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unfmt (opposite of "fmt")
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"fmt" formats text into lines which are approximately 72 characters
each, avoiding breaking words. I like using line oriented text but
Apple and Windows have popularized the concept of a single line per
paragraph.

The following sed line is the opposite of fmt: it converts each
paragraph into a line, whilst keeping blank lines.

$ cat a.txt
Hello
Mitty

Meet
Kitty who is from Japan



#          1      2               3      4        5 6
$ gsed -n '/^$/!H;/^$/{g;s/\n/ /g;s/^ //;s/$/\n/p;z;x;}' a.txt
Hello Mitty

Meet  Kitty who is from Japan

Explanation:

1
2
3
4
5
6