RHCSA Series: How to Perform File and Directory Management - Part 2 - Part 2

Archiving, Compressing, Unpacking, and Uncompressing Files

If you need to transport, backup, or send via email a group of files, you will use an archiving (or grouping) tool such as tar, typically used with a compression utility like gzip, bzip2, or xz.

Your choice of a compression tool will be likely defined by the compression speed and rate of each one. Of these three compression tools, gzip is the oldest and provides the least compression, bzip2 provides improved compression, and xz is the newest and provides the best compression. Typically, files compressed with these utilities have .gz, .bz2, or .xz extensions, respectively.

Command Abbreviation Description –create c Creates a tar archive –concatenate A Appends tar files to an archive –append r Appends non-tar files to an archive –update u Appends files that are newer than those in an archive –diff or –compare d Compares an archive to files on disk –list t Lists the contents of a tarball –extract or –get x Extracts files from an archive Operation modifier Abbreviation Description —directory dir C Changes to directory dir before performing operations —same-permissions and —same-owner p Preserves permissions and ownership information, respectively. –verbose v Lists all files as they are read or extracted; if combined with –list, it also displays file sizes, ownership, and timestamps —exclude file — Excludes file from the archive. In this case, file can be an actual file or a pattern. —gzip or —gunzip z Compresses an archive through gzip –bzip2 j Compresses an archive through bzip2 –xz J Compresses an archive through xz Example 5: Creating a tarball and then compressing it using the three compression utilities

You may want to compare the effectiveness of each tool before deciding to use one or another. Note that while compressing small files, or a few files, the results may not show much differences, but may give you a glimpse of what they have to offer.

# tar cf ApacheLogs-$(date +%Y%m%d).tar /var/log/httpd/*        # Create an ordinary tarball
# tar czf ApacheLogs-$(date +%Y%m%d).tar.gz /var/log/httpd/*    # Create a tarball and compress with gzip
# tar cjf ApacheLogs-$(date +%Y%m%d).tar.bz2 /var/log/httpd/*   # Create a tarball and compress with bzip2
# tar cJf ApacheLogs-$(date +%Y%m%d).tar.xz /var/log/httpd/*    # Create a tarball and compress with xz

tar command examples Example 6: Preserving original permissions and ownership while archiving and when

If you are creating backups from users’ home directories, you will want to store the individual files with the original permissions and ownership instead of changing them to that of the user account or daemon performing the backup. The following example preserves these attributes while taking the backup of the contents in the /var/log/httpd directory:

# tar cJf ApacheLogs-$(date +%Y%m%d).tar.xz /var/log/httpd/* --same-permissions --same-owner

Create Hard and Soft Links

In Linux, there are two types of links to files: hard links and soft (aka symbolic) links. Since a hard link represents another name for an existing file and is identified by the same inode, it then points to the actual data, as opposed to symbolic links, which point to filenames instead.

In addition, hard links do not occupy space on disk, while symbolic links do take a small amount of space to store the text of the link itself. The downside of hard links is that they can only be used to reference files within the filesystem where they are located because inodes are unique inside a filesystem. Symbolic links save the day, in that they point to another file or directory by name rather than by inode, and therefore can cross filesystem boundaries.

The basic syntax to create links is similar in both cases:

# ln TARGET LINK_NAME               # Hard link named LINK_NAME to file named TARGET
# ln -s TARGET LINK_NAME            # Soft link named LINK_NAME to file named TARGET

Example 7: Creating hard and soft links

There is no better way to visualize the relation between a file and a hard or symbolic link that point to it, than to create those links. In the following screenshot you will see that the file and the hard link that points to it share the same inode and both are identified by the same disk usage of 466 bytes.

On the other hand, creating a hard link results in an extra disk usage of 5 bytes. Not that you’re going to run out of storage capacity, but this example is enough to illustrate the difference between a hard link and a soft link.

Difference between a hard link and a soft link

A typical usage of symbolic links is to reference a versioned file in a Linux system. Suppose there are several programs that need access to file fooX.Y, which is subject to frequent version updates (think of a library, for example). Instead of updating every single reference to fooX.Y every time there’s a version update, it is wiser, safer, and faster, to have programs look to a symbolic link named just foo, which in turn points to the actual fooX.Y.

Thus, when X and Y change, you only need to edit the symbolic link foo with a new destination name instead of tracking every usage of the destination file and updating it.

Summary

In this article we have reviewed some essential file and directory management skills that must be a part of every system administrator’s tool-set. Make sure to review other parts of this series as well in order to integrate these topics with the content covered in this tutorial.

Feel free to let us know if you have any questions or comments. We are always more than glad to hear from our readers.

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