From d5da86cecf37e2b02a847e5c9d1eafea94c2078e Mon Sep 17 00:00:00 2001 From: geekpi Date: Thu, 3 Aug 2023 08:40:50 +0800 Subject: [PATCH] translated --- ...⭐️ Bash Basics Series 7 If Else Statement.md | 214 ----------------- ...⭐️ Bash Basics Series 7 If Else Statement.md | 217 ++++++++++++++++++ 2 files changed, 217 insertions(+), 214 deletions(-) delete mode 100644 sources/tech/20230724.0 ⭐️⭐️ Bash Basics Series 7 If Else Statement.md create mode 100644 translated/tech/20230724.0 ⭐️⭐️ Bash Basics Series 7 If Else Statement.md diff --git a/sources/tech/20230724.0 ⭐️⭐️ Bash Basics Series 7 If Else Statement.md b/sources/tech/20230724.0 ⭐️⭐️ Bash Basics Series 7 If Else Statement.md deleted file mode 100644 index f5ab0c78d8..0000000000 --- a/sources/tech/20230724.0 ⭐️⭐️ Bash Basics Series 7 If Else Statement.md +++ /dev/null @@ -1,214 +0,0 @@ -[#]: subject: "Bash Basics Series #7: If Else Statement" -[#]: via: "https://itsfoss.com/bash-if-else/" -[#]: author: "Abhishek Prakash https://itsfoss.com/author/abhishek/" -[#]: collector: "lkxed" -[#]: translator: "geekpi" -[#]: reviewer: " " -[#]: publisher: " " -[#]: url: " " - -Bash Basics Series #7: If Else Statement -====== - -Bash supports if-else statements so that you can use logical reasoning in your shell scripts. - -The generic if-else syntax is like this: - -``` -if [ expression ]; then - - ## execute this block if condition is true else go to next - -elif [ expression ]; then - - ## execute this block if condition is true else go to next - -else - - ## if none of the above conditions are true, execute this block - -fi -``` - -As you can notice: - -- `elif` is used for "else if" kind of condition -- The if else conditions always end with `fi` -- the use of semicolon `;` and `then` keyword - -Before I show the examples of if and else-if, let me share common comparison expressions (also called test conditions) first. - -### Test conditions - -Here are the test condition operators you can use for numeric comparison: - -| Condition | Equivalent to true when | -| :- | :- | -| $a -lt $b | $a < $b ($a is **l**ess **t**han $b) | -| $a -gt $b | $a > $b ($a is **g**reater **t**han $b) | -| $a -le $b | $a <= $b ($a is **l**ess or **e**qual than $b) | -| $a -ge $b | $a >= $b ($a is **g**reater or **e**qual than $b) | -| $a -eq $b | $a is equal to $b | -| $a -ne $b | $a is not equal to $b | - -If you are comparing strings, you can use these test conditions: - -| Condition | Equivalent to true when | -| :- | :- | -| "$a" = "$b" | $a is same as $b | -| "$a" == "$b" | $a is same as $b | -| "$a" != "$b" | $a is different from $b | -| -z "$a" | $a is empty | - -There are also conditions for file type check: - -| Condition | Equivalent to true when | -| :- | :- | -| -f $a | $a is a file | -| -d $a | $a is a directory | -| -L $a | $a is a link | - -Now that you are aware of the various comparison expressions let's see them in action in various examples. - -### Use if statement in bash - -Let's create a script that tells you if a given number is even or not. - -Here's my script named `even.sh`: - -``` -#!