Tar File Format: Advantages and Uses Today

The Enduring Legacy of the Tar Archiving Format

In the realm of computing, the tar archiving format possesses a remarkable longevity, akin to that of Methuselah. Despite its age, it remains a widely utilized tool in modern systems.

The continued relevance of the tar format begs the question: what attributes contribute to its sustained usefulness even decades after its initial development?

Origins of the Question

This particular inquiry and its subsequent answer were sourced from SuperUser.

SuperUser is a specialized segment within Stack Exchange, a collaborative network of question-and-answer websites.

Stack Exchange functions as a community-driven platform where users can pose questions and receive answers from fellow members.

Why Tar Remains Relevant

The tar format’s persistence isn’t accidental. It offers a simple, yet effective, method for bundling multiple files and directories into a single archive.

While not inherently a compression format, tar is frequently paired with compression algorithms like gzip or bzip2 to reduce file size.

Key Advantages of Using Tar

• Portability: Tar archives are highly portable across different operating systems.
• Simplicity: The format is relatively straightforward to understand and implement.
• Ubiquity: Tar utilities are pre-installed on most Unix-like systems.

These characteristics have cemented tar’s position as a fundamental tool for system administrators, developers, and everyday computer users alike.

Its ability to reliably package and transfer files makes it an indispensable component of many workflows.

A Reader's Inquiry Regarding Tar

A SuperUser user, MarcusJ, has posed an insightful question concerning the tar format. He wonders about its continued relevance given the emergence of more modern archive formats.

I understand that tar was originally designed for tape archives. However, contemporary archive formats integrate both file aggregation and compression into a single file structure.

My questions are as follows:

• Does utilizing tar in conjunction with gzip or bzip2 introduce a performance overhead during aggregation, compression, and decompression compared to formats that combine these processes natively, assuming equivalent compressor speeds (like gzip and Deflate)?
• Are there specific capabilities within the tar file format that are absent in other formats, such as .7z and .zip?
• Considering tar’s age and the availability of newer formats, why does it remain so prevalent across GNU/Linux, Android, BSD, and other UNIX-like systems for file transfers, software distribution, and even package management?

This is a valid point to raise. The computing landscape has evolved significantly over the past three decades, yet tar persists. Let's explore the reasons behind its enduring popularity.

Performance Considerations

The question of performance is a crucial one. When tar is paired with compression tools like gzip or bzip2, it does introduce an extra step. Data must first be archived by tar, and then compressed.

Formats like .zip and .7z perform both operations simultaneously. This integrated approach can, in theory, be more efficient. However, the actual performance difference is often negligible in practice.

Modern hardware and optimized compression algorithms minimize the overhead. Furthermore, the speed of the compression algorithm itself (gzip, bzip2, xz) typically has a greater impact on overall performance than the layering of tar.

Unique Features of Tar

Tar possesses certain characteristics that distinguish it from other archive formats. One key feature is its focus on preserving file metadata.

Tar reliably stores information such as file permissions, ownership, timestamps, and symbolic link structure. This is particularly important in UNIX-based systems where these attributes are integral to file functionality.

While .zip supports some metadata, it's often less comprehensive. .7z offers better metadata support, but historically hasn't been as universally compatible as tar.

The Enduring Legacy of Tar

Despite the existence of newer formats, tar remains widely used for several compelling reasons.

• Ubiquity: Tar is a standard utility on virtually all UNIX-like operating systems. It's a core component of the toolchain.
• Simplicity: The tar format itself is relatively simple. This simplicity contributes to its robustness and ease of implementation.
• Portability: Tar archives are highly portable across different systems and architectures.
• Historical Momentum: A vast amount of existing software and data is distributed in tar archives. Changing this would require a massive undertaking.
• Scripting Friendliness: Tar integrates seamlessly with shell scripting, making it ideal for automated tasks and build processes.

The combination of these factors explains why tar, often coupled with gzip, bzip2, or xz, continues to be a dominant force in the world of archiving and distribution, even in the 21st century.

Understanding the Tar Format: Performance and Features

A SuperUser contributor, Allquixotic, provides valuable insights into the enduring relevance and functionality of the tar format.

Performance Considerations

Let's examine two distinct workflows to understand their implications.

Consider a file, blah.tar.gz, occupying 1 GB in compressed form, which expands to 2 GB upon decompression – representing a 50% compression ratio.

If archiving and compression were performed separately, the process would begin with:

tar cf blah.tar files ...

This command creates blah.tar, a simple aggregation of the files ... in an uncompressed state.

Subsequently, you would execute:

gzip blah.tar

This action reads the contents of blah.tar from the disk, compresses them using the gzip algorithm, and writes the result to blah.tar.gz, finally deleting the original blah.tar file.

Decompression – Method 1

Assuming you have blah.tar.gz, you might run:

gunzip blah.tar.gz

This process will:

• READ the 1GB of compressed data from blah.tar.gz.
• PROCESS the compressed data using the gzip decompressor in memory.
• WRITE the uncompressed data to the file blah.tar on disk as the memory buffer fills, repeating until all data is processed.
• Delete the file blah.tar.gz.

This results in blah.tar on disk, which is uncompressed and contains the original files with minimal overhead. The file size will be slightly larger than the combined size of the original files.

Then, you would run:

tar xvf blah.tar

This will:

• READ the 2GB of uncompressed data and the tar file format’s data structures, including file permissions and names.
• WRITE the 2GB of data and associated metadata to disk, creating or overwriting files and directories as needed.

The total data READ from disk in this method is 1GB (for gunzip) + 2GB (for tar) = 3GB.

The total data WROTE to disk is 2GB (for gunzip) + 2GB (for tar) plus a small amount for metadata, totaling approximately 4GB.

Decompression – Method 2

Again, starting with blah.tar.gz, you could run:

tar xvzf blah.tar.gz

This will:

• READ the 1GB of compressed data from blah.tar.gz in blocks.
• PROCESS the compressed data through the gzip decompressor in memory.
• PIPE the decompressed data directly to the tar file format parser.
• WRITE the uncompressed data to disk, creating files and directories with their contents.

The total data READ from disk in this process is simply 1GB of compressed data.

The total data WROTE to disk is 2GB of uncompressed data plus a small amount for metadata, approximately 2GB.

Notably, the disk I/O in Method 2 is comparable to that of Zip or 7-Zip programs, adjusted for compression ratio differences.

For superior compression, consider using the Xz compressor with tar, creating an LZMA2-based TAR archive that rivals the efficiency of 7-Zip.

Key Features of the Tar Format

The tar format preserves UNIX permissions within its metadata, making it a reliable choice for archiving directories with diverse permissions and symbolic links.

It's also useful for combining files into a single stream without necessarily compressing them, although compression is frequently employed.

Compatibility and Modern Alternatives

tar.gz and tar.bz2 are often used for distribution because they represent a "lowest common denominator" file format. Similar to how most Windows users can decompress .zip or .rar files, most Linux installations, even minimal ones, include tar and gunzip.

Modern projects may opt for newer formats like tar.xz (using Xz compression, which offers better compression than gzip or bzip2) or .7z.

The limited adoption of formats like .7z mirrors the situation with audio codecs like Opus or video codecs like WebM – compatibility with older systems is a primary concern.

Do you have additional insights to share? Please contribute in the comments. For a more extensive discussion, visit the original Stack Exchange thread here.