Shell 循环详解

#!/bin/bash

# for循环示例

echo "=== 基本for循环 ==="
# 遍历单词列表
for fruit in apple banana orange grape; do
echo "水果: $fruit"
done

echo -e "\n=== 遍历数组 ==="
fruits=("apple" "banana" "orange" "grape")
for fruit in "${fruits[@]}"; do
echo "数组元素: $fruit"
done

echo -e "\n=== 遍历数字序列 ==="
# 方法1: 使用序列表达式
for i in {1..5}; do
echo "数字: $i"
done

# 方法2: 使用seq命令
for i in $(seq 1 5); do
echo "seq数字: $i"
done

# 方法3: 指定步长
for i in {1..10..2}; do
echo "步长为2: $i"
done

echo -e "\n=== 遍历文件 ==="
# 遍历当前目录的所有.sh文件
for file in *.sh; do
if [ -f "$file" ]; then
    echo "Shell脚本: $file"
    # 可以在这里处理文件
fi
done

echo -e "\n=== 遍历命令输出 ==="
# 遍历ls命令的输出
for item in $(ls /var/log/ | head -5); do
echo "日志文件/目录: $item"
done

echo -e "\n=== C风格for循环 ==="
# C风格for循环
for ((i=1; i<=5; i++)); do
echo "C风格循环: $i"
done

# 复杂的C风格循环
for ((i=0, j=10; i<=j; i++, j--)); do
echo "i=$i, j=$j"
done

echo -e "\n=== 遍历参数 ==="
# 遍历所有命令行参数
echo "参数个数: $#"
for arg in "$@"; do
echo "参数: $arg"
done

# 使用$*（不推荐，可能有问题）
for arg in $*; do
echo "参数(\$*): $arg"
done

echo -e "\n=== 遍历带空格的字符串 ==="
# 处理包含空格的字符串
names="Alice Bob Charlie"
# 错误的方式（会被空格分割）
echo "错误方式:"
for name in $names; do
echo "名字: $name"
done

# 正确的方式（使用数组）
echo -e "\n正确方式:"
name_array=("Alice" "Bob" "Charlie")
for name in "${name_array[@]}"; do
echo "名字: $name"
done

echo -e "\n=== 遍历目录 ==="
# 遍历目录及其子目录（需要Bash 4.0+）
for file in /var/log/*; do
if [ -d "$file" ]; then
    echo "目录: $file"
elif [ -f "$file" ]; then
    echo "文件: $file"
fi
done

echo -e "\n=== 使用IFS控制分割 ==="
# 修改IFS（内部字段分隔符）
data="apple,banana,orange"
OLD_IFS="$IFS"
IFS=','
for item in $data; do
echo "CSV项: $item"
done
IFS="$OLD_IFS"  # 恢复原来的IFS

echo -e "\n=== 无限for循环 ==="
# 无限循环（需要break退出）
# for ((;;)); do
#     echo "这是一个无限循环"
#     sleep 1
# done

#!/bin/bash

# while循环示例

echo "=== 基本while循环 ==="
counter=1
while [ $counter -le 5 ]; do
echo "计数: $counter"
((counter++))
done

echo -e "\n=== 读取文件行 ==="
# 方法1: 使用read和重定向
filename="example.txt"
# 创建示例文件
echo -e "第一行\n第二行\n第三行" > "$filename"

echo "读取文件内容:"
while read line; do
echo "行内容: $line"
done < "$filename"

echo -e "\n=== 读取命令输出 ==="
# 读取命令的每一行输出
echo "进程信息:"
ps aux | head -5 | while read line; do
echo "进程: $line"
done

echo -e "\n=== 使用管道和子shell ==="
# 注意：在管道中的while循环会在子shell中运行
counter=1
echo -e "a\nb\nc" | while read item; do
echo "项目 $counter: $item"
((counter++))
done
echo "计数器值（子shell中修改不会影响父shell）: $counter"

echo -e "\n=== 无限while循环 ==="
# 无限循环的几种方式

# 方式1: 使用true命令
# while true; do
#     echo "无限循环..."
#     sleep 1
# done

# 方式2: 使用冒号命令（比true更高效）
# while :; do
#     echo "无限循环..."
#     sleep 1
# done

# 方式3: 使用永远为真的条件
# while [ 1 -eq 1 ]; do
#     echo "无限循环..."
#     sleep 1
# done

echo -e "\n=== 使用break退出循环 ==="
counter=1
while [ $counter -le 10 ]; do
if [ $counter -eq 5 ]; then
    echo "在计数5时退出循环"
    break
fi
echo "计数: $counter"
((counter++))
done

