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[#]: subject: "Bash Basics Series #7: If Else Statement"
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[#]: via: "https://itsfoss.com/bash-if-else/"
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[#]: author: "Abhishek Prakash https://itsfoss.com/author/abhishek/"
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[#]: collector: "lkxed"
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[#]: translator: "geekpi"
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[#]: reviewer: " "
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[#]: publisher: " "
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[#]: url: " "
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Bash Basics Series #7: If Else Statement
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======
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Bash supports if-else statements so that you can use logical reasoning in your shell scripts.
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The generic if-else syntax is like this:
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```
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if [ expression ]; then
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## execute this block if condition is true else go to next
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elif [ expression ]; then
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## execute this block if condition is true else go to next
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else
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## if none of the above conditions are true, execute this block
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fi
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```
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As you can notice:
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- `elif` is used for "else if" kind of condition
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- The if else conditions always end with `fi`
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- the use of semicolon `;` and `then` keyword
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Before I show the examples of if and else-if, let me share common comparison expressions (also called test conditions) first.
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### Test conditions
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Here are the test condition operators you can use for numeric comparison:
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| Condition | Equivalent to true when |
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| :- | :- |
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| $a -lt $b | $a < $b ($a is **l**ess **t**han $b) |
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| $a -gt $b | $a > $b ($a is **g**reater **t**han $b) |
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| $a -le $b | $a <= $b ($a is **l**ess or **e**qual than $b) |
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| $a -ge $b | $a >= $b ($a is **g**reater or **e**qual than $b) |
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| $a -eq $b | $a is equal to $b |
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| $a -ne $b | $a is not equal to $b |
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If you are comparing strings, you can use these test conditions:
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| Condition | Equivalent to true when |
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| :- | :- |
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| "$a" = "$b" | $a is same as $b |
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| "$a" == "$b" | $a is same as $b |
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| "$a" != "$b" | $a is different from $b |
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| -z "$a" | $a is empty |
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There are also conditions for file type check:
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| Condition | Equivalent to true when |
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| :- | :- |
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| -f $a | $a is a file |
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| -d $a | $a is a directory |
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| -L $a | $a is a link |
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Now that you are aware of the various comparison expressions let's see them in action in various examples.
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### Use if statement in bash
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Let's create a script that tells you if a given number is even or not.
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Here's my script named `even.sh`:
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```
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#!/bin/bash
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read -p "Enter the number: " num
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mod=$(($num%2))
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if [ $mod -eq 0 ]; then
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echo "Number $num is even"
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fi
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```
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The modulus operation (%) returns zero when it is perfectly divided by the given number (2 in this case).
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> 🚧 Pay special attention to space. There must be space between the opening and closing brackets and the conditions. Similarly, space must be before and after the conditional operators (-le, == etc).
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Here's what it shows when I run the script:
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![Running a script with if statement example in bash][1]
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Did you notice that the script tells you when a number is even but it doesn't display anything when the number is odd? Let's improve this script with the use of else.
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### Use if else statement
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Now I add an else statement in the previous script. This way when you get a non-zero modulus (as odd numbers are not divided by 2), it will enter the else block.
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```
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#!/bin/bash
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read -p "Enter the number: " num
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mod=$(($num%2))
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if [ $mod -eq 0 ]; then
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echo "Number $num is even"
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else
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echo "Number $num is odd"
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fi
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```
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Let's run it again with the same numbers:
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![Running a bash script that checks odd even number][2]
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As you can see, the script is better as it also tells you if the number is odd.
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### Use elif (else if) statement
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Here's a script that checks whether the given number is positive or negative. In mathematics, 0 is neither positive nor negative. This script keeps that fact in check as well.
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```
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#!/bin/bash
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read -p "Enter the number: " num
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if [ $num -lt 0 ]; then
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echo "Number $num is negative"
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elif [ $num -gt 0 ]; then
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echo "Number $num is positive"
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else
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echo "Number $num is zero"
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fi
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```
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Let me run it to cover all three cases here:
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![Running a script with bash elif statement][3]
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### Combine multiple conditions with logical operators
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So far, so good. But do you know that you may have multiple conditions in a single by using logical operators like AND (&&), OR (||) etc? It gives you the ability to write complex conditions.
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Let's write a script that tells you whether the given year is a leap year or not.
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Do you remember the conditions for being a leap year? It should be divided by 4 but if it is divisible by 100, it's not a leap year. However, if it is divisible by 400, it is a leap year.
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Here's my script.
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```
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#!/bin/bash
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read -p "Enter the year: " year
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if [[ ($(($year%4)) -eq 0 && $(($year%100)) != 0) || ($(($year%400)) -eq 0) ]]; then
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echo "Year $year is leap year"
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else
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echo "Year $year is normal year"
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fi
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```
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> 💡 Notice the use of double brackets [[ ]] above. It is mandatory if you are using logical operators.
