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Translated:20140730 How to use variables in shell Scripting.md
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How to use variables in shell Scripting
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================================================================================
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In every **programming** language **variables** plays an important role , in Linux shell scripting we are using two types of variables : **System Defined Variables** & **User Defined Variables**.
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A variable in a shell script is a means of **referencing** a **numeric** or **character value**. And unlike formal programming languages, a shell script doesn't require you to **declare a type** for your variables
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In this article we will discuss variables, its types and how to set & use variables in shell scripting.
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### System Defined Variables : ###
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These are the variables which are created and maintained by **Operating System(Linux) itself**. Generally these variables are defined in **CAPITAL LETTERS**. We can see these variables by using the command "**$ set**". Some of the system defined variables are given below :
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<table width="100%" cellspacing="1" cellpadding="1">
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<tbody>
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<tr>
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<td><strong> System Defined Variables </strong></td>
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<td><strong> Meaning </strong></td>
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</tr>
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<tr>
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<td> BASH=/bin/bash </td>
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<td> Shell Name </td>
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</tr>
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<tr>
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<td> BASH_VERSION=4.1.2(1) </td>
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<td> Bash Version </td>
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</tr>
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<tr>
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<td> COLUMNS=80 </td>
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<td> No. of columns for our screen </td>
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</tr>
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<tr>
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<td> HOME=/home/linuxtechi </td>
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<td> Home Directory of the User </td>
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</tr>
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<tr>
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<td> LINES=25 </td>
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<td> No. of columns for our screen </td>
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</tr>
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<tr>
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<td> LOGNAME=LinuxTechi </td>
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<td> LinuxTechi Our logging name </td>
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</tr>
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<tr>
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<td> OSTYPE=Linux </td>
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<td> OS type </td>
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</tr>
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<tr>
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<td> PATH=/usr/bin:/sbin:/bin:/usr/sbin </td>
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<td> Path Settings </td>
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</tr>
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<tr>
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<td> PS1=[\u@\h \W]\$ </td>
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<td> Prompt Settings </td>
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</tr>
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<tr>
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<td> PWD=/home/linuxtechi </td>
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<td> Current Working Directory </td>
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</tr>
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<tr>
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<td> SHELL=/bin/bash </td>
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<td> Shell Name </td>
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</tr>
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<tr>
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<td> USERNAME=linuxtechi </td>
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<td> User name who is currently login to system </td>
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</tr>
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</tbody>
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</table>
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To Print the value of above variables, use **echo command** as shown below :
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# echo $HOME
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# echo $USERNAME
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We can tap into these environment variables from within your scripts by using the environment variable's name preceded by a dollar sign. This is demonstrated in the following script:
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$ cat myscript
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#!/bin/bash
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# display user information from the system.
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echo “User info for userid: $USER”
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echo UID: $UID
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echo HOME: $HOME
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Notice that the **environment variables** in the echo commands are replaced by their current values when the script is run. Also notice that we were able to place the **$USER** system variable within the double quotation marks in the first string, and the shell script was still able to figure out what we meant. There is a **drawback** to using this method, however. Look at what happens in this example:
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$ echo “The cost of the item is $15”
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The cost of the item is 5
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That is obviously not what was intended. Whenever the script sees a dollar sign within quotes, it assumes you're referencing a variable. In this example the script attempted to display the **variable $1** (which was not defined), and then the number 5. To display an actual dollar sign, you **must precede** it with a **backslash character**:
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$ echo “The cost of the item is \$15”
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The cost of the item is $15
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That's better. The backslash allowed the shell script to interpret the **dollar sign** as an actual dollar sign, and not a variable.
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### User Defined Variables: ###
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These variables are defined by **users**. A shell script allows us to set and use our **own variables** within the script. Setting variables allows you to **temporarily store data** and use it throughout the script, making the shell script more like a real computer program.
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**User variables** can be any text string of up to **20 letters, digits**, or **an underscore character**. User variables are case sensitive, so the variable Var1 is different from the variable var1. This little rule often gets novice script programmers in trouble.
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Values are assigned to user variables using an **equal sign**. No spaces can appear between the variable, the equal sign, and the value (another trouble spot for novices). Here are a few examples of assigning values to user variables:
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var1=10
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var2=-57
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var3=testing
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var4=“still more testing”
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The shell script **automatically determines the data type** used for the variable value. Variables defined within the shell script maintain their values throughout the life of the shell script but are deleted when the shell script completes.
