linux shell脚本之获取用户输入指定范围的单个字符的两种方法

如下需求：
要求获取用户输一个字符a-zA-Z：
方法一：

echo `echo {a..z}``echo {A..Z}` > /tmp/letterfile
while read -p "please input a letter: " letter
do
        ((`awk -v var="$letter" 'BEGIN{print length(var)}'` == 1)) && grep -q $letter /tmp/tmpfile && break
done
rm -f /tmp/letterfile
echo "you input letter $letter "

[root@station1 ~]# cat a.sh
echo `echo {a..z}``echo {A..Z}` > /tmp/letterfile
while read -p "please input a letter: " letter
do
((`awk -v var="$letter" 'BEGIN{print length(var)}'` == 1)) && grep -q $letter /tmp/tmpfile && break
done
rm -f /tmp/letterfile
echo "you input letter $letter "
[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 ~]#

我已开始是使用expr length $letter 替代wk -v var="$letter" 'BEGIN{print length(var)}'的但是，当你输入*等特殊字符时，会报错。如下：

[root@station1 ~]# cat data
while read -p "please input a letter: " letter
do
((`expr length $letter` == 1)) && expr $letter : [[:alnum:]] > /dev/null && break
done
echo "you input letter $letter "
[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:

方法二：
while read -p "please input a letter: " letter
do
    [[ `echo $letter | awk '/^[[:alpha:]]$/{print "1"}'` -eq 1 ]] && break
done
echo "you input letter $letter "

如下：

[root@station1 ~]# cat b.sh
while read -p "please input a letter: " letter
do
[[ `echo $letter | awk '/^[[:alpha:]]$/{print "1"}'` -eq 1 ]] && break
done
echo "you input letter $letter "
[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 ~]#