Convert RAR to TAR.BZ2 Online (Proprietary Compression Swapped for an Open, Block-Sorted One)

Bzip2 Publishes Exactly How It Compresses; RAR Never Has

Bzip2, written by Julian Seward and first released in 1996, compresses data through a fully documented, three-stage pipeline: the Burrows-Wheeler Transform rearranges each block of input to group similar byte sequences together, move-to-front encoding follows, and Huffman coding finishes the job. Every stage of that pipeline is openly specified, which is why independent implementations of bzip2 exist across essentially every platform without any licensing negotiation required. RAR's compression algorithm, by contrast, has never been published as an open specification by RARLAB — only the separate unRAR code for extraction is released, and its license explicitly bars using it to build a RAR-compatible archiver. Converting RAR to tar.bz2 means using WinRAR or unRAR-licensed extraction to recover the original files, then compressing them fresh with bzip2's openly documented method inside a tar archive.

Bzip2 processes data in blocks of up to 900 KB, selectable from 100 KB up in increments controlled by its -1 through -9 flags, and each block carries its own independent compression state. RAR5 instead uses a proprietary dictionary that can reach up to 4 GB, letting its algorithm reference matches across a much wider span of the input than bzip2's block-based approach allows — a real, structural difference in how the two formats find redundancy, not just a difference in compression ratio.

Every bzip2 stream is identifiable by its own fixed header: the three bytes "BZh" followed by a single ASCII digit from 1 to 9 that records the block size actually used, in units of 100 KB. GNU tar's -j flag, the one that invokes bzip2 for a .tar.bz2 file, hands off to bzip2 with its default setting of level 9 (the full 900 KB block size) unless a different level is explicitly requested, so most .tar.bz2 files created through tar carry a "BZh9" header. RAR's own file signature works differently: RAR4 archives open with the seven-byte sequence 52 61 72 21 1A 07 00, while RAR5 archives use an eight-byte version that swaps the final byte to 01 and adds a trailing 00 — a one-byte difference at a fixed offset that file-identification tools use to tell RAR4 and RAR5 apart without opening WinRAR at all.

RAR's Solid Archiving Against Bzip2's Independent Block Structure

A solid RAR archive treats every file inside it as one continuous data stream, letting the algorithm find repeated patterns across file boundaries — genuinely effective for a batch of many small, similar files. Bzip2's block structure works differently: since each 900 KB (or smaller) block is compressed and checksummed independently, a bzip2-compressed tarball doesn't get the same cross-file pattern matching that a solid RAR archive offers, though tar's own bundling still groups the files together before bzip2 ever sees them, so files similar enough to fit within the same block boundary can still compress well together.

The practical consequence of bzip2's independent blocks is resilience rather than ratio: because each block is self-contained, a single corrupted block in a .tar.bz2 file doesn't necessarily prevent recovering the rest of the archive, whereas a solid RAR archive's cross-file dependencies mean damage in one place can affect the ability to extract files that come later in the stream, RAR5's optional recovery record notwithstanding.

What Rebuilding RAR Content as a Bzip2 Tarball Changes

• Gain — an openly documented compression method: bzip2's Burrows-Wheeler Transform and Huffman coding are fully specified, unlike RAR's proprietary algorithm.
• Lose — RAR5's Reed-Solomon recovery record: neither bzip2 nor plain tar has a built-in equivalent for repairing archive damage.
• Gain — a format freely creatable without licensing: bzip2 tools exist on every major platform with no restriction resembling RARLAB's extraction-only unRAR terms.
• Lose — cross-file solid compression: bzip2's independent block structure doesn't match RAR's ability to find patterns across an entire archive's worth of files at once.
• Gain — native Unix permission and ownership fields: tar's header stores this data natively, rather than depending on RAR's proprietary, tool-dependent attribute handling.
• Lose — RAR's optional password encryption: neither tar nor bzip2 provides encryption on its own; a separate step such as GPG is needed for that afterward.
• Gain — decades of format stability: bzip2's on-disk stream format hasn't changed since Julian Seward's original 1996 release, while RAR replaced its entire container structure once already, in the 2013 move to RAR5.

