.Z File Extension

What kind of file is a .Z?

A .Z file is one data stream compressed with the historical Unix compress utility. Compress uses adaptive Lempel-Ziv-Welch coding and conventionally adds an uppercase .Z to the original filename.¹ The commonly used MIME label is application/x-compress, although IANA does not register that media type.²

The stream begins with hexadecimal bytes 1F 9D. Check those bytes and attempt a full decode when a filename may be misleading. The suffix alone cannot identify or validate the contents.

How do I open a .Z on my computer?

• On Windows, 7-Zip can unpack Unix Compress (Z) data.³
• On macOS, Linux, and other Unix-like systems, ncompress provides uncompress, while GNU gzip can decode traditional .Z streams.¹⁴
• For a .tar.Z package, GNU tar can recognize the compression and inspect or extract the TAR payload.⁵

Keep the compressed original and write the result to a separate file:

gzip -cd report.txt.Z > report.txt

Do not assume lowercase .z identifies the same format. Case-sensitive Unix conventions use uppercase .Z for Compress, while lowercase .z has also been used by other compression tools.

Poly support for .Z

Poly safely stores, syncs, shares, versions, downloads, and finds .Z files by filename and ordinary file properties. Poly does not currently decompress the stream, generate a thumbnail, provide an archive preview, index the payload, or make the compressed content available to the Poly agent.

If you need content search, preview, or agent access, decompress the file with a trusted local utility and add the recovered file to Poly separately. Keep the .Z original when provenance or exact preservation matters.

History of .Z

Spencer W. Thomas released the first compress implementation in July 1984. Joseph M. Orost produced portable updates, and version 4.0 entered the BSD lineage in 1985.⁶ The tool became common for Unix software distribution, backups, and online archives.

Patent concerns around LZW later helped motivate GNU Gzip, which uses DEFLATE instead. GNU describes gzip as a replacement for compress, and modern tools retain .Z decoding mainly for compatibility with older collections.⁷

Working with .Z and alternatives

Unix Compress has no checksum over the original uncompressed bytes. A successful traditional decode therefore does not prove that the result is intact.⁴ Verify an external digest when available, limit output size for untrusted streams, and inspect any recovered TAR member list before extraction.

Conversion means decompressing the .Z stream and compressing the recovered data with the chosen format. Renaming archive.Z to .gz, .xz, or .zst changes no bytes and does not convert the file.