Slackware Linux Essentials
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15.3 `tar`

+ +

tar(1) is the GNU tape archiver. It takes several files or +directories and creates one large file. This allows you to compress an entire directory +tree, which is impossible by just using gzip or bzip2. tar has many command line options, +which are explained in its man page. This section will just cover the most common uses of +tar.

+ +

The most common use for tar is to decompress and unarchive a +package that you've downloaded from a web site or ftp site. Most files will come with a +.tar.gz extension. This is commonly known as a +“tarball”. It means that several files were archived using tar and then compressed using gzip. You +might also see this listed as a .tar.Z file. It means the same +thing, but this is usually encountered on older Unix systems.

+ +

Alternatively, you might find a .tar.bz2 file somewhere. +Kernel source is distributed as such because it is a smaller download. As you might have +guessed, this is several files archived with tar and then +bzipped.

+ +

You can get to all the files in this archive by making use of tar and some command line arguments. Unarchiving a tarball makes use +of the -z flag, which means to first run the file through gunzip and decompress it. The most common way to decompress a +tarball is like so:

+ + + + + +

+% tar -xvzf filename.tar.gz
+

+ +

That's quite a few options. So what do they all mean? The -x +means to extract. This is important, as it tells tar exactly +what to do with the input file. In this case, we'll be splitting it back up into all the +files that it came from. -v means to be verbose. This will list +all the files that are being unarchived. It is perfectly acceptable to leave this option +off, if somewhat boring. Alternatively, you could use -vv to be +very verbose and list even more information about each file being unarchived. The -z option tells tar to run filename.tar.gz through gunzip first. And +finally, the -f option tells tar that +the next string on the command line is the file to operate on.

+ +

There are a few other ways to write this same command. On older systems lacking a +decent copy of GNU tar, you might see it written like so:

+ + + + + +

+% gunzip filename.tar.gz | tar -xvf -
+

+ +

This command line will uncompress the file and send the output to tar. Since gzip will write its output to +standard out if told to do so, this command will write the decompressed file to standard +out. The pipe then sends it to tar for unarchiving. The +“-” means to operate on standard input. It will unarchive the stream of data +that it gets from gzip and write that to the disk.

+ +

Another way to write the first command line is to leave off the dash before the +options, like so:

+ + + + + +

+% tar xvzf filename.tar.gz
+

+ +

You might also encounter a bzipped archive. The version of tar that comes with Slackware Linux can handle these the same as +gzipped archives. Instead of the -z command line option, you'd +use -j:

+ + + + + +

+% tar -xvjf filename.tar.bz2
+

+ +

It is important to note that tar will place the unarchived +files in the current directory. So, if you had an archive in /tmp that you wanted to decompress into your home directory, there +are a few options. First, the archive could be moved into your home directory and then +run through tar. Second, you could specify the path to the +archive file on the command line. Third, you can use the -C +option to “explode” the tarball in a specified directory.

+ + + + + +

+% cd $HOME
+% cp /tmp/filename.tar.gz .
+% tar -xvzf filename.tar.gz
+
+% cd $HOME
+% tar -xvzf /tmp/filename.tar.gz
+
+% cd /
+% tar -xvzf /tmp/filename.tar.gz -C $HOME
+

+ +

All the above statements are equivalent. In each case, the archive is unpacked inside +your home directory and the original uncompressed archive is left in place.

+ +

So what good is being able to uncompress these archives if you can't make them? Well, +tar handles that too. In most cases it's as easy as removing the +“-x” option and replacing it with the “-c” option.

+ + + + + +

+% tar -cvzf filename.tar.gz .
+

+ +

In this command line, the -c option tells tar to create an archive, while the -z +option runs the resulting archive file through gzip to compress +it. filename.tar.gz is the file that you want to create.

+ +

Specifying the “-f” option isn't always +necessary, but is typically good practice anyway. Without it, tar writes to standard output, which is usually desired for piping +tar's output to another program, like so.

+ + + + + +

+% tar -cv filename.tar . | gpg --encrypt
+

+ +

That command creates an non-compressed tar archive of the current directory, pipes the +tarball through gpg which encrypts and compresses the tarball, +making it realistically impossible to read by anyone other than the person knowing the +secret key.

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15.3 tar

15.3 `tar`