|
|
Эта страница руководства для версии 10.9 Mac OS XЕсли Вы выполняете различную версию Mac OS X, просматриваете документацию локально:
Читать страницы руководстваСтраницы руководства предназначаются как справочник для людей, уже понимающих технологию.
|
MAGIC(5) BSD File Formats Manual MAGIC(5)
NAME
magic -- file command's magic pattern file
DESCRIPTION
This manual page documents the format of the magic file as used by the file(1) command, version 5.04.
The file(1) command identifies the type of a file using, among other tests, a test for whether the file
contains certain ``magic patterns''. The file /usr/share/file/magic specifies what patterns are to be
tested for, what message or MIME type to print if a particular pattern is found, and additional infor-mation information
mation to extract from the file.
Each line of the file specifies a test to be performed. A test compares the data starting at a partic-ular particular
ular offset in the file with a byte value, a string or a numeric value. If the test succeeds, a mes-sage message
sage 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 in this machine's native byte order.
long A four-byte value in this machine's native byte order.
quad An eight-byte value in this machine's native byte order.
float A 32-bit single precision IEEE floating point number in this machine's native byte
order.
double A 64-bit double precision IEEE floating point number 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 insen-sitive insensitive
sitive matching: lowercase characters in the magic match both lower and upper case
characters in the target, whereas upper case characters in the magic only match
uppercase characters in the target.
pstring A Pascal-style string where the first byte is interpreted as the an unsigned
length. The string is not NUL terminated.
date A four-byte value interpreted as a UNIX date.
qdate A eight-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.
qldate An eight-byte value interpreted as a UNIX-style date, but interpreted as local
time rather than UTC.
beid3 A 32-bit ID3 length in big-endian byte order.
beshort A two-byte value in big-endian byte order.
belong A four-byte value in big-endian byte order.
bequad An eight-byte value in big-endian byte order.
befloat A 32-bit single precision IEEE floating point number in big-endian byte order.
bedouble A 64-bit double precision IEEE floating point number in big-endian byte order.
bedate A four-byte value in big-endian byte order, interpreted as a Unix date.
beqdate An eight-byte value in big-endian byte order, interpreted as a Unix date.
beldate A four-byte value in big-endian byte order, interpreted as a UNIX-style date, but
interpreted as local time rather than UTC.
beqldate An eight-byte value in big-endian byte order, interpreted as a UNIX-style date,
but interpreted as local time rather than UTC.
bestring16 A two-byte unicode (UCS16) string in big-endian byte order.
leid3 A 32-bit ID3 length in little-endian byte order.
leshort A two-byte value in little-endian byte order.
lelong A four-byte value in little-endian byte order.
lequad An eight-byte value in little-endian byte order.
lefloat A 32-bit single precision IEEE floating point number in little-endian byte order.
ledouble A 64-bit double precision IEEE floating point number in little-endian byte order.
ledate A four-byte value in little-endian byte order, interpreted as a UNIX date.
leqdate An eight-byte value in little-endian byte order, interpreted as a UNIX date.
leldate A four-byte value in little-endian byte order, interpreted as a UNIX-style date,
but interpreted as local time rather than UTC.
leqldate An eight-byte value in little-endian byte order, interpreted as a UNIX-style date,
but interpreted as local time rather than UTC.
lestring16 A two-byte unicode (UCS16) string in little-endian byte order.
melong A four-byte value in middle-endian (PDP-11) byte order.
medate A four-byte value in middle-endian (PDP-11) byte order, interpreted as a UNIX
date.
meldate A four-byte value in middle-endian (PDP-11) byte order, interpreted as a UNIX-style UNIXstyle
style date, but interpreted as local time rather than UTC.
indirect Starting at the given offset, consult the magic database again.