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Verify the script by running it with different data:
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![Example of running bash script with logical operators in if statement][4]
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### 🏋️ Exercise time
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Let's do some workout :)
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**Exercise 1**: Write a bash shell script that checks the length of the string provided to it as an argument. If no argument is provided, it prints 'empty string'.
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**Exercise 2**: Write a shell script that checks whether a given file exists or not. You can provide the full file path as the argument or use it directly in the script.
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**Hint**: Use -f for file
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**Exercise 3**: Enhance the previous script by checking if the given file is regular file, a directory or a link or if it doesn't exist.
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**Hint**: Use -f, -d and -L
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**Exercise 3**: Write a script that accepts two string arguments. The script should check if the first string contains the second argument as a substring.
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**Hint**: Refer to the previous chapter on [bash strings][5]
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You may discuss your solution in the Community:
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I hope you are enjoying the Bash Basics Series. In the next chapter, you'll learn about using loops in Bash. Keep on bashing!
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--------------------------------------------------------------------------------
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via: https://itsfoss.com/bash-if-else/
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作者：[Abhishek Prakash][a]
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选题：[lkxed][b]
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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) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://itsfoss.com/author/abhishek/
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[b]: https://github.com/lkxed/
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[1]: https://itsfoss.com/content/images/2023/07/bash-if-example.png
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[2]: https://itsfoss.com/content/images/2023/07/bash-if-else-example.png
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[3]: https://itsfoss.com/content/images/2023/07/bash-elif.png
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[4]: https://itsfoss.com/content/images/2023/07/bash-logical-operators-in-if-else.png
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[5]: https://itsfoss.com/bash-strings/
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@ -0,0 +1,217 @@
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[#]: subject: "Bash Basics Series #7: If Else Statement"
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[#]: via: "https://itsfoss.com/bash-if-else/"
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[#]: author: "Abhishek Prakash https://itsfoss.com/author/abhishek/"
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[#]: collector: "lkxed"
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[#]: translator: "geekpi"
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[#]: reviewer: " "
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[#]: publisher: " "
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[#]: url: " "
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Bash 基础知识系列 #7：If Else 语句
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======
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Bash 支持 if-else 语句，以便你可以在 shell 脚本中使用逻辑推理。
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通用的 if-else 语法如下：
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```
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if [ expression ]; then
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## 如果条件为真则执行此块，否则转到下一个
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elif [ expression ]; then
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## 如果条件为真则执行此块，否则转到下一个
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else
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## 如果以上条件都不成立，则执行此块
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fi
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```
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正如你所注意到的：
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- `elif` 用于 “else if” 类型的条件。
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- if else 条件始终以 `fi` 结尾。
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- 使用分号 `;`和 `then`关键字
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在展示 if 和 else-if 的示例之前，我先分享一下常见的比较表达式（也称为测试条件）。
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### 测试条件
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以下是可用于数字比较的测试条件运算符：
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| 条件 | 当满足以下条件时为 true |
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| :-| :-|
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| $a -lt $b | $a < $b（$a 是 **小于** $b）|
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| $a -gt $b | $a > $b（$a 是 **大于** $b）|
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| $a -le $b | $a <= $b（$a **小于等于** $b ）|
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| $a -ge $b | $a >= $b （$a **大于等于** $b）
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| $a -eq $b | $a 等于 $b |
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| $a -ne $b | $a 不等于 $b |
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如果你要比较字符串，可以使用以下测试条件：
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| 条件 | 当满足以下条件时为 true |
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| :- | :- |
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| “$a”=“$b”| $a 与 $b 相同 |
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| “$a”==“$b”| $a 与 $b 相同 |
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| “$a”！=“$b”| $a 与 $b 不同 |
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| -z“$a”| $a 为空 |
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文件类型检查也有条件：
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| 条件 | 当满足以下条件时为 true |
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| :- | :- |
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| -f $a | $a 是一个文件 |
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| -d $a | $a 是一个目录 |
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| -L $a | $a 是一个链接 |
