Re: Command "tail" printing of '\b' and '\t' Characters

[Kevin R Frank]

---------------------- Forwarded by Kevin R Frank/Raleigh/IBM on 09/05/2001
09:31 AM ---------------------------



Kevin R Frank <address@hidden> on 09/05/2001 02:18:08 PM


Please respond to Kevin R Frank  address@hidden

To:   Kevin R Frank/Raleigh/address@hidden
cc:   Timothy Smith/Raleigh/address@hidden, GM Kump/Raleigh/address@hidden, 
Kevin R
      Frank/Raleigh/address@hidden, John Lloyd/Raleigh/address@hidden, Robert
      Kantner/Raleigh/address@hidden, Burt Silverman/Raleigh/address@hidden, 
John F
      Davis/Raleigh/address@hidden, David D Smith/Raleigh/address@hidden, Truc C
      Nguyen/Raleigh/address@hidden

Subject:  Re: Command "tail" printing of '\b' and '\t' Characters




Hello Bob:

Thank you for the response.  Here are the version strings:

mrx:/home/krfrank/tail> tail --version
tail (GNU textutils) 2.0
Written by Paul Rubin, David MacKenzie, Ian Lance Taylor, and Jim Meyering.

Copyright (C) 1999 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

mrx:/home/krfrank/tail> cat /proc/version
Linux version 2.2.14-5.0 (address@hidden) (gcc version egcs-2.91.66
19990314/Linux (egcs-1.1.2 release)) #4 Wed Dec 20 17:51:31 EST 2000

A program that reproduces the problem follows these directions.

Copy the source to a file called tail.c.

Build the program with the following command:

cc -c -Wall -O tail.c

Issue a tail on the /var/log/messages file on another tty:

tail -f /var/log/messages

Load the module into your kernel after a "switch user to root":

insmod tail.o

Remove the module with the following:

rmmod tail

The tail command will output:
tail -f /var/log/messages
Sep  5 13:51:05 mrx kernel: tail.c^H^H::init_module() line 20
Sep  5 13:51:05 mrx kernel: ^IThe back space and tab characters show up as:
Sep  5 13:51:05 mrx kernel: ^I^I- backspace -> ^H
Sep  5 13:51:05 mrx kernel: ^I^I- tab       -> ^I
Sep  5 13:51:16 mrx kernel: tail.c^H^H::cleanup_module() line 34
Sep  5 13:51:16 mrx kernel: ^IUnloading...

If the output does not show up on the tail tty, do a:

ps -e -f

Issue a kill on the syslogd and klogd deamons.  Then restart the deamons:

syslogd -m O
klogd

Then repeat the tail, insmod, and rmmod commands.

Thank you,
Kevin R Frank
IBM Microelectronics
address@hidden

Here is a program to reproduce the problem:
===========================================================================

#ifndef __KERNEL__
#define __KERNEL__
#endif
#ifndef MODULE
#define MODULE
#endif

#include <linux/module.h>
#include <linux/version.h>
#include <linux/kernel.h>

int
init_module (
    void)
{
    printk(KERN_WARNING "%s\b\b::%s() line %u\n",
                                             __FILE__, __FUNCTION__,
__LINE__);
    printk(KERN_WARNING "\tThe back space and tab characters show up as:
\n");
    printk(KERN_WARNING "\t\t- backspace -> ^H\n");
    printk(KERN_WARNING "\t\t- tab       -> ^I\n");

    return (0);
}


void
cleanup_module (
    void)
{
    printk(KERN_WARNING "%s\b\b::%s() line %u\n",
                                             __FILE__, __FUNCTION__,
__LINE__);
    printk(KERN_WARNING "\tUnloading...\n");
    return;
} /* end cleanup_module() */


MODULE_DESCRIPTION("Example Error When Using \"tail\" on Kernel Message
Log");
MODULE_SUPPORTED_DEVICE("None");
MODULE_AUTHOR("Kevin R Frank -- IBM");

===========================================================================

>
> ---------------------- Forwarded by Kevin R Frank/Raleigh/IBM on
09/05/2001
> 04:46 AM ---------------------------
>
> Bob Proulx <address@hidden> on 09/04/2001 08:06:13 PM
>
> To:   Kevin R Frank/Raleigh/address@hidden
> cc:   address@hidden
> Subject:  Re: Command "tail" printing of '\b' and '\t' Characters
>
>
>
> Kevin
>
> > Kevin R Frank <address@hidden> on 09/04/2001 03:12:38 PM
> > Please respond to Kevin R Frank/Raleigh/address@hidden
>
> Unlikely that anyone outside of IBM would be able to use that
> address.  :-)
>
> > When using the command tail <file> on a file that contain the ASCII
> > characters '\b' (i.e backspace) or '\t' (tab), we get the following
> > characters printed to the terminal rather than the expected characters:
> >
> >         '\b' --> ^H
> >         '\t' --> ^I
> >
> > The output tty is formated incorrectly because these characters are
> > not expanded correctly.
>
> Thanks for the report.  But in the future could you provide the
> version number of the program such as provided by the 'tail --version'
> output and also the smallest possible test case to recreate the
> problem.  There is not yet enough information to determine what is
> happening for you and so we can only guess.
>
> Since backspace is a control-H and tab is a control-I there is a
> relationship there.  But it does not make sense that you would see
> them on your screen.  Your terminal should be tracking those
> characters and operating on them.  You can see the output I see.
>
> Try this experiment:
>
>   echo 'abc'
>   abc
>   echo 'abc\b\bdef'
>   adef
>   echo 'abc\b\bdef' | tail
>   adef
>   echo 'abc\b\bdef' | cat
>   adef
>
> I cannot recreate your problem using tail.  I can forcibly print
> non-printing characters.
>
>   echo 'abc\b\bdef' | cat -v
>   abc^H^Hdef
>
> The cat -v'isible option shows that those control characters are
> there.  The tail command does do character mapping.  Perhaps you are
> seeing the output inside of a less/more pager or inside of an emacs
> shell terminal or perhaps in some other way that displays nonprinting
> characters in a mapped to visible manor?
>
> Bob Proulx
>
>