regex A regular expression match in extended POSIX regular expression syntax (like
egrep). Regular expressions can take exponential time to process, and their per-formance performance
formance is hard to predict, so their use is discouraged. When used in production
environments, their performance should be carefully checked. The type specifica-tion specification
tion can be optionally followed by /[c][s]. The ``c'' flag makes the match case
insensitive, while the ``s'' flag update the offset to the start offset of the
match, rather than the end. The regular expression is tested against line N + 1
onwards, where N is the given offset. Line endings are assumed to be in the
machine's native format. ^ and $ match the beginning and end of individual lines,
respectively, not beginning and end of file.
search A literal string search starting at the given offset. The same modifier flags can
be used as for string patterns. The modifier flags (if any) must be followed by
/number the range, that is, the number of positions at which the match will be
attempted, starting from the start offset. This is suitable for searching larger
binary expressions with variable offsets, using \ escapes for special characters.
The offset works as for regex.
default This is intended to be used with the test x (which is always true) and a message
that is to be used if there are no other matches.
Each top-level magic pattern (see below for an explanation of levels) is classified as text or
binary according to the types used. Types ``regex'' and ``search'' are classified as text
tests, unless non-printable characters are used in the pattern. All other tests are classified
as binary. A top-level pattern is considered to be a test text when all its patterns are text
patterns; otherwise, it is considered to be a binary pattern. When matching a file, binary
patterns are tried first; if no match is found, and the file looks like text, then its encod-ing encoding
ing is determined and the text patterns are tried.
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 spec-
ify 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 ~, the value specified after is negated before tested. x, to specify that any value
will match. If the character is omitted, it is assumed to be =. Operators &, ^, and ~ don't
work with floats and doubles. The operator ! specifies that the line matches if the test does
not succeed.
Numeric values are specified in C form; e.g. 13 is decimal, 013 is octal, and 0x13 is hexa-decimal. hexadecimal.
decimal.
For string values, the string from the file must match the specified 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 non-empty string
(usually used to then print the string), with >\_ (because all non-empty strings are greater
than the empty string).
The special test x always evaluates to true. message The message to be printed if the compar-ison comparison
ison 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.
If the string begins with ``\b'', the message printed is the remainder of the string with no
whitespace added before it: multiple matches are normally separated by a single space.
An APPLE 4+4 character APPLE creator and type can be specified as:
!:apple CREATYPE
A MIME type is given on a separate line, which must be the next non-blank or comment line after the
magic line that identifies the file type, and has the following format:
!:mime MIMETYPE
i.e. the literal string ``!:mime'' followed by the MIME type.
An optional strength can be supplied on a separate line which refers to the current magic description
using the following format:
!:strength OP VALUE
The operand OP can be: +, -, *, or / and VALUE is a constant between 0 and 255. This constant is
applied using the specified operand to the currently computed default magic strength.
Some file formats contain additional information which is to be printed along with the file type or
need additional tests to determine the true file type. These additional tests are introduced by one or
more > characters preceding the offset. 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. Tests are arranged in a tree-like
hierarchy: If a the test on a line at level n succeeds, all following tests at level n+1 are performed,
and the messages printed if the tests succeed, untile a line with level n (or less) appears. For more
complex files, one can use empty messages to get just the "if/then" effect, in the following way:
0 string MZ
>0x18 leshort <0x40 MS-DOS executable
>0x18 leshort >0x3f extended PC executable (e.g., MS Windows)
Offsets do not need to be constant, but can also be read from the file being examined. If the first
character following the last > is a ( then the string after the parenthesis is interpreted as an indi-rect indirect
rect 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 [.[bislBISL]][+-][ y ]). The value of x is used as an offset in the file. A byte, id3
length, short or long is read at that offset depending on the [bislBISLm] type specifier. The capital-ized capitalized
ized types interpret the number as a big endian value, whereas the small letter versions interpret the
number as a little endian value; the m type interprets the number as a middle endian (PDP-11) 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.
