#ROZ# File Documentation

Summary

A .0 file is almost always the first part of a split file — one large file cut into numbered pieces (.0, .1, .2…) so it could be e-mailed, uploaded, or stored on size-limited media. It is not a format you open on its own; you put every numbered part in one folder and rejoin them. If they were made with Hacha or HJSplit, use that tool to join; 7-Zip joins HJSplit-style sets, and copy /b (Windows) or cat (Mac/Linux) concatenates raw slices back into the original.

Technical details

FeatureValue
Full nameNumbered split-archive volume (first part)
File extension.0 (first of .0, .1, .2…)
MIME typeapplication/octet-stream
Format typeFirst (zero-indexed) volume of a split file
What it isA raw slice of another file — not a format of its own
Common creatorHacha (Spanish freeware splitter), HJSplit
Magic numberNone — the bytes are whatever the original file started with
Companion fileHacha writes a small .hac control file describing the join
How to identifyBy sibling parts and the original name before .0 (e.g. movie.avi.0)
CompressionNone — splitting is not compression; parts sum to the original size
To use itRejoin all parts; output keeps the original name and type
RequiresEvery numbered part — a missing part cannot be recovered
Join toolsHacha, HJSplit, 7-Zip, copy /b, cat
Related extensions.001, .z01, .hac, .rar
Also used byRotated logs / app backups (e.g. app.log.0) — unrelated

What is a .0 file?

A .0 file is not really a file format — it is a part number. It appears when a program splits one large file into a sequence of smaller volumes named .0, .1, .2, and so on (some tools start the count at .001 instead). The point of splitting is purely logistical: fitting a big file under an e-mail attachment limit, spreading it across several USB sticks or discs, or making a large download easier to resume. One common source is Hacha, a free Spanish-language file splitter and joiner created around 2000 and, unusually for a tool that old, still maintained (version 3.5.0 was released in October 2025).

The .0 part on its own does nothing useful. To get your file back you rejoin every numbered part into the original, which then carries whatever type it always was — often a ZIP archive, an AVI video, or an ISO disc image. The strongest clue to what you will get is the name sitting before the number: holiday.avi.0 rejoins to a video, project.zip.0 to an archive.

What is actually inside a .0 part

A Hacha- or HJSplit-style split part is a raw slice of the original file with no header of its own. The bytes at the start of a .0 are simply the first bytes of the original file, so a .0 cut from a ZIP begins with PK, and one cut from an MP4 begins with that file's ftyp box. There is no magic number that identifies a .0 as a split part, because the format deliberately adds nothing — that is what lets the parts be concatenated back together byte-for-byte.

Two things follow from this. First, splitting is not compression. The parts together are exactly the same size as the original; you rejoin them, you do not decompress them. Second, every part must be present. If any numbered part in the series is missing, the set cannot be rejoined and the missing bytes cannot be reconstructed — there is no redundancy or recovery record in a plain split. Hacha additionally writes a small .hac control file that records the original filename, size, and part count, which is why a Hacha set is best rejoined with Hacha itself.

How to rejoin the parts

Put all the numbered parts in one folder first, then pick the method that matches how they were made:

  • Hacha — if there is a companion .hac file, open Hacha, choose Unir (Join), and point it at the .0; it rebuilds the original from the .hac description.
  • HJSplit — for generic numbered parts with no .hac, choose Join and select the .0; it concatenates the series in order.
  • 7-Zip — for HJSplit-style .001/.002 sets, open the first part and it joins the rest.
  • Command line — for plain raw slices, Windows copy /b file.0 + file.1 + file.2 original.zip, or macOS/Linux cat file.0 file.1 file.2 > original.zip, concatenate the slices. This only works when the parts have no per-part header.

After joining you get a single file with the original name and extension. Open it with the normal application for that type. If it will not open, the most likely cause is a missing or out-of-order part rather than a broken tool.

References