linux man page for magic

MAGIC(5)						      BSD File Formats Manual							  MAGIC(5)

NAME
     magic -- file command's magic pattern file

DESCRIPTION
     This manual page documents the format of the magic file as used by the file(1) command, version 5.04.  The file(1) command identifies the
     type of a file using, among other tests, a test for whether the file contains certain ``magic patterns''.	The file /usr/share/misc/magic
     specifies what patterns are to be tested for, what message or MIME type to print if a particular pattern 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 byte
     value, a string or a numeric value.  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 in this machine's native byte order.

	      long	  A four-byte value in this machine's native byte order.

	      quad	  An eight-byte value in this machine's native byte order.

	      float	  A 32-bit single precision IEEE floating point number in this machine's native byte order.

	      double	  A 64-bit double precision IEEE floating point number 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 target, whereas upper case characters in the magic only match uppercase characters in
			  the target.

	      pstring	  A Pascal-style string where the first byte is interpreted as the an unsigned length.	The string is not NUL terminated.

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

	      qdate	  A eight-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.

	      qldate	  An eight-byte value interpreted as a UNIX-style date, but interpreted as local time rather than UTC.

	      beid3	  A 32-bit ID3 length in big-endian byte order.

	      beshort	  A two-byte value in big-endian byte order.

	      belong	  A four-byte value in big-endian byte order.

	      bequad	  An eight-byte value in big-endian byte order.

	      befloat	  A 32-bit single precision IEEE floating point number in big-endian byte order.

	      bedouble	  A 64-bit double precision IEEE floating point number in big-endian byte order.

	      bedate	  A four-byte value in big-endian byte order, interpreted as a Unix date.

	      beqdate	  An eight-byte value in big-endian byte order, interpreted as a Unix date.

	      beldate	  A four-byte value in big-endian byte order, interpreted as a UNIX-style date, but interpreted as local time rather than
			  UTC.

	      beqldate	  An eight-byte value in big-endian byte order, interpreted as a UNIX-style date, but interpreted as local time rather
			  than UTC.

	      bestring16  A two-byte unicode (UCS16) string in big-endian byte order.

	      leid3	  A 32-bit ID3 length in little-endian byte order.

	      leshort	  A two-byte value in little-endian byte order.

	      lelong	  A four-byte value in little-endian byte order.

	      lequad	  An eight-byte value in little-endian byte order.

	      lefloat	  A 32-bit single precision IEEE floating point number in little-endian byte order.

	      ledouble	  A 64-bit double precision IEEE floating point number in little-endian byte order.

	      ledate	  A four-byte value in little-endian byte order, interpreted as a UNIX date.

	      leqdate	  An eight-byte value in little-endian byte order, interpreted as a UNIX date.

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

	      leqldate	  An eight-byte value in little-endian byte order, interpreted as a UNIX-style date, but interpreted as local time rather
			  than UTC.

	      lestring16  A two-byte unicode (UCS16) string in little-endian byte order.

	      melong	  A four-byte value in middle-endian (PDP-11) byte order.

	      medate	  A four-byte value in middle-endian (PDP-11) byte order, interpreted as a UNIX date.

	      meldate	  A four-byte value in middle-endian (PDP-11) byte order, interpreted as a UNIX-style date, but interpreted as local time
			  rather than UTC.

	      indirect	  Starting at the given offset, consult the magic database again.

	      regex	  A regular expression match in extended POSIX regular expression syntax (like egrep). Regular expressions can take expo-
			  nential time to process, and their performance is hard to predict, so their use is discouraged. When used in production
			  environments, their performance should be carefully checked. The type specification can be optionally followed by
			  /[c][s].  The ``c'' flag makes the match case insensitive, while the ``s'' flag update the offset to the start offset of
			  the match, rather than the end.  The regular expression is tested against line N + 1 onwards, where N is the given off-
			  set.	Line endings are assumed to be in the machine's native format.	^ and $ match the beginning and end of individual
			  lines, respectively, not beginning and end of file.

	      search	  A literal string search starting at the given offset. The same modifier flags can be used as for string patterns. The
			  modifier flags (if any) must be followed by /number the range, that is, the number of positions at which the match will
			  be attempted, starting from the start offset. This is suitable for searching larger binary expressions with variable
			  offsets, using  escapes for special characters. The offset works as for regex.

	      default	  This is intended to be used with the test x (which is always true) and a message that is to be used if there are no
			  other matches.

	      Each top-level magic pattern (see below for an explanation of levels) is classified as text or binary according to the types used.
	      Types ``regex'' and ``search'' are classified as text tests, unless non-printable characters are used in the pattern. All other
	      tests are classified as binary. A top-level pattern is considered to be a test text when all its patterns are text patterns; other-
	      wise, it is considered to be a binary pattern. When matching a file, binary patterns are tried first; if no match is found, and the
	      file looks like text, then its encoding is determined and the text patterns are tried.

	      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 ~, the value specified after is negated before tested.  x, to specify that any value will
	      match.  If the character is omitted, it is assumed to be =.  Operators &, ^, and ~ don't work with floats and doubles.  The operator
	      ! specifies that the line matches if the test does not succeed.

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

	      For string values, the string from the file must match the specified 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 non-
	      empty string (usually used to then print the string), with > (because all non-empty strings are greater than the empty string).

	      The special test x always evaluates to true.

     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.  If the string begins with ``'',
	      the message printed is the remainder of the string with no whitespace added before it: multiple matches are normally separated by a
	      single space.

