MAGIC(FILES)

NAME
magic - file command's magic number file

DESCRIPTION
The file(1) 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 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).

    long
        A four-byte value (on most systems).

    string
        A string of bytes.

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, or &, to specify that the value is to be AND'ed with the numeric value before any comparisons are done. Numeric values are specified in C form; e.g. 13 is decimal, 013 is octal, and 0x13 is hexadecimal. 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 operators =, < and > (but not &) can be applied to strings. The length used for matching is that of the string argument in the magic file.

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 which begins with the character > indicates additional tests and messages 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 printed if the tests succeed. The next line which does not begin with a > terminates this.

BUGS
The formats long and short are system-dependant; 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 should be more than one level of subtests, with the level possibly indicated by the number of > at the beginning of the line.

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