Free GZ test files and sample generator
Gzip-compressed stream, common on Unix systems. Frequently wraps a tarball (.tar.gz) or single files.
What is a .gz file?
GZ: Gzip Compressed. Gzip compresses a single stream. It is not a multi-file archive. It is the compression behind HTTP Content-Encoding, log rotation and .tar.gz, so decompression behavior is worth testing on its own.
Why generate a test GZ file?
- Test a decompression step, including its size limits.
- Verify handling of compressed HTTP payloads or rotated logs.
- Check that a truncated stream fails cleanly.
How a .gz file is identified
Every .gz file begins with 1F 8B 08, the gzip magic number and the deflate method. That is what a validator inspecting content rather than the file name looks for.
How the size lands on the exact byte
The gzip header can carry an optional null-terminated comment field of any length. Decompressors ignore it, so it takes the padding and the file still round-trips through gunzip.
Negative testing with broken GZ files
- Invalid. Identifiable, but wrong inside, so it fails during parsing.
- Corrupted. The leading bytes are destroyed, so the
1F 8B 08identifying a .gz is gone. Same length as the valid file. - Incomplete. Truncated part-way, so it is genuinely shorter.
- Empty. Chosen under Content rather than File Condition: valid structure, no data.
Which failure to reach for, and what to assert for each, is covered in how to make a corrupted file for testing. For upload limits and type checks see testing file upload validation.
Sample GZ content
[Compressed], gzip, deflate, from access.log
This shows the shape of the file. Your download is generated fresh at the size you choose, so the values will differ.
How to open and verify the file
Decompress with `gunzip file.gz` or `gzip -t file.gz` to test integrity.
Media type application/gzip · extension .gz · category Archives
Generate a .gz file → · questions about sizes, privacy or cost are answered on the site FAQ.
Frequently asked questions about GZ files
What identifies a .gz file?
A .gz file begins with the bytes 1F 8B 08, the gzip magic number and the deflate method. Those bytes are what a content-sniffing validator looks for, and what the Corrupted condition destroys.
How can a .gz file be an exact number of bytes?
The gzip header can carry an optional null-terminated comment field of any length. Decompressors ignore it, so it takes the padding and the file still round-trips through gunzip.
Can I download several broken GZ files at once?
Yes, select more than one File Condition and you get one ZIP containing a .gz per condition, each named after its condition.
Can the GZ file use UTF-16 or a byte-order mark?
No, and deliberately. A .gz file is a container rather than plain text, so rewriting its bytes as UTF-16 would not produce a UTF-16 document. It would produce a broken file. Use a text format such as CSV, JSON or XML for encoding tests.
How do I open or verify the GZ file?
Decompress with gunzip file.gz or gzip -t file.gz to test integrity.