magic(4)                                                 magic(4)



NAME
     magic - magic number file for file command

DESCRIPTION
     The file(1) command identifies the type of a file by  using,
     among  other  tests,  a  test  to ascertain whether the file
     begins with a certain magic  number.   The  file  /etc/magic
     specifies  the  magic  numbers  are  to  be tested for, what
     message to print if a particular magic number is found,  and
     what  additional  information  is  to  be extracted 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.  A line
     consists of the following fields:

     offset
          A number specifying the offset, in bytes, into the file
          of the data 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.

          long A four-byte value.

          string
               A string of bytes.

          The types byte,  short,  and  long  may  optionally  be
          followed by a mask specifier of the form &number.  If a
          mask specifier is given, the value is AND'ed  with  the
          number  before any comparisons are done.  The number is
          specified in C form; for example, 13 is decimal, 013 is
          octal, and 0x13 is hexadecimal.

     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 the type is a string, it is specified as  a  C
          string  with  the usual escapes permitted (for example,
          \n for newline).

          Numeric  values  may  be  preceded   by   a   character
          indicating   the   operation   to  be  performed.   The
          character may be an = to specify that  the  value  from
          the file must equal the specified value, a < to specify



January 1992                                                    1

magic(4)                                                 magic(4)



          that the value from the file  must  be  less  than  the
          specified value, a > to specify that the value from the
          file must be greater than the specified value, or an  x
          to specify that any value will match.  If the character
          is omitted, it is assumed to be =.

          For string values, the byte string from the  file  must
          match  the  specified byte string; the byte string from
          the file that is matched is  the  same  length  as  the
          specified byte string.

     message
          The message to be printed if the  comparison  succeeds.
          If    the   string   contains   a   printf(3S)   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.  A line that begins
     with the character > indicates  that  additional  tests  and
     messages  are  to  be  printed.   If  the  test  on the line
     preceding the first  line  with  a  >  succeeds,  the  tests
     specified  in  all the subsequent lines beginning with > are
     performed,  and  the  messages  are  printed  if  the  tests
     succeed.   The  next  line  that  does  not  begin  with a >
     terminates this command.

LIMITATIONS
     There should be more than one level of  subtests,  with  the
     level  indicated  by the number of > at the beginning of the
     line.

FILES
     /etc/magic
          Magic number file

SEE ALSO
     file(1) in A/UX Command Reference
















2                                                    January 1992