NetCDF (classic) file signature
Magic number at the start of the file · CDF.
A classic NetCDF file starts with the ASCII letters CDF and a version byte. NetCDF-4 files are HDF5 instead and carry the HDF5 signature.
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 43 44 46 01 (the text CDF.), it is a NetCDF (classic) file.
What each byte of 43 44 46 01 means
Here is the signature byte by byte, the way a hex editor shows it: the position in the file, the value in hex and in decimal, and the character that value stands for in ASCII. Bytes with no printable character show a dot.
| Byte offset | Hex | Decimal | ASCII |
|---|---|---|---|
| 0 | 43 | 67 | C |
| 1 | 44 | 68 | D |
| 2 | 46 | 70 | F |
| 3 | 01 | 1 | . |
Signature variants
NetCDF (classic) appears with more than one byte pattern. These all identify the same family:
| Hex signature | Offset | Note |
|---|---|---|
| 43 44 46 01 | 0 | classic 32-bit offset format |
| 43 44 46 02 | 0 | 64-bit offset format |
Formats that use the 43 44 46 01 signature
These file types in our database carry this signature:
How to check a file's signature
You can read the bytes of any file yourself. They are shown in hex, the same way this page lists them.
Linux & macOS
- xxd -l 16 example.nc
- hexdump -C -n 16 example.nc
- file example.nc
Windows (PowerShell)
- Format-Hex -Path example.nc -Count 16
Python
- open("example.nc","rb").read(4).hex()
In your browser
- Drop the file into the WhatFileType identifier, which reads the signature without uploading it.
Frequently asked questions
What is the NetCDF (classic) file signature?
NetCDF (classic) files start with the hex bytes 43 44 46 01 (CDF. in ASCII). 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.nc on Linux or macOS, or Format-Hex -Path example.nc -Count 16 in Windows PowerShell, and read the first bytes.
Can a file fake the NetCDF (classic) signature?
Renaming a file does not change its bytes, so the extension can lie but the signature usually cannot. A genuine NetCDF (classic) file has these exact bytes; a file with the wrong bytes is not really NetCDF (classic), whatever its name says.
Related signatures
Sources
Reuse this signature
This entry is part of the WhatFileType file signature reference, published under a CC BY 4.0 licence with the full provenance for every entry, a copy-paste embed and a JSON export. Credit WhatFileType and the data is yours to republish.