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MAGIC(5) BSD File Formats Manual MAGIC(5)
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, ver-
sion 4.26. 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/file/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 /[Bbc]*. The ``B'' 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 ``b'' flag treats every blank in the target as an optional
blank. Finally the ``c'' flag, specifies case insensitive matching:
lowercase characters in the magic match both lower and upper case char-
acters in the target, whereas upper case characters in the magic only
match uppercase characters in the target.
pstring A Pascal-style string where the first byte is interpreted as the an
unsigned length. The string is not NUL terminated.
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.
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.
bestring16 A two-byte unicode (UCS16) string in big-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.
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.
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 care-
fully checked. The type specification can be optionally followed by
/[c][s]. 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 regular expression is tested against line N + 1
onwards, where N is the given offset. Line endings are assumed to be
in the machine's native format. ^ 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 modifier flags (if any)
must be followed by /number the range, 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 offset works as
for regex.
default This is intended to be used with the test x (which is always true) and
a message that is to be used if there are no other matches.
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 per-
formed. It may be =, to specify that the value from the file must equal the speci-
fied value, <, to specify that the value from the file must be less than the speci-
fied 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.
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).
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.
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 con-
stant 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 a 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, untile 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 [.[bslBSL]][+-][ y ]). The value of x
is used as an offset in the file. A byte, short or long is read at that offset depending on
the [bslBSLm] 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
Finally, 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 parenthe-
ses. 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
SEE ALSO
file(1) - the command that reads this file.
BUGS
The formats long, belong, lelong, melong, short, beshort, leshort, date, bedate, medate,
ledate, beldate, leldate, and meldate are system-dependent; perhaps they should be specified
as a number of bytes (2B, 4B, etc), since the files being recognized typically come from a
system on which the lengths are invariant.
BSD August 30, 2008 BSD
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On Apache
Under GNU General Public License
2012-05-25 22:35 @38.107.179.237 Crawled by CCBot/1.0 (+http://www.commoncrawl.org/bot.html)