Re: How to read \000?

[Greg Wooledge]

On Wed, May 15, 2024 at 06:01:04PM -0400, Jeffrey Walton wrote:
> On Wed, May 15, 2024 at 3:48 PM Peng Yu <pengyu.ut@gmail.com> wrote:
> 
> > No. I need to just read one character.
> >
> 
> `tail -c +1` will read just one character.

I think you have the wrong command.  Perhaps you meant "head -c 1"?

Whether head reads one byte, or more than one byte, will be implementation
dependent.  I don't think you can guarantee its behavior across platforms.

On Debian 12, "head -c 1" appears to consume exactly one byte, but
"head -n 1" consumes a full 8k buffer:

hobbit:~$ seq 2000 | wc
   2000    2000    8893
hobbit:~$ seq 2000 | { head -c 1 >/dev/null; wc; }
   2000    1999    8892
hobbit:~$ seq 2000 | { head -n 1 >/dev/null; wc; }
    141     140     701

On other platforms, "head -c 1" may use a buffered read.

If you want a standard command that reads 1 byte *without* reading a
whole buffer, on all platforms, I suggest "dd bs=1 count=1".

> I guess the problem is, what do
> you want to do afterwards.

This is Peng Yu.  The one guarantee you have is that you will *not*
be told what they want to do afterward.  Or before.  Or during.