没有执行条件检查，它只是打印echo语句以进行合法输入

Question

read a
if [[ $a != 'A' || $a != 'a']] && [[ $a != 'B' || $a != 'b' ]] && [[ $a !='C' || $a != 'c']]; then 
       echo "ABC only"
fi

Answer 1

read -rN1 a
if [[ "$a" =~ [^AaBbCc] ]]; then
    echo "ABC only"
fi