Basics of Linux and Web Server

ls: lists the files and directories in a directory

cd: changes the current working directory

pwd: displays the current working directory

cat: concatenates and displays files

cp: copies files and directories 

1. cp [...file/directory-sources] [destination]

mv: moves or renames files and directories

rm: deletes files and directories

mkdir: creates a new directory

rmdir: removes an empty directory

touch: creates a new file or updates the modification time of an existing file

echo: displays a message or the contents of an environment variable

ps: displays information about the currently running processes

top: provides real-time information about system resource usage

ping: tests connectivity to a network host

ssh: opens a secure shell connection to a remote host

scp: securely copies files between hosts

tar: creates or extracts archive files

grep: searches for a pattern in text files

find: searches for files and directories based on certain criteria

chmod: changes the permissions of files and directories.

Root Directory: The top-level directory in the Linux file system hierarchy is the root directory, represented by a forward slash "/".

Subdirectories: The root directory branches out into several subdirectories, each serving a specific purpose.

Standard Directories: Some of the standard directories in the Linux file system hierarchy include:

• /bin: contains essential user command binaries
• /boot: contains boot loader files and the Linux kernel
• /dev: contains device files that represent system devices
• /etc: contains configuration files for the system and applications
• /home: contains user home directories
• /lib: contains libraries required by the system and applications
• /media: contains mount points for removable media devices
• /mnt: used as a mount point for temporarily mounting file systems
• /opt: contains optional software packages
• /root: contains the home directory of the root user
• /srv: contains data for services provided by the system
• /tmp: contains temporary files
• /usr: contains user-related data and applications
• /var: contains variable data, such as log files and spool directories

File Permissions: In Linux, every file and directory has permissions that determine who can access and modify them. These permissions can be viewed and modified using the "ls -l" and "chmod" commands.

Hidden Files: In Linux, files and directories that start with a dot (.) are considered hidden and are not displayed by default when using the "ls" command.

Symbolic Links: Symbolic links, also known as symlinks, are special files that point to other files or directories. They can be created using the "ln -s" command.

Mount Points: In Linux, different file systems can be mounted at various mount points in the file system hierarchy to make them accessible. The "mount" and "umount" commands can be used to mount and unmount file systems, respectively.

Current Directory: The current working directory is the directory that you are currently in and is indicated by the prompt in the terminal.

Absolute Path: An absolute path is the full path to a file or directory, starting from the root directory "/".

Relative Path: A relative path is a path to a file or directory relative to the current working directory.

cd Command: The "cd" (change directory) command is used to navigate to different directories in the file system hierarchy.

.. and . : The ".." (dot dot) symbol represents the parent directory of the current working directory, while the "." (dot) symbol represents the current working directory itself.

Tab Completion: In the terminal, you can use tab completion to automatically complete the names of files and directories.

~ (Tilde): The "~" symbol represents the home directory of the current user.

Shortcuts: Some common directory navigation shortcuts include:

• cd ~: goes to the current user's home directory
• cd /: goes to the root directory
• cd ..: goes to the parent directory of the current working directory

Pushd and Popd: The "pushd" and "popd" commands can be used to save and switch between directories, respectively. They are part of the dirstack, a stack of directories that you can navigate.

Package: A package is a collection of files, scripts, and metadata that make up a software program. Packages are typically distributed in a pre-compiled and compressed format, such as a .deb or .rpm file.

Package Manager: A package manager is a program that handles the installation, removal, and updating of packages. Some popular package managers in Linux include apt (for Debian-based systems), yum (for Red Hat-based systems), and dnf (for Fedora-based systems).

Repositories: Repositories are centralized collections of packages that can be accessed and installed through the package manager. Official repositories are typically maintained by the Linux distribution, while third-party repositories may be created and maintained by individuals or organizations.

Searching and Installing Packages: The package manager allows you to search for packages in the repositories, view package information, and install packages. The specific commands for searching and installing packages vary depending on the package manager in use.

Updating and Removing Packages: The package manager also provides commands for updating packages to the latest version and removing packages that are no longer needed.

Package Dependencies: Packages often have dependencies, which are other packages that must be installed for the package to work correctly. The package manager handles the installation of dependencies automatically.

Package Management in Virtual Environments: Package management can also be performed in virtual environments, such as virtualenv for Python or conda for data science. This allows you to manage packages and dependencies for individual projects, rather than system-wide.

