Re: [Bug-tar] (forw) tar --listed-incremental bogous?

[Sergey Poznyakoff]

Hi Andreas,

> then tar is broken in some othe way. as my modified demo script
> shows, it does not honor the snapshot file and does a full backup
> again, including all files in the backup. it should have scipped
> file1 in the second backup.

Incremental backups the way they are implemented now, have
given raise to quite a number of bug reports and notices. This is
partly because of the intrinsic inconsistencies in the implementation
itself (see below), partly because of the lack of documentation.

Notice, that listed-incremental, in its present state, assumes that
the names given to tar are actually *directory* names (that's because
it is based on comparing the directory mtime).  So, currently,
instead of `tar --listed=name ... $DIR/file*' one should use
`tar --listed=name ... $DIR'.

Another point is that the first line of the snapshot file contains the
timestamp of the last run of tar. Thus the lines 17-21 of your test script:

   mkdir $DIR
   touch $F1
   echo "$DIR/*1" > $INCF

   tar --verbose \

will, in the majority of runs, cause the timestamp saved in
$SNAPSHOT_FILE to equal the mtime of the file $F1, and therefore
this file will be considered new when invoking tar second time.
That is the reason while file1 gets included twice. Simply adding `sleep 1',
after `touch' (along with the above modification) will make tar behave
as it should.

Now I am doing a major rewrite of tar, which among other issues will
address incremental backups. In the meantime, however, I hope that the
above description will help you in using current snapshots of tar.

Regards,
Sergey