How to break string into N 80 character lines?

Use fold.

echo "Running $0 $@" | fold -sw 80

-s wraps on spaces. -w 80 sets width to 80 columns.

Of course fold is the best option, but you can also achieve this with grep or sed:

echo "Running $0 $@" | grep -o '.\{80\}[^ ]* '
#or
echo "Running $0 $@" | sed -E 's/(.{80}[^ ]*) /\1\n/g'

Note, this will break at 80 chars + 1 word, so it might not be suitable for you.

Although others mentioned tools that break the lines just before the word that hits the 80-character limit, there is also another tool that tries to not break the lines just before end of after start of a sentence and does other things for better readability. Its usage is similar:

echo "…" | fmt -w 80

The -w option specifies the maximum width of the text, but the text will use only 93 percent of the line (this can be overridden by using -g).

This is how fold formats the text:

Although others mentioned tools that break the lines just before the word that 
hits the 80-character limit, there is also another tool that tries to not break 
the lines just before end of after start of a sentence and does other things 
for better readability. Its usage is similar:

    echo "…" | fmt -w 80

The `-w` option specifies the maximum width of the text, but the text will use 
only 93 percent of the line (this can be overridden by using `-g`).

The long sentence that hits the 80 character column count is this one and here.

And this is output from fmt:

Although others mentioned tools that break the lines just before the word
that hits the 80-character limit, there is also another tool that tries
to not break the lines just before end of after start of a sentence and
does other things for better readability. Its usage is similar:

    echo "…" | fmt -w 80

The `-w` option specifies the maximum width of the text, but the text will
use only 93 percent of the line (this can be overridden by using `-g`).

The long sentence that hits the 80 character column count is this one
and here.

You can notice that the last (exactly 80-char long) get wrapped, but not at column 75 (the 93 % of 80), but a word before it, because we do not want to have one word on the line alone (in printed text etc.).