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

Provided by: libmagic1_5.14-2ubuntu3.4_amd64

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 5.14.
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.

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. 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.

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).

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:

!: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
[.[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

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 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

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.

Debian                                           January 7, 2013                                        MAGIC(5)

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BUGS