echo -e "\n=== 使用continue跳过迭代 ==="
counter=0
while [ $counter -lt 5 ]; do
((counter++))
if [ $counter -eq 3 ]; then
    echo "跳过计数3"
    continue
fi
echo "计数: $counter"
done

echo -e "\n=== 处理用户输入 ==="
# 简单的菜单系统
while true; do
echo "=== 菜单 ==="
echo "1. 显示当前时间"
echo "2. 显示系统信息"
echo "3. 退出"

read -p "请选择 [1-3]: " choice

case $choice in
    1)
        echo "当前时间: $(date)"
        ;;
    2)
        echo "系统信息: $(uname -a)"
        ;;
    3)
        echo "再见!"
        break
        ;;
    *)
        echo "无效选择，请重新输入"
        ;;
esac

echo
done

echo -e "\n=== 监控文件变化 ==="
# 监控文件变化的简单示例
watch_file="watch.txt"
echo "初始内容" > "$watch_file"
last_mtime=$(stat -c %Y "$watch_file")

echo "监控文件 $watch_file 的变化 (Ctrl+C 退出)"
while true; do
current_mtime=$(stat -c %Y "$watch_file" 2>/dev/null)

if [ "$current_mtime" != "$last_mtime" ]; then
    echo "文件已修改: $(date)"
    echo "新内容: $(cat "$watch_file")"
    last_mtime="$current_mtime"
fi

sleep 2
done

# 清理示例文件
rm -f "$filename" "$watch_file"

#!/bin/bash

# until循环示例

echo "=== 基本until循环 ==="
counter=1
until [ $counter -gt 5 ]; do
echo "计数: $counter"
((counter++))
done

echo -e "\n=== 等待条件满足 ==="
# 等待文件创建
target_file="target.txt"
rm -f "$target_file"

echo "等待文件 $target_file 被创建..."
until [ -f "$target_file" ]; do
echo "文件还不存在，等待 1 秒..."
sleep 1
done
echo "文件已创建!"

# 在后台创建文件
(sleep 3; touch "$target_file") &

echo -e "\n=== 等待服务启动 ==="
# 等待端口可用（模拟）
port_available=false
attempt=1
max_attempts=5

until [ "$port_available" = true ] || [ $attempt -gt $max_attempts ]; do
echo "尝试检查端口 (尝试 $attempt/$max_attempts)..."

# 模拟端口检查（随机成功）
if [ $((RANDOM % 3)) -eq 0 ]; then
    port_available=true
    echo "端口现在可用!"
else
    echo "端口还不可用，等待 2 秒..."
    sleep 2
fi

((attempt++))
done

if [ "$port_available" = false ]; then
echo "错误: 在 $max_attempts 次尝试后端口仍然不可用"
fi

echo -e "\n=== 等待用户输入特定值 ==="
# 等待用户输入特定值
valid_input=false
until [ "$valid_input" = true ]; do
read -p "请输入 'yes' 或 'no': " user_input

case $user_input in
    yes|YES|Yes)
        echo "你选择了: yes"
        valid_input=true
        ;;
    no|NO|No)
        echo "你选择了: no"
        valid_input=true
        ;;
    *)
        echo "无效输入，请重新输入"
        ;;
esac
done

echo -e "\n=== 超时控制 ==="
# 带超时的等待
timeout=10
start_time=$(date +%s)
service_ready=false

echo "等待服务准备就绪 (超时: ${timeout}秒)..."
until [ "$service_ready" = true ]; do
current_time=$(date +%s)
elapsed=$((current_time - start_time))

if [ $elapsed -ge $timeout ]; then
    echo "错误: 在 ${timeout} 秒后服务仍未就绪"
    break
fi

# 模拟服务检查
if [ $((RANDOM % 4)) -eq 0 ]; then
    service_ready=true
    echo "服务已就绪! 耗时: ${elapsed}秒"
else
    echo "服务还未就绪，等待 1 秒... (已等待 ${elapsed}秒)"
    sleep 1
fi
done

echo -e "\n=== 处理多个条件 ==="
# 多个条件的until循环
counter=0
max_count=8
target_value=5

until [ $counter -ge $max_count ] || [ $counter -eq $target_value ]; do
echo "计数器: $counter"
((counter++))
done

if [ $counter -eq $target_value ]; then
echo "达到目标值: $target_value"
else
echo "达到最大计数: $max_count"
fi

echo -e "\n=== 与while循环对比 ==="
echo "while循环示例:"
count=1
while [ $count -le 3 ]; do
echo "while: $count"
((count++))
done

echo -e "\nuntil循环示例:"
count=1
until [ $count -gt 3 ]; do
echo "until: $count"
((count++))
done

