Shell编程

echo $SHELL          # /bin/bash
cat /etc/shells      # 查看系统已有的shell

vim test.sh

#!/bin/bash
echo "Hello shell"
ls /

chmod +x test.sh
./test.sh

VARIABLE="value"         # 定义全局变量
echo $VARIABLE           # 输出变量值

readonly CONSTANT=42     # 定义常量

#!/bin/bash
FILE_NAME="readme"
FILE_PATH="${HOME}/my_folder/${FILE_NAME}.txt"
echo $FILE_PATH

# 1. 声明整数变量
declare -i num=10

# 2. 声明只读变量
declare -r name="Alice"

# 3. 声明数组变量
declare -a fruits=("Apple" "Banana" "Cherry")

# 4. 声明环境变量
declare -x MY_VAR="Hello"
bash -c 'echo $MY_VAR'     # 输出 Hello

total_files=$(ls | wc -l)
echo "Total files: $total_files"

$(ls | wc -l) 等同于 `ls | wc -l`

# 显示所有环境变量
printenv
env
export

# 查看特定环境变量
printenv PATH
echo $PATH

# 临时生效（仅当前 shell）
export PATH=$PATH:/opt/bin

# 永久生效（当前用户）
echo 'export PATH=$PATH:/opt/bin' >> ~/.bashrc
source ~/.bashrc

# 永久生效（所有用户）
echo 'export PATH=$PATH:/opt/bin' >> /etc/profile
source /etc/profile

# 在 systemd 服务配置中手动指定环境变量
vim /etc/systemd/system/myapp.service
# 添加/修改为类似下面的配置
[Service]
Environment="JAVA_HOME=/usr/lib/jvm/java-11-openjdk"
Environment="PATH=/usr/local/bin:/usr/bin:/bin:$JAVA_HOME/bin"
ExecStart=/usr/bin/java -jar /opt/myapp/app.jar
# 然后重载 systemd 并重启服务
sudo systemctl daemon-reexec && sudo systemctl restart myapp

#!/bin/bash

echo "================ Shell 特殊变量示例 ================"

# 显示脚本名称
echo "\$0（当前脚本名称）: $0"

# 显示传递给脚本的参数
echo "\$1（第一个参数）: $1"
echo "\$2（第二个参数）: $2"
echo "\$3（第三个参数）: $3"
echo "..."

# 显示所有参数
echo "\$*（所有参数，作为一个整体）: $*"
echo "\$@（所有参数，分开显示）: $@"

# 显示参数个数
echo "\$#（参数个数）: $#"

# 显示当前进程 ID
echo "\$$（当前脚本的进程 ID）: $$"

# 启动一个后台任务
sleep 2 &
echo "\$!（最近后台任务的进程 ID）: $!"

# 显示上一个命令的退出状态
ls /nonexistent_dir 2>/dev/null
echo "\$?（上一个命令的返回值）: $?"

echo "================= 结束 ================="

chmod +x test.sh

./test.sh arg1 arg2 arg3

================ Shell 特殊变量示例 ================
$0（当前脚本名称）: ./test.sh
$1（第一个参数）: arg1
$2（第二个参数）: arg2
$3（第三个参数）: arg3
...
$*（所有参数，作为一个整体）: arg1 arg2 arg3
$@（所有参数，分开显示）: arg1 arg2 arg3
$#（参数个数）: 3
$$（当前脚本的进程 ID）: 3229
$!（最近后台任务的进程 ID）: 3230
$?（上一个命令的返回值）: 2
================= 结束 =================

#!/bin/bash

# 读取用户输入
read -p "Enter your name: " name
echo "Hello, $name!"

# 读取密码（静默模式）
read -s -p "Enter your password: " password
echo -e "\nPassword entered."

# 不输出换行符
echo -n "Enter your name: ";read name

# 启用转义字符
echo -e "Hello,\nWorld!"

# 1. (( expr )) 适用于整数运算，不支持浮点数, 
(( sum = a + b ))
# 作为条件语句时，表达式 结果非零为真，0 为假
if (( a > b )); then

# 2. let：和 (( )) 类似
let sum=a+b

# 3. $[]：旧式整数运算（已被 (( )) 替代）
sum=$[a + b]

# 4. expr：POSIX 兼容但较低效
sum=$(expr $a + $b)
# 计算乘法（`*` 号要加引号防止被 Shell 解析）
product=$(expr $a \* $b)

# 5. bc：支持浮点运算，计算需要通过管道 | 传输表达式
echo "3.5 + 2.1" | bc       # 输出：5.6
echo "scale=2; 5 / 3" | bc  # 设置精度 scale
# 计算 π * r² 示例
area=$(echo "scale=4; 3.1415 * 2.5 * 2.5" | bc)
echo "圆的面积: $area"  # 输出: 19.6344

