MAGIC(5)                  OpenBSD Programmer's 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 4.24.  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 /etc/magic specifies 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 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 specifi-
                                  cation can be optionally followed by
                                  /[Bbc]*.  The ``B'' flag compacts whitespace
                                  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 tar-
                                  get, 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           An 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.

                  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 or-
                                  der.

                  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 or-
                                  der, interpreted as a Unix date.

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

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

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

                  leshort         A two-byte value in little-endian byte or-
                                  der.

                  lelong          A four-byte value in little-endian byte or-
                                  der.

                  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 or-
                                  der, 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 or-
                                  der, 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.

                  regex           A regular expression match in extended POSIX
                                  regular expression syntax (like egrep).
                                  Regular expressions can take exponential
                                  time to process, and their performance is
                                  hard to predict, so their use is discour-
                                  aged.  When used in production environments,
                                  their performance should be carefully
                                  checked.  The type specification can be op-
                                  tionally 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 offset.  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 giv-
                                  en 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, start-
                                  ing from the start offset.  This is suitable
                                  for searching larger binary expressions with
                                  variable offsets, using \ escapes for spe-
                                  cial 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; otherwise, it is considered
                  to be a binary pattern.  When matching a file, binary pat-
                  terns 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 nu-
                  meric value, to specify that the value is to be AND'ed with
                  the numeric value before any comparisons are done.  Prepend-
                  ing 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. \n 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 >\0 (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 ``\b'', the message printed is the re-
                  mainder of the string with no whitespace added before it:
                  multiple matches are normally separated by a single space.

     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.

     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 > char-
     acters preceding the offset.  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.  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 per-
     formed, and the messages printed if the tests succeed, untile 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 off-
     set 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 [bslBSLm] 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\0\0  PE executable (MS-Windows)
           >>(0x3c.l)  string  LX\0\0  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\0\0 nor LE\0\0 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 us-
     ing `&' as a prefix to the offset:

           0           string  MZ
           >0x18       leshort >0x3f
           >>(0x3c.l)  string  PE\0\0    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\0\0  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     \b, UPX compressed

     Or even both!

           0                string  MZ
           >0x18            leshort >0x3f
           >>(0x3c.l)       string  LE\0\0 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  \b, 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 in-
     direct offset is always relative to the start of the main indirect off-
     set.

           0                 string       MZ
           >0x18             leshort      >0x3f
           >>(0x3c.l)        string       PE\0\0 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       PK\3\4 \b, 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-depen-
     dent; 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.

OpenBSD 4.7                    November 26, 2009                             6
