方法一：
[root@station1 ~]# cat a.sh
[root@station1 ~]# chmod +x a.sh
[root@station1 ~]# ./a.sh
please input a letter: 123
please input a letter: abc
please input a letter: 4
please input a letter: &
please input a letter: (
please input a letter: a
you input letter a
[root@station1 ~]#
最初使用expr length $letter 替代 wk -v var="$letter" 'BEGIN{print length(var)}'的，但是，当输入*等特殊字符时，会报错。
如下：
[root@station1 ~]# cat data
[root@station1 ~]# bash data
please input a letter: 123
please input a letter: abc
please input a letter: a
you input letter a
[root@station1 ~]# bash data
please input a letter: *
expr: 语法错误
data: line 3: ((: == 1: syntax error: operand expected (error token is "== 1")
please input a letter:
来看方法二：
如下：
[root@station1 ~]# cat b.sh
[root@station1 ~]# chmod +x b.sh
[root@station1 ~]# ./b.sh
please input a letter: *
please input a letter: abc
please input a letter: 234
please input a letter: )
please input a letter: T
you input letter T
[root@station1 ~]#