Re: Formatless files

On 09/08/2022 21:01, Roger L Costello wrote:
> A file has no inherent format.
> 
> The format of a file is determined by the programs that use it.
> 
> Since file types are not determined by the file system, the "kernel"
> can't tell you the type of file: it doesn't know.

Yet the Unix 'file' [1] command has been doing a pretty good job of it
since 1973. [2]

The 'file' command firstly uses filesystem tests to determine if a file
is empty or is a special file, such as a socket or a symbolic link.

It secondly uses 'magic' tests to detect the file type.  The 'file'
manpage includes:

      The magic tests are used to check for files with data in particular
      fixed formats.

The magic tests use "magic patterns" from a 'magic' file. [3]

The 'magic' file on my Linux system includes 11 patterns that start with
'<?xml'.  They are mostly followed by other tests to try to determine
the type of XML, e.g.:

0	string		\<?xml\ version="
 >15	string		>\0
 >>19	search/4096	\<svg			SVG Scalable Vector Graphics image

There's even a test for '<?XML' that will be reported as 'broken XML 
document'.

Regards,


Tony Graham.
-- 
Senior Architect
XML Division
Antenna House, Inc.
----
Skerries, Ireland
[email protected]

[1] https://www.man7.org/linux/man-pages/man1/file.1.html
[2] https://www.man7.org/linux/man-pages/man1/file.1.html#HISTORY
[3] https://man7.org/linux/man-pages/man4/magic.4.html

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