Linux Learning note--bash scripting one variable, variable type

Bash scripting one variable, variable type

Shell Programming:

Compilers, interpreters

Programming languages: Machine language, assembly language, advanced languages

Static language: Compiled language

Strongly typed (variable) language

convert into executable format in advance

C,C + +,JAVA,C #

Dynamic Language: interpreted language On the fiy

Weak type

Edge interpretation Side Execution

ASP,asp, PHP, Shell,Python,Perl

process oriented:shell,C

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

Variables: Memory space, naming

Memory: Compiled storage unit

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

character

value

integral type:

floating point type:

Boolean type: True, False

Logical operations: with, or, not (!) , XOR, or

Shell: weakly typed programming languages

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

Empty ( NULL)

Weak type: variable time declaration, not even type;

variable Assignment:

Bash Variable Type:

Environment type:

local variable: (local variable)

positional variables:

Special variables (System variables):

Local variables:

Varname=value : scoped to entire bash process;

Local variables:

Localvarname=valuw : The scope is the current code snippet;

environment variable: scope is current shell process and its child processes

Setexport Varname=value :

"Export"

The script starts a child shell process when it executes ;

scripts that are started on the command line inherit the current shell environment variable

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

Bash :

Positional variables:

$1,$2,...

Special variables:

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

There are two possible types of return values for program execution:

Program execution Results

Program Execution status return code (255 )

0 : Execute correctly

(1-255): Error execution,1,2,127 system reserved;

Output redirection:

: Overwrite output

>>: Append output

2>: Redirect Error output

2>>: Append method

&>: simultaneous redirection (redirected error output)

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

undo Variable:

Unset VARNAME}

View current shell The variables in:

Set

View current shell Environment Variables in:

Printenv

Env

Export

Script: The stuffing of the command, as the actual need, combined with

shebang : Magic number

# ! /bin/bash

# Comment lines , do not execute

Variable reference:${varname}, parentheses can sometimes be omitted

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Linux Learning note--bash scripting one variable, variable type