Redirecting log files to null writing junk into log files Post: 303008204

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Top Forums Shell Programming and Scripting Redirecting log files to null writing junk into log files Post 303008204 by bakunin on Tuesday 28th of November 2017 02:05:59 PM

11-28-2017

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Quote:

Originally Posted by greenworld123

the process prints some garbage and then with proper text. it is not the process writing garbage i think.

I think you have already gotten very good advice about how to solve it but it might be helping you to understand what is going on:

When a process "opens" a file it calls some OS function (namely fopen()) and part of this "opening" is that the OS sets up an environment through which the process can access the file. Part of this is to find out how big (=how many bytes) a file is. The process also gets a "place" where it "stands" right now. This "place" can be moved forward, backwards, etc., but only within the limits of the length of the file.

Say, a program opens a file and is told that the file is 10 bytes long. Right now it "stands" on byte 1 and it can read it, which would move the place it stands forward to byte 2, etc.. It can also do things like "go forward 3 bytes and then read (or write) 2 bytes from there". It can also add to the file, which would increase the size so that now it can position its place to byte 11. But if it tries to do something impossible (like "go to byte number <behind the current length>" it would receive an error because the OS "knows" that the file is only as long as it is.

All this works well as long as one process accesses a file. But in your case a process opened a file and wrote lots of bytes into it, making its length some big number in the "internal bookkeeping" of the OS. Now a second process (your shell command) truncated the file and but for the first program it is still as long as it was when it last added to it. If it tries to read something from further up (like when it tries to print the content) of course it will get garbage because what it reads is some random block on a disk which is not part of the file any more - but the program won't know that.

Log writing processes should therefore NOT write into log files continuously but open and close the log for every write action separately.

I hope this helps.

bakunin

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scanlogs

SCANLOGS(8)						      System Manager's Manual						       SCANLOGS(8)

NAME

       scanlogs - summarize INN log files.

SYNOPSIS

       scanlogs [ norotate ] [ nonn ]

DESCRIPTION

       Scanlogs  summarizes  the information recorded in the INN log files (see newslog(5)).  By default, it also rotates and cleans out the logs.
       It is normally invoked by the news.daily(8) script.

KEYWORDS

       The following keywords are accepted:

       norotate
	      Using this keyword disables the rotating and cleaning aspect of the log processing: the logs files are only scanned for  information
	      and no contents are altered.

       nonn   Normally the nn log file is scanned and rotated.	Using this keyword disables this function.

       If scanlogs is invoked more than once a day, the ``norotate'' keyword should be used to prevent premature log cleaning.

HISTORY

       Written by Landon Curt Noll <chongo@toad.com> and Rich $alz <rsalz@uunet.uu.net> for InterNetNews.  This is revision 1.1, dated 1996/10/29.

SEE ALSO

       innd(8) newslog(5), news.daily(8), nnrpd(8).

																       SCANLOGS(8)

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