Basic Linux knowledge

Linux was first designed by a student called Linus Torvalds at the University of Helsinki, Finland. It is free to use and spread freely.
Such as nuix system, source code development, can be run on various hardware platforms (including intel X86 series and RISC processors.
 
Why Linux is favored by many computer enthusiasts:
1. Linux has implemented all UNIX features on PC computers, with multi-task and multi-user capabilities, and is stable and efficient in many aspects.
2. Linux is a free software. Users can get it and other original code without paying any fee. They can modify it and use it for free without any restrictions.
 
Linux has the following features:
Openness
Openness means that the system complies with world standards, especially the OSI International Standards. All hardware and software developed in compliance with international standards are compatible with each other and can be easily interconnected.
Multiple users
Multi-User means that system resources can be used by different users, that is, each user has specific permissions on their own resources (such as files and devices), without affecting each other. Both Linux and UNIX have the characteristics of multiple users.
Multi-task
Multi-task is a major feature of modern computers. It means that the computer executes multiple programs at the same time, and each program runs independently of each other. In Linux, every process is scheduled to access the microprocessor equally. Because the CPU processing speed is very fast, the result is that the started application seems to be running in parallel. In fact, there is only a short delay between the processor executing a group of commands in an application and the Linux scheduling microprocessor running this program again, which is invisible to users.
Good user interface
Linux provides users with two interfaces: 'User interface and system call.
The traditional linux user interface is a text-based command line interface (Shell), which can be used both online and offline. Shell has strong programming capabilities, allowing users to easily use it to program, thus providing users with more advanced means to expand system functions. A Programmable shell combines multiple commands to form a shell program, which can be run independently or simultaneously with other programs.
The system calls the interface used for programming. Users can directly use the System Call commands provided by the system during programming. The system provides low-level and efficient services for user programs through this interface.
Linux also provides a graphical user interface. It uses the mouse, menu, window, scroll bar and other facilities to present a user with an intuitive, easy to operate, interactive and friendly graphical interface.
Device independence
Device independence means that the operating system treats all external devices as files. Any user can manipulate and use these devices as long as their drivers are installed, you don't have to know their specific forms of existence.
An operating system with device independence simplifies the process of adding new devices by treating each peripheral device as an independent file. When a new device needs to be added, the system administrator adds the necessary connections to the kernel. This connection, also known as a device driver, ensures that the kernel processes the device in the same way each time it calls the device to provide services. When new and better peripherals are developed and delivered to users, operations allow immediate access to these devices without restrictions after they are connected to the kernel. The key to device independence lies in kernel adaptability. Other operating systems only allow a certain number of external devices or some types of external devices to connect. The device-independent operating system can accommodate any type and any number of devices, because each device is accessed independently through its dedicated connection to the kernel.
Linux is an operating system with device independence. Its kernel is highly adaptive. As more programmers join Linux programming, more hardware devices are added to various linux kernels and release versions. In addition, because you can get the Linux kernel source code for free, you can modify the kernel source code to adapt to the new external devices.
Provides a wide range of network functions
A complete built-in network is a major feature of Linux. Linux is superior to other operating systems in terms of communication and network functions. Other operating systems do not have the ability to connect networks so closely integrated with the kernel, nor have the flexibility to build in these networking features. Linux provides users with comprehensive and powerful network functions.
Internet is one of its network functions. Linux provides a lot of software that supports the Internet for free. The Internet is established and prosperous in the Unix field. In this regard, Linux is quite convenient to use, users can use Linux to communicate with others in the world over the Internet.
File Transfer is the second of its network function. You can use some Linux commands to transmit internal information or files.
Remote access is the third of its network functions. Linux not only allows file and program transmission, but also provides system administrators and technicians with windows to access other systems. With this remote access feature, a technician can effectively serve multiple systems, even if those systems are located far apart.
Reliable System Security
Linux has adopted many security technical measures, including permission Control for read and write, and core authorization for audit tracking of protected subsystems, which provides necessary security protection for users in multi-user network environments.
Good portability
Portability refers to the ability to move an operating system from one platform to another so that it can still run in its own way.
Linux is a portable operating system that can run in any environment from a microcomputer to a large computer and on any platform.
Portability provides a means for different computer platforms running Linux to communicate accurately and effectively with any other machine without additional special and expensive communication interfaces.
When talking about Linux features, let alone its shortcomings. Linux has no core team and fans from all over the world can release their own patches ), the disadvantage is that the source code may be disorganized and may conflict with each other. in fact, it is also an advantage from another aspect. enthusiasts can release their own patches, which makes Linux dynamic and constantly updated and improved.

