This manual page documents the format of the magic file as used by the
file(1) command, version 5.03. 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 /[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 characters 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 inter-
preted as the an unsigned length. The string is not
NUL terminated.
date A four-byte value interpreted as a UNIX date.
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
local time rather than UTC.
beqldate An eight-byte value in big-endian byte order, inter-
preted 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.
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.
preted as local time rather than UTC.
indirect Starting at the given offset, consult the magic
database again.
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
type specification can be optionally followed by
/[c][s]. The ``c'' flag makes the match case insen-
sitive, 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 off-
set. 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 pat-
terns; 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 deter-
mined 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
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 spec-
ified 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 mes-
sage 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 sepa-
rated 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 > char-
acters 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 per-
the file. The value at that offset is read, and is used again as an off-
set 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)
# 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 off-
set.
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-depen-
dent; 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.
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