• Choose a category
• All
• Classic-Fiction

By Root 1099 0

Therefore, when the archive is extracted, the subdirectory is also created and any files placed there. This way you can ensure that the files won't be placed directly in your current working directory; they will be tucked out of the way and prevent confusion. This also saves the person doing the extraction the trouble of having to create a separate directory (should they wish to do so) to unpack the tar file. Of course, there are plenty of situations where you wouldn't want to do this. So much for etiquette.

When creating archives, you can, of course, give tar a list of files or directories to pack into the archive. In the first example, we have given tar the single directory mt, but in the previous paragraph we used the wildcard *, which the shell expands into the list of filenames in the current directory.

Before extracting a tar file, it's usually a good idea to take a look at its table of contents to determine how it was packed. This way you can determine whether you do need to create a subdirectory yourself where you can unpack the archive. A command such as:

tar tvf tarfile

lists the table of contents for the named tarfile. Note that when using the t function, only one v is required to get the long file listing, as in this example:

courgette% tar tvf mt.tar

drwxr-xr-x root/root 0 Nov 16 19:03 2004 mt/

-rw-r--r-- root/root 11204 Sep 5 13:10 2004 mt/st_info.txt

-rw-r--r-- root/root 847 Sep 21 16:37 2004 mt/README

-rw-r--r-- root/root 2775 Aug 7 09:50 2004 mt/mt.1

-rw-r--r-- root/root 24 Sep 21 16:03 2004 mt/Makefile

-rw-r--r-- root/root 6421 Aug 7 09:50 2004 mt/mt.c

-rw-r--r-- root/root 3948 Nov 16 19:02 2004 mt/mt.o

-rwxr-xr-x root/root 9220 Nov 16 19:03 2004 mt/mt

No extraction is being done here; we're just displaying the archive's table of contents. We can see from the filenames that this file was packed with all files in the subdirectory mt, so that when we extract the tar file, the directory mt will be created and the files placed there.

You can also extract individual files from a tar archive. To do this, use the command:

tar xvf tarfile files

where files is the list of files to extract. As we've seen, if you don't specify any files, tar extracts the entire archive.

When specifying individual files to extract, you must give the full pathname as it is stored in the tar file. For example, if we wanted to grab just the file mt.c from the previous archive mt.tar, we'd use the command:

tar xvf mt.tar mt/mt.c

This would create the subdirectory mt and place the file mt.c within it.

tar has many more options than those mentioned here. These are the features that you're likely to use most of the time, but GNU tar, in particular, has extensions that make it ideal for creating backups and the like. See the tar manual page and the following section for more information.

Using tar with gzip and bzip2

tar does not compress the data stored in its archives in any way. If you are creating a tar file from three 200 K files, you'll end up with an archive of about 600 K. It is common practice to compress tar archives with gzip (or the older compress program). You could create a gzipped tar file using the commands:

tar cvf tarfile files...

gzip -9tarfile

But that's so cumbersome, and requires you to have enough space to store the uncompressed tar file before you gzip it.

A much trickier way to accomplish the same task is to use an interesting feature of tar that allows you to write an archive to standard output. If you specify - as the tar file to read or write, the data will be read from or written to standard input or output. For example, we can create a gzipped tar file using the command:

tar cvf - files... | gzip -9 > tarfile.tar.gz

Here, tar creates an archive from the named files and writes it to standard output; next, gzip reads the data from standard input, compresses it, and writes the result to its own standard output; finally, we redirect the gzipped tar file to tarfile.tar.gz.

We could extract such a tar file using the command:

gunzip -c tarfile.tar.gz | tar xvf -

gunzip