Files as files

On Jun 10, 14:56, Max Eskin wrote: Not really. Certainly not consistently across all file types. Often command scripts have a comment at the top, and some versions of unix (eg, Irix) embed a "tag" number into executables so they can distinguish individual programs/versions quickly, but other than that, filetype determination is done by looking at various parts of a file and comparing what's found ("magic numbers") to a database (the "magic" file). So, for example, my system "knows" a certain file is a command script because the first 256 characters are all ASCII (which means it's probably a text file of some sort) and the file permissions are set such that it's executable (not merely readable). It also knows that a certain file is an ELF-format executable for a 32-bit little-endian MIPS processor with a version 1 architecture (ie it will run on *old* MIPS cpus as well as newer ones) because the bytes at offset 1 are "ELF", at offset 4 there's a binary "1" (which in this context means 32-bit not 64-bit), at offset 5 there's another "1" (little-endian), at offset 16 "2" means "executable", and at offset 18 "0" means "MIPS" (not Sparc, 80x86, 68000, etc). Some magic numbers are much simpler to decode, of course: a file that begins with "GIF89a" is a GIF file, surprise, surprise. And the more you dig, the more detail you can work out. -- Pete Peter Turnbull Dept. of Computer Science University of York