Unix compress (.Z) file signature
Magic number at the start of the file
A .Z file made by the classic Unix compress tool starts with the two magic bytes 1F 9D, marking its LZW-compressed data. It predates gzip (1F 8B).
What this magic number means
A magic number is a short, fixed run of bytes at a known position that tells a program what a file really is, no matter what the filename says. If a file starts with the bytes 1F 9D, it is a Unix compress (.Z) file.
Formats that use the 1F 9D signature
These file types in our database carry this signature:
How to check a file's signature
You can read the first bytes of any file yourself. The magic bytes are shown in hex, the same way this page lists them.
Linux & macOS
- xxd -l 16 example.z
- hexdump -C -n 16 example.z
- file example.z
Windows (PowerShell)
- Format-Hex -Path example.z -Count 16
Python
- open("example.z","rb").read(2).hex()
In your browser
- Drop the file into the WhatFileType identifier, which reads the signature without uploading it.
Frequently asked questions
What is the Unix compress (.Z) file signature?
Unix compress (.Z) files start with the hex bytes 1F 9D. This magic number identifies the format regardless of the file's name or extension.
How do I check a file's magic number?
Open the file in a hex editor, or run a command such as xxd -l 16 example.z on Linux or macOS, or Format-Hex -Path example.z -Count 16 in Windows PowerShell, and read the first bytes.
Can a file fake the Unix compress (.Z) signature?
Renaming a file does not change its bytes, so the extension can lie but the signature usually cannot. A genuine Unix compress (.Z) file has these exact bytes; a file with the wrong bytes is not really Unix compress (.Z), whatever its name says.