Let's say foo.txt contains a very long string, like this.

asdfakjdslkfjkllk343lk434l3klkjvsdfsiwqepoqwpisdfsddslkjpizndf3498sdf

The fold command with the -w option can be used to wrap the content of foo.txt. In this example, the string will be wrapped at every 10th character.

fold -w 10 foo.txt

asdfakjdsl
kfjkllk343
lk434l3klk
jvsdfsiwqe
poqwpisdfs
ddslkjpizn
df3498sdf

Note that fold will not actual update the file in question. Redirection can be used to create a new, folded file.

fold -w 10 foo.txt > bar.txt