Re: [OT] Magic numbers
On Sunday 06 February 2011 23:58:50 Bob Proulx wrote:
> Lisi wrote:
> > Thanks, Andy - yes, it told me what the files were, and its best guess
> > was good. (I should know - I created the files!)
> >
> > But no actual magic numbers. :-(
>
> Help us out. Can you phrase an example of what you want? Otherwise
> the only magic number anyone can give you is 42.
>
> "Magic" is relative here. The file program simply looks at parts here
> and parts there of the file and then makes a best-guess about it. The
> magic file database in libmagic1 is a documentation of that best guess
> effort. See the man page for information about the format of the
> information there.
>
> man magic
>
> So for example most files don't have a single magic number. Most types
> of files have many possible combinations. And when files are layered
> it is more difficult such as a tar of script, or a gzip'd file that is
> a tar of a script. The file program will quickly try to undo those
> layers and make a guess. It isn't perfect.
>
> Since the file program tries to work across any possibility in the
> universe it has a very hard task. If you have a more restricted set
> of input files you might be able to do better.
>
> For example '#!/' and #! /' denote scripts. Any file that starts with
> the binary data 23 21 20 2f or 23 21 2f is almost certainly a shell
> script. What type? For that you need to look further and see what
> characters follow.
>
> Note that in Squeeze /usr/share/misc/magic is now an empty directory
> instead of a file. The source isn't installed anymore. That is a
> shame. For squeeze you will need to install the source package to get
> to raw data that you can look at.
>
> apt-get source libmagic1
> cd file-5.04
> ls -log magic/Magdir/
Your questions and detailed analysis, and everyone else's answers, have
answered my questions and teh penny has dropped. I was asking teh wrong
question, which illustrates teh depth of my ignorance!
So thank you very much indeed for taking so much trouble. You and teh others
have turned a light on in my almost dark brain.
Lisi
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