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现在你已经了解了各种比较表达式，让我们在各种示例中看看它们的实际应用。
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### 在 bash 中使用 if 语句
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让我们创建一个脚本来告诉你给定的数字是否为偶数。
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这是我的脚本，名为 `even.sh`：
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```
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#!/bin/bash
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read -p "Enter the number: " num
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mod=$(($num%2))
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if [ $mod -eq 0 ]; then
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echo "Number $num is even"
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fi
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```
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当模数运算 (%) 整除给定数字（本例中为 2）时，它返回零。
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> 🚧 特别注意空格。左括号和右括号与条件之间必须有空格。同样，条件运算符（-le、== 等）前后必须有空格。
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这是我运行脚本时显示的内容：
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![Running a script with if statement example in bash][1]
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你是否注意到，当数字为偶数时，脚本会告诉你，但当数字为奇数时，脚本不会显示任何内容？ 让我们使用 else 来改进这个脚本。
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### 使用 if else 语句
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现在我在前面的脚本中添加了一条 else 语句。这样，当你得到一个非零模数（因为奇数不能除以 2）时，它将进入 else 块。
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```
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#!/bin/bash
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read -p "Enter the number: " num
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mod=$(($num%2))
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if [ $mod -eq 0 ]; then
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echo "Number $num is even"
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else
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echo "Number $num is odd"
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fi
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```
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让我们用相同的数字再次运行它：
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![Running a bash script that checks odd even number][2]
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正如你所看到的，该脚本更好，因为它还告诉你该数字是否为奇数。
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### 使用 elif（else if）语句
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这是一个检查给定数字是正数还是负数的脚本。在数学中，0 既不是正数也不是负数。该脚本也检查了这一事实。
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```
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#!/bin/bash
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read -p "Enter the number: " num
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if [ $num -lt 0 ]; then
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echo "Number $num is negative"
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elif [ $num -gt 0 ]; then
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echo "Number $num is positive"
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else
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echo "Number $num is zero"
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fi
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```
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让我运行它来涵盖这里的所有三种情况：
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![Running a script with bash elif statement][3]
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### 用逻辑运算符组合多个条件
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到目前为止，一切都很好。但是你是否知道通过使用 AND (&&)、OR (||) 等逻辑运算符可以在一个条件中包含多个条件？ 它使你能够编写复杂的条件。
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让我们编写一个脚本来告诉你给定的年份是否是闰年。
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你还记得闰年的条件吗？ 它应该被 4 整除，但如果它能被 100 整除，那么它就不是闰年。但是，如果能被 400 整除，则为闰年。
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这是我的脚本。
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```
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#!/bin/bash
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read -p "Enter the year: " year
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if [[ ($(($year%4)) -eq 0 && $(($year%100)) != 0) || ($(($year%400)) -eq 0) ]]; then
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echo "Year $year is leap year"
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else
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echo "Year $year is normal year"
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fi
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```
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> 💡 注意上面双括号 [[ ]] 的使用。如果你使用逻辑运算符，则这是强制性的。
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通过使用不同的数据运行脚本来验证脚本：
|
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|
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![Example of running bash script with logical operators in if statement][4]
|
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|
||||
### 🏋️ 练习时间
|
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|
||||
让我们做一些练习吧 :)
|
||||
|
||||
**练习 1**：编写一个 bash shell 脚本，检查作为参数提供给它的字符串的长度。如果未提供参数，它将打印 “empty string”。
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|
||||
**练习 2**：编写一个 shell 脚本来检查给定文件是否存在。你可以提供完整的文件路径作为参数或直接在脚本中使用它。
|
||||
|
||||
**提示**：文件使用 -f 选项
|
||||
|
||||
**练习 3**：通过检查给定文件是否是常规文件、目录或链接或者是否不存在来增强之前的脚本。
|
||||
|
||||
**提示**：使用 -f、-d 和 -L
|
||||
|
||||
**练习 3**：编写一个接受两个字符串参数的脚本。脚本应检查第一个字符串是否包含第二个参数的子串。
|
||||
|
||||
**提示**：请参阅上一章 [bash 字符串][5]
|
||||
|
||||
你可以在社区中讨论你的解决方案：
|
||||
|
||||
[Bash 基础系列练习 #7：If Else 语句][6]
|
||||
|
||||
我希望你喜欢 Bash 基础知识系列。在下一章中，你将了解如何在 Bash 中使用循环。继续编写 bash！
|
||||
|
||||
--------------------------------------------------------------------------------
|
||||
|
||||
via: https://itsfoss.com/bash-if-else/
|
||||
|
||||
作者：[Abhishek Prakash][a]
|
||||
选题：[lkxed][b]
|
||||
译者：[geekpi](https://github.com/geekpi)
|
||||
校对：[校对者ID](https://github.com/校对者ID)
|
||||
|
||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
|
||||
|
||||
[a]: https://itsfoss.com/author/abhishek/
|
||||
[b]: https://github.com/lkxed/
|
||||
[1]: https://itsfoss.com/content/images/2023/07/bash-if-example.png
|
||||
[2]: https://itsfoss.com/content/images/2023/07/bash-if-else-example.png
|
||||
[3]: https://itsfoss.com/content/images/2023/07/bash-elif.png
|
||||
[4]: https://itsfoss.com/content/images/2023/07/bash-logical-operators-in-if-else.png
|
||||
[5]: https://itsfoss.com/bash-strings/
|
||||
[6]: https://itsfoss.community/t/practice-exercise-in-bash-basics-series-7-if-else-statements/10926?ref=itsfoss.com
|
||||
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Block a user