magic - file command's magic number file
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 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.
date A four-byte value interpreted as a UNIX
date.
beshort A two-byte value (on most systems) in bigendian byte
order.
belong A four-byte value (on most systems) in bigendian byte
order.
bedate A four-byte value (on most systems) in bigendian 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 permitted
(e.g., `0 for newline).
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 `> ' (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[.[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 specified
using `&' as a prefix to the offset.
/etc/magic
file(1)
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
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