Split very long line of words into separate lines of max length

What you want does fold:

fold -s -w80 file

• -s: tells fold to break only at spaces
• -w80: declared the width
• fold: also reads from stdin: echo "..." | fold -s -w80

I would prefer fmt, because fmt leaves long words intact:

echo "a a a a a a  123456789 xyz" | fmt -6
a a a
a a a
123456789
xyz