magic -- file command's magic number file
This manual page documents the format of the magic file as used by the
magic command, version 3.41. 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 /usr/share/misc/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.
date A four-byte value interpreted as a UNIX date.
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.
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.
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 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 > 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 ( 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[.[bslBSL]][+-][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 [bslBSL] type specifier. The capitalized types
interpret the number as a big endian value, whereas a small letter versions
interpret the number as a little endian 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.
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 specified
using & as a prefix to the offset.
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.
file(1)
FreeBSD 5.2.1 February 27, 2003 FreeBSD 5.2.1 [ Back ] |
