Piping in and out of tar and gzip

Question

I am trying to get a handle on how to pipe from a command into something like gzip, cpio or tar.

The commands in question belong to the ZFS system. I am running ZFS on Ubuntu Linux 10.04.3.

The commands I working with are;

To create a snapshot;
zfs snapshot media/mypictures@20070607

To copy the snapshot to a directory;
zfs send media/mypictures@20070607 > ~/backups/20070607

Then I can also pipe into gzip
zfs send media/mypictures@20070607 | gzip > ~/backups/20070607.gz

These parts I understand.

But my first question is, what would I do to pipe into tar + gzip?

This?

zfs send media/mypictures@20070607 | tar czvf > ~/backups/20070607.tar.gz

And my other question is how would I get the data out of the tarball or gzip?

I have to use zfs recieve media/mypictures@20070607 < ~/backups/20070607

So would it just be this if I was using tar?

zfs recieve media/mypictures@20070607 | tar xzvf < ~/backups/20070607.tar.gz

Any idea?

Solution 1

tar expects to be given a list of filenames as command line arguments. So far as I know, it cannot be used to read an anonymous pipeline (what would it store as filename and file-metadata?)

$ echo aaa | tar cv > f.tar
tar: Cowardly refusing to create an empty archive
Try `tar --help' for more information.

If your data is a single logical item or stream, there is no need to use something like tar, which is used to group together a set of separate files.

Otherwise you will have to write the data to a file first, tar the file, then delete the file.

Solution 2

The -f option specifies a file, else the output goes to stdout. So, either drop the redirection:

zfs send media/mypictures@20070607 | tar czvf ~/backups/20070607.tar.gz

Or drop the f option:

zfs send media/mypictures@20070607 | tar czv > ~/backups/20070607.tar.gz

Similarly with untarring.

Comments

[ianc1215]

Really? I though tar was just another form of compression. So skip tar and just gzip?

[user5249203]

Yes, just use zfs send media/mypictures@20070607 | gzip -c > ~/backups/20070607.gz

[drevicko]

Tar DOES NOT expect a list of file names unless you specify the -f option (tar also accepts f without the "-").

[user5249203]

@drevicko: The context of the question is creating a tar archive. The list of files I mean is the list of files (and/or directories) to be included in the written archive. For example: cd /etc; TAPE=/tmp/rgb.tar tar c hosts passwd where the list of files is hosts passwd

[drevicko]

@RedGrittyBrick but the context of the question is to tar from a pipe, ie: no list of files. Saying tar expects a list of files is not true. The problem is that it expects a (single) output file name when the OP specifies f, and he didn't provide it.

[425nesp]

Does this actually work? Both echo "hello" | tar czvf foo.tar.gz and echo "hello" | tar czv > foo.tar.gz fail for me.