Shell 脚本中常用的循环结构有哪些？如何使用 for、while 和 until 循环？ - 面试题

Shell 脚本中常用的循环结构包括 for 循环、while 循环和 until 循环。

for 循环

基本语法

bash
for variable in list
do
    commands
done

遍历列表

bash
# 遍历字符串列表
for fruit in apple banana orange
do
    echo "Fruit: $fruit"
done

# 遍历数字序列
for i in 1 2 3 4 5
do
    echo "Number: $i"
done

# 使用 seq 命令生成序列
for i in $(seq 1 10)
do
    echo "Number: $i"
done

# 使用 {start..end} 语法（Bash 3.0+）
for i in {1..10}
do
    echo "Number: $i"
done

# 指定步长
for i in {0..10..2}
do
    echo "Number: $i"
done

C 风格 for 循环

bash
# C 风格语法
for (( i=0; i<10; i++ ))
do
    echo "Number: $i"
done

# 多变量
for (( i=0, j=10; i<10; i++, j-- ))
do
    echo "i=$i, j=$j"
done

遍历文件和目录

bash
# 遍历当前目录下的文件
for file in *.txt
do
    echo "Processing: $file"
done

# 遍历命令输出
for user in $(cut -d: -f1 /etc/passwd)
do
    echo "User: $user"
done

# 递归遍历目录
for file in $(find . -name "*.sh")
do
    echo "Shell script: $file"
done

while 循环

基本语法

bash
while condition
do
    commands
done

while 循环示例

bash
# 计数器
count=0
while [ $count -lt 5 ]
do
    echo "Count: $count"
    count=$((count + 1))
done

# 读取文件行
while IFS= read -r line
do
    echo "Line: $line"
done < input.txt

# 读取用户输入
while true
do
    read -p "Enter 'quit' to exit: " input
    if [ "$input" = "quit" ]; then
        break
    fi
    echo "You entered: $input"
done

# 等待进程结束
while ps -p $PID > /dev/null
do
    echo "Process is running..."
    sleep 1
done
echo "Process finished"

until 循环

基本语法

bash
until condition
do
    commands
done

until 循环示例

bash
# 等待条件成立
count=0
until [ $count -ge 5 ]
do
    echo "Count: $count"
    count=$((count + 1))
done

# 等待服务启动
until curl -s http://localhost:8080 > /dev/null
do
    echo "Waiting for service..."
    sleep 2
done
echo "Service is ready"

循环控制语句

break 语句

bash
# 跳出循环
for i in {1..10}
do
    if [ $i -eq 5 ]; then
        break
    fi
    echo "Number: $i"
done
# 输出: 1 2 3 4

continue 语句

bash
# 跳过当前迭代
for i in {1..10}
do
    if [ $((i % 2)) -eq 0 ]; then
        continue
    fi
    echo "Odd number: $i"
done
# 输出: 1 3 5 7 9

break 和 continue 指定层级

bash
# 跳出多层循环
for i in {1..3}
do
    for j in {1..3}
    do
        if [ $i -eq 2 ] && [ $j -eq 2 ]; then
            break 2
        fi
        echo "i=$i, j=$j"
    done
done

实际应用示例

bash
#!/bin/bash

# 示例 1: 批量处理文件
echo "Processing text files..."
for file in *.txt
do
    if [ -f "$file" ]; then
        echo "Processing: $file"
        # 在这里添加处理逻辑
        cp "$file" "backup_$file"
    fi
done

# 示例 2: 监控进程
echo "Monitoring process..."
while true
do
    if ps aux | grep -q "[m]yprocess"; then
        echo "Process is running"
    else
        echo "Process stopped"
        break
    fi
    sleep 5
done

# 示例 3: 等待多个服务
services=("service1" "service2" "service3")
for service in "${services[@]}"
do
    echo "Waiting for $service..."
    until systemctl is-active --quiet "$service"
    do
        sleep 2
    done
    echo "$service is ready"
done

# 示例 4: 读取配置文件
echo "Reading configuration..."
while IFS='=' read -r key value
do
    # 跳过注释和空行
    [[ $key =~ ^#.*$ ]] && continue
    [[ -z $key ]] && continue
    
    echo "Config: $key = $value"
done < config.txt

# 示例 5: 并行处理
max_parallel=3
count=0
for task in task1 task2 task3 task4 task5
do
    # 启动后台任务
    echo "Starting $task..."
    (
        sleep 2
        echo "$task completed"
    ) &
    
    count=$((count + 1))
    
    # 控制并发数
    if [ $((count % max_parallel)) -eq 0 ]; then
        wait
    fi
done
wait
echo "All tasks completed"

循环最佳实践

使用引号包裹变量: 防止空格和特殊字符问题
设置 IFS 处理特殊分隔符: IFS= read -r line
使用 break 和 continue 控制循环: 提高代码可读性
避免无限循环: 确保 while 和 until 循环有退出条件
使用 wait 等待后台进程: 确保所有任务完成
优先使用 C 风格 for 循环: 处理数字序列时更清晰
使用数组遍历: for item in "${array[@]}"