Types of Text Editors: There are two main types of text editors in Linux: graphical and command-line. Graphical text editors, such as Gedit and Sublime Text, provide a graphical user interface for editing text. Command-line text editors, such as nano and vi, are text-based and run in the terminal.

nano: nano is a simple, easy-to-use text editor that is often used as a beginner-friendly alternative to vi. nano has a menu-based interface and provides basic editing features, such as cut and paste.

vi: vi is a powerful, but more complex, text editor that is included with most Linux distributions. vi operates in two modes: command mode and insert mode. In command mode, you can navigate and manipulate the text, while in insert mode, you can insert and edit the text.

Syntax Highlighting: Many text editors, including nano and vi, support syntax highlighting, which displays different elements of the code, such as keywords and comments, in different colors. This can make the code easier to read and understand.

Customization: Many text editors, including vi, can be customized with plugins and configurations to add additional functionality and tailor the editor to your needs.

Choosing a Text Editor: The text editor you choose will depend on your needs and preferences. If you're new to Linux, nano may be a good place to start. If you're looking for a more powerful and customizable option, vi may be a better choice.

Purpose: Environment variables store information such as system paths, user settings, and program configuration options. They can be accessed and used by both the operating system and various programs and scripts.

Setting Environment Variables: Environment variables can be set in various ways, including in shell profiles, shell scripts, and program configuration files. The syntax for setting environment variables varies depending on the shell in use.

Viewing Environment Variables: The command echo $VAR_NAME can be used to view the value of an environment variable in the terminal, where VAR_NAME is the name of the environment variable. The printenv command can be used to display all environment variables and their values.

Common Environment Variables: Some common environment variables include PATH, which stores the directories where the system should search for executable programs, and HOME, which stores the path to the user's home directory.

Persistence: Environment variables are typically set for a specific session, and their values will be lost when the session is closed. To persist environment variable values across sessions, they must be set in a shell profile or configuration file that is executed every time the system starts.

Modifying Environment Variables: Environment variables can be modified, added, or removed as needed. This can be done through the terminal or by editing configuration files.

What is a Process: A process is an instance of a running program, including the program's code and data in memory. Each process has a unique process ID (PID) that is assigned by the system.

ps Command: The ps command displays information about the processes running on the system. It can be used to view the PID, status, and other information about processes.

top Command: The top command provides an interactive interface for viewing information about processes running on the system, including the CPU and memory usage of each process.

kill Command: The kill command can be used to send signals to processes, such as the SIGTERM signal, which terminates the process. The kill command is often used to stop unresponsive or rogue processes.

nohup Command: The nohup command can be used to run a process in the background, even if the terminal is closed. This is useful for running long-running processes that should continue running even if the user logs out.

& Operator: The & operator can be used to run a process in the background from the terminal. For example, command & will run command in the background.

fg and bg Commands: The fg and bg commands can be used to bring a background process to the foreground or to move a foreground process to the background, respectively.

Apache: Apache is the most widely-used web server software, with a large user community and a wealth of documentation and support resources. Apache supports a wide range of operating systems and is known for its flexibility and stability.

Nginx: Nginx is a high-performance web server that is often used to serve static content, such as images and videos. Nginx is known for its efficiency and can handle a large number of concurrent connections with a low memory footprint.

Microsoft IIS: Microsoft IIS is a web server designed to run on Windows-based servers. IIS is often used in enterprise environments and integrates well with other Microsoft products, such as Windows Active Directory.

Lighttpd: Lighttpd is a lightweight web server that is often used for smaller websites and applications. Lighttpd is known for its low resource usage and high performance.

Node.js: Node.js is a JavaScript runtime that can be used as a web server. Node.js is known for its fast performance and event-driven architecture, which makes it well-suited for real-time applications and web sockets.

Tomcat: Tomcat is a web server and servlet container, designed to run Java applications. Tomcat is widely used for Java-based web applications and integrates well with other Java technologies, such as JSP and JSF.

HTTP Requests: A web server serves web pages in response to HTTP requests from clients, such as web browsers. The HTTP request specifies the web page the client wants to access, and the web server returns the requested web page in the HTTP response.

Retrieving Web Pages: The web server retrieves the requested web page from the file system and processes any dynamic content, such as PHP or Python scripts. The result of this processing is a complete web page, ready to be sent to the client.

