unixdev.net


Switch to SpeakEasy.net DSL

The Modular Manual Browser

Home Page
Manual: (OpenBSD-3.6)
Page:
Section:
Apropos / Subsearch:
optional field

MAGIC(5)                  OpenBSD Programmer's 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 3.22.  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 /etc/magic specifies what magic numbers are to be tested for,
     what message to print if a particular magic number is found, and addi-
     tional 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 na-
                      tive byte order.

             long     A four-byte value (on most systems) in this machine's
                      native byte order.

             string   A string of bytes.

             date     A four-byte value interpreted as a UNIX date.

             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.

     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 per-
             mitted (e.g., `\n' for newline).

             Numeric values may be preceded by a character indicating the op-
             eration 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 val-
             ue, `^' 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 `='.

             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.  A line which begins with the character `>' in-
     dicates additional tests and messages to be printed.  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.

     Each line at level n+1 is under the control of the line at level n most
     closely preceding it in the magic file.  If the test on a line at level n
     succeeds, the tests specified in all the subsequent lines at level n+1
     are performed, and the messages printed if the tests succeed.  The next
     line at level n terminates this.

     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[.[bsl]][+-][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 ``[bsl]'' type specifier.  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.

     Sometimes you do not know the exact offset as this depends on the length
     of preceding fields.  You can specify an offset relative to the end of
     the last uplevel field (of course this may only be done for sublevel
     tests, i.e., test beginning with `>').  Such a relative offset is speci-
     fied using `&' as a prefix to the offset.

FILES
     /etc/magic

SEE ALSO
     file(1)

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.

OpenBSD 3.6                    September 3, 1994                             3