Provided by: libmagic1_4.21-3_i386 bug

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 4.21.  The file(1) 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 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 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.

              quad        An eight-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 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 target, whereas upper case characters in the
                          magic, only much 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.

              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.

              bequad      An eight-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.

              beqdate     An eight-byte value (on most systems) in big-endian
                          byte order, interpreted as a Unix date.

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

              beqldate    An eight-byte value (on most systems) 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.

              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.

              lequad      An eight-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.

              leqdate     An eight-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.

              leqldate    An eight-byte value (on most systems) 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 (on most systems) in middle-endian
                          (PDP-11) byte order.

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

              meldate     A four-byte value (on most systems) 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 (much like egrep).  The type
                          specification can be optionally followed by /[cse]*.
                          The “c” flag makes the match case insensitive, while
                          the “s” or “e” flags update the offset to the
                          starting or ending offsets of the match (only one
                          should be used).  By default, regex does not update
                          the offset.  The regular expression is always tested
                          against the first N lines, where N is the given
                          offset, thus it is only useful for (single-byte
                          encoded) text.  ^ and $ will 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.  It must be followed by <number> which
                          specifies how many matches shall be attempted (the
                          range).  This is suitable for searching larger
                          binary expressions with variable offsets, using \
                          escapes for special characters.

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

     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. \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 =.  For all tests
              except string and regex, operation !  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 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 >\0 (because all strings are greater than the null
              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 remainder of the string with no
              whitespace added before it: multiple matches are normally
              separated by a single space.

     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 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
     performed, 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
     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 [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 one 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 as-is, there are simple
     calculations 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\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
     indirect offset is always relative to the start of the main indirect
     offset.

           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-
     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.