     An APPLE 4+4 character APPLE creator and type can be specified as:

	   !:apple CREATYPE

     A MIME type is given on a separate line, which must be the next non-blank or comment line after the magic line that identifies the file type,
     and has the following format:

	   !:mime  MIMETYPE

     i.e. the literal string ``!:mime'' followed by the MIME type.

     An optional strength can be supplied on a separate line which refers to the current magic description using the following format:

	   !:strength OP VALUE

     The operand OP can be: +, -, *, or / and VALUE is a constant between 0 and 255.  This constant is applied using the specified operand to the
     currently computed default magic strength.

     Some file formats contain additional information which is to be printed along with the file type or need additional tests to determine the
     true file type.  These additional tests are introduced by one or more > characters preceding the offset.  The number of > on the line indi-
     cates the level of the test; a line with no > at the beginning is considered to be at level 0.  Tests are arranged in a tree-like hierarchy:
     If a the test on a line at level n succeeds, all following tests at level n+1 are performed, and the messages printed if the tests succeed,
     until a line with level n (or less) appears.  For more complex files, one can use empty messages to get just the "if/then" effect, in the
     following way:

	   0	  string   MZ
	   >0x18  leshort  <0x40   MS-DOS executable
	   >0x18  leshort  >0x3f   extended PC executable (e.g., MS Windows)

     Offsets do not need to be constant, but can also be read from the file being examined.  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
     [.[bislBISL]][+-][ y ]).  The value of x is used as an offset in the file.  A byte, id3 length, short or long is read at that offset depend-
     ing on the [bislBISLm] 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; the m type interprets the number as a middle endian (PDP-11) 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.

     That way variable length structures can be examined:

	   # MS Windows executables are also valid MS-DOS executables
	   0	       string  MZ
	   >0x18       leshort <0x40   MZ executable (MS-DOS)
	   # skip the whole block below if it is not an extended executable
	   >0x18       leshort >0x3f
	   >>(0x3c.l)  string  PE  PE executable (MS-Windows)
	   >>(0x3c.l)  string  LX  LX executable (OS/2)

     This strategy of examining has a drawback: You must make sure that you eventually print something, or users may get empty output (like, when
     there is neither PE nor LE in the above example)

     If this indirect offset cannot be used directly, simple calculations are possible: appending [+-*/%&|^]number inside parentheses allows one
     to modify the value read from the file before it is used as an offset:

	   # MS Windows executables are also valid MS-DOS executables
	   0	       string  MZ
	   # sometimes, the value at 0x18 is less that 0x40 but there's still an
	   # extended executable, simply appended to the file
	   >0x18       leshort <0x40
	   >>(4.s*512) leshort 0x014c  COFF executable (MS-DOS, DJGPP)
	   >>(4.s*512) leshort !0x014c MZ executable (MS-DOS)

     Sometimes you do not know the exact offset as this depends on the length or position (when indirection was used before) of preceding fields.
     You can specify an offset relative to the end of the last up-level field using '&' as a prefix to the offset:

	   0	       string  MZ
	   >0x18       leshort >0x3f
	   >>(0x3c.l)  string  PE	 PE executable (MS-Windows)
	   # immediately following the PE signature is the CPU type
	   >>>&0       leshort 0x14c	 for Intel 80386
	   >>>&0       leshort 0x184	 for DEC Alpha

     Indirect and relative offsets can be combined:

	   0		 string  MZ
	   >0x18	 leshort <0x40
	   >>(4.s*512)	 leshort !0x014c MZ executable (MS-DOS)
	   # if it's not COFF, go back 512 bytes and add the offset taken
	   # from byte 2/3, which is yet another way of finding the start
	   # of the extended executable
	   >>>&(2.s-514) string  LE	 LE executable (MS Windows VxD driver)

     Or the other way around:

	   0		     string  MZ
	   >0x18	     leshort >0x3f
	   >>(0x3c.l)	     string  LE  LE executable (MS-Windows)
	   # at offset 0x80 (-4, since relative offsets start at the end
	   # of the up-level match) inside the LE header, we find the absolute
	   # offset to the code area, where we look for a specific signature
	   >>>(&0x7c.l+0x26) string  UPX     , UPX compressed

     Or even both!

	   0		    string  MZ
	   >0x18	    leshort >0x3f
	   >>(0x3c.l)	    string  LE LE executable (MS-Windows)
	   # at offset 0x58 inside the LE header, we find the relative offset
	   # to a data area where we look for a specific signature
	   >>>&(&0x54.l-3)  string  UNACE  , ACE self-extracting archive

     Finally, if you have to deal with offset/length pairs in your file, even the second value in a parenthesized expression can be taken from the
     file itself, using another set of parentheses.  Note that this additional indirect offset is always relative to the start of the main indi-
     rect offset.

	   0		     string	  MZ
	   >0x18	     leshort	  >0x3f
	   >>(0x3c.l)	     string	  PE PE executable (MS-Windows)
	   # search for the PE section called ".idata"...
	   >>>&0xf4	     search/0x140 .idata
	   # ...and go to the end of it, calculated from start+length;
	   # these are located 14 and 10 bytes after the section name
	   >>>>(&0xe.l+(-4)) string	  PK34 , ZIP self-extracting archive

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

BUGS
     The formats long, belong, lelong, melong, short, beshort, leshort, date, bedate, medate, ledate, beldate, leldate, and meldate 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.

BSD
								  August 30, 2008							       BSD