file的内容如下：
ADSFADSFSDFASDFJKJK----------------------------------------ADSFADSFSDFASDFJKJK----------------------------------------ADSFADSFSDFASDFJKJK
linux 用grep匹配第2行和第4行横短线：
grep "^-*$" file
linux 用grep匹配不含横短线的行：grep -v "^-*$" file