/bin/bash - -read -p "Enter the number: " num - -mod=$(($num%2)) - -if [ $mod -eq 0 ]; then - echo "Number $num is even" -fi -``` - -The modulus operation (%) returns zero when it is perfectly divided by the given number (2 in this case). - -> 🚧 Pay special attention to space. There must be space between the opening and closing brackets and the conditions. Similarly, space must be before and after the conditional operators (-le, == etc). - -Here's what it shows when I run the script: - -![Running a script with if statement example in bash][1] - -Did you notice that the script tells you when a number is even but it doesn't display anything when the number is odd? Let's improve this script with the use of else. - -### Use if else statement - -Now I add an else statement in the previous script. This way when you get a non-zero modulus (as odd numbers are not divided by 2), it will enter the else block. - -``` -#!/bin/bash - -read -p "Enter the number: " num - -mod=$(($num%2)) - -if [ $mod -eq 0 ]; then - echo "Number $num is even" -else - echo "Number $num is odd" -fi -``` - -Let's run it again with the same numbers: - -![Running a bash script that checks odd even number][2] - -As you can see, the script is better as it also tells you if the number is odd. - -### Use elif (else if) statement - -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. - -``` -#!/bin/bash - -read -p "Enter the number: " num - -if [ $num -lt 0 ]; then - echo "Number $num is negative" -elif [ $num -gt 0 ]; then - echo "Number $num is positive" -else - echo "Number $num is zero" -fi -``` - -Let me run it to cover all three cases here: - -![Running a script with bash elif statement][3] - -### Combine multiple conditions with logical operators - -So far, so good. But do you know that you may have multiple conditions in a single by using logical operators like AND (&&), OR (||) etc? It gives you the ability to write complex conditions. - -Let's write a script that tells you whether the given year is a leap year or not. - -Do you remember the conditions for being a leap year? It should be divided by 4 but if it is divisible by 100, it's not a leap year. However, if it is divisible by 400, it is a leap year. - -Here's my script. - -``` -#!/bin/bash - -read -p "Enter the year: " year - -if [[ ($(($year%4)) -eq 0 && $(($year%100)) != 0) || ($(($year%400)) -eq 0) ]]; then - echo "Year $year is leap year" -else - echo "Year $year is normal year" -fi -``` - -> 💡 Notice the use of double brackets [[ ]] above. It is mandatory if you are using logical operators. - -Verify the script by running it with different data: - -![Example of running bash script with logical operators in if statement][4] - -### 🏋️ Exercise time - -Let's do some workout :) - -**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'. - -**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. - -**Hint**: Use -f for file - -**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. - -**Hint**: Use -f, -d and -L - -**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. - -**Hint**: Refer to the previous chapter on [bash strings][5] - -You may discuss your solution in the Community: - -I hope you are enjoying the Bash Basics Series. In the next chapter, you'll learn about using loops in Bash. Keep on bashing! - --------------------------------------------------------------------------------- - -via: https://itsfoss.com/bash-if-else/ - -作者：[Abhishek Prakash][a] -选题：[lkxed][b] -译者：[译者ID](https://github.com/译者ID) -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/ diff --git a/translated/tech/20230724.0 ⭐️⭐️ Bash Basics Series 7 If Else Statement.md b/translated/tech/20230724.0 ⭐️⭐️ Bash Basics Series 7 If Else Statement.md new file mode 100644 index 0000000000..f40cff2174 --- /dev/null +++ b/translated/tech/20230724.0 ⭐️⭐️ Bash Basics Series 7 If Else Statement.md @@ -0,0 +1,217 @@ +[#]: subject: "Bash Basics Series #7: If Else Statement" +[#]: via: "https://itsfoss.com/bash-if-else/" +[#]: author: "Abhishek Prakash https://itsfoss.com/author/abhishek/" +[#]: collector: "lkxed" +[#]: translator: "geekpi" +[#]: reviewer: " " +[#]: publisher: " " +[#]: url: " " + +Bash 基础知识系列 #7：If Else 语句 +====== + +Bash 支持 if-else 语句，以便你可以在 shell 脚本中使用逻辑推理。 + +通用的 if-else 语法如下： + +``` +if [ expression ]; then + + ## 如果条件为真则执行此块，否则转到下一个 + +elif [ expression ]; then + + ## 如果条件为真则执行此块，否则转到下一个 + +else + + ## 如果以上条件都不成立，则执行此块 + +fi +``` + +正如你所注意到的： + +- `elif` 用于 “else if” 类型的条件。 +- if else 条件始终以 `fi` 结尾。 +- 使用分号 `;`和 `then`关键字 + +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）。 + +### 测试条件 + +以下是可用于数字比较的测试条件运算符： + +| 条件 | 当满足以下条件时为 true | +| :-| :-| +| $a -lt $b | $a < $b（$a 是 **小于** $b）| +| $a -gt $b | $a > $b（$a 是 **大于** $b）| +| $a -le $b | $a <= $b（$a **小于等于** $b ）| +| $a -ge $b | $a >= $b （$a **大于等于** $b） +| $a -eq $b | $a 等于 $b | +| $a -ne $b | $a 不等于 $b | + +如果你要比较字符串，可以使用以下测试条件： + +| 条件 | 当满足以下条件时为 true | +| :- | :- | +| “$a”=“$b”| $a 与 $b 相同 | +| “$a”==“$b”| $a 与 $b 相同 | +| “$a”！=“$b”| $a 与 $b 不同 | +| -z“$a”| $a 为空 | + +文件类型检查也有条件： + +| 条件 | 当满足以下条件时为 true | +| :- | :- | +| -f $a | $a 是一个文件 | +| -d $a | $a 是一个目录 | +| -L $a | $a 是一个链接 | + +现在你已经了解了各种比较表达式，让我们在各种示例中看看它们的实际应用。 + +### 在 bash 中使用 if 语句 + +让我们创建一个脚本来告诉你给定的数字是否为偶数。 + +这是我的脚本，名为 `even.sh`： + +``` +#!/bin/bash + +read -p "Enter the number: " num + +mod=$(($num%2)) + +if [ $mod -eq 0 ]; then + echo "Number $num is even" +fi +``` + +当模数运算 (%) 整除给定数字（本例中为 2）时，它返回零。 + +> 🚧 特别注意空格。左括号和右括号与条件之间必须有空格。同样，条件运算符（-le、== 等）前后必须有空格。 + +这是我运行脚本时显示的内容： + +![Running a script with if statement example in bash][1] + +你是否注意到，当数字为偶数时，脚本会告诉你，但当数字为奇数时，脚本不会显示任何内容？ 让我们使用 else 来改进这个脚本。 + +### 使用 if else 语句 + +现在我在前面的脚本中添加了一条 else 语句。这样，当你得到一个非零模数（因为奇数不能除以 2）时，它将进入 else 块。 + +``` +#!/bin/bash + +read -p "Enter the number: " num + +mod=$(($num%2)) + +if [ $mod -eq 0 ]; then + echo "Number $num is even" +else + echo "Number $num is odd" +fi +``` + +让我们用相同的数字再次运行它： + +![Running a bash script that checks odd even number][2] + +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。 + +### 使用 elif（else if）语句 + +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。 + +``` +#!/bin/bash + +read -p "Enter the number: " num + +if [ $num -lt 0 ]; then + echo "Number $num is negative" +elif [ $num -gt 0 ]; then + echo "Number $num is positive" +else + echo "Number $num is zero" +fi +``` + +让我运行它来涵盖这里的所有三种情况： + +![Running a script with bash elif statement][3] + +### 用逻辑运算符组合多个条件 + +到目前为止，一切都很好。但是你是否知道通过使用 AND (&&)、OR (||) 等逻辑运算符可以在一个条件中包含多个条件？ 它使你能够编写复杂的条件。 + +让我们编写一个脚本来告诉你给定的年份是否是闰年。 + +你还记得闰年的条件吗？ 它应该被 4 整除，但如果它能被 100 整除，那么它就不是闰年。但是，如果能被 400 整除，则为闰年。 + +这是我的脚本。 + +``` +#!/bin/bash + +read -p "Enter the year: " year + +if [[ ($(($year%4)) -eq 0 && $(($year%100)) != 0) || ($(($year%400)) -eq 0) ]]; then + echo "Year $year is leap year" +else + echo "Year $year is normal year" +fi +``` + +> 💡 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。 + +通过使用不同的数据运行脚本来验证脚本： + +![Example of running bash script with logical operators in if statement][4] + +### 🏋️ 练习时间 + +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”。 + +**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