opendarwin man page for magic

MAGIC(5)							File Formats Manual							  MAGIC(5)

NAME
       magic - file command's magic number file

DESCRIPTION
       This manual page documents the format of the magic file as used by the file(1) command, version 4.02.  The file command identifies the type
       of a file using, among other tests, a test for whether the file begins with a certain magic number.  The file /usr/share/file/magic  speci-
       fies  what  magic  numbers are to be tested for, what message to print if a particular magic number is found, and additional information to
       extract from the file.

       Each line of the file specifies a test to be performed.	A test compares the data starting at a	particular  offset  in	the  file  with  a
       1-byte,	2-byte,  or  4-byte  numeric  value  or a string.  If the test succeeds, a message is printed.	The line consists of the following
       fields:

       offset	A number specifying the offset, in bytes, into the file of the data which is to be tested.

       type	The type of the data to be tested.  The possible values are:

		byte	 A one-byte value.

		short	 A two-byte value (on most systems) in this machine's native byte order.

		long	 A four-byte value (on most systems) in this machine's native byte order.

		string	 A string of bytes.  The string type specification can be optionally followed by /[Bbc]*.  The ``B'' flag compacts  white-
			 space	in the target, which must contain at least one whitespace character.  If the magic has "n" consecutive blanks, the
			 target needs at least "n" consecutive blanks to match.  The ``b'' flag treats every blank in the target  as  an  optional
			 blank.   Finally  the ``c'' flag, specifies case insensitive matching: lowercase characters in the magic match both lower
			 and upper case characters in the targer, whereas upper case characters in the magic, only much  uppercase  characters	in
			 the target.

		date	 A four-byte value interpreted as a UNIX date.

		ldate	 A four-byte value interpreted as a UNIX-style date, but interpreted as local time rather than UTC.

		beshort  A two-byte value (on most systems) in big-endian byte order.

		belong	 A four-byte value (on most systems) in big-endian byte order.

		bedate	 A four-byte value (on most systems) in big-endian byte order, interpreted as a unix date.

		leshort  A two-byte value (on most systems) in little-endian byte order.

		lelong	 A four-byte value (on most systems) in little-endian byte order.

		ledate	 A four-byte value (on most systems) in little-endian byte order, interpreted as a UNIX date.

		leldate  A  four-byte  value  (on  most systems) in little-endian byte order, interpreted as a UNIX-style date, but interpreted as
			 local time rather than UTC.

       The numeric types may optionally be followed by & and a numeric value, to specify that the value is to be AND'ed  with  the  numeric  value
       before any comparisons are done.  Prepending a u to the type indicates that ordered comparisons should be unsigned.

       test   The  value  to  be  compared  with  the  value from the file.  If the type is numeric, this value is specified in C form; if it is a
	      string, it is specified as a C string with the usual escapes permitted (e.g. 
 for new-line).

	      Numeric values may be preceded by a character indicating the operation to be performed.  It may be =, to specify that the value from
	      the  file  must  equal  the specified value, <, to specify that the value from the file must be less than the specified value, >, to
	      specify that the value from the file must be greater than the specified value, &, to specify that the value from the file must  have
	      set  all of the bits that are set in the specified value, ^, to specify that the value from the file must have clear any of the bits
	      that are set in the specified value, or x, to specify that any value will match.	If the character is omitted, it is assumed  to	be
	      =.

	      Numeric values are specified in C form; e.g.  13 is decimal, 013 is octal, and 0x13 is hexadecimal.

	      For  string values, the byte string from the file must match the specified byte string.  The operators =, < and > (but not &) can be
	      applied to strings.  The length used for matching is that of the string argument in the magic file.  This  means	that  a  line  can
	      match any string, and then presumably print that string, by doing > (because all strings are greater than the null string).

       message
	      The  message  to be printed if the comparison succeeds.  If the string contains a printf(3) format specification, the value from the
	      file (with any specified masking performed) is printed using the message as the format string.

       Some file formats contain additional information which is to be printed along with the file type.  A line which begins with the character >
       indicates  additional  tests  and messages to be printed.  The number of > on the line indicates the level of the test; a line with no > at
       the beginning is considered to be at level 0.  Each line at level n+1 is under the control of the line at level n most closely preceding it
       in  the magic file.  If the test on a line at level n succeeds, the tests specified in all the subsequent lines at level n+1 are performed,
       and the messages printed if the tests succeed.  The next line at level n terminates this.  If the first character following the last > is a
       (  then the string after the parenthesis is interpreted as an indirect offset.  That means that the number after the parenthesis is used as
       an offset in the file.  The value at that offset is read, and is used again as an offset in the file.  Indirect offsets are  of	the  form:
       ((x[.[bslBSL]][+-][y]).	 The  value  of  x is used as an offset in the file. A byte, short or long is read at that offset depending on the
       [bslBSL] type specifier.  The capitalized types interpret the number as a big endian value, whereas the small letter versions interpret the
       number as a little endian value.  To that number the value of y is added and the result is used as an offset in the file.  The default type
       if one is not specified is long.

       Sometimes you do not know the exact offset as this depends on the length of preceding fields.  You can specify an offset  relative  to  the
       end  of	the last uplevel field (of course this may only be done for sublevel tests, i.e.  test beginning with > ).  Such a relative offset
       is specified using & as a prefix to the offset.

BUGS
       The formats long, belong, lelong, short, beshort, leshort, date, bedate, and ledate are system-dependent; perhaps they should be  specified
       as a number of bytes (2B, 4B, etc), since the files being recognized typically come from a system on which the lengths are invariant.

       There is (currently) no support for specified-endian data to be used in indirect offsets.

SEE ALSO
       file(1) - the command that reads this file.

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