Shell Script—Conditional Statements

1.if statement

In Shell scripts, it is a common requirement to determine whether a condition is true. In Bash, you can use the following statement to implement conditional judgment:

1.1 if

ifThe statement ifis the keyword of the conditional judgment statement, and its grammatical structure is:

if [ condition ]
then
    # do something if condition is true
fi

[ condition ]is a conditional expression, usually combined using comparison operators ( <, >, <=, >=, =, !=) and logical operators ( &&, ). ||Then, between thenand fiis a piece of code to be executed when the condition is true, which can be a command or a multi-line statement block.

For example, here is a sample script that prints a message when the value of an integer variable numis greater than 10:

#!/bin/bash

num=15

if [ $num -gt 10 ]
then
    echo "num value is $num"
fi

output

num value is 15

1.2 if-else

if-elseStatements can be used to perform some actions if a condition is true and other actions if the condition is not true. The grammatical structure is:

if [ condition ]
then
    # do something if condition is true
else
    # do something if condition is false
fi

For example, here is a sample script that, depending on numthe value of an integer, determines its parity and prints a message:

#!/bin/bash

num=7

if [ $((num % 2)) -eq 0 ]
then
  echo "The number is even"
else
  echo "The number is odd"
fi

output

The number is odd

1.3 if-elif-else

if-elif-elseStatements can be used to select a condition to operate on among multiple conditions. The grammatical structure is:

if [ condition1 ]
then
    # do something if condition1 is true
elif [ condition2 ]
then
    # do something if condition2 is true
else
    # do something if all conditions are false
fi

For example, the following is a sample script that determines numwhich interval the value of an integer variable is in, and then prints the corresponding message:

#!/bin/bash

num=25

if [ $num -lt 20 ]
then
    echo "The number is less than 20"
elif [ $num -ge 20 ] && [ $num -lt 30 ]
then
    echo "The number is between 20 and 29"
else
    echo "The number is greater than or equal to 30"
fi

output

The number is between 20 and 29

In short, in Bash, you can use if, if-else, if-elif-elseand other statements to make conditional judgments, and select corresponding statements to write according to different application requirements.

2. switch statement

switchStatements, also called statements, in shell scripts casecan match different blocks of code based on one or more values. Following is switchthe basic syntax of the statement:

case expression in
pattern1)
    # 匹配 pattern1 时执行的代码
    ;;
pattern2|pattern3)
    # 匹配 pattern2 或 pattern3 时执行的代码
    ;;
*)
    # 如果所有模式都不匹配时执行的代码
    ;;
esac

caseThe execution of the statement is:

1. caseThe statement expressiontests each pattern one by one against the value of pattern.
2. If the pattern expressionmatches, the block of code associated with it is executed, and other patterns and code are skipped.
3. If no pattern is matched, *the block of code in the pattern is executed.
4. Two semicolons are required after each code block to stop any matching process of the pattern.

Here is an caseexample of a usage statement:

#!/bin/bash
echo "input string"
read str

case $str in
    "apple")
        echo "苹果"
        ;;
    "banana")
        echo "香蕉"
        ;;
    "cherry"||"cranberry")
        echo "樱桃"
        ;;
    *)  # 匹配除了上面所列所有情况外的任何字符
        echo "无法识别的水果"
        ;;
esac

echo -e "\n"
echo "input number"
read num

case $num in
	1)
		echo "number 1"
		;;
	2)
		echo "number 2"
		;;
	3)
		echo "number 3"
		;;
	*)
		echo "other number"
		;;
esac

The above example will prompt the user to enter a string, and then match the code segment associated with the entered string.

In short, switcha statement is a flow control statement used in Shell scripts to match different code blocks based on one or more values, which can be used to simplify code and improve readability.