WHAT IS A SHELL?

shell [Concise English-Chinese Dictionary].

fel]n.Shells, shells, shapes, shells.

vt.Hulled, shedding, shelled.

vi.Peeling, shelling.

Sets the position of the command-line interpreter.

The main interface between a text operating system and the outside is called a shell

The shell is one that acts as an interface between the user and the systeminterfaceIt allows the user to enter the commands that need to be executed into the operating system. This is similar to the Windows Command Prompt and DOS commands, but the Linux shell is much more powerful. Windows now also has a powerful PowerShell.

The shell is like an interface, and for security and convenience, use it as much as possible to operate and avoid directly manipulating the underlying kernel.

Shell infrastructure host means: the host shell of the infrastructure.

All of them are devices within the computer console. 80% of the hardware equipment needs to be replaced every time you upgrade, and this computer terminal only needs to replace 15% of the hardware equipment! The computer mini computer effectively reduces the cost of the first set-up, maintenance fee, electricity cost, venue fee, upgrade fee, etc.

The cost performance is unprecedentedly improved, saving power energy and space, which is conducive to environmental protection, and can realize multiple users of a single machine.

High stability performance: Plug the graphics card into the computer host, install the mini computer, 1-7 computer users can use the computer at the same time, can run the same or different programs, do not interfere with each other, and the processing speed is not affected. The mini computer can achieve dynamic allocation of system resources and stably execute applications based on win2000 or winxp system.

The overall cost is less: due to the rapid upgrading of computer CPUs, expert statistics, more than 99% of computers are upgraded every 2 3 years on average, and each upgrade of hardware equipment, motherboards, hard disks, CPUs, and memory are the main objects of upgrades.

Shell language refers to the command language of the UNIX operating system, and is also the abbreviation of the interpreter of the command language.

The shell itself is a program written in C, which is a bridge for users to use UNIX Linux, and most of the user's work is done through the shell. Shell is both a command language and a programming language. As a command language, it interactively interprets and executes commands entered by the user; As a programming language, it defines a variety of variables and parameters, and provides many control structures that are only available in high-level languages, including loops and branches.

Although it is not part of the Unix Linux system kernel, it invokes most of the functions of the system core to execute programs, establish files, and coordinate the operation of individual programs in parallel. Therefore, for users, the shell is the most important utility, and in-depth understanding and proficiency in the characteristics of the shell and how to use it are the key to using the UNIX Linux system well.

The shell is the user interface of the system, which provides an interface for the user to interact with the kernel. It receives the command input from the user and sends it into the kernel for execution.

In effect, the shell is a command interpreter that interprets commands entered by the user and sends them to the kernel. Not only that, but the shell has its own programming language for commands, which allows users to write programs consisting of shell commands. The shell programming language has many characteristics of ordinary programming languages, such as it also has a loop structure and a branch control structure, and shell programs written in this programming language have the same effect as other applications.

Linux provides a visual command input interface like Microsoft Windows - the graphical user interface (GUI) of X Window. It provides a lot of desktop environment systems, and its operation is just like Windows, with windows, icons, and menus, and all management is controlled through the mouse. The more popular desktop environment systems are KDE and GNOME.

Each Linux user can have their own user interface or shell to meet their own specialized shell needs.

Like Linux itself, there are many different versions of the shell. There are currently the following versions of the shell: Bourne Shell: developed by Bell Labs.

bash: is the bourne again shell of GNU, which is the default shell on the GNU operating system.

Korn Shell: is a development of the Bourne Shell, and is compatible with the Bourne Shell in most respects. C Shell: is the BSD version of Sun's shell.

The shell can be understood as an interpreter between the user and the operating system core, which is responsible for parsing the instructions entered by the user and handing them over to the operating system core for processing.

A shell script is a command or commands that are combined through a specific logical structure to complete complex management tasks, similar to the BAT script on the Windows system.

A shell is a command processor — a program that reads in and interprets the commands you type. In addition to being a command interrupter, the shell is also a programming language. You can write programs that can be interpreted by the shell (called source programs), which can contain shell program design commands, and so on.

The shell does other things besides interpreting commands, and it can also be configured and programmed. The shell has its own language, allowing users to write programs and run them in a complex way. The shell programming language has the characteristics of many commonly used programming languages, such as:

circulation and control structures, etc. Users can generate shell programs as complex as other applications.

Additional note: To put it simply: The shell is an interactive command interpreter.

The shell is independent of the operating system, and this design gives users the flexibility to choose the command that suits you, and you can type commands on the command line, which are interpreted by the shell and then transmitted to the operating system (kernel) for execution.

Here's a summary of the shell's features:

Find the location of the command and execute the associated program.

Assign a new value to the shell variable.

Execute the command instead.

Handle ioRedirection and Pipeline functionality.

Provides an interpretive programming language interface, including statements such as tests, branches, and loops.

Shell purpose. Block control.

Shell contents. Easy to control.

$ in shell has a lot of meanings.

1. After logging in to the shell successfully, you will find $ on the left side of the cursor, which is a command prompt, which represents ordinary account login, and is on behalf of root account login.

2. $ can be used as a variable operator, such as echo $path will output the content of the path variable.

3. a=$((1+2)) can assign a to 3 instead of the string "1+2", which is the arithmetic function.

4. a=$(ls home) can assign the command execution result in parentheses to a, which means that the command is executed first.

5. In regular expressions, $ means the end of the line, such as"a$"Represents a line ending in a.

And if you use vi, in normal mode, $ is the command to move the cursor to the end of the line.

That's all I need to think about for now.

Scripting languages in Linux and UNIX are similar to the commands you can use in CMD under Windows.