Linux beginners, linux beginners

Linux beginners, linux beginners

Linux features

1. Free and Open-Source

2. multithreading is supported. Multi-User

3. Good Security

4. superior memory and file management

5. linux requires at least 4 m-> embedded development

Disadvantages:

Relatively difficult operations

Command:

Shutdown-h now shutdown immediately

Shutdown-r now restart the computer

RebootRestart the computer now

Logout

Use vi to develop hello. java in linux

1 vi Hello. java

2 input I [enter the insert mode]

3 public class Hello {

Public static void main (String [] args ){

System. out. println ("Hello ");

}

}

4. Enter esc to enter the command mode]

5 input: wq [Save and quit] q! [Exit and do not save]

Enter ls [Current Directory]

Ls-l [Details]

6 compile Hello. java

Javac Hello. java

7 run

Java Hello

How to Develop c/c ++ programs in linux

Vi hello. cpp

# Include <stdio. h>

Int main (){

Printf ("hello! ");

Return 0;

}

Gcc Hello. cpp

./A. out

 

Add User useradd xiaoming

Add passwd xiaoming

I/o redirection

Output> overwrite output. May the content be overwritten> append output

Input <

2>: redirect error output

&> Redirect all outputs

Weak shell programming language

Strong: variables must be declared or even initialized before they can be used.

Weak: variables are declared, directly used, or even non-differentiated.

'Strong references do not replace variables

"" Weak reference for variable replacement

Variable VALUE assignment: VAR_NAME = VALUE

 

Variable type:

Environment Variable

Local variable)

Location variable

Special Variables

Local variable:

NAME = Jerry (entire bash process)

Echo $ NAME echo display characters

Variable reference: $ {}{} can be omitted sometimes.

Local variables:

Local VARNAME = VALUE (current code segment)

Environment variable: the scope is the current shell process and its sub-processes.

Export VARNAME = VALUE "export"

Starting any script in the command line will start a shell Process

The script automatically executed by the system (not started by the command line) requires self-defined environment variables

Location variable:

$1 $2...

Special Data

$? : Return Value of the execution status of the previous command

Program execution may have two types of return values.

Program Execution result

Code returned from the program status (0-255)

0: Correct

1-255: execution error 127, system reserved

/Dev/null software device bit bucket, data black hole

Undo variable

Unset VARNAME

View the current shell variable:

Set

View the environment variables in the Current shell

Printenv env export

Add string variable

Script: the source program implemented by the command flow control mechanism based on actual needs.

Shebang: Magic number

#! /Bin/bash

# Comment row, not executed

Nano fisrt. sh write script

Programming Language: machine language, assembly language, advanced language

Static language: compiled language strong type (variable) is converted into executable format in advance

Dynamic Language: interpreted and executed on the fly

Asp. Php, python Shell

Variable: memory space, name

Memory: the storage unit.

Process:

1 + 100

Variable type: Determine the data storage format and length in advance

Character

Value

Integer

Floating Point: 11.23 1.123*10 ^ 1,

Logic: 1 + 1> 2

Logical operation: And, or, not, exclusive or

Bash: script interpreter

Programming capability:

Script Programming

Condition judgment

If the user does not exist

Add a user to the password and the user is successfully added

Otherwise

Show if not, not added

How to implement conditional judgment in Bash

Condition judgment type:

Integer Test

Character Test

File Test

Conditional test expression:

[Expression]

[[Expression] keywords

Test expression

 

Integer comparison (binocular operation ):

-Eq: test whether two integers are equal: $ A-eq $ B

 

Variable name: it can only contain letters, numbers, and underscores. It cannot start with a number.

It should not be the same as the existing environment variable in the system

It is best to be knowledgeable

-Ne: test whether two integers are unequal: unequal, true; equal, false;

-Gt: test whether one number is greater than the other. If the value is greater than true, otherwise the value is false.

-Lt: test whether one number is smaller than the other.

-Ge: greater than or equal

-Le: less than or equal

 

The logical relationship between commands:

Logic and: & when the first condition is false, the second condition does not need to be judged. The final result already exists. When the first condition is true, the second condition must be judged.

Logic or: |

 

Condition judgment and control structure

Single branch if statement

If judgment condition; then

Statement1

Statrment2

Fi

If statement with two branches;

If judgment condition; then

Statement1

Statement2

Else

Statement3

Statement4

Fi

How to perform arithmetic operations in shell

A = 3

B = 6

Let C = $ A + $ B

2. $ [arithmetic expression]

C = $ [$ A + $ B]

3. $ (arithmetic expression ))

C =$ ($ A + $ B ))

4. expr arithmetic expression. There must be spaces between the operands and operators in the expression, and commands must be used to reference

C = 'expr $ A + $ B'