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.

NAME

DESCRIPTION

SEE ALSO

BUGS