Where Bzip2 Already Exists by Default Compared With RAR Support

Bzip2 support ships by default on virtually every Linux distribution and inside macOS's command line, and 7-Zip opens .tar.bz2 archives on Windows without any additional plugin. RAR support is never bundled the same way — Windows Explorer only gained native extraction (not creation, and not for encrypted archives) with the 24H2 update in 2024, macOS's Archive Utility has no RAR support at all, and most Linux distributions need the unrar or unar package added separately.

That asymmetry is the real, practical reason this conversion has value on Unix-centric systems specifically: content locked in a RAR archive can be extracted once using whatever RAR-capable tool is available, then rebuilt as a .tar.bz2 that behaves like any other standard tarball afterward, needing no further RAR-specific software for anyone downstream who receives it.

Documented Problems Moving RAR Content Into a Bzip2 Tarball

A recurring complaint involves the conversion taking noticeably longer than expected on large archives — bzip2's Burrows-Wheeler Transform is considerably more CPU-intensive than RAR's own matching-based algorithm, and machines with limited processing power can see compression times stretch out well past what the original RAR creation took, particularly on archives holding many gigabytes of data.

A second common issue involves the resulting file's compression ratio disappointing someone expecting bzip2 to automatically beat RAR5 — it depends heavily on content type; bzip2 tends to do very well on text and source code thanks to its block-sorting approach, but on already-compressed media like photos or video, neither format saves much, and RAR5's larger dictionary can sometimes edge out bzip2's block-limited window on those same files.

A third pattern involves multi-volume RAR archives specifically — every .part001.rar, .part002.rar volume has to be present before extraction begins at all, since RAR5's splitting scheme spreads the archive's structure across the full set rather than keeping each volume independently readable, so a missing volume stops the entire conversion rather than producing a partial result.

A fourth, more specialized problem traces back to bzip2 itself rather than anything RAR-related: CVE-2019-12900, a bug in bzip2 1.0.6 where a specially crafted archive containing an excessive number of selectors could trigger a buffer overflow during decompression. The fix landed in bzip2 1.0.7 by rejecting files with too many selectors, then was deliberately relaxed again in 1.0.8 (released July 2019) once maintainers determined some legitimately created files also used higher selector counts than the stricter check allowed. The practical upshot for anyone converting RAR content into .tar.bz2 today is straightforward: keeping the bzip2 tool itself updated matters just as much as trusting the source RAR archive, since the vulnerability lived in the decompression step, not in anything RAR did upstream.

RAR's Closed Algorithm Against Bzip2's Documented One

Questions About Turning RAR Content Into a Bzip2 Tarball

Why does the conversion take longer than opening the RAR did?
Because bzip2's Burrows-Wheeler Transform is considerably more CPU-intensive than RAR's own matching algorithm, so the compression step of the conversion genuinely takes more processing time, especially on large archives.

Will the tar.bz2 file be smaller than the original RAR?
It depends on the content. Bzip2 often does well on text and code, but RAR5's larger compression dictionary can sometimes beat bzip2's block-limited window on the same data, and neither saves much on already-compressed media.

Does converting to bzip2 keep RAR's solid-archive space savings?
Not the same way. Bzip2 compresses in independent blocks of up to 900 KB, so it doesn't match RAR's ability to find patterns across an entire archive's worth of files the way solid mode does.

Do I need every volume of a split RAR archive to convert it?
Yes. RAR5's volume-splitting scheme spreads the archive's internal structure across the whole set of .part001.rar, .part002.rar files, so a missing volume stops extraction entirely.

Is bzip2 actually free to use, unlike RAR?
Yes. Bzip2's algorithm and reference implementation are openly documented and freely implementable, with no licensing restriction comparable to RARLAB's extraction-only terms for RAR.

How can I tell a RAR4 archive from a RAR5 one without opening it?
Check the first bytes of the file. RAR4 archives start with the seven-byte sequence 52 61 72 21 1A 07 00, while RAR5 archives use an eight-byte signature with the value at that same offset changed to 01 — a fixed marker any hex viewer or file-identification tool can read directly.