файл волшебного шаблона команды file (file command's magic pattern file)
Описание (Description)
This manual page documents the format of magic files as used by the
file(1) command, version 5.40. 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 database of these
'magic patterns' is usually located in a binary file in
/usr/local/share/misc/magic.mgc or a directory of source text magic
pattern fragment files in /usr/local/share/misc/magic. The
database specifies what patterns are to be tested for, what message
or MIME type to print if a particular pattern is found, and
additional information to extract from the file.
The format of the source fragment files that are used to build this
database is as follows: Each line of a fragment file specifies a
test to be performed. A test compares the data starting at a
particular offset in the file with a byte value, a string or a
numeric value. 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. This offset can be a
negative number if it is:
• The first direct offset of the magic entry (at
continuation level 0), in which case it is interpreted
an offset from end end of the file going backwards.
This works only when a file descriptor to the file is
available and it is a regular file.
• A continuation offset relative to the end of the last
up-level field (&).
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
/[WwcCtbTf]*. The 'W' 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 'w' flag treats every blank in the magic
as an optional blank. The 'f' flags requires
that the matched string is a full word, not a
partial word match. The 'c' flag specifies
case insensitive matching: lower case
characters in the magic match both lower and
upper case characters in the target, whereas
upper case characters in the magic only match
upper case characters in the target. The 'C'
flag specifies case insensitive matching:
upper case characters in the magic match both
lower and upper case characters in the target,
whereas lower case characters in the magic
only match upper case characters in the
target. To do a complete case insensitive
match, specify both 'c' and 'C'. The 't' flag
forces the test to be done for text files,
while the 'b' flag forces the test to be done
for binary files. The 'T' flag causes the
string to be trimmed, i.e. leading and
trailing whitespace is deleted before the
string is printed.
pstring A Pascal-style string where the first
byte/short/int is interpreted as the unsigned
length. The length defaults to byte and can
be specified as a modifier. The following
modifiers are supported:
B A byte length (default).
H A 2 byte big endian length.
h A 2 byte little endian length.
L A 4 byte big endian length.
l A 4 byte little endian length.
J The length includes itself in its count.
The string is not NUL terminated. 'J' is used
rather than the more valuable 'I' because this
type of length is a feature of the JPEG
format.
date A four-byte value interpreted as a UNIX date.
qdate An 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.
qwdate An eight-byte value interpreted as a Windows-
style date.
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.
beqwdate An eight-byte value in big-endian byte order,
interpreted as a Windows-style date.
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.
leqwdate An eight-byte value in little-endian byte
order, interpreted as a Windows-style date.
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 date,
but interpreted as local time rather than UTC.
indirect Starting at the given offset, consult the
magic database again. The offset of the
indirect magic is by default absolute in the
file, but one can specify /r to indicate that
the offset is relative from the beginning of
the entry.
name Define a 'named' magic instance that can be
called from another use magic entry, like a
subroutine call. Named instance direct magic
offsets are relative to the offset of the
previous matched entry, but indirect offsets
are relative to the beginning of the file as
usual. Named magic entries always match.
use Recursively call the named magic starting from
the current offset. If the name of the
referenced begins with a ^ then the endianness
of the magic is switched; if the magic
mentioned leshort for example, it is treated
as beshort and vice versa. This is useful to
avoid duplicating the rules for different
endianness.
regex A regular expression match in extended POSIX
regular expression syntax (like egrep).
Regular expressions can take exponential time
to process, and their performance is hard to
predict, so their use is discouraged. When
used in production environments, their
performance should be carefully checked. The
size of the string to search should also be
limited by specifying /<length>, to avoid
performance issues scanning long files. The
type specification can also be optionally
followed by /[c][s][l]. 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 'l' modifier,
changes the limit of length to mean number of
lines instead of a byte count. Lines are
delimited by the platforms native line
delimiter. When a line count is specified, an
implicit byte count also computed assuming
each line is 80 characters long. If neither a
byte or line count is specified, the search is
limited automatically to 8KiB. ^ 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 search expression
must contain the range in the form /number,
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 order of modifier and number
is not relevant.
default This is intended to be used with the test x
(which is always true) and it has no type. It
matches when no other test at that
continuation level has matched before.
Clearing that matched tests for a continuation
level, can be done using the clear test.
clear This test is always true and clears the match
flag for that continuation level. It is
intended to be used with the default test.
der Parse the file as a DER Certificate file. The
test field is used as a der type that needs to
be matched. The DER types are: eoc, bool,
int, bit_str, octet_str, null, obj_id,
obj_desc, ext, real, enum, embed, utf8_str,
rel_oid, time, res2, seq, set, num_str,
prt_str, t61_str, vid_str, ia5_str, utc_time,
gen_time, gr_str, vis_str, gen_str, univ_str,
char_str, bmp_str, date, tod, datetime,
duration, oid-iri, rel-oid-iri. These types
can be followed by an optional numeric size,
which indicates the field width in bytes.
guid A Globally Unique Identifier, parsed and
printed as XXXXXXXX-XXXX-XXXX-XXXX-
XXXXXXXXXXXX. It's format is a string.
offset This is a quad value indicating the current
offset of the file. It can be used to
determine the size of the file or the magic
buffer. For example the magic entries:
-0 offset x this file is %lld bytes
-0 offset <=100 must be more than 100 \
bytes and is only %lld
For compatibility with the Single UNIX Standard, the type
specifiers dC and d1 are equivalent to byte, the type
specifiers uC and u1 are equivalent to ubyte, the type
specifiers dS and d2 are equivalent to short, the type
specifiers uS and u2 are equivalent to ushort, the type
specifiers dI, dL, and d4 are equivalent to long, the type
specifiers uI, uL, and u4 are equivalent to ulong, the
type specifier d8 is equivalent to quad, the type
specifier u8 is equivalent to uquad, and the type
specifier s is equivalent to string. In addition, the
type specifier dQ is equivalent to quad and the type
specifier uQ is equivalent to uquad.
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 encoding 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 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 ~, 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 hexadecimal.
Numeric operations are not performed on date types,
instead the numeric value is interpreted as an offset.
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 >\0 (because all non-
empty strings are greater than the empty string).
Dates are treated as numerical values in the respective
internal representation.
The special test x always evaluates to true.
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. 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 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, until 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
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 [[.,][bBcCeEfFgGhHiIlmsSqQ]][+-][ 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
[bBcCeEfFgGhHiIlmsSqQ] type specifier. The value is treated as
signed if '', is specified or unsigned if ''. is specified. The
capitalized 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. The following types are recognized:
Type Sy Mnemonic Sy Endian Sy Size
bcBc Byte/Char N/A 1
efg Double Little 8
EFG Double Big 8
hs Half/Short Little 2
HS Half/Short Big 2
i ID3 Little 4
I ID3 Big 4
m Middle Middle 4
q Quad Little 8
Q Quad Big 8
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 (such
as 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 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
If you have to deal with offset/length pairs in your file, even the
second value in a parenthesized 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
If you have a list of known values at a particular continuation
level, and you want to provide a switch-like default case:
# clear that continuation level match
>18 clear
>18 lelong 1 one
>18 lelong 2 two
>18 default x
# print default match
>>18 lelong x unmatched 0x%x
