Refer

1. https://blog.csdn.net/mfkjq/article/details/80377114

gsed ":a;N;s/\n/,/g;ta"

Or

gsed ":d;N;s/\n/,/g;td"

It means it will jump to the tag :a or :d, if one time failed, then it will break.

gsed ":a;N;s/\n/,/g;ba"

Or

gsed ":d;N;s/\n/,/g;bd"

Refer

1. http://kodango.com/sed-and-awk-notes-part-5

sed '/^$/{N;/^\n$/D}'

E.g. Remove all the redundant empty lines in the Java source code

find app/src/ -iname "*.java" -exec gsed -i '/^$/{N;/^\n$/D}' {} +

sed 'N;N;s/\n/ /g'