expr 计算整数变量值
使用方法如下：
linux-zpycfm:/home/test/shell # s=expr 2+ 3 -bash: 2+: command not found linux-zpycfm:/home/test/shell # s=expr 2+3 -bash: 2+3: command not found linux-zpycfm:/home/test/shell # s=`expr 2 + 3` linux-zpycfm:/home/test/shell # echo $s 5 linux-zpycfm:/home/test/shell # `expr 2 + 3` -bash: 5: command not found linux-zpycfm:/home/test/shell # expr 2 + 3 5 linux-zpycfm:/home/test/shell # expr 2+3 2+3 linux-zpycfm:/home/test/shell # expr $s * 4 expr: syntax error linux-zpycfm:/home/test/shell # expr $s * 4 20 linux-zpycfm:/home/test/shell # expr `expr 2 + 3` * 4 20 linux-zpycfm:/home/test/shell # expr 2 "*" 3 6 linux-zpycfm:/home/test/shell # expr 2 '*' 3 6 linux-zpycfm:/home/test/shell # expr 2 * 3 expr: syntax error linux-zpycfm:/home/test/shell # expr 2.3 + 3.4 expr: non-integer argument
运算符号和参数之间要有空格分开；
通配符号(*),在作为乘法运算符时要用、“”、‘’符号修饰
–:expr 3 * 2 expr 3 “*” 2 expr 3 ‘*’ 2
`(反引号)与键盘上的~同一个键上的符号