|
NAME | DESCRIPTION | SEE ALSO | BUGS | COLOPHON |
MAGIC(4) BSD Kernel Interfaces Manual MAGIC(4)
magic — file command's magic pattern file
This manual page documents the format of magic files as used by the
file(1) command, version 5.31. The file(1) command identifies the type
of a file using, among other tests, a test for whether the file con‐
tains 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 fol‐
lowing 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 num‐
ber in this machine's native byte order.
double A 64-bit double precision IEEE floating point num‐
ber in this machine's native byte order.
string A string of bytes. The string type specification
can be optionally followed by /[WwcCtbT]*. 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 “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 tar‐
get, 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 4 byte big endian length.
h A 2 byte big endian length.
L A 4 byte little endian length.
l A 2 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 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.
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 num‐
ber in big-endian byte order.
bedouble A 64-bit double precision IEEE floating point num‐
ber in big-endian byte order.
bedate A four-byte value in big-endian byte order, inter‐
preted 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, inter‐
preted 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 num‐
ber in little-endian byte order.
ledouble A 64-bit double precision IEEE floating point num‐
ber 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 regu‐
lar 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 envi‐
ronments, 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 spec‐
ified, an implicit byte count also computed assum‐
ing 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, respec‐
tively, not beginning and end of file.
search A literal string search starting at the given off‐
set. The same modifier flags can be used as for
string patterns. The search expression must con‐
tain 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 expres‐
sions 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.
For compatibility with the Single UNIX Standard, the type
specifiers dC and d1 are equivalent to byte, the type speci‐
fiers 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 pat‐
tern. 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 inter‐
nal 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
[[.,][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 depend‐
ing on the [bislBISLm] 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.
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
file(1) - the command that reads this file.
The formats long, belong, lelong, melong, short, beshort, and leshort
do not depend on the length of the C data types short and long on the
platform, even though the Single UNIX Specification implies that they
do. However, as OS X Mountain Lion has passed the Single UNIX Specifi‐
cation validation suite, and supplies a version of file(1) in which
they do not depend on the sizes of the C data types and that is built
for a 64-bit environment in which long is 8 bytes rather than 4 bytes,
presumably the validation suite does not test whether, for example long
refers to an item with the same size as the C data type long. There
should probably be type names int8, uint8, int16, uint16, int32,
uint32, int64, and uint64, and specified-byte-order variants of them,
to make it clearer that those types have specified widths.
This page is part of the file (a file type guesser) project. Informa‐
tion about the project can be found at http://www.darwinsys.com/file/.
If you have a bug report for this manual page, see
http://bugs.gw.com/my_view_page.php. This page was obtained from the
project's upstream Git read-only mirror of the CVS repository
https://github.com/glensc/file on 2017-07-05. If you discover any ren‐
dering problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have correc‐
tions or improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to man-pages@man7.org
BSD February 12, 2017 BSD