Does tar's z option compress on the fly?

[rharder]

When using tar (or gnutar or star) does the z parameter compress the data all in one fell swoop or does it tar the directories and then zip it?

The reason I'm wondering is that when I backup my OS X to my Win98 box, my 100baseT network only gets 200K throughput, so if I could compress the data that would help me out.

In other words, if I execute

% <b>sudo gnutar cpvzf /backup/usr.tar /usr</b>

does that compress the stream across the network (to the /backup mount) or does it tar across the network, read it back from the network, compress it, and write it back across the network?

BTW, has anyone gotten star to compile on OS X? It's supposed to be an ultra-fast tar.

-Rob

[RacerX]

I have always needed to add the zip (.z) or gnuzip (.gz) after creating the my tar archive. gnutar never gave me all that much in the way of space savings. I had moved most of my things off my original drive to my Indy when I got my 30 GB drive.

[howardm4]

It does neither of the things you describe but it APPEARS to do #1.

What it does in reality is setup a pipeline so that the gzip binary is transparently called and the final output is sent to the final
destination.

It is equiv to "tar cvf - <directory> | gzip -c </destination/tarball.tar.gz>

[genecutl]

Originally posted by howardm4
It is equiv to "tar cvf - <directory> | gzip -c </destination/tarball.tar.gz>

The benefit of using the syntax listed above, is you can add additional flags for gzip such as this:

tar -cvf <dir> | gzip -c --best > <dir.tgz>

which gives you the maximal compression out of gzip.

[genecutl]

Originally posted by howardm4
It is equiv to "tar cvf - <directory> | gzip -c </destination/tarball.tar.gz>

The benefit of using the syntax listed above, is you can add additional flags for gzip such as this:

tar -cvf <dir> | gzip -c --best > <dir.tgz>

which gives you the maximal compression out of gzip.

[howardm4]

--best is usually a pretty waste of time. The change in compression vs. the
increased runtime isn't worth it.

Just use a better/different tool such as bzip2

[rharder]

So when I pipe the tar command to gzip or bzip2 or whatever, does <em>that</em> happen on the fly or does it write the whole tar file to /tmp and <em>then</em> compress it?

-Rob

[offets]

You have to use the 'z' first, because its all done in order.

Example tar zxvf file.tar.gz