shell运算符：算数，关系，布尔，字符串，文件测试
原生bash不支持简单的数学运算，可通过awk expr实现，expr最常用
expr是一款表达式计算工具，使用它能完成表达式的求值操作
示例如下：
1 val=` expr 2 + 2 `
2 echo " Total value : ${val} "
注意
1.表达式和运算符之间要有空格
2.完整表达式要被``包含
算数运算符
示例如下： + - * / % == ！=
1 a=10
2 b=20
3
4 val=` expr ${a} + ${b} `
5 echo " a + b : ${val} "
6
7 val=` expr ${a} - ${b} `
8 echo " a - b : ${val} "
9
10 val=` expr ${a} \* ${b} ` #乘号前必须加\才能实现惩罚运算#
11 echo " a * b : ${val}"
12
13 val=` expr ${b} / ${a} `
14 echo " b / a : ${val} "
15
16 val=` expr ${b} % ${a} `
17 echo " b % a : ${val} "
18
19 if [ ${a} == ${b} ] #条件语句写法，后续加入.此条件返回的是true or false，空格不能少#
20 then
21 echo " a is equal to b "
22 fi
23
24 if [ ${a} != ${b} ]
25 then
26 echo " a is not equal to b"
27 fi
关系运算符
关系运算符：只支持数字，不支持字符串，除非字符串的值是数字
关系运算符列表
1. -eq 检测两个数是否相等，返回true [ ${a} -eq ${b} ] equal
2. -ne 检测两个数是否相等，返回false [ ${a} -ne ${b} ] not euqal
3. -gt 检测左边是否大于右边，返回true [ ${a} -gt ${b} ] greater than
4. -lt 检测左边是否小于右边，返回false [ ${a} -lt ${b} ] less than
5. -ge 检测左边是否大于等于右边，返回true [ ${a} -ge ${b} ] greater or equal
6. -le 检测左边是否小于等于右边，返回false [ ${a} -le ${b} ] less or equal
示例如下：
1 a=10
2 b=20
3 if [ ${a} -eq ${b} ]
4 then
5 echo " a is equal to b "
6 else
7 echo " a is not equal to b "
8 fi
布尔运算符
布尔运算符列表
1. ！ 非 [ !false ]
2. -o 或 [ ${a} -lt 20 -o ${b} -gt 100] a<20或b>100 返回true
3. -a 与 [ ${a} -lt 20 -a ${b} -gt 100] a<20并且b>100 返回true
示例如下：
1 a=10
2 b=110
3
4 if [ ${a} -lt 20 -a ${b} -lt 100 ] #a<20并且b<100#
5 then
6 echo " a<20或b<100成立！"
7 else
8 echo " 不成立！ "
9 fi
字符串运算符
字符串运算符列表
1. = 检测两个字符串是否相等 [ ${a} = ${b} ]
2. != 检测两个字符串是否相等 [ ${a} = ${b} ]
3. -z 检测字符串长度是否为0,返回true [ -z ${a} ]
4. -n 检测字符串长度是否为0,返回false [ -n ${a} ]
5. str 检测字符串是否为空，不为空返回true [ ${a} ]
示例如下：
1 a="abc"
2 b="abc"
3
4 if [ ${a} = ${b} ]
5 then
6 echo " ${a} = ${b} : a is equal to b "
7 else
8 echo " ${a} = ${b} : a is not equal to b "
9 fi
10
11 if [ ${a} != ${b} ]
12 then
13 echo " ${a} != ${b} : a is not equal to b "
14 else
15 echo " ${a} != ${b} : a is equal to b "
16 fi
17
18 if [ -z ${a} ]
19 then
20 echo " -z ${a} : String length is zero "
21 else
22 echo " -z ${a} : String length is not zero "
23 fi
24
25 if [ -n ${b} ]
26 then
27 echo " -n ${b} : String length is not zero "
28 else
29 echo " -n ${b} : String length is zero"
30 fi