magic






magic − file command’s magic number file

This manual page documents the format of the magic file as
used by the command, version 4.17.  The command identifies
the type of a file using, among other tests, a test for
whether the file begins with a certain 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.










                             ‐2‐


         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.










                             ‐3‐


         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.  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.











                             ‐4‐


     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 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 succeeds, all
following tests at level are performed, and the messages
printed if the tests succeed, untile a line with level (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: [.[bslBSL]][+−][((x 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:










                             ‐5‐


    # 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)









                             ‐6‐


    # 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

The formats 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.

− the command that reads this file.