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Find file types by using a modified "magic" file

Based on file v3.22 by Ian F. Darwin (see libfile/LEGAL.NOTICE and
libfile/README.dist - File v3.22 can be found at many archive sites)

For each entry in the magic file, the "message" for the initial offset MUST
be 4 characters for the CREATOR and 4 characters for the TYPE - white space is
optional between them. Any other characters on this line are ignored.
Continuation lines (starting with a '>') are also ignored i.e. only the initial
offset lines are used.

e.g magic entry for a GIF file:

# off	type		test		message
#
# GIF image
0       string          GIF8            8BIM GIFf
>4      string          7a              \b, version 8%s,
>4      string          9a              \b, version 8%s,
>6      leshort         >0              %hd x
>8      leshort         >0              %hd,
#>10    byte            &0x80           color mapped,
#>10    byte&0x07       =0x00           2 colors
#>10    byte&0x07       =0x01           4 colors
#>10    byte&0x07       =0x02           8 colors
#>10    byte&0x07       =0x03           16 colors
#>10    byte&0x07       =0x04           32 colors
#>10    byte&0x07       =0x05           64 colors
#>10    byte&0x07       =0x06           128 colors
#>10    byte&0x07       =0x07           256 colors

Just the "8BIM" "GIFf" will be used whatever the type of GIF file it is.
The continuation lines are used by the "file" command, but ignored by
mkhybrid. They could be left out completely.

The complete format of the magic file is given in the magic man page (magic.5).

See the file "magic" for other examples

Use with the -magic magic_file option, where magic_file is a file
described above.

The magic file can be used with the mapping file (option -map) - the order
these options appear on the command line is important.  mkhybrid will try to
detect if the file is one of the Unix/Mac files (e.g. a CAP or Netatalk
file) first. If that fails, it will then use the magic and/or mapping
file e.g:

mkhybrid -o output.raw -map mapping -magic magic src_dir

The above will check filename extensions first, if that fails to set the
CREATOR/TYPE, the magic file will be used. To check the magic file
before the filename extensions, use:

mkhybrid -o output.raw -magic magic -map mapping src_dir


Using just a magic file - filename extensions will not be checked e.g:

mkhybrid -o output.raw -magic magic src_dir

For the magic method to work, each file must be opened and read twice
(once to find it's CREATOR/TYPE, and a second time to actually copy the
file to the CD image). Therefore the -magic option may significantly
increase processing time.

If a file's CREATOR/TYPE is not set via the magic and mapping matches,
then the file is given the default CREATOR/TYPE.