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Re: possible bug in coreutils?

From: Bob Proulx
Subject: Re: possible bug in coreutils?
Date: Wed, 22 Mar 2006 10:11:28 -0700
User-agent: Mutt/1.5.9i

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

> 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,

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