Convert TAR.Z to 7Z Online (Trading 1984 LZW Coding for 1999's LZMA2 Container)

Fifteen Years of Compression Research Between These Two Formats

A .tar.Z file pairs tar's file-bundling structure with the Unix compress command's LZW compression, implemented by Spencer Thomas in 1984 and finalized as version 4.0 in 1985. A .7z file, developed by Igor Pavlov and first released in 1999, is a completely different kind of container that stores its own file list and metadata directly rather than relying on tar, and typically compresses using LZMA2, a considerably more advanced algorithm whose own container specification wasn't finalized until December 2008 — meaning the compression method most 7z archives actually use postdates compress's original LZW implementation by roughly a quarter century.

Converting from .tar.Z to .7z means fully decompressing the older LZW-compressed stream to recover the plain tarball, then rebuilding that content inside 7z's own container using LZMA2 or another of its supported methods. The tar layer disappears entirely in this process, since 7z has its own native way of recording file names, folder structure, and metadata that doesn't depend on tar's header format as an intermediate step the way compress's simpler design always required.

Because compress can only shrink one file at a time, exactly the same limitation 7z doesn't share, tar was always a required first step for bundling multiple files under the old .tar.Z pairing. 7z has no equivalent requirement, since its container natively handles multiple files and folders without needing a separate bundling tool first — a structural simplification that didn't exist yet when compress was designed.

Dictionary Substitution Against a Vastly Larger Dictionary Window

Compress's LZW encoding scans forward through data, building a dictionary of repeated byte sequences and replacing them with progressively wider codes, starting at 9 bits and growing to a configurable maximum, typically 16 bits, as that dictionary fills. LZMA2, the algorithm 7-Zip most commonly uses, works on a similar basic principle of matching repeated sequences, but its dictionary can scale far beyond anything LZW's 16-bit code ceiling allows — up to 64 MiB at the highest standard preset in general xz-style tooling, letting it spot repetition across an enormously wider span of a file than compress's dictionary, bounded by its own bit-width limit, ever could.

LZMA2 also finishes its compression with range coding, a more precise form of entropy coding than anything in compress's original pipeline, which has no dedicated entropy-coding stage after its LZW substitution step at all. This combination — a far larger dictionary plus a genuinely more sophisticated final encoding stage — is the specific, technical reason 7z archives built with LZMA2 routinely compress meaningfully tighter than the same content compressed with compress's older LZW scheme, not simply a vague "newer is better" claim.

That compression advantage comes at a real cost in the other direction: LZMA2 requires substantially more memory and processing time to compress than LZW's comparatively lightweight 1980s-era design ever needed, which was appropriate for the hardware available when compress was written but would be considered minimal by any current standard.

What Trading LZW for LZMA2 Actually Changes

• Gain — a meaningfully smaller archive on most typical data: LZMA2's much wider dictionary and range coding generally outperform LZW's older, narrower dictionary substitution by a real margin.
• Gain — optional AES-256 encryption: 7z supports built-in password protection, something compress's format has never offered at all.
• Gain — solid-block compression across files: 7z can group multiple files into shared compression blocks, letting LZMA2 find repetition across file boundaries the way tar's single continuous stream did for compress, but with configurable granularity.
• Lose — the magic-number safety check compress used: 7z relies on its own distinct header signature and per-file CRC32 checks rather than compress's simple 0x1F 0x9D header.
• Lose — native, always-on Unix permission storage: tar's Unix-first header design carried permission bits directly; 7z's native model centers on Windows-style attributes, with Unix support only as an extension.
• Unchanged — every actual file's content: both LZW and LZMA2 are lossless, so extracted files come out identical regardless of which algorithm compressed them.

Where Each Format Actually Survives in Practice Today

Compress remains standard, built-in tooling on several current commercial Unix systems specifically — Oracle Solaris and illumos, IBM AIX, and HP-UX all continue to ship a working compress binary by default, unlike most Linux distributions, which generally treat it as optional legacy software at best. 7z, by contrast, is overwhelmingly a Windows desktop format in practice, since 7-Zip itself began and remains primarily a Windows application, even though compatible ports like p7zip extend it to Linux and macOS as well.

Opening an existing .tar.Z file on a modern system generally works through gzip's own bundled uncompress and zcat commands, the standalone ncompress package, or general tools like 7-Zip and The Unarchiver, though almost none of these default to creating new .Z-compressed output. Opening a .7z file on Linux, meanwhile, requires explicitly installing p7zip, since it isn't part of a typical default installation there any more than compress creation tools are on most current Linux systems.

For anyone converting a genuinely old .tar.Z archive specifically to make it easier for Windows-based colleagues to open, 7z's native Windows integration through 7-Zip's shell context menu is the practical benefit driving this direction of conversion — the recipient never needs to know what LZW or compress even is, unlike the original .Z format, which almost always requires them to install something extra first.

Mobile platforms follow a similar pattern worth checking before assuming either format travels equally well: free archive apps on both iOS and Android generally handle 7z extraction reliably, while support for compress's older LZW format on those same mobile apps is considerably less consistent, since compress simply isn't a format most current app developers prioritize supporting at all.

The Real Complaint Behind "My Decades-Old Archive Won't Even Open Anymore"

A documented, recurring issue reported when working with genuinely old .tar.Z archives on modern, minimal, or containerized Linux environments is a missing compress-compatible decompressor entirely, since some slimmed-down base images deliberately exclude less commonly used compression tools to reduce their footprint. The fix reported in these threads is explicitly installing ncompress or an equivalent package rather than assuming every environment that handles gzip or bzip2 automatically handles compress as well — it often doesn't.

A second real pattern involves the code-width byte compress stores in its header: some very old .tar.Z files were built with a lower maximum code width than the common 16-bit default, and a decompressor that assumes the newer, more common setting instead of reading that specific byte can misinterpret those older archives — a narrow but genuinely documented compatibility gap that correctly implemented tools, including most current 7-Zip builds, handle without issue by actually checking the header.

A third issue shows up around expected size after this conversion: someone converting a .tar.Z archive to 7z using default settings and expecting a dramatic size reduction sometimes needs to explicitly select LZMA2 as the compression method and a reasonably high dictionary setting, since 7-Zip's own default settings for a given archive don't automatically guarantee the largest possible improvement over the much older LZW-compressed original.

1984's LZW Coding Set Beside 1999's LZMA2-Capable Container

Questions About Rebuilding an Old Compress Archive as 7z

How much smaller will my file get converting from .tar.Z to .7z?
Often meaningfully smaller, since LZMA2's much wider dictionary and range coding generally outperform LZW's older, narrower dictionary substitution on typical data.

Can I add a password when rebuilding as 7z?
Yes. 7z supports built-in AES-256 encryption directly, a feature compress's original format has never offered at all.

Why won't my old .tar.Z file open on a new Linux server?
Some modern, minimal Linux environments don't include compress-compatible decompression by default. Installing ncompress or a similar package typically resolves it.

Do Unix permissions survive this conversion?
Only partially by default. 7z's native attribute model is built around Windows conventions, though Unix permission data can be stored through an extension depending on the tool used.

Is there any reason to keep using compress instead of 7z today?
Mainly compatibility with specific legacy systems, particularly commercial Unix platforms like Solaris, AIX, or HP-UX that still ship compress as standard tooling and may have scripts expecting it by name.

Does converting from .tar.Z to .7z change the order files appear in?
It can. 7z's own indexing, and its solid-block grouping if enabled, may reorder entries internally to improve compression, while tar simply lists files in the order they were originally added, so a raw listing comparison between the two isn't meaningful even when the actual contents match exactly.