magic

     This manual page documents the format of the magic file as used by the
     file(1) command, version 5.09.  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 /[WwcCtb]*.  The ``W''
                          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 ``w'' flag
                          treats every blank in the target as an optional
                          blank.  The ``c'' flag, specifies case insensitive
                          matching: lower case characters in the magic match
                          both lower and upper case characters in the target,
                          whereas upper case characters in the magic only
                          match upper case characters in the target.  The
                          ``C'' flag, specifies case insensitive matching:
                          upper case characters in the magic match both lower
                          and upper case characters in the target, whereas
                          lower case characters in the magic only match upper
                          case characters in the target.  To do a complete
                          case insensitive match, specify both ``c'' and
                          ``C''.  The ``t'' flag, forces the test to be done
                          J  The length includes itself in its count.
                          The string is not NUL terminated.  ``J'' is used
                          rather than the more valuable ``I'' because this
                          type of length is a feature of the JPEG format.

              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, inter-
                          preted as a Unix date.

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

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

              beqldate    An eight-byte value in big-endian byte order, inter-
                          preted 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.

              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 inter-
                          preted 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 exponential time to process, and their per-
                          formance is hard to predict, so their use is dis-
                          couraged.  When used in production environments,
                          their performance should be carefully checked.  The
                          size of the string to search should also be limited
                          by specifying /<length>, to avoid performance issues
                          scanning long files.  The type specification can
                          also be optionally followed by /[c][s][l].  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 ``l'' modi-
                          fier, changes the limit of length to mean number of
                          lines instead of a byte count.  Lines are delimited
                          by the platforms native line delimiter.  When a line
                          count is specified, an implicit byte count also com-
                          puted assuming each line is 80 characters long.  If
                          neither a byte or line count is specified, the
                          search is limited automatically to 8KiB.  The regu-
                          lar 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 given off-
                          set.  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 num-

              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 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. \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 spec-
              ify 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.

              Numeric operations are not performed on date types, instead the
              numeric value is interpreted as an offset.

              For string values, the string from the file must match the spec-
              ified 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

           !: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 > 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 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, 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 off-
     set 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 depending
     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\0\0  PE executable (MS-Windows)
           >>(0x3c.l)  string  LX\0\0  LX executable (OS/2)

           >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 off-
     set.

           0                 string       MZ
           >0x18             leshort      >0x3f
           >>(0x3c.l)        string       PE\0\0 PE executable (MS-Windows)

     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.

BSD                             April 20, 2011                             BSD
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