后端

if [ 条件 ]; then
    # 条件为真时执行的命令
fi

if [ 条件 ]; then
    # 条件为真时执行的命令
else
    # 条件为假时执行的命令
fi

if [ 条件1 ]; then
    # 条件1为真时执行的命令
elif [ 条件2 ]; then
    # 条件2为真时执行的命令
else
    # 所有条件都为假时执行的命令
fi

if (( 条件 )); then
    # 条件为真时执行的命令
fi

if [[ 条件 ]]; then
    # 条件为真时执行的命令
fi

case "变量" in
    值1)
        # 匹配值1时执行的命令
        ;;
    值2)
        # 匹配值2时执行的命令
        ;;
    *)
        # 匹配所有其他值时执行的命令
        ;;
esac

for 变量 in 值1 值2 值3...; do
    # 循环体命令
    echo $变量
done

for 文件 in *.txt; do
    # 处理每个txt文件
    echo $文件
done

for ((i=1; i<=10; i++)); do
    # 循环体命令
    echo $i
done

while [ 条件 ]; do
    # 循环体命令
done

until [ 条件 ]; do
    # 循环体命令（条件为假时执行）
done

for i in {1..10}; do
    if [ $i -eq 5 ]; then
        break  # 跳出循环
    fi
    echo $i
done

for i in {1..10}; do
    if [ $((i % 2)) -eq 0 ]; then
        continue  # 跳过当前循环，进入下一次循环
    fi
    echo $i
done

#!/bin/bash

# 算术运算符示例
a=10
b=5

echo "a = $a, b = $b"
echo "a + b = $((a + b))"
echo "a - b = $((a - b))"
echo "a * b = $((a * b))"
echo "a / b = $((a / b))"
echo "a % b = $((a % b))"
echo "a ** b = $((a ** b))"

# 自增自减
((a++))
echo "a++ = $a"
((b--))
echo "b-- = $b"

#!/bin/bash

# 条件判断示例
read -p "Enter a number: " num

if [ $num -gt 0 ]; then
    echo "Positive number"
elif [ $num -lt 0 ]; then
    echo "Negative number"
else
    echo "Zero"
fi

# 字符串比较
read -p "Enter yes or no: " answer

if [[ "$answer" == "yes" ]]; then
    echo "You entered yes"
elif [[ "$answer" == "no" ]]; then
    echo "You entered no"
else
    echo "Invalid input"
fi

# 文件测试
read -p "Enter a filename: " filename

if [ -e "$filename" ]; then
    if [ -f "$filename" ]; then
        echo "$filename is a file"
    elif [ -d "$filename" ]; then
        echo "$filename is a directory"
    fi
else
    echo "$filename does not exist"
fi

#!/bin/bash

# for循环示例
echo "For loop with list:"
for fruit in apple banana cherry; do
    echo "I like $fruit"
done

echo "\nFor loop with numbers:"
for ((i=1; i<=5; i++)); do
    echo "Number: $i"
done

# while循环示例
echo "\nWhile loop:"
counter=1
while [ $counter -le 5 ]; do
    echo "Counter: $counter"
    ((counter++))
done

# until循环示例
echo "\nUntil loop:"
counter=1
until [ $counter -gt 5 ]; do
    echo "Counter: $counter"
    ((counter++))
done

# case语句示例
echo "\nCase statement:"
read -p "Enter a day (1-7): " day

case $day in
    1)
        echo "Monday"
        ;;
    2)
        echo "Tuesday"
        ;;
    3)
        echo "Wednesday"
        ;;
    4)
        echo "Thursday"
        ;;
    5)
        echo "Friday"
        ;;
    6)
        echo "Saturday"
        ;;
    7)
        echo "Sunday"
        ;;
    *)
        echo "Invalid day"
        ;;
esac

#!/bin/bash

# break示例
echo "Break example:"
for i in {1..10}; do
    if [ $i -eq 6 ]; then
        echo "Breaking at $i"
        break
    fi
    echo "$i"
done

# continue示例
echo "\nContinue example:"
for i in {1..10}; do
    if [ $((i % 2)) -eq 0 ]; then
        continue
    fi
    echo "Odd number: $i"
done