# 3. 条件判断

* 格式 \[ 表达式 ]
* 表达式前后各有一个空格

## 文件(夹),路径相关

* -e:(exist)目标是否存在
* -d:(directory)是否为文件夹
* -f:(file)是否为文件

### 例子

1. 判断当前目录下的 1.txt 是否存在,是则输出true

   ```bash
   [ -e 1.txt ]&& echo "true"
   ```
2. 判断dir1是否是目录,是则输出true

   ```bash
   [ -d dir1 ]&& echo "true"
   ```
3. 判断1.txt是否为文件,是则输出true

   ```bash
   [ -f 1.txt ]&& echo "true"
   ```
4. 如果1.txt不存在则创建一个1.txt

   ```bash
   [ -e 1.txt ] || touch 1.txt
   ```

## 权限相关

* -r:(read)是否具有读取权限
* -w:(write)是否具有写入权限
* -x:(excute)是否具有运行权限

### 例子

1. 判断1.txt 是否具有写权限,有则输出success

   ```bash
   [ -w 1.txt ]&& echo "success"
   ```
2. 判断1.txt 是否具有读权限

   ```bash
   [ -r 1.txt ]
   ```
3. 判断1.txt 是否具有执行权限

   ```bash
   [ -x 1.txt ]
   ```

## 整数比较

* -eq:(equal) 等于
* -ne:(not equal) 不等于
* -gt:(greater than) 大于
* -lt:(lesser than) 小于
* -ge:(greater or equal) 大于等于
* -le:(lesser or equal) 小于等于

### 例子

```bash
# 1>2
[ 1 -gt 2 ]

# 1<3
[ 1 -lt 3 ]

# 1<=4
[ 1 -le 4 ]

# 4>=3
[ 4 -ge 3 ]

# 1=2
[ 1 -eq 2 ]

# 2!=3
[ 2 -ne 3 ]
```

## 字符串比较

* \=  :相等
* != :不相等
* -z :判断字符串长度是否为0
* -n :判断字符串长度是否不为0
* $  :判断字符串是否不为空

### 例子

```
# "hello" 与 "hello"比较
[ "hello" = "hello" ]
[ "hello" != "hello" ]
```

demo2

```bash
# 代码
a="abc"
b="efg"
if [ $a = $b ]
then
    echo "equal"
fi

if [ $a != $b ]
then
    echo "not equal"
fi

if [ -z $a ]
then
    echo "zero"
fi

if [ -n $a ]
then
    echo "not zero"
fi

if [ $a ]
then 
    echo "not empty"
fi

# 输出

not equal
not zero
not empty
```

## 小数(浮点型)比较

利用bc计算器比较

* 满足返回 1
* 不满足返回 0

### 例子

5 是否 大于 1.3

```bash
[ `echo "5>1.3"| bc` -eq 1 ]
```

## 逻辑比较

* || :或
* && :且
* !  :非
* -o :或
* -a :且

  **例子**

  \`\`\`bash

  a=123

  b=456

  c=123

  if \[\[ $a -eq $c || $a -lt $b ]]

  then

  &#x20; echo "a = c 或 a < b"

  fi

if \[\[ $a -eq $c && $b -gt $a ]] then echo "a = c 且 b > a" fi

if \[\[ ! $a -ne $c ]] then echo "a = c" fi

if \[ $a -eq $c -o $a -lt $b ] then echo "a = c 或 a < b" fi

if \[ $a -eq $c -a $b -gt $a ] then echo "a = c 且 b > a" fi \`\`\`


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