That way variable length structures can be examined:
# MS Windows executables are also valid MS-DOS executables
0 string MZ
>0x18 leshort <0x40 MZ executable (MS-DOS)
# skip the whole block below if it is not an extended executable
>0x18 leshort >0x3f
>>(0x3c.l) string PE\0\0 PE executable (MS-Windows)
>>(0x3c.l) string LX\0\0 LX executable (OS/2)
This strategy of examining has a drawback: You must make sure that you eventually print something, or
users may get empty output (like, when there is neither PE\0\0 nor LE\0\0 in the above example)
If this indirect offset cannot be used directly, simple calculations are possible: appending
[+-*/%&|^]number inside parentheses allows one to modify the value read from the file before it is used
as an offset:
# MS Windows executables are also valid MS-DOS executables
0 string MZ
# sometimes, the value at 0x18 is less that 0x40 but there's still an
# extended executable, simply appended to the file
>0x18 leshort <0x40
>>(4.s*512) leshort 0x014c COFF executable (MS-DOS, DJGPP)
>>(4.s*512) leshort !0x014c MZ executable (MS-DOS)
Sometimes you do not know the exact offset as this depends on the length or position (when indirection
was used before) of preceding fields. You can specify an offset relative to the end of the last up-level uplevel
level field using `&' as a prefix to the offset:
0 string MZ
>0x18 leshort >0x3f
>>(0x3c.l) string PE\0\0 PE executable (MS-Windows)
# immediately following the PE signature is the CPU type
>>>&0 leshort 0x14c for Intel 80386
>>>&0 leshort 0x184 for DEC Alpha
Indirect and relative offsets can be combined:
0 string MZ
>0x18 leshort <0x40
>>(4.s*512) leshort !0x014c MZ executable (MS-DOS)
# if it's not COFF, go back 512 bytes and add the offset taken
# from byte 2/3, which is yet another way of finding the start
# of the extended executable
>>>&(2.s-514) string LE LE executable (MS Windows VxD driver)
Or the other way around:
0 string MZ
>0x18 leshort >0x3f
>>(0x3c.l) string LE\0\0 LE executable (MS-Windows)
# at offset 0x80 (-4, since relative offsets start at the end
# of the up-level match) inside the LE header, we find the absolute
# offset to the code area, where we look for a specific signature
>>>(&0x7c.l+0x26) string UPX \b, UPX compressed
Or even both!
0 string MZ
>0x18 leshort >0x3f
>>(0x3c.l) string LE\0\0 LE executable (MS-Windows)
# at offset 0x58 inside the LE header, we find the relative offset
# to a data area where we look for a specific signature
>>>&(&0x54.l-3) string UNACE \b, ACE self-extracting archive
Finally, if you have to deal with offset/length pairs in your file, even the second value in a paren-thesized parenthesized
thesized expression can be taken from the file itself, using another set of parentheses. Note that
this additional indirect offset is always relative to the start of the main indirect offset.
0 string MZ
>0x18 leshort >0x3f
>>(0x3c.l) string PE\0\0 PE executable (MS-Windows)
# search for the PE section called ".idata"...
>>>&0xf4 search/0x140 .idata
# ...and go to the end of it, calculated from start+length;
# these are located 14 and 10 bytes after the section name
>>>>(&0xe.l+(-4)) string PK\3\4 \b, ZIP self-extracting archive
SEE ALSO
file(1) - the command that reads this file.
BUGS
The formats long, belong, lelong, melong, short, beshort, leshort, date, bedate, medate, ledate,
beldate, leldate, and meldate 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.
BSD August 30, 2008 BSD
|
Сообщение о проблемах
Способ сообщить о проблеме с этой страницей руководства зависит от типа проблемы:
- Ошибки содержания
- Ошибки отчета в содержании этой документации со ссылками на отзыв ниже.
- Отчеты об ошибках
- Сообщите об ошибках в функциональности описанного инструмента или API через Генератор отчетов Ошибки.
- Форматирование проблем
- Отчет, форматирующий ошибки в интерактивной версии этих страниц со ссылками на отзыв ниже.