Linux-shell Programming (I.)

Shell Programming Script 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: Interpreted language, on the fly

Weak type

Edge Interpretation Side Execution

PHP, SHELL, Python, Perl

Process oriented: Shell, C

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

Variables: Memory space, naming

Memory: Addressable storage unit

Process:

1 + 100:

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

Character

Numerical

Integral type

Float type: 11.23 1.123*10^1, 0.1123*10^2

Shell: Weakly typed programming language

Strong: Variables must be declared beforehand and even initialized before they are used;

Weak: variable time declaration, even the type is not differentiated;

Variable assignment: Var_name = value

Bash Variable type:

Environment variables

Local variables (local variables)

Positional variables

Special variables

Local variables:

Set VARNAME = VALUE: scope for the entire bash 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

Varname=value

Export VARNAME

Export

Positional variables:

$,...

Special variables

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

Program execution. There may be two types of return values:

Program execution Results

Program Status return codes (0-255)

0: Correct execution

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

The script starts a child shell process when it executes

Scripts that are started on the command line inherit the environment variables of the current shell

Scripts that are automatically executed by the system (not the command line) need to self-define the required environment variables

Reference variable

${varname}: Parentheses can be omitted

Output redirection:

>

>>

2>

&>

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

Linux-shell Programming (I.)