Merge pull request #29811 from lkxed/20230724-0-Bash-Basics-Series-7-If-Else-Statement

[手动选题][tech]: 20230724.0 ⭐️⭐️ Bash Basics Series 7 If Else Statement.md

sources/tech/20230724.0 ⭐️⭐️ Bash Basics Series 7 If Else Statement.md

@@ -0,0 +1,214 @@
+[#]: 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: " "
+[#]: 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/