只取出字符串长度大于某一长度的行（比如用于删除一些垃圾行的情况）

直接贴代码了：

You could use sed. The following would remove lines that are 3 characters long or smaller:

sed -r '/^.{,3}$/d' filename
In order to save the changes to the file in-place, supply the -i option.

If your version of sed doesn't support extended RE syntax, then you could write the same in BRE:

sed '/^.\{,3\}$/d' filename
which would work with all sed variants.

You could also use awk:

awk 'length($0)>3' filename
Using perl:

perl -lne 'length()>3 && print' filename