Shells - Linux (linux.md)

👉 Overview

👀 What ?

A shell in Linux is a software interface that's often used as a command-line interpreter and it allows users to access the services of a kernel. The shell gets the command from the user and instructs the kernel what to do, which then sends the output to the shell to display it to the user.

🧐 Why ?

Understanding and using a Linux shell is important because it provides a powerful and extensible environment for managing and interacting with your computer. It offers greater control and scripting capabilities compared to graphical user interfaces, enabling users to perform complex tasks more efficiently and automate repetitive tasks.

⛏️ How ?

To start using a Linux shell, open the terminal application on your Linux system. You can enter commands directly into the shell and also write these commands into a file and execute the file as a shell script. Basic commands include 'ls' for listing directory contents, 'cd' for changing the current directory, and 'rm' for removing files or directories.

⏳ When ?

Shells have been a part of Unix-like systems like Linux since their inception in the 1970s. They are just as relevant today as they were then, with many users still relying on them for their speed, power, and versatility.

⚙️ Technical Explanations

A Linux shell functions by reading and interpreting either command-line input or scripts, and subsequently executing them. It uses a standardized syntax to receive and process instructions. When a command is inputted, the shell breaks it down into individual units known as tokens. The first token is recognized as the command, and the remaining tokens are treated as arguments for that command. The shell then creates a new process, known as forking, and executes the command within this new process environment. It pauses until the command has fully run its course, after which it displays the final output.

In more detail, the process of tokenization separates the input into meaningful components, such as variables, operators, and structures, which helps in interpreting the command correctly. The creation of a new process ensures that the command execution doesn't interfere with the shell's process and the user's current working environment.

The shell acts as a bridge between the user and the Linux kernel, receiving commands from the user and communicating them to the kernel to be executed. This allows users to interact with the system, manipulate files and directories, and run scripts without having to understand the complexities of the underlying system.

Here are some of the crucial commands you might use, along with more detailed explanations and examples:

1. pwd: "Print Working Directory". This command shows the full pathname of the current directory you're in. It is useful when you're navigating through directories and want to know your exact location in the filesystem.
2. ls: This command lists the contents of a directory. By default, ls will show you the files and directories in your current location. There are several options you can add to ls to modify its behavior, such as l (long format, showing permissions, ownership, size, and modification date), a (show all files, including hidden ones), R (recursively list directory tree), and t (sort by modification time, newest first).
3. cd: "Change Directory". This command allows you to move between directories. Just type cd followed by the directory name. For example, cd Documents would move you into the Documents directory. cd .. allows you to move up one level in the directory structure, and cd / takes you to the root directory regardless of your current location in the filesystem.
4. mkdir: "Make Directory". This command allows you to create a new directory. For example, mkdir Project would create a new directory named 'Project' in your current location.
5. rmdir: "Remove Directory". This command allows you to delete an empty directory. For instance, rmdir Project would delete a directory named 'Project' if it is empty.
6. touch: This command allows you to create a new, empty file. It's often used when you need to quickly create a file for various purposes. For example, touch example.txt would create a file named 'example.txt'.
7. mv: "Move". This command is used to move or rename files and directories. For instance, mv old_file.txt new_file.txt would rename the file 'old_file.txt' to 'new_file.txt'. mv file.txt new_directory/ would move 'file.txt' to the directory 'new_directory'.
8. cp: "Copy". This command is used to copy files or directories from one location to another. For example, cp source_file.txt destination_file.txt would create a copy of 'source_file.txt' and name it 'destination_file.txt'. If you're copying directories, you would use the r option to recursively copy all files and subdirectories.
9. rm: "Remove". This command is used to delete files or directories. rm file.txt would delete the file 'file.txt'. rm -r directory/ would delete the directory 'directory' and everything within it.
10. cat: This command concatenates and displays file content. For example, cat file.txt would display the contents of 'file.txt' on the screen.
11. less: This command allows you to view the contents of a file one screen at a time. It's especially useful when dealing with large files.
12. grep: This command is used to search for specific patterns within files. For example, grep 'keyword' file.txt would search for 'keyword' within 'file.txt' and print any matching lines on the screen.
13. find: This command is used to search for files and directories within the file system. For example, find / -name 'file.txt' would search for a file named 'file.txt' starting from the root directory.
14. chmod: This command changes the permissions of a file or a directory. For example, chmod 755 file.txt would set the permissions of 'file.txt' to '755' (read, write, and execute permissions for the user, and read and execute permissions for the group and others).
15. chown: This command changes the owner of a file or a directory. For example, chown user:group file.txt would change the owner of 'file.txt' to 'user' and the group to 'group'.
16. nano, vi, or emacs: These commands open a text editor. You can use these to write and edit shell scripts or any text file. For example, nano script.sh would open the file 'script.sh' in the nano text editor.

This overview just scratches the surface of what you can do with a Linux shell. The real power of the shell comes from its flexibility and the control it offers. By learning more commands and how to use them together, you can accomplish complex tasks efficiently, automate repetitive tasks, and interact directly with the system.

🖇️ Références