Shell入门（二）之Shell运算符

旧城等待， 2022-06-01 02:24 280阅读 0赞

### Shell入门（二）之Shell运算符 ###

#### Shell基本运算符 ####

Shell和其他编程语言一样，支持多种运算符，包括：

*  算术运算符
 *  关系运算符
 *  布尔运算符
 *  字符串运算符

原生的bash不支持简单的数学运算，但是可以通过其他命令来实现，例如：`awk`和`expr`，`expr`最为常用。

如：

val=`expr 2 + 3`
    echo $val

输出：5

*注意，`2 + 3`之间需要有一个空格*

**算术运算符**

a变量为10，b变量为20。

<table> 
 <thead> 
  <tr> 
   <th>运算符</th> 
   <th>说明</th> 
   <th>举例</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>+</td> 
   <td>加号</td> 
   <td>`expr $a + $b`，结果：30</td> 
  </tr> 
  <tr> 
   <td>-</td> 
   <td>减号</td> 
   <td>`expr $a - $b`,结果：-10</td> 
  </tr> 
  <tr> 
   <td>*</td> 
   <td>乘号</td> 
   <td>`expr $a * $b`,结果：200</td> 
  </tr> 
  <tr> 
   <td>/</td> 
   <td>除号</td> 
   <td>`expr $b / $a`,结果：2</td> 
  </tr> 
  <tr> 
   <td>%</td> 
   <td>取余</td> 
   <td>`expr $b % $a`,结果：0</td> 
  </tr> 
  <tr> 
   <td>=</td> 
   <td>赋值</td> 
   <td>var=$a，把变量a赋值给var</td> 
  </tr> 
  <tr> 
   <td>==</td> 
   <td>等于</td> 
   <td>[$a == $b]，结果为0</td> 
  </tr> 
  <tr> 
   <td>!=</td> 
   <td>不等于</td> 
   <td>[$a != $b]，结果为1</td> 
  </tr> 
 </tbody> 
</table>

eg:

[zhang@localhost home]$ a=10
    [zhang@localhost home]$ b=20
    [zhang@localhost home]$ echo $a + $b = `expr $a + $b`
    10 + 20 = 30
    [zhang@localhost home]$ echo $a - $b = `expr $a - $b`
    10 - 20 = -10
    [zhang@localhost home]$ echo $a \* $b = `expr $a \* $b`
    10 * 20 = 200
    [zhang@localhost home]$ echo $a / $b = `expr $a / $b`
    10 / 20 = 0
    [zhang@localhost home]$ echo $b / $a = `expr $b / $a`
    20 / 10 = 2
    [zhang@localhost home]$ echo $b % $a = `expr $b % $a`
    20 % 10 = 0
    
    [zhang@localhost home]$ var=$a
    [zhang@localhost home]$ echo $var
    10
    
    [zhang@localhost home]$ echo $[$a == $b]
    0
    [zhang@localhost home]$ echo $[$a != $b]
    1
    [zhang@localhost home]$ echo $[$a == 10]
    1

注：`$[$a == $b]`的写法，可以使用`$[a == b]`替代，如：

[zhang@localhost ~]$ a=10
    [zhang@localhost ~]$ b=20
    [zhang@localhost ~]$ echo $[a + b]
    30
    [zhang@localhost ~]$ echo $[a - b]
    -10
    [zhang@localhost ~]$ echo $[a * b]
    200
    [zhang@localhost ~]$ echo $[a / b]
    0
    [zhang@localhost ~]$ echo $[a == b]
    0
    [zhang@localhost ~]$ echo $[a != b]
    1
    [zhang@localhost ~]$ echo $[a >= b]
    0
    [zhang@localhost ~]$ echo $[a <= b]
    1
    [zhang@localhost ~]$ echo $[a % b]
    10

**关系运算符**

<table> 
 <thead> 
  <tr> 
   <th>运算符</th> 
   <th>说明</th> 
   <th>举例</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>-eq</td> 
   <td>检测2个数是否相等，相等返回true。</td> 
   <td>[$a -eq $b]返回false</td> 
  </tr> 
  <tr> 
   <td>-ne</td> 
   <td>检测2个数是否不相等，相等返回true。</td> 
   <td>[$a -ne $b]返回true</td> 
  </tr> 
  <tr> 
   <td>-gt</td> 
   <td>检测左边的数是否大于右边的，如果是，则返回true</td> 
   <td>[$a -gt $b]返回false</td> 
  </tr> 
  <tr> 
   <td>-lt</td> 
   <td>检测左边的数是否小于右边的，如果是，则返回true</td> 
   <td>[$a -lt $b]返回true</td> 
  </tr> 
  <tr> 
   <td>-ge</td> 
   <td>检测左边的数是否大于等于右边的，如果是，则返回true</td> 
   <td>[$a -ge $b]返回false</td> 
  </tr> 
  <tr> 
   <td>-le</td> 
   <td>检测左边的数是否小于等于右边的，如果是，则返回true</td> 
   <td>[$a -le $b]返回true</td> 
  </tr> 
 </tbody> 
</table>

eg:

