Bash Script programming (1/4) variables, variable types, and other notes

Shell Programming:

Compilers, interpreters

Programming languages: Machine language, assembly language, advanced languages

Static language: Compiled language

Strongly typed (variable)

Convert into executable format in advance

c\c++\java\c#

Dynamic Language: Explanatory language

Weak type

Edge Interpretation Side Execution

Asp/asp.net/php/shell/python/perl

Shell: Weakly typed programming language

Strongly typed: The variable must be declared beforehand before it is used, and even need to be initialized:

Weak types: Variable time declarations, or even non-differentiated types:

Process oriented: Shell C

Object-oriented: JAVA Python, Perl, C + +

Variables: Memory space, naming

Memory: Addressable storage unit

Process:

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

Character

Numerical

Plastic

Floating point Type

　

Programming Ability:

Script programming

Variable assignment: Var_name=value

Bash Variable type:

Environment variables

Local variables (local variables)

Positional variables

Special variables

Local variables:

Var_name=value: Scope for the entire script process:

Local variables:

Local Varname=value scope is the current code snippet

Environment variable: scope is the current shell process and its child processes

Export Varname=value

Export

Two different ways:

1. Export Varname=value

2. Varname=value

Export VARNAME

Positional variables

$, $, ...

Special variables:

$? The execution status return value of the previous command:

Program execution Results

Program Status return codes (0-255)

0: Correct execution

1-255: Error execution, 1,2,127 system reserved

Output redirection:

>

>>

2>

2>>

&> redirect standard output or error output to the same file

/dev/null: Software device, bit bucket data black hole

Order execution, there may be two types of return values:

When the script executes, a child shell process is started:

Script that is started on the command line inherits the current shell environment variable

Scripts that are automatically executed by the system (not command-line startup) require a self-defined environment variable

Undo Variable:

Unset VARNAME (never add $)

View variables in the current shell

Set (including environment variables and local variables)

To view environment variables in the current shell:

Printenv

Env

Export

Script: Command stack, according to the actual needs, combined with the command flow control mechanism realization of the source program

Shebang: Magic number

#!/bin/bash

Exercise: Write a script to complete the following tasks

1. Add 5 users, User1,user2,... user5

2. Each user's password is the same as the user name, and requires that the execution result information of the passwd command not be displayed after the Add password is completed

3. After each user is added, it is shown that the user XX has been successfully added

Exercise: Write a script to complete the following tasks

1. Use a variable to save a user name:

2. Delete the user from this variable and delete its home directory

3. Display the "User Delete complete" class information

This article is from the "Richier" blog, make sure to keep this source http://richier.blog.51cto.com/1447532/1631396

Bash Script programming (1/4) variables, variable types, and other notes