名称：expr
### 字串长度
shell expr length "this is a test"
14
### 数字商数
shell expr 14 % 9
5
### 从位置处抓取字串
shell expr substr "this is a test" 3 5
is is
### 数字串 only the first character
shell expr index "testforthegame" e
2
### 字串真实重现
shell expr quote thisisatestformela
thisisatestformela