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Just like system variables, user variables can be referenced using the dollar sign:
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$ cat test3
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#!/bin/bash
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# testing variables
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days=10
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guest="Katie"
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echo "$guest checked in $days days ago"
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days=5
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guest="Jessica"
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echo "$guest checked in $days days ago"
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$
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Running the script produces the following output:
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$ chmod u+x test3
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$ ./test3
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Katie checked in 10 days ago
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Jessica checked in 5 days ago
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$
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Each time the variable is **referenced**, it produces the value currently assigned to it. It's important to remember that when referencing a variable value you use the **dollar sign**, but when referencing the variable to assign a value to it, you do not use the dollar sign. Here's an example of what I mean:
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$ cat test4
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#!/bin/bash
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# assigning a variable value to another variable
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value1=10
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value2=$value1
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echo The resulting value is $value2
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$
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When you use the **value** of the **value1** variable in the assignment statement, you must still use the dollar sign. This code produces the following output:
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$ chmod u+x test4
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$ ./test4
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The resulting value is 10
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$
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If you forget the dollar sign, and make the value2 assignment line look like:
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value2=value1
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you get the following output:
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$ ./test4
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The resulting value is value1
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$
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Without the dollar sign the **shell interprets** the variable name as a **normal text string**, which is most likely not what you wanted.
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### Use of Backtick symbol (`) in shell variables : ###
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The **backtick allows** you to assign the output of a shell command to a variable. While this doesn't seem like much, it is a major building block in **script programming**.You must surround the entire command line command with backtick characters:
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**testing=`date`**
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The shell runs the command within the **backticks** and assigns the output to the variable testing. Here's an example of creating a variable using the output from a normal shell command:
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$ cat test5
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#!/bin/bash
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# using the backtick character
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testing=`date`
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echo "The date and time are: " $testing
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$
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The variable testing receives the output from the date command, and it is used in the echo statement to display it. Running the shell script produces the following output:
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$ chmod u+x test5
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$ ./test5
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The date and time are: Mon Jan 31 20:23:25 EDT 2011
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**Note** : In bash you can also use the alternative $(…) syntax in place of backtick (`),which has the advantage of being re-entrant.
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Example :
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$ echo " Today’s date & time is :" $(date)
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Today’s date & time is : Sun Jul 27 16:26:56 IST 2014
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--------------------------------------------------------------------------------
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via: http://www.linuxtechi.com/variables-in-shell-scripting/
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作者：[Pradeep Kumar][a]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
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[a]:http://www.linuxtechi.com/author/pradeep/
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@ -0,0 +1,200 @@
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如何在shell脚本中使用变量
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================================================================================
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在每种**编程**语言中，**变量**都扮演了一个重要的角色。在Linux shell脚本编程中，我们使用两种类型的变量：**系统定义的变量**和**用户定义的变量**。
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shell脚本中的变量是用来**调用**一个**数值**或者**字符值**的手段。与正规的编程语言不同的是，shell脚本不要求你去为变量**声明一个类型**。
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在本文中，我们将讨论shell脚本编程中的变量及其类型，以及如何设置和使用这些变量。
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### 系统定义的变量： ###
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这些变量由**操作系统（Linux）自身**创建并维护，通常它们以**大写字母**定义，我们可以通过命令“**$ set**”来查看这些变量。下面列出了部分系统定义的变量：
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<table width="100%" cellspacing="1" cellpadding="1">
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<tbody>
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<tr>
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<td><strong> System Defined Variables </strong></td>
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<td><strong> Meaning </strong></td>
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</tr>
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<tr>
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<td> BASH=/bin/bash </td>
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<td> Shell Name </td>
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</tr>
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<tr>
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<td> BASH_VERSION=4.1.2(1) </td>
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<td> Bash Version </td>
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</tr>
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<tr>
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<td> COLUMNS=80 </td>
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<td> No. of columns for our screen </td>
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</tr>
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<tr>
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<td> HOME=/home/linuxtechi </td>
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<td> Home Directory of the User </td>
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</tr>
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<tr>
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<td> LINES=25 </td>
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<td> No. of columns for our screen </td>
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</tr>
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<tr>
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<td> LOGNAME=LinuxTechi </td>
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<td> LinuxTechi Our logging name </td>
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</tr>
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<tr>
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<td> OSTYPE=Linux </td>
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<td> OS type </td>
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</tr>
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<tr>
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<td> PATH=/usr/bin:/sbin:/bin:/usr/sbin </td>
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<td> Path Settings </td>
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</tr>
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<tr>
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<td> PS1=[\u@\h \W]\$ </td>
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<td> Prompt Settings </td>
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</tr>
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<tr>
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<td> PWD=/home/linuxtechi </td>
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<td> Current Working Directory </td>
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</tr>
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<tr>
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<td> SHELL=/bin/bash </td>
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<td> Shell Name </td>
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</tr>
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<tr>
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<td> USERNAME=linuxtechi </td>
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<td> User name who is currently login to system </td>
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</tr>
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</tbody>
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</table>
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要打印以上变量的值，可以使用**echo command**命令，如下：
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# echo $HOME
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# echo $USERNAME
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我们可以通过在环境变量名前前置一个美元符号来从你的脚本里输入环境变量。请看下面脚本演示：
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$ cat myscript
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#!/bin/bash
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# display user information from the system.
