I have a document with a lot of empty lines.How can I remove them when there are 2 or more together.I tried sed s/\n\n// file but it didn't work.  No error. 
Can sed remove 'double' newline characters?
sed
Just to remove empty lines:sed  '/^$/d'sed is line oriented, so thinking in terms of 2 or more of a particular byte works except when that byte is a newline.  Then you have to think of something that works for the whole line.