Provided by: libmagic1_4.16-0ubuntu3_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 4.16.  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 target,
                         whereas upper case characters in the magic, only much
                         uppercase characters in the target.

                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.

                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.

                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 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
       [bslBSL] 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.  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,  short,  beshort,  leshort, date,
       bedate,  and  ledate  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.

       There is (currently) no support for specified-endian data to be used in
       indirect offsets.

SEE ALSO

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

                                 Public Domain                        MAGIC(5)