Ubuntu Manpage: magic — file command's magic pattern file

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 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:
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 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

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.

NAME

DESCRIPTION

SEE ALSO

BUGS