Slackware Linux Essentials
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15.3 `tar`

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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.

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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.

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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.

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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:

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-% tar -xvzf filename.tar.gz
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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.

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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:

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

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

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-% tar xvzf filename.tar.gz
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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:

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-% tar -xvjf filename.tar.bz2
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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.

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

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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.

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

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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.

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