Linux Basic Variables Learning: Local variables, environment variables, special variables, positional variables

Shell: weakly typed programming languages

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: the scope is the entire bash process;  

Local variables:

Local Varname=value : The 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 code (0-255)

0: Execute correctly

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

Output redirection:

> Output override Redirection

>> Output Append redirect

2> error output overwrite redirect

2>> error output append redirect

&> correct error output redirection

Undo Variable:

unset VARNAME varname is the variable name

To view variables in the shell:

Set VARNAME but set can omit VARNAME is the variable name

View the Environment Variables command in the current shell as follows:

Printenv

Env

Export

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

shebang: magic number

#!/bin/bash

# comment lines, do not execute

/dev/null: software devices, bit buckets, data black holes      

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 (non-command-line startup) require a self-defined environment variable;

This article is from the "Learn Linux history" blog, please be sure to keep this source http://woyaoxuelinux.blog.51cto.com/5663865/1863045

Linux Basic Variables Learning: Local variables, environment variables, special variables, positional variables