magic(5)                                                       magic(5)

  magic

  NAME

    magic - file command's magic number file

  DESCRIPTION

    The file(1) command identifies the type of a file using a number of tests,
    including a test for whether the file begins with a certain magic number.
    The 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 one-byte, two-
    byte, or four-byte numeric value or a string. If the test succeeds, a
    message is printed. The line consists of the following fields:

    offset  type  test  message

    The fields are separated by white space; the message filed is the
    remainder of the line.

    byte
        A one-byte value.

    short
        A two-byte value (on most systems) in this computer's native byte
        order.

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

    string
        A string of bytes.

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

    The numeric types can 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.

    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 (such as \n for
    newline).

    Numeric values can be preceded by a character indicating the operation to
    be performed.

    Character     Description

    =             The test succeeds if the value from the file equals the
                  specified value.

    <             The test succeeds if the value from the file is less
                  than the specified value.

    >             The test succeeds if the value from the file is greater
                  than the specified value.

    &             The test succeeds if the value from the file has set all
                  of the bits that are set in the specified value.

    ^             The test succeeds if the value from the file has clear
                  any of the bits that are set in the specified value.

    x             The test succeeds if there is any value in the file.

    Numeric values are specified in C form; that is, 13 is decimal, 013 is
    octal, and 0x13 is hexadecimal. 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 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 that is to be printed
    along with the file type. A line that begins with the character >
    indicates 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 (, the string after the
    parenthesis is interpreted as an indirect offset. That means that the
    number after the parenthesis is used as a offset in the file. The value at
    that offset is read and 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.

  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, and so on), because 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)