#!/bin/bash 
    a=10
    b=20
    if [ $a -eq $b ]
    then
        echo "$a -eq $b : a is equal to b"
    else
        echo "$a -eq $b: a is not equal to b"
    fi
    
    if [ $a -ne $b ]
    then
        echo "$a -ne $b : a is not equal to b"
    else
        echo "$a -ne $b : a is equal to b"
    fi
    
    if [ $a -gt $b ]
    then
        echo "$a -gt $b : a is greater than b"
    else
        echo "$a -gt $b : a is not greater than b"
    fi
    
    if [ $a -lt $b ]
    then
        echo "$a -lt $b : a is less than b"
    else
        echo "$a -lt $b : a is not less than b"
    fi
    
    if [ $a -ge $b ]
    then
        echo "$a -ge $b : a is greater or equal to b"
    else
        echo "$a -ge $b : a is not greater or equal to b"
    fi
    
    if [ $a -le $b ]
    then
        echo "$a -le $b : a is less or equal to b"
    else
        echo "$a -le $b : a is not less or equal to b"
    fi

输出结果：

10 -eq 20: a is not equal to b
    10 -ne 20 : a is not equal to b
    10 -gt 20 : a is not greater than b
    10 -lt 20 : a is less than b
    10 -ge 20 : a is not greater or equal to b
    10 -le 20 : a is less or equal to b

**布尔运算符**

<table> 
 <thead> 
  <tr> 
   <th>运算符</th> 
   <th>说明</th> 
   <th>举例</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>!</td> 
   <td>非运算，表达式为true，则返回false，否则返回true</td> 
   <td>[!false]返回true</td> 
  </tr> 
  <tr> 
   <td>-o</td> 
   <td>或运算，有一个表达式为true，则返回true</td> 
   <td>[ $a -lt 20 -o $b -gt 100]，返回true</td> 
  </tr> 
  <tr> 
   <td>-a</td> 
   <td>与运算，2个表达式都为true，才返回true</td> 
   <td>[$a -lt 20 -a $b -gt 100]，返回false</td> 
  </tr> 
 </tbody> 
</table>

eg:

#!/bin/bash 
    a=10
    b=20
    
    if [ $a != $b ]
    then
        echo "$a != $b"
    else
        echo "$a == $b"
    fi
    
    
    if [ $a -lt 20 -o $b -gt 100 ]
    then
        echo "$a less than 20 or $b greater than 100 : return true"
    else
        echo "$a less than 20 or $b greater than 100 : return false"
    fi
    
    
    if [ $a -lt 20 -a $b -gt 100 ]
    then
        echo "$a less than 20 and greater than 100 : return true"
    else
        echo "$a less than 20 and greater than 100 : return false"
    fi

输出结果：

10 != 20
    10 less than 20 or 20 greater than 100 : return true
    10 less than 20 and greater than 100 : return false

**逻辑运算符**

<table> 
 <thead> 
  <tr> 
   <th>运算符</th> 
   <th>说明</th> 
   <th>举例</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>&amp;&amp;</td> 
   <td>逻辑的and</td> 
   <td>[[ $a -lt 100 &amp;&amp; $b -gt 100 ]] 返回 false</td> 
  </tr> 
  <tr> 
   <td>||</td> 
   <td>逻辑的 OR</td> 
   <td>[[ $a -lt 100 || $b -gt 100 ]] 返回 true</td> 
  </tr> 
 </tbody> 
</table>

eg:

#!/bin/bash 
    a=10
    b=20
    
    if [[ $a -lt 100 && $b -gt 100 ]]
    then
        echo "return true"
    else
        echo "return false"
    fi
    
    
    if [[ $a -lt 100 || $b -gt 100 ]]
    then
        echo "return true"
    else
        echo "return false"
    fi

输出结果：

return false
    return true

**字符串运算符**

a变量为”abc”，b变量为”efg”。

<table> 
 <thead> 
  <tr> 
   <th>运算符</th> 
   <th>说明</th> 
   <th>举例</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>=</td> 
   <td>检测两个字符串是否相等，相等返回true</td> 
   <td>[ $a = $b ]，返回false</td> 
  </tr> 
  <tr> 
   <td>!=</td> 
   <td>检测两个字符串是否相等，不相等返回true</td> 
   <td>[ $a != $b ]，返回true</td> 
  </tr> 
  <tr> 
   <td>-z</td> 
   <td>检测字符串长度是否为0，不为0返回true</td> 
   <td>[ -z $a ]返回false</td> 
  </tr> 
  <tr> 
   <td>-n</td> 
   <td>检测字符串长度是否为0，不为0返回true</td> 
   <td>[ -n $a ]返回true</td> 
  </tr> 
  <tr> 
   <td>str</td> 
   <td>检测字符串是否为空，不为空返回true</td> 
   <td>[$a]返回true</td> 
  </tr> 
 </tbody> 
</table>

eg:

#!/bin/bash 
    a="abc"
    b="efg"
    
    if [ $a = $b ]
    then
        echo "$a equal $b"
    else
        echo "$a not equal $b"
    fi
    
    
    if [ $a != $b ]
    then
        echo "$a != $b"
    else
        echo "$a == $b"
    fi
    
    
    if [ -n $b ]
    then
        echo "$b length ${#b}"
    else
        echo "$b length 0"
    fi
    
    # eg: str
    if [ $a ]
    then
        echo "$a not null"
    else
        echo "$a is null"
    fi

结果：

abc not equal efg
    abc != efg
    abc length 3
    efg length 3
    abc not null