Shell 编程基础（下）：条件、循环、函数

#!/bin/bash

# test 命令
if test -f file.txt; then
    echo "File exists"
fi

# [ ] 是 test 的简写（注意：括号内必须有空格！）
if [ -f file.txt ]; then
    echo "File exists"
fi

# [[ ]] 是 [ ] 的增强版（推荐，支持正则）
if [[ -f file.txt ]]; then
    echo "File exists"
fi

#!/bin/bash

# 基本 if
if [ -f file.txt ]; then
    echo "File exists"
fi

# if-else
if [ -f file.txt ]; then
    echo "File exists"
else
    echo "File does not exist"
fi

# if-elif-else
if [ $score -ge 90 ]; then
    echo "A"
elif [ $score -ge 80 ]; then
    echo "B"
elif [ $score -ge 70 ]; then
    echo "C"
elif [ $score -ge 60 ]; then
    echo "D"
else
    echo "F"
fi

# 多条件组合（-a = and, -o = or）
if [ -f file.txt -a -r file.txt ]; then
    echo "File exists and is readable"
fi

# [[ ]] 支持 && 和 ||
if [[ -f file.txt && -r file.txt ]]; then
    echo "File exists and is readable"
fi

# 命令返回值判断
if grep -q "error" log.txt; then
    echo "Found error in log"
fi

#!/bin/bash

case $1 in
    start)
        echo "Starting service..."
        ;;
    stop)
        echo "Stopping service..."
        ;;
    restart)
        echo "Restarting service..."
        ;;
    status)
        echo "Checking status..."
        ;;
    *)
        echo "Usage: $0 {start|stop|restart|status}"
        exit 1
        ;;
esac

#!/bin/bash

# 遍历列表
for i in 1 2 3 4 5; do
    echo "Number: $i"
done

# 遍历范围（Bash 4.0+）
for i in {1..5}; do
    echo "Number: $i"
done

# 带步长
for i in {1..10..2}; do
    echo "Number: $i"  # 1, 3, 5, 7, 9
done

# C 风格
for ((i=0; i<5; i++)); do
    echo "Index: $i"
done

# 遍历文件
for file in *.txt; do
    echo "Processing: $file"
done

# 遍历命令输出
for user in $(cat /etc/passwd | awk -F: '{print $1}'); do
    echo "User: $user"
done

# 遍历数组
fruits=("apple" "banana" "cherry")
for fruit in "${fruits[@]}"; do
    echo "Fruit: $fruit"
done

#!/bin/bash

# 基本 while
i=1
while [ $i -le 5 ]; do
    echo "Count: $i"
    i=$((i + 1))
done

# 读取文件（逐行）
while read line; do
    echo "Line: $line"
done < file.txt

# 读取文件（带行号）
line_num=0
while read line; do
    line_num=$((line_num + 1))
    echo "$line_num: $line"
done < file.txt

# 无限循环（配合 break）
while true; do
    read -p "Enter command (q to quit): " cmd
    if [ "$cmd" = "q" ]; then
        break
    fi
    echo "Executing: $cmd"
done

# 读取用户输入直到空行
while read -p "Enter line (empty to finish): " line && [ -n "$line" ]; do
    echo "You entered: $line"
done

#!/bin/bash

# until 条件为真时停止（与 while 相反）
i=1
until [ $i -gt 5 ]; do
    echo "Count: $i"
    i=$((i + 1))
done

#!/bin/bash

# 定义函数
hello() {
    echo "Hello, World!"
}

# 带参数的函数
greet() {
    echo "Hello, $1!"
    echo "You are $2 years old."
}

# 带返回值的函数（只能返回 0-255 的整数）
add() {
    return $(($1 + $2))
}

# 使用 echo 返回字符串结果
get_date() {
    echo "$(date +%Y-%m-%d)"
}

# 调用函数
hello
greet "Alice" 25

add 3 5
result=$?    # 获取返回值（$? 是上一个命令的退出状态）
echo "3 + 5 = $result"

# 获取 echo 返回的结果
today=$(get_date)
echo "Today is $today"

#!/bin/bash

# 局部变量（local）
counter() {
    local count=0    # 局部变量，不影响外部
    count=$((count + 1))
    echo "Local count: $count"
}

count=100
counter
echo "Global count: $count"    # 仍然是 100

# 检查参数个数
process_file() {
    if [ $# -ne 1 ]; then
        echo "Usage: process_file <filename>"
        return 1
    fi
    
    if [ ! -f "$1" ]; then
        echo "Error: File '$1' not found"
        return 2
    fi
    
    echo "Processing: $1"
    wc -l "$1"
}

process_file file.txt

#!/bin/bash

# backup.sh - 自动备份脚本

# 配置
SOURCE_DIR="$HOME/Documents"
BACKUP_DIR="$HOME/Backups"
DATE=$(date +%Y%m%d_%H%M%S)
BACKUP_FILE="backup_${DATE}.tar.gz"
RETENTION_DAYS=7

# 检查源目录
if [ ! -d "$SOURCE_DIR" ]; then
    echo "Error: Source directory '$SOURCE_DIR' not found"
    exit 1
fi

# 创建备份目录
if [ ! -d "$BACKUP_DIR" ]; then
    mkdir -p "$BACKUP_DIR"
    echo "Created backup directory: $BACKUP_DIR"
fi

# 执行备份
echo "Starting backup..."
echo "Source: $SOURCE_DIR"
echo "Destination: $BACKUP_DIR/$BACKUP_FILE"

if tar -czf "$BACKUP_DIR/$BACKUP_FILE" -C "$SOURCE_DIR" .; then
    echo "Backup completed: $BACKUP_FILE"
    ls -lh "$BACKUP_DIR/$BACKUP_FILE"
else
    echo "Backup failed!"
    exit 1
fi

# 清理旧备份
echo "Cleaning up backups older than $RETENTION_DAYS days..."
find "$BACKUP_DIR" -name "backup_*.tar.gz" -mtime +$RETENTION_DAYS -delete
echo "Cleanup completed"

echo "All done!"