
\j Number of jobs managed by the shell
\n Newline
\t Current time 24-hour (HH:MM:SS)
\@ Current time
\u Current username
\w Full path of the current working directory

System Information

Command Description
uname Prints basic information about the operating system name and system hardware.
ifconfig The ifconfig utility is used to assign or to view an address to a network interface and/or configure network interface parameters.
ip ip is a utility to show or manipulate routing, network devices, interfaces and tunnels.
netstat Shows network status.
ss Another utility to investigate sockets.
ps Shows process status.
env Prints environment or sets and executes command.
lsblk Lists block devices.
lsusb Lists USB devices
lsof Lists opened files.
lspci Lists PCI devices.

SSH

Secure Shell (SSH) is a network protocol that allows the secure transmission of data and commands over a network. The most commonly used SSH server is the OpenSSH server. OpenSSH is a free and open-source implementation of the Secure Shell (SSH) protocol that allows the secure transmission of data and commands over a network. OpenSSH can be configured and customized by editing the file /etc/ssh/sshd_config with a text editor.

ssh [username]@[IP address]

System Logs

System logs on Linux are a set of files that contain information about the system and the activities taking place on it. There are several different types of system logs on Linux, including:

  • Kernel Logs contain information about the system's kernel, including hardware drivers, system calls, and kernel events. They are stored in the /var/log/kern.log file.
  • System Logs contain information about system-level events, such as service starts and stops, login attempts, and system reboots. They are stored in the /var/log/syslog file.
  • Authentication Logs contain information about user authentication attempts, including successful and failed attempts. They are stored in the /var/log/auth.log file.
  • Application Logs contain information about the activities of specific applications running on the system. They are often stored in their own files, such as /var/log/apache2/error.log for the Apache web server or /var/log/mysql/error.log for the MySQL database server.
  • Security Logs and their events are often recorded in a variety of log files, depending on the specific security application or tool in use.