# 2. shell中的四则运算符

* n1,n2 :常量数字
* char:运算符号 加,减,乘,除,取余(+,-,\*,/,%)
* $a,$b:变量a,变量b

## 方法1

* 数字与符号之间需要有空格
* 不支持小数

> expr n1 char n2
>
> expr $a char $b

* 此种格式中乘号 *需要写成 \\*

  ```bash
  # 例如:100*100
  expr 100 \* 100
  ```

## 方法2

* 数字与符号之间可以不加空格
* 不支持小数 

> echo $\[n1 char n2]
>
> expr $\[$a char $b]

## 方法3

* 数字与符号之间可以不加空格
* 不支持小数

> echo $((n1 char n2))
>
> echo $(($a char $b))

## 例子

计算:1\*100

```bash
expr 1 \* 100
```

等价于

```bash
echo $[1 * 100]
```

等价于

```
echo $((1 * 100))
```

## linux自带计算器

控制台中使用bc命令进入

成功进入后会显示

```
bc 1.06.95
Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'.
```

输出quit然后回车退出bc计算器

## 方法4

使用bc计算器进行四则运算

* 支持小数

  > echo "n1 char n2" | bc

计算1.2+3.4

```bash
echo "1.2+3.4" | bc
```

### 保留指定小数位数

```bash
  echo "scale=num;n1 / n2" | bc
```

* 加减法无效
* num:代表小数位数
* 结果在(-1,1) 区间里面不显示0

#### 例子

```bash
# 1.2/3 保留２位  = .40
echo "scale=2;1.2/3" | bc

# 1.2 -5   = .40 = -3.8
echo "1.2-5" | bc

# 3 * 5  = 15
echo "3*5" | bc

# 1+2.345 保留两位　
# 3.345
echo "scale=2;1+2.345" | bc 
# 3.34
echo "scale=2;(1+2.345)/1" | bc
```


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