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 5.22. 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/misc/magic 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.
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 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.
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 /[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 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 big little length.
- L
- A 4 byte big endian length.
- l
- A 4 byte big little 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 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 th
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.
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:
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
[.[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 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 (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 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 avalues 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
SEE ALSO¶
file(1) - the command that reads this file.
BUGS¶
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.