# 清理
rm -f "$target_file"

#!/bin/bash

# 循环控制示例

echo "=== break基本用法 ==="
for i in {1..10}; do
if [ $i -eq 5 ]; then
    echo "在 i=5 时退出循环"
    break
fi
echo "i = $i"
done
echo "循环结束"

echo -e "\n=== continue基本用法 ==="
for i in {1..5}; do
if [ $i -eq 3 ]; then
    echo "跳过 i=3"
    continue
fi
echo "i = $i"
done

echo -e "\n=== break n (退出多层循环) ==="
# 退出指定层数的循环
for i in {1..3}; do
echo "外层循环 i=$i"
for j in {1..3}; do
    echo "  内层循环 j=$j"
    if [ $j -eq 2 ]; then
        echo "  退出两层循环"
        break 2  # 退出两层循环
    fi
done
done

echo -e "\n=== continue n (跳过多层迭代) ==="
# 跳过指定层数的迭代
for i in {1..3}; do
echo "外层循环 i=$i"
for j in {1..3}; do
    if [ $j -eq 2 ]; then
        echo "  跳过外层循环的剩余迭代"
        continue 2  # 跳过外层循环的当前迭代
    fi
    echo "  内层循环 j=$j"
done
done

echo -e "\n=== 在while循环中使用break ==="
count=1
while true; do  # 无限循环
echo "计数: $count"
if [ $count -eq 5 ]; then
    echo "达到目标计数，退出循环"
    break
fi
((count++))
done

echo -e "\n=== 在until循环中使用continue ==="
count=0
until [ $count -gt 5 ]; do
((count++))
if [ $count -eq 3 ]; then
    echo "跳过计数3"
    continue
fi
echo "当前计数: $count"
done

echo -e "\n=== 实际应用：查找文件 ==="
# 在目录树中查找文件
search_file="target.txt"
found=false

# 创建测试目录结构
mkdir -p test_dir/sub1 test_dir/sub2
echo "content" > "test_dir/sub1/file1.txt"
echo "content" > "test_dir/sub2/$search_file"
echo "content" > "test_dir/file2.txt"

echo "查找文件: $search_file"
for dir in test_dir/*; do
if [ -d "$dir" ]; then
    echo "搜索目录: $dir"
    for file in "$dir"/*; do
        if [ -f "$file" ] && [ "$(basename "$file")" = "$search_file" ]; then
            echo "找到文件: $file"
            found=true
            break 2  # 找到文件，退出两层循环
        fi
    done
fi
done

if [ "$found" = false ]; then
echo "未找到文件: $search_file"
fi

echo -e "\n=== 实际应用：处理数据 ==="
# 处理数据，跳过无效条目
data=("正常数据1" "" "正常数据2" "无效数据" "正常数据3")

echo "处理数据列表:"
for item in "${data[@]}"; do
# 跳过空字符串
if [ -z "$item" ]; then
    echo "跳过空数据"
    continue
fi

# 跳过包含"无效"的数据
if [[ "$item" == *"无效"* ]]; then
    echo "跳过无效数据: $item"
    continue
fi

echo "处理数据: $item"
done

echo -e "\n=== 实际应用：菜单系统 ==="
# 带退出选项的菜单系统
while true; do
echo "=== 系统菜单 ==="
echo "1. 显示磁盘使用情况"
echo "2. 显示内存使用情况"
echo "3. 显示进程信息"
echo "4. 退出系统"

read -p "请选择选项 [1-4]: " choice

case $choice in
    1)
        echo "磁盘使用情况:"
        df -h | head -5
        ;;
    2)
        echo "内存使用情况:"
        free -h
        ;;
    3)
        echo "进程信息 (前5个):"
        ps aux | head -6
        ;;
    4)
        echo "退出系统..."
        break
        ;;
    *)
        echo "无效选项，请重新选择"
        continue  # 跳过本次迭代，重新显示菜单
        ;;
esac

echo
read -p "按回车键继续..."
echo
done

# 清理测试文件
rm -rf test_dir