# 6. awk：既支持整数也支持浮点
awk 'BEGIN {print 3.5 + 2.1}'  # 输出：5.6
awk 'BEGIN {print sqrt(16)}'   # 输出：4 （计算平方根）

if [ $a -gt $b ] && [ $a -lt $c ]; then
    echo "$a is between $b and $c"
fi

if [ $a -gt $b -a $a -lt $c ]; then
    echo "$a is between $b and $c"
fi

FILE="/path/to/file"

if test -f "$FILE"; then
    echo "File exists and is a regular file."
else
    echo "File does not exist or is not a regular file."
fi

FILE="/path/to/file"

if [ -f "$FILE" ]; then
    echo "File exists and is a regular file."
else
    echo "File does not exist or is not a regular file."
fi

FILE="/path/to/newfile"

if [ ! -f "$FILE" ]; then
    echo "File does not exist. Creating it now..."
    touch "$FILE"
else
    echo "File already exists."
fi

FILE="/path/to/newfile"

[ ! -f "$FILE" ] && touch "$FILE" || echo "File already exists."

if [ condition ]; then
    # do something
elif [ another_condition ]; then
    # do something else
else
    # default action
fi

#!/bin/bash

read -p "Enter a number: " num

if [[ $num -gt 10 ]]; then
    echo "Number is greater than 10."
elif [[ $num -eq 10 ]]; then
    echo "Number is exactly 10."
else
    echo "Number is less than 10."
fi

case $variable in
    pattern1)
        # code block for pattern1
        ;;
    pattern2)
        # code block for pattern2
        ;;
    *)
        # default action
        ;;
esac

#!/bin/bash

read -p "Enter fruit name: " fruit

case $fruit in
    apple|orange)
        echo "It's an orange or apple."
        ;;
    banana)
        echo "It's a banana."
        ;;
    *)
        echo "Unknown fruit."
        ;;
esac

select choice in option1 option2 option3; do
    case $choice in
        option1) echo "You selected option 1";;
        option2) echo "You selected option 2";;
        option3) echo "You selected option 3";;
        *) echo "Invalid option";;
    esac
done

#!/bin/bash

echo "Select your favorite color:"
select color in red blue green yellow; do
    case $color in
        red)
            echo "You selected red."
            break
            ;;
        blue)
            echo "You selected blue."
            break
            ;;
        green)
            echo "You selected green."
            break
            ;;
        yellow)
            echo "You selected yellow."
            break
            ;;
        *)
            echo "Invalid option, please select again."
            ;;
    esac
done

while [ condition ]; do
    # do something
done

until [ condition ]; do
    # do something
done

#!/bin/bash

# While loop example
counter=0
while [ $counter -lt 5 ]; do
    echo "Counter is $counter"
    ((counter++))
done

# Until loop example
counter=0
until [ $counter -ge 5 ]; do
    echo "Counter is $counter"
    ((counter++))
done

#!/bin/bash

# For loop with sequence
for i in {1..5}; do
    echo "Number: $i"
done

# For loop with file patterns
for file in *.txt; do
    if [ -f "$file" ]; then
        echo "Processing $file"
    else
        echo "No .txt files found."
    fi
done

function function_name {
    commands  # 函数体
}

function_name arg1 arg2 ...

#!/bin/bash

# 函数：加法
add() {
    echo $(($1 + $2))
}

# 函数：减法
subtract() {
    echo $(($1 - $2))
}

# 函数：乘法
multiply() {
    echo $(($1 * $2))
}

# 函数：除法
divide() {
    if [ $2 -eq 0 ]; then
        echo "Error: Division by zero"
        return 1
    else
        echo "scale=2; $1 / $2" | bc
    fi
}

# 函数：显示菜单并获取用户输入
show_menu_and_get_input() {
    echo "Simple Calculator"
    echo "1. Add"
    echo "2. Subtract"
    echo "3. Multiply"
    echo "4. Divide"
    echo "5. Exit"
    
    read -p "Choose an option (1-5): " choice
    
    case $choice in
        1|2|3|4)
            read -p "Enter first number: " num1
            read -p "Enter second number: " num2
            ;;
        5)
            echo "Exiting..."
            exit 0
            ;;
        *)
            echo "Invalid option, please try again."
            show_menu_and_get_input
            ;;
    esac
    
    return $choice
}

# 主程序逻辑
while true; do
    show_menu_and_get_input
    choice=$?

    case $choice in
        1)
            result=$(add $num1 $num2)
            ;;
        2)
            result=$(subtract $num1 $num2)
            ;;
        3)
            result=$(multiply $num1 $num2)
            ;;
        4)
            result=$(divide $num1 $num2)
            if [ $? -ne 0 ]; then continue; fi  # 如果除法出错则跳过输出结果
            ;;
        *)
            continue  # 忽略其他情况（理论上不会到达这里）
            ;;
    esac

    echo "Result: $result"
done

cut -d',' -f1,3 file.csv

sed 's/foo/bar/' input.txt

awk '{print $1}' file.txt

sort -n -k2 file.txt