magic —
file
command's magic pattern file
This manual page documents the format of magic files as used by the
file(1) command, version 5.44. 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/share/misc/magic.mgc or a directory of
source text magic pattern fragment files in
/usr/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 a /<width> option and optionally followed
by a set of flags /[bCcftTtWw]*. The width limits the number of
characters to be copied. Zero means all characters. The following
flags are supported:
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:
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:
octal
- A string representing an octal number.
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.
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.
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:
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:
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:
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 |
o |
Octal |
Textual |
Variable |
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
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 Specification 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.