Re: sed -> needs to wait for EOF?

[Stepan Kasal]

Hallo,

On Thu, 16 May 2002 12:11:45 +1000, address@hidden wrote:
> It seems that you can't use sed when following a file.
> 
> Example:
> 
>   tail -f /tmp/foo | sed -e s/foo/bar/g &
>   echo 123foo456 > /tmp/foo
> 
> You would expect that 123bar456 would be printed to your screen somewhere by
> the tail command, but it's not.
> 
> My only conclusion here is that sed needs the EOF before it can print
> anyhting, I came to this conclcusion because if you are not doing a -f on your
> tail, then it works fine.

Your conclusion is wrong--try this:
$ sed -e s/foo/bar/g
123foo456
123bar456
 
No EOF but immediate response!

The reason is that input is buffered unless it's read from terminal.
On my host (linux/i386) this means that you have to write at least 8KB of data
to see any output. Try this (with bash shell):

$ touch /tmp/foo
$ tail -f /tmp/foo | sed -e s/foo/bar/g &
$ for f in {0,1}{,1}{,1}{,1}{,1}{,1}{,1}{,1}{,1}{,1}; do
        echo 123foo456 >>/tmp/foo; done

You need to tell sed to process the input unbuffered, as if it was a terminal.
This was recently discussed in gnu.utils.bug under the subject
"[PATCH] sed unbuffered."
(I hope you won't mind if I mail you the posts.)

In short: you can either use a patch to sed, or use ptybandage from ptyget
package.  (http://multivac.cwru.edu./ptyget/)

Hope this helps,
        Stepan