MAGIC(5) BSD File Formats Manual MAGIC(5)
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 5.44. 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 usu-
ally 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 data-
base 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 num-
ber 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 or-
der.
long A four-byte value in this machine's native byte or-
der.
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 tar-
get.
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 tar-
get. 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, inter-
preted as a Unix date.
beqdate An eight-byte value in big-endian byte order, inter-
preted as a Unix date.
beldate A four-byte value in big-endian byte order, inter-
preted as a UNIX-style date, but interpreted as lo-
cal time rather than UTC.
beqldate An eight-byte value in big-endian byte order, inter-
preted as a UNIX-style date, but interpreted as lo-
cal time rather than UTC.
beqwdate An eight-byte value in big-endian byte order, inter-
preted 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, in-
terpreted as a UNIX date.
leqdate An eight-byte value in little-endian byte order, in-
terpreted as a UNIX date.
leldate A four-byte value in little-endian byte order, in-
terpreted as a UNIX-style date, but interpreted as
local time rather than UTC.
leqldate An eight-byte value in little-endian byte order, in-
terpreted as a UNIX-style date, but interpreted as
local time rather than UTC.
leqwdate An eight-byte value in little-endian byte order, in-
terpreted 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 or-
der.
medate A four-byte value in middle-endian (PDP-11) byte or-
der, interpreted as a UNIX date.
meldate A four-byte value in middle-endian (PDP-11) byte or-
der, interpreted as a UNIX-style date, but inter-
preted 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 rela-
tive 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 be-
gins with a ^ then the endianness of the magic is
switched; if the magic mentioned leshort for exam-
ple, it is treated as beshort and vice versa. This
is useful to avoid duplicating the rules for differ-
ent endianness.
regex A regular expression match in extended POSIX regular
expression syntax (like egrep). Regular expressions
can take exponential time to process, and their per-
formance is hard to predict, so their use is dis-
couraged. 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 nei-
ther 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 off-
set. 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 vari-
able offsets, using \ escapes for special charac-
ters. The order of modifier and number is not rele-
vant.
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 continua-
tion 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 equiva-
lent 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 charac-
ters 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 oc-
tal, 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 repre-
sentation.
The special test x always evaluates to true.
The message to be printed if the comparison succeeds. If the string con-
tains 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 re-
mainder 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 > charac-
ters 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 be-
ing 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 off-
set 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 mod-
ify 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, us-
ing 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 plat-
form, 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.
BSD October 9, 2022 BSD
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