[9-13] Shell Series 2--Variable Base

This article is intended to review shell variable definitions, use variables, delete variables, read-only variables, variable types, special variable instances

Knowledge Reserve

• The variable is essentially a specified area in the memory space, and its data content will change;

• The variable name essence is a pointer to the memory space of this data change, then in the command execution or operation, only need to pass the variable name, can call that change of data at any time, very convenient

• The variables in the shell script, like most programming languages, have global and local [for example, a function function]

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• It is worth mentioning that the shell is a weakly typed programming language

  • Weak type: variable without prior declaration, directly available [null]

  • Strongly typed: Before using variables, you must specify the variable name and store the format [data type] before you can use

Defining variables and redefining

• Definition Format: Var_name=value

• There can be no spaces between the variable name and the equals sign

• The variable name follows this format ^[_a-za-z] middle section [_a-za-z0-9]*
  

• Variable names cannot use punctuation, keywords

• In a bash environment, you can use set to define variables, but in shell scripts, set commands have special uses!

Example: Define a variable and call

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#!/bin/bash var1= "I like" _var2= "computer." echo  "$var 1 $_var2"

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Some people on the internet say that the shell script variable name cannot begin with an underscore, this example illustrates the possible!

Redefining A variable is no different in nature from defining a variable, it only covers it, does not demonstrate

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#!/bin/bash var= "1 Yuan qian!" var="Ten Yuan qian!" Echo $var

Using variables

• Direct use ($Var _name), such as:

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Var="I like Fruit" echo "$var"

• You can also use ${var_name}, which works the same, but sometimes you can only use this usage!

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Var="I like Fruit" echo "${var}s."

• When you want to add a string directly to a variable, you need to use {}, otherwise the variable name error will be resolved. It is recommended to use this usage!

Delete a variable

It's a lot easier to delete variables.

• Unset var_name

Read-only variables

Some variables once defined do not want to be changed, as read only, can be modified with Read only

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#!/bin/bash mylove="mageedu" ReadOnly Mylove mylove="Niaoge" echo "$mylove"

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Variable type

When you run the shell, there are three different variables:

Local Variables

A local variable is defined in a script or command and is valid only in the current shell instance.

Environment variables

All programs, including shell-initiated programs, can access environment variables, and some programs require environment variables to keep them running properly. Shell scripts can also define environment variables when necessary.

Shell variables

Shell variables are special variables that are set by the shell program. Some of the shell variables are environment variables, some of which are local variables that guarantee the shell's normal operation.

Special variable instances and delivery

The list of special variables is listed in the [9-13]shell series of basic concepts article, where a demo

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#!/bin/bash echo  "shell script filename ---> $0" echo  "current process pid   ---> $$" Span style= "color: #000000;" > echo  "the number of parameter -> $#" echo  " First parameter        ---> $1 " echo  echo  " last command  state    ---> $? "

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This example briefly illustrates the use of special parameters, where 1 2 3 4 5 is passed as the script executes, and I'll talk about

There are 2 command results: Command execution Results and command execution status results

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Reference all passed arguments can be in $* and [email protected], or a bit different

When not enclosed by double quotation marks (""), all are "$" and "$" ... All parameters are output in the form "$n".

But when they are enclosed in double quotation marks (""), "$*" takes all the parameters as a whole and outputs all parameters in the form of "$ $ ... $n"; "[email protected]" separates the various parameters to "$" "$" ... All parameters are output in the form "$n".

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Implementation code

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#!/bin/bash Echo"\[email protected] like this--" forvarinch "[email protected]" Do Echo"$var" Done Echo"\$* " forvarinch "$*" Do Echo"$var" Done

This article is from the "Blue Warehouse" blog, be sure to keep this source http://bluebox.blog.51cto.com/8852456/1696771

[9-13] Shell Series 2--Variable Base