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，當(dāng)輸入*等特殊字符時(shí)，會(huì)報(bào)錯(cuò)。
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: 語(yǔ)法錯(cuò)誤
data: line 3: ((: == 1: syntax error: operand expected (error token is "== 1")
please input a letter:

來(lái)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 ~]#