MAGIC(4) Device and Network Interfaces MAGIC(4)

NAME

magic - file command's magic pattern file

DESCRIPTION

This manual page documents the format of magic files as used by the
file(1) command, version 5.45. 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:
+o 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.
+o 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:
b Force binary file test.
C Use upper 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.
c Use lower 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. To do a complete case
insensitive match, specify both "c"
and "C".
f Require that the matched string is
a full word, not a partial word
match.
T Trim the string, i.e. leading and
trailing whitespace
t Force text file test.
W Compact 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.
w Treat every blank in the magic as
an optional blank. 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

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:

!:apple CREATYPE

A slash-separated list of commonly found filename extensions can be
specified as:

!:ext ext[/ext...]

i.e. the literal string "!:ext" followed by a slash-separated list of
commonly found extensions; for example for JPEG images:

!:ext jpeg/jpg/jpe/jfif

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
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

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.

illumos Arpil 18, 2023 illumos