Differences between Linux and other operating systems
Linux can coexist with other operating systems such as MS-DOS and Windows on the same machine. They are all operating systems and have some commonalities, but they are distinctive and different from each other. MS-DOS and Windows are very familiar to our computer hobbies. Only the differences between Linux and UNIX are listed here.
UNIX and Linux on some PCs are similar in implementation. Almost all commercial UNIX versions basically support the same software, programming environment, and network features. However, there are still many differences between commercial Linux and UNIX versions. Linux supports different hardware scopes than commercial UNIX. Generally, commercial Unix supports more hardware, but Linux supports more hardware. Note that Linux is at least as stable as commercial UNIX. For many users, the most important factor is price. Linux is a free software that can be downloaded from the Internet. If the Internet access is inconvenient, you can obtain the Linux disk or CD-Rom through mail order very cheaply. Of course, you can also get it directly from your friends. The value of commercial Unix should not be devalued. In addition to the price of the software, you also need to pay the documents, after-sales support and quality assurance fee. These are important for large organizations, but PC users may not benefit from them. Many commercial companies, universities, and other organizations have found that running Linux on cheap pcs in laboratories is better than running commercial UNIX on workstations. Linux can provide the workstation function on a PC, and the price of a PC is a fraction of that of a workstation. There are also some cheap UNIX for PC, the most famous of which is ipvbsd. In many aspects, the iisbsd package is compatible with Linux, but Linux is more suitable for user needs. Most importantly, Linux development is open, and any volunteers can contribute to the development process. In contrast, objective BSD is developed by closed teams. In this way, there is a serious conceptual and Design difference between the two products: the objective of Linux is to develop a complete UNIX system from the beginning; objective of objective BSD is to modify the existing BSD to fit the 80386 system. Of course, there are also many free Unix running on PCs, such as NetBSD, FreeBSD, OpenBSD, NetBSD, FreeBSD, and OpenBSD, which are not as good as Linux in terms of system performance, with expert analysis, FreeBSD has better network performance, software porting, and system standardization than Linux.

Directory tree in Linux
/
This is the root directory. In Windows, DOS, or other similar operating systems, each partition has a corresponding root directory. But Linux and other UNIX systems put all the files in a directory tree, And/is the only root directory. Generally, only one kernel image is stored in the root directory.
/Boot
Many Linux systems put kernel images and other startup-related files here.
/Tmp
Generally, only temporary files generated during startup are stored in this place. All our own temporary files are stored in/var/tmp.
/Mnt
The directory contains subdirectories used to mount other devices, such as/mnt/CDROM or/mnt/floppy.
/Lib
All the library files required for startup are stored in this directory. All non-startup library files are stored in/usr/lib. The kernel module is stored in/lib/modules/(kernel version.
/Proc
This directory does not exist on the disk. The files are all about the current system status, including running processes, hardware status, memory usage ......
/Dev
All device files are stored in this directory. There are some special files created by the Linux kernel to control hardware devices. Note that the NIC device file (eth0, ppp0, etc.) is not here.
/Var
Here there are some data that has been changed by the system. For example,/var/tmp is used to store temporary files. There are also many other processes and modules that place their record files here. The following lists some important sub-directories:
/Var/log
There are a vast majority of record files. This directory may become very bloated as time goes on. So we need to clean it regularly.
/Var/run
Contains a variety of small information during running.
/Var/lib
Including the files required for running the system. If you use a laptop to access the Internet,/var/lib/PCMCIA/stab may be useful to you.
/Var/spool
The location of the email, news, and print sequence.
/Root
Root user's home directory.
/Home
Generally, users' home directories are stored in this directory. You can use $ Cd ~ To enter your home directory.
/Etc
If you are a root user, this directory should be the most common place for you. The vast majority of system configuration files are saved here. Relatively speaking, the system configuration file of a single user will be saved in the user's home directory. The files here will be increased or decreased Based on the software you have installed. Important sub-directories are listed below:
/Etc/X11
Here is the configuration file required by X system. Xf86config stores the configuration in this place. /Etc/X11/fonts contains some fonts required by the server. Some configuration files stored in the window manager are also stored in this place.
/Etc/init. d
This directory stores the startup description file, including the loading description of various modules and services. So if you are not clear about it, it is best not to delete anything here. The files here are all configured by the system, not by the user.
/Etc/RCs. d
Here are some connection files connected to/etc/init. d, which are described based on runlevel. The file names start with "S", followed by two digits, indicating the order in which various services are started. For example, s24foo is executed before s42bar. Then the name of the file connected to/etc/init. d is displayed.
/Etc/rc0.d-/etc/rc6.d
There are also some connection files, which are similar to/etc/RCs. D. The difference is that these will only run the corresponding description under the specified runlevel. 0 indicates shutdown, and 6 indicates restart. All files starting with K are closed, and all files starting with S are restarted. Currently, the file naming method is the same as/etc/rc. d. Note: In Debian, 2 indicates start.
/Bin,/sbin
Here are the common programs and system programs required for startup respectively. Many programs are also useful after being started. They are placed in this directory so that they are often called by other programs.
/Usr
This is a very large directory, and almost all the files are placed under this directory, except those mentioned above. The following lists some important sub-directories:
/Usr/x11r6,/usr/X11,/usr/xfree86
The directory structure of X system is the same as that of/usr.
/Usr/bin
The directory where the V binary executable files are stored. Most applications are stored here.
/Usr/sbin
There are a vast majority of system programs.
/Usr/games
Game programs and corresponding data will be placed here.
/Usr/include
This directory stores the header files of C and C ++. Unless you are a programmer, you may not be interested.
/Usr/lib
Library files that are not used at startup will be placed here.
/Usr/info
The data required by the GNU info program is saved here.
/Usr/man
The data required by the man program is saved here.
/Usr/src
The source code file is saved here. The source code of the Linux kernel is stored in/usr/src/Linux.
/Usr/doc
Various document files are saved here. These files help you understand Linux, solve problems, and provide some tips. Or you can live more knowledge by visiting http://www.linuxdoc.org.
/Usr/local
The files required by the local computer are saved. It is particularly meaningful for users to perform remote access. In some Linux systems, this directory is a separate partition that stores the user files that the sub-host belongs. The structure is the same as/usr.
/Usr/shared,/usr/share
Various shared files are stored here, such as the sound files required by an application.