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 4.17.  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 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.

                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 targer,
                         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.

                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.

                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.

                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.

                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.

                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 /c for
                         case-insensitive matches.  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.

       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).

       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 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
       uplevel 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 uplevel 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 parenthesed 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

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.

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



                                 Public Domain                        MAGIC(5)