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GZ
What is a GZ file?
Compresses a single file. Combined with TAR it becomes the standard Unix archive.
What GZ is
GZ is a stream format, written and read from front to back. It is used for the web and archiving.
The extension is .gz, and the full name is Gzip Archive. Both matter less than what the file can hold, which is what the rest of this page is about.
Where GZ came from
It dates from 1992. The specification is RFC 1952.
A format that has been readable for that long is a format worth trusting with something you want back in ten years.
The specification is public
Anyone can implement it without asking permission or paying a licence, which is why it turns up in so many programs and why files written twenty years ago still open.
Nothing is thrown away
GZ stores its content exactly. Saving it again changes nothing, so it can be opened, edited and re-saved as often as you like without accumulating damage — which is what makes it a working format rather than a delivery one.
It holds exactly one file
GZ compresses a single stream and has no concept of a filename or a folder. That is why it is so often seen paired with TAR, which supplies the structure it lacks.
What else it can carry
GZ can hold a layout that lets it start playing before it has finished arriving.
That matters mostly when converting: whatever the target cannot hold is dropped, usually without a warning.
Where its compression ratio comes from
It works with a window of 32 KB: the amount of already-seen data it can refer back to when it meets a repeat.
That single number explains most of the difference between one archive format and another. A larger window finds more repetition and takes longer and more memory to do it, which is the whole trade.
What it does to protect itself
GZ carries a checksum, so a damaged file is detected rather than silently mis-read.
A checksum will not repair anything. It tells you the file is wrong, which is the difference between finding out now and finding out from whoever you sent it to.
What opens GZ
gzip, 7-Zip and Keka read it, and so do most programs of the same kind.
If a file will not open, the format is rarely the problem — it is more often that the program predates it. Converting to something older is the reliable way past that, and it is what the rest of this site is for.
Opening it in a browser
Every current browser reads it.
That makes it a safe thing to put on a page or attach to a message without wondering what the other end has installed.
It is a delivery format
GZ is meant to be handed over rather than worked in. Editing one is possible and rarely pleasant; the sane approach is to change the source and export again.
What goes wrong with it
The recurring complaints: it holds one stream and cannot describe a folder.
None of these is a reason to avoid the format. They are the things worth knowing before you are surprised by one, which is a different claim and a more useful one.
The facts, in one place
| Extension | .gz, .tgz |
|---|---|
| Media type | application/gzip |
| First published | 1992 |
| Specification | RFC 1952 |