Dynamic Content: Dynamic content refers to web pages that are generated in real-time based on user input or other data. For example, a PHP script might generate a web page that shows the current time, or a Python script might retrieve data from a database and use it to generate a web page.

Caching: To improve performance, a web server may cache frequently-requested web pages in memory. When a client requests a cached web page, the web server can return the cached page instead of retrieving it from the file system and processing dynamic content, which saves time.

Performance Optimization: A web server can be optimized for performance in several ways, such as serving static content efficiently, compressing content before sending it to the client, and reducing the number of HTTP round-trips between the client and server.

HTTP Requests: When a client requests a web page, it sends an HTTP request to the web server. The request specifies the URL of the requested page, as well as any parameters or data that should be sent to the server.

Dynamic Content: Some web pages contain dynamic content, which is generated on the fly by the server in response to the request. For example, a server-side scripting language such as PHP or Python can be used to generate dynamic content, based on user input or other variables.

Static Content: Other web pages contain static content, which does not change from one request to the next. For example, images, videos, and CSS files are typically static content that can be served directly by the web server.

File System: The web server retrieves the requested web page from the file system, based on the URL in the request. The web server must be configured to map URLs to file paths on the file system, so that it knows where to find the requested page.

Processing: The web server may need to process the requested web page before sending it to the client. For example, if the page contains server-side scripts, the web server must execute those scripts and include their output in the response.

Response: Once the web server has processed the requested web page, it sends an HTTP response back to the client. The response includes the contents of the requested web page, as well as headers that provide information about the response (such as the content type, encoding, and length).

Caching: Web browsers often cache web pages, so that they don't need to request the same page multiple times. Web servers can also cache web pages, either on the server itself or in a proxy server. Caching can improve performance by reducing the number of requests that need to be processed by the web server.

Server Settings: The web server must be configured with various settings, such as the IP address and port to listen on, the document root (the location on the file system where the web content is stored), and the default file to serve when a directory is requested. These settings can be configured in the web server's configuration file, or through a graphical user interface provided by the web server software.

Virtual Hosts: Many web servers support virtual hosting, which allows multiple websites to be hosted on a single server. Virtual hosting enables different websites to have different IP addresses, domains, and configurations, even though they are all running on the same physical server.

SSL/TLS Certificates: If your website needs to support secure connections (such as for online transactions), you'll need to obtain an SSL/TLS certificate and configure the web server to use it. The certificate is used to encrypt traffic between the client and server, and to verify the identity of the website.

Access Control: Web servers can be configured to restrict access to certain parts of the website, based on criteria such as the IP address of the client, the URL being requested, or the credentials provided by the client. This is typically done through the use of access control rules, which can be specified in the web server's configuration file.

Logging: Web servers log information about incoming requests and other events, such as errors or warnings. Logs can be useful for debugging and monitoring the performance of the web server. Web server configurations often include settings for logging, such as the location of the log files and the format of the logs.

Performance: Web server configurations can also be optimized for performance, to handle a high volume of traffic or to improve the response time for clients. This can involve tuning various settings, such as the maximum number of concurrent connections, the buffer size for incoming requests, or the number of worker processes.

Patch Management: Keeping your web server software up to date is one of the most important steps in maintaining security. Software vendors frequently release updates that fix security vulnerabilities, so it's important to apply these updates in a timely manner.

Firewall Configuration: A firewall can be used to block incoming traffic that is not necessary for the functioning of the web server. This helps to reduce the attack surface of the server and makes it more difficult for attackers to exploit vulnerabilities.

Secure Configuration: The web server software should be configured securely to minimize the risk of attack. For example, the software should be configured to run with the minimum privileges necessary, to disable unnecessary services and features, and to use strong authentication and encryption mechanisms.

Input Validation: Web servers must be designed to validate all incoming data to prevent attacks such as SQL injection or cross-site scripting (XSS). This involves checking the format and content of incoming requests, and rejecting requests that contain unexpected or malicious data.

Access Control: The web server should be configured to restrict access to sensitive resources, such as configuration files, system logs, and user data. This can be achieved through the use of authentication and authorization mechanisms, such as passwords, certificates, or tokens.

Monitoring: Regularly monitoring the web server for signs of attack or compromise is an important aspect of security. This includes monitoring logs for unusual activity, using intrusion detection systems (IDS) or intrusion prevention systems (IPS) to detect and block attacks, and performing regular security audits.