Feature request for head command

[Robert McKay]

Synopsis of the problem:

When you use head to read a certain number of lines out of a pipe
sometimes it eats more data than you ask it to,

For example:

address@hidden ~]$ ( echo  hello;  echo world; ) | (  head -1 > /dev/null; cat)
address@hidden ~]$ ( echo  hello; sleep 1;  echo world; ) | (  head -1
> /dev/null; cat)
world

So.. why does the first command not print anything but the second
command prints out "world"?

Well in the first case hello and world were immediately available to
be read and head -1 read them both into it's buffer before discovering
that actually it should have stopped at the first newline.

In the second instance the sleep 1 in the middle causes head's read()
call to return early, (because no more data was immediately available
to be read), and head realized that it already had a newline and
exited, leaving the "world" for cat to read.

What to do about this? head can't unread data that it's already read
so the only solution is for it to read the input one byte at a time -
constantly checking for a \n (newline). Is this inefficient? Yes, of
course it is but it would also be very useful. I'm not proposing that
this be made the default behavior, simply that a new option be added
to support it.

I eagerly await your flames :-)

Robert McKay.