How to detect type of compression used on the file? (if no file extension is specified)

Question

How can one detect the type of compression used on the file? (assuming that .zip, .gz, .xz or any other extension is not specified).

Is this information stored somewhere in the header of that file?

Answer 1

You can determine that it is likely to be one of those formats by looking at the first few bytes. You should then test to see if it really is one of those, using an integrity check from the associated utility for that format, or by actually proceeding to decompress.

You can find the header formats in the descriptions:

• Zip (.zip) format description, starts with 0x50, 0x4b, 0x03, 0x04 (unless empty — then the last two are 0x05, 0x06 or 0x06, 0x06)
• Gzip (.gz) format description, starts with 0x1f, 0x8b, 0x08
• xz (.xz) format description, starts with 0xfd, 0x37, 0x7a, 0x58, 0x5a, 0x00

Others:

• zlib (.zz) format description, starts with two bytes (in bits) 0aaa1000 bbbccccc, where ccccc is chosen so that the first byte viewed as a int16 times 256 plus the second byte viewed as a int16 is a multiple of 31. e.g: 01111000(bits) = 120(int16), 10011100(bits) = 156(int16), 120 * 256 + 156 = 30876 which is a multiple of 31
• compress (.Z) starts with 0x1f, 0x9d
• bzip2 (.bz2) starts with 0x42, 0x5a, 0x68

Answer 2

If you're on a Linux box just use the 'file' command.

http://en.wikipedia.org/wiki/File_(command)

$ mv foo.zip dink
$ file dink
dink: gzip compressed data, from Unix, last modified: Sat Aug  6 08:08:57 2011,
max compression
$

Answer 3

As an alternative to inspecting the file header by hand, you could use some utility like TrID. The link points to the cross-platform command line version; for Windows there's a GUI, too.