Use tar to append?

[Mike McCarty]

Les Mikesell wrote:
> Mike McCarty wrote:
> 
>> I have a backup script which I run on some sort of regular
>> basis. I use tar to create an archive, which I then split
>> into pieces of CDROM size (703MB) and write to CDROMs.

[snip]

>> Is there a way to get tar to use the archive it is adding to
>> "in place"? I've read man and info, and I see --append
>> (which is what I was using) and --catenate (which looks
>> marginally faster, perhaps, since I compress), but see
>> no way to make it "just do it" without doing an implicit
>> copy.
>>
> 
> You can't append to an already gzipped file, so it must be copying the 
> previous section by uncompressing from the start and recompressing a new 
> copy so it can continue with the compressor in the right state.  Have 
> you tried not using -z with tar while creating the archive, then piping 
> through gzip and split at the end?

I have tried just using cat to put them together, then telling
tar -i (ignore zeroes) and can successfully get a TOC back out.

Further, the man page for gzip states:

[QUOTE MODE ON]

Multiple  compressed  files  can be concatenated. In this case, gunzip
will extract all members at once. For example:
 

       gzip -c file1  > foo.gz
       gzip -c file2 >> foo.gz
 
                Then
 

       gunzip -c foo
 

is equivalent to
 

       cat file1 file2

[QUOTE MODE OFF]

So, I trow you are not quite correct on that point. I suspect
that sth like

for path in $backupdirs
do
	tar cvz $badkupdir >> backup.tgz
...

will work, but need the -i for recovery. This is not quite
what I would like, of course. Also, I'm not quite clear
whether usig >> in this wise obviates the copy.

Mike
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