Bash 支持很多运算符，包括算数运算符、关系运算符、布尔运算符、字符串运算符和文件测试运算符。
原生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

算数运算符

var=`expr 2 + 2`
echo "total value:$var"

a=10
b=20
var=`expr $a + $b`
echo "a + b :$var"

var=`expr $a - $b`
echo "a - b :$var"

var=`expr $a \* $b`
echo "a * b :$var"

var=`expr $b / $a`
echo "b / a :$var"

var=`expr $b % $a`
echo "b % a :$var"


if [ $a == $b ]
then
    echo "a is equal to b"
fi

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

两点注意：

1、表达式和运算符之间要有空格，例如 2+2 是不对的，必须写成 2 + 2，这与我们熟悉的大多数编程语言不一样。
2、完整的表达式要被 包含，注意这个字符不是常用的单引号，在 Esc 键下边。

关系运算符

3、乘号(*)前边必须加反斜杠()才能实现乘法运算；

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 -gt $b : a is less than b"
else
    echo "$a -gt $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

关系运算符列表

布尔运算符

if [ $a -lt 100 -a $b -gt 15 ]
then
    echo "$a -lt 100 -a $b -gt 15 : returns true"
else
    echo "$a -lt 100 -a $b -gt 15 : returns false"
fi

if [ $a -lt 100 -o $b -gt 100 ]
then
    echo "$a -lt 100 -o $b -gt 100 : returns true"
else
    echo "$a -lt 100 -o $b -gt 100 : returns false"
fi

if [ $a -lt 5 -o $b -gt 100 ]
then
    echo "$a -lt 100 -o $b -gt 100 : returns true"
else
    echo "$a -lt 100 -o $b -gt 100 : returns false"
fi

布尔运算符列表

字符串运算符

a="abc"
b="efg"

if [ $a = $b ]
then
echo "$a = $b : a is equal to b"
else
echo "$a = $b: a is not equal to b"
fi

if [ $a != $b ]
then
echo "$a != $b : a is not equal to b"
else
echo "$a != $b: a is equal to b"
fi

if [ -z $a ]
then
echo "-z $a : string length is zero"
else
echo "-z $a : string length is not zero"
fi

if [ -n $a ]
then
echo "-n $a : string length is not zero"
else
echo "-n $a : string length is zero"
fi

if [ $a ]
then
echo "$a : string is not empty"
else
echo "$a : string is empty"
fi

字符串运算符列表

文件测试运算符

file="/Users/ios/Desktop/shell教程/shell运算符.sh"

if [ -r $file ]
then
echo "File has read access"
else
echo "File does not have read access"
fi

if [ -w $file ]
then
echo "File has write permission"
else
echo "File does not have write permission"
fi

if [ -x $file ]
then
echo "File has execute permission"
else
echo "File does not have execute permission"
fi

if [ -f $file ]
then
echo "File is an ordinary file"
else
echo "This is sepcial file"
fi

if [ -d $file ]
then
echo "File is a directory"
else
echo "This is not a directory"
fi

if [ -s $file ]
then
echo "File size is not zero"
else
echo "File size is zero"
fi

if [ -e $file ]
then
echo "File exists"
else
echo "File does not exist"
fi

文件测试运算符列表