Archive converter

Windows, macOS and every Linux desktop open ZIP with nothing installed. 7Z compresses better and RAR often better still, but both need software the recipient may not have — which is why an archive that will not open is a far more common problem than an archive that is slightly too large.

Repacking does not recompress what is inside the archive in any way that loses data. The files that come out are byte-for-byte the files that went in; only the container around them changes.

We can open RAR archives and we deliberately cannot create them. This is a licensing constraint rather than a technical one: the licence for the RAR decoder explicitly permits extracting archives and forbids using it to build a compatible packer. That is a line we will not cross, so RAR appears here as a source and never as a target.

In practice this costs nothing. Nobody needs to *create* a RAR; what people need is to get out of one, and that works.

Converting between archive formats means extracting every file and compressing it again. For a large archive that is real time, and if the target compresses harder — 7z or `.tar.xz` — considerably more of it.

The result is not a repair or an improvement of the contents; it is the same files in a different wrapper. Where the archive is only being opened once, extracting it and working with the files directly is usually quicker than converting and then extracting anyway.

Unix permissions, ownership and symbolic links live in TAR and have no dependable home in ZIP, so a `.tar.gz` repacked as ZIP arrives with the executable bit gone. RAR recovery records — the redundant data that lets a damaged archive repair itself — have no equivalent anywhere else and are simply lost.

Neither is announced by the tool doing the conversion. If an archive is going back to a Unix system or was made specifically to survive unreliable storage, converting it away is giving up the property it was built for.

Converting one would mean decrypting it, and there is nowhere to type the password: these pages send 7-Zip an empty one, so an encrypted archive fails at the listing step instead of hanging on a prompt. You get a sentence saying so rather than a broken output.

There is also no way to put the protection back. The conversion offers no options at all, so even if the contents could be read the result would be an unencrypted archive. Extract it locally with the password and repack it there if the protection was the point.

ZIP when the archive is going to a person whose setup you do not know — it is the one format every operating system opens without installing anything, and sending anything else is quietly asking them to solve a problem.

`.tar.gz` when it is going back to a server, because that is what preserves the permissions. 7z when the archive is large, full of similar files, and going somewhere you know has the software. And often nothing at all: an archive being kept rather than sent has no reason to be converted.