一、基本语法

1、“$((运算式))”或“$[运算式]”
2、expr + , - , *, /, %：加，减，乘，除，取余；注意：expr运算符间要有空格
3、$(($a + $b))：变量中的$符号可以去掉 $((a + b))
4、比较运算：echo $((a<b))
5、交互方式：bc

🌙加法：

zhilong@zhilong-virtual-machine:~$ expr 2 + 3
2+3

或者：

zhilong@zhilong-virtual-machine:~$ a=10
zhilong@zhilong-virtual-machine:~$ b=8
zhilong@zhilong-virtual-machine:~$ c=6
zhilong@zhilong-virtual-machine:~$ echo $(($a + $b))
18

🌙减法：

zhilong@zhilong-virtual-machine:~$ expr 2 - 3
-1

或者：

zhilong@zhilong-virtual-machine:~$ a=10
zhilong@zhilong-virtual-machine:~$ b=8
zhilong@zhilong-virtual-machine:~$ echo $(($a - $b))
2

🌙乘法：

乘法中*前面一定加\,不加语法错误

zhilong@zhilong-virtual-machine:~$ expr 2 * 3
expr: 语法错误：未预期的参数 “公共的”
zhilong@zhilong-virtual-machine:~$ expr 2 \* 3
6

或者：

zhilong@zhilong-virtual-machine:~$ a=10
zhilong@zhilong-virtual-machine:~$ b=8
zhilong@zhilong-virtual-machine:~$ echo $(($a * $b))
80

🌙除法：

除法默认取整

zhilong@zhilong-virtual-machine:~$ expr 6 / 2
3
zhilong@zhilong-virtual-machine:~$ expr 6 / 4
1

或者：

zhilong@zhilong-virtual-machine:~$ a=10
zhilong@zhilong-virtual-machine:~$ b=8
zhilong@zhilong-virtual-machine:~$ echo $(($a / $b))
1

🌙取余：

zhilong@zhilong-virtual-machine:~$ expr 6 % 4
2

🌙混合运算：

zhilong@zhilong-virtual-machine:~$ expr `expr 2 + 4` / 2
3

或者

zhilong@zhilong-virtual-machine:~$ S=$[(2+4)/2]
zhilong@zhilong-virtual-machine:~$ echo $S
3

🌙比较大小

如果成立：打印1；不成立打印0；与$?恰恰相反

zhilong@zhilong-virtual-machine:~$ a=10
zhilong@zhilong-virtual-machine:~$ b=6
zhilong@zhilong-virtual-machine:~$ echo $((a>b))
1
zhilong@zhilong-virtual-machine:~$ echo $((a<b))
0

二、条件表达式

🌙1、返回值

条件成立，返回0
条件不成立，返回1

🌙2、文件表达式

-f：判断输入文件是否是一个文件
-d：判断输入的内容是否是一个目录
-x：判断输入的内容是否可执行
-e：判断文件是否存在

zhilong@zhilong-virtual-machine:~$ [ -f /home/zhilong ];echo $?
1
zhilong@zhilong-virtual-machine:~$ [ -d /home/zhilong ];echo $?
0
zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ [ -x /home/zhilong/passwd ];echo $?
1
zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ [ -e /home/zhilong/Desktop/shell_text ];echo $?
0

🌙3、数值操作符

n1 -eq n2：相等
n1 -gt n2：大于
n1 -lt n2：小于
n1 -ne n2：不等于

zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ a=10
zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ b=8
zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ c=6
zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ [ $a -eq $b ];echo $?
1
zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ [ $a -gt $b ];echo $?
0
zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ [ $a -lt $b ];echo $?
1
zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ [ $a -ne $b ];echo $?
0

🌙4、字符串比较

str1 == str2：str1和str2字符串内容一致，字符串与符号之间加空格
str1 != str2：str1和str2字符串内容不一致，字符串与符号之间加空格

zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ a='hello'
zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ b='hell'
zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ c='hello'

zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ [ $a == $b ];echo $?
1
zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ [ $a == $c ];echo $?
0
zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ [ $a != $c ];echo $?
1

🌙5、判断字符串的长度是否为0

特别注意：如果字符串包含空格，需要包裹起来用

zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ a="qweqw  qweqwe"
zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ [ -z $a ];echo $?
bash: [: qweqw：需要二元表达式
2

zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ [ -z "$a" ];echo $?
1
zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ [ -z "$q" ];echo $?
0

🌙6、判断字符串的长度是否不为0

zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ [ -n "$q" ];echo $?
1
zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ [ -n "$a" ];echo $?
0

🌙7、逻辑表达式

&&： 表示前一条命令执行成功时，才执行后一条命令；
|| ：表示上一条命令执行失败后，才执行下一条命令
-a：多个条件和的判断
-o：多个条件或的判断

zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ username="kobe"
zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ [ "$username" == "kobe" ] && echo "成功"
成功
zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ [ "$username" == "kobe1" ] && echo "成功" || echo "失败"
失败
zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ [ "$username" == "kobe" ] && echo "成功" || echo "失败"
成功

zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ username="kobe"
zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ password=123

zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ [ "$username" == "kobe" -a "$password" == "123" ] && echo "成功" || echo "失败"
成功
zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ [ "$username" == "kobe" -a "$password" == "123123" ] && echo "成功" || echo "失败"
失败
zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ [ "$username" == "kobe" -o "$password" == "123123" ] && echo "成功" || echo "失败"
成功
zhilong@zhilong-virtual-machine:~/Desktop/shell_text$ [ "$username" == "kobe1" -o "$password" == "123123" ] && echo "成功" || echo "失败"