This is slightly inelegant, as the old style
echo foo
echo blah
chmod a+x $3
doesn't work anymore; the stuff you wrote to stdout didn't end up in $3.
You can rewrite it as:
exec >$3
echo foo
echo blah
chmod a+x $3
Anyway, it's better this way, because now we can tell the difference between
a zero-length $3 and a nonexistent one. A .do script can thus produce
either one and we'll either delete the target or move the empty $3 to
replace it, whichever is right.
As a bonus, this simplifies our detection of whether you did something weird
with overlapping changes to stdout and $3.
