First knowledge of the shell

1. What is a shell?

The shell is in the outer layer of the kernel, which is responsible for receiving commands entered by the user, interprets the commands as kernel can understand, then executes by kernel and returns the results back to the default output perimeter. The shell is a command interpreter and also a programming language. There are many types of shells, and in many Linux distributions, the standard shell is bash and the main object of our learning.

Basic structure of 2.shell program

The shell structure is largely set by the variables, built-in commands, shell syntax structure, function composition .

Usage Example Description: test.sh

1. #!/bin/bash

2. #说明使用/bin/bash as the interpreter for this script

3.

4. #定义一个函数

5. Function My_fun () {

6. Echo "Hello, $1,today is $"

7.}

8.

9. #定义连个变量

Ten. name=$1

today= ' Date '

12.

#函数调用.

My_fun "$name" "$today"

The above script will need to do some work to run, first give the execution permission

chmod +x test.sh

And then execute

./test.sh Willis

Output

Hello, Willis,today is Tue June 1 14:51:46 CST 2010

3. Parent shell and child shell

Before executing the script, the environment in which it is located is the parent shell. When the script is executed, the parent shell comes out with a new shell environment based on #!/bin/bash,fork, then executes in the child shell, executes after execution, and ends with the child shell, still returning to the parent shell, which does not affect the environment of the parent shell.

4.login Shell and Non-login shell

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This picture is the process of the login shell, and when it is non-login the shell, only the part of the callout in the box is executed. By this diagram we can know, in the following cases, the process of execution.

4.1 Landing (login)

/etc/profile

~/.bash_profile

4.2 Logout (logout)

~/.bash_logout

5.Shell Interactive mode

5.1 Automatic Interaction Method One

Automatic interaction is the most important thing is the automatic input of interactive information, first Lenovo to file redirection, in shell programming has such a method of "command << delimiter (delimiter) is read from the standard input until the delimiter delimiter is encountered . "

The redirect operator Command << delimiter is a very useful order, and the shell will know that the input is finished by delimiter the delimiter until all the content before the next same delimiter is entered as input and the next delimiter is encountered. The most common delimiter delimiter is EOF, which can, of course, be self-defined for other characters.

Example: Disk auto-Partitioning script

#!/bin/bash

fdisk << EOF

N

P

1

( space)

+1g   

W

Eof

MKFS.XFS/DEV/VDB1  

Automatically modify user password scripts

#!/bin/bash

passwd << EOF

Willis

Willis

Eof

5.2 Automatic Interaction Method Two

another automatic input method that reminds you of interactive information: pipe, via echo + |

Example: Automatically modify user password scripts

#!/bin/bash

(echo "redhat"

Sleep 1

echo "redhat ") |passwd

Automatic partitioning

#!/bin/bash

Echo '

N

P

1

+1g

W ' | Fdisk $

Mkfs.xfs/dev/vdb1

5.3 Automatic Interaction Method Three

Expect is a dedicated tool to implement automatic interaction, expect syntax can refer to the relevant information, the code is as follows:

automatically switch to root user

#!/usr/bin/expect

Spawn Su Root

Expect "Password:"

Send "Willis \ r "

Expect EOF

Exit

The test passes, runs the script, and logs directly from the current user to the root user.

Shell Interactive Method Summary:

Method One (redirection) is simple, intuitive, and often practical, but has limited functionality in the field of auto-interaction.

Method Two (pipeline) is also very simple and intuitive, and sometimes even without sleep coordination can show a powerful automatic interactive power

Method Three (expect) is the most powerful in function, expect originally is to realize automatic interactive function, but the disadvantage is to install expect package, in the embedded environment difficult to install.

Three methods each have advantages and disadvantages, good application, can complete the Linux shell automatic interaction

This article from the "Technology life, Simple not simple" blog, please be sure to keep this source http://willis.blog.51cto.com/11907152/1845287

First knowledge of the shell