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echo “User info for userid: $USER”
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echo UID: $UID
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echo HOME: $HOME
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注意：echo命令中的**环境变量**在脚本运行时会被它们的值替代。同时注意，我们可以再第一个字符串的双引号中放置**$USER**系统变量，而shell脚本仍然可以明白我们的意思。然而，该方法有一个**缺点**。看下面这个例子：
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$ echo “The cost of the item is $15”
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The cost of the item is 5
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很明显，那不是我们说希望的。无论何时，当脚本遇见引号中的美元符号时，它都会认为你是在调用一个变量。在本例中，改脚本试着显示**变量$1**（而这个变量并没有定义），然后显示数字5。要显示实际上的美元符号，你**必须前置**一个**反斜线字符**：
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$ echo “The cost of the item is \$15”
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The cost of the item is $15
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那样好多了。反斜线允许shell脚本将**美元符号**解释成为实际的美元符号，而不是变量。
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### 用户定义的变量： ###
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这些变量由**用户**定义。shell脚本允许我们在脚本中设置并使用我们**自己的变量**。设置变量允许你**临时存储数据**并在脚本中使用，让shell脚本看起来像一个真正的计算机程序。
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**用户变量**可以是任何不超过**20个字母，数字**的文本字符串，或者**一个下划线字符**。用户变量是大小写敏感的，因此，变量Var1和变量var1是不同的变量。这个小规则常常让新手编写脚本时麻烦重重。
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我们可以通过**等于号**为变量赋值。变量，等于号和值（对于新手又是个麻烦的地方）之间不能有空格。下面是几个给用户变量赋值的例子：
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var1=10
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var2=-57
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var3=testing
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var4=“still more testing”
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shell脚本为变量值**自动确定数据类型**。shell脚本内定义的变量会在脚本运行时保留它们的值，当脚本完成后则删除这些值。
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就像系统变量一样，用户变量也可以使用美元符号来调用：
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$ cat test3
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#!/bin/bash
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# testing variables
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days=10
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guest="Katie"
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echo "$guest checked in $days days ago"
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days=5
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guest="Jessica"
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echo "$guest checked in $days days ago"
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$
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运行脚本会产生以下输出：
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$ chmod u+x test3
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$ ./test3
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Katie checked in 10 days ago
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Jessica checked in 5 days ago
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$
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每次变量被**调用**，它都会产生当前分配给它的值。记住这一点很重要，当调用一个变量值时，你使用**美元符号**，但是当调用一个变量来为其分配一个值时，你不能用美元符号。下面用例子来说明：
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$ cat test4
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#!/bin/bash
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# assigning a variable value to another variable
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value1=10
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value2=$value1
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echo The resulting value is $value2
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$
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当你使用赋值语句中**value1**变量的**值**时，你仍然必须使用美元符号。这段代码产生了如下输出：
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$ chmod u+x test4
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$ ./test4
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The resulting value is 10
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$
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如果你忘了美元符号，而又让value2赋值行看起来像这样：
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value2=value1
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你会获得下面的输出：
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$ ./test4
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The resulting value is value1
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$
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没有美元符号，**shell解释**变量名为**普通文本字符串**，这极有可能不是你想要的。
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### 在shell变量中使用反引号（`）： ###
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**反引号允许**你将shell命令的输出赋值给变量。虽然这似乎没什么大不了，但它是**脚本编程**中主要的构建基块。你必须使用反引号将整个命令行包含起来：
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**testing=`date`**
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shell会在**反引号**中运行命令，然后将输出结果赋值给变量testing。下面的例子给出了如何使用一个常规shell命令的输出结果来创建一个变量：
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$ cat test5
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#!/bin/bash
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# using the backtick character
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testing=`date`
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echo "The date and time are: " $testing
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$
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变量testing接收来自date命令的输出结果，而它又在echo语句中被调用。运行脚本会产生如下输出：
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$ chmod u+x test5
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$ ./test5
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The date and time are: Mon Jan 31 20:23:25 EDT 2011
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**注**：在bash中，你也可以选用$(...)语法来替换反引号(`)，它有个优点就是可以重用。
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例：
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$ echo " Today’s date & time is :" $(date)
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Today’s date & time is : Sun Jul 27 16:26:56 IST 2014
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--------------------------------------------------------------------------------
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via: http://www.linuxtechi.com/variables-in-shell-scripting/
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作者：[Pradeep Kumar][a]
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译者：[GOLinux](https://github.com/GOLinux)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
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||||
[a]:http://www.linuxtechi.com/author/pradeep/
|
||||
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