Bob,
Thank you very much for your response. I was just looking at the
man page for grep, and wouldn't you know it.. --line-buffer is not
very well documented. Infact, all it says about that is:
(excerpt from man page)
--line-buffering
Use line buffering, it can be a performance penality.
Your solution seems to be the missing piece. When I piped in 350
lines of tail, your's worked where as mine still seems to hang there.
I even tried to send my output to a file, and still nothing. Maybe the
man page could get an update describing what --line-buffer actually
does? Just a thought.
Once again, Thank you very much for your assistance. My work
around did the job, but I would much rather have it done in real time.
(or as close as possible without actually writing a C/C++ program)
Sincerely,
Scott Griffith
[EMAIL PROTECTED]
--- Bob Proulx <[EMAIL PROTECTED]> wrote:
> You sent your message to bug-textutils which may indicate that you
> have a very old version. The current programs are in the coreutils
> package and would have directed you to the bug-coreutils mailing
> list. The current stable version is 5.94. But that won't change
> anything for the purposes of your problem.
>
> ftp://ftp.gnu.org/gnu/coreutils/coreutils-5.94.tar.gz (7.6MB)
> ftp://ftp.gnu.org/gnu/coreutils/coreutils-5.94.tar.bz2 (4.9MB)
>
> Scott Griffith wrote:
> > I am not sure if this is as designed, but the following produce
> results
> > that you would not expect:
>
> I think standard I/O file buffering is the reason here.
>
> > cat /var/log/messages | grep -i fail | cut -f 9- -d ' '
>
> The 'cat' exits and buffers are flushed, the grep exits and buffers
> are flushed. The cut writes to a tty and so output is not buffered.
>
> > using the following produces nothing, yet it should continually
> yield
> > the above messages when watching the system log:
> >
> > tail -f /var/log/messages | grep -i fail | cut -f 9- -d ' '
>
> I think they would, if you waited long enough, generated enough input
> to fill a large I/O buffer and allow the data to be written out. But
> until that happens the data is in a standard I/O buffer pending more
> input.
>
> > however, I am able to use the following command and get output:
> >
> > tail -f /var/log/messages | grep -i fail
>
> In this command 'grep' writes to a tty instead of a pipe. The C
> stdio
> library does not buffer when writing to a tty because it assumes a
> user is there. But when writing to anything else, file or pipe, it
> does buffer.
>
> Try this:
>
> tail -f /var/log/messages | grep --line-buffer -i fail | cut -f 9-
> -d ' '
>
> > cat /var/log/messages | grep -i fail | cut -f 9- -d ' '
>
> By the way... You don't need 'cat' in your above example. Just use
> grep, remove the extraneous process, and redirect the input from the
> file directly.
>
> grep -i fail < /var/log/messages | cut -f 9- -d ' '
>
> Also, because logs tend to get rotated by the system, you might want
> to look at the tail follow options. Try --follow=name so that it can
> follow the rotations of the log file.
>
> tail --follow=name /var/log/messages | grep --line-buffer -i fail |
> cut -f 9- -d ' '
>
> Hope that helps,
> Bob
>
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