MAGIC(5) MAGIC(5)
NAME
magic - file commands magic number file
DESCRIPTION
This manual page documents the format of the magic file as used by the
file(1) command, version 4.17. The file command identifies the type of
a file using, among other tests, a test for whether the file begins
with a certain magic number. The file /usr/share/file/magic specifies
what magic numbers are to be tested for, what message to print if a
particular magic number is found, and additional information to extract
from the file.
Each line of the file specifies a test to be performed. A test com
pares the data starting at a particular offset in the file with a
1-byte, 2-byte, or 4-byte numeric value or a string. If the test suc
ceeds, 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 (on most systems) in this machines
native byte order.
long A four-byte value (on most systems) in this machines
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 much
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.
ldate A four-byte value interpreted as a UNIX-style date,
but interpreted as local time rather than UTC.
beshort A two-byte value (on most systems) in big-endian byte
order.
belong A four-byte value (on most systems) in big-endian
byte order.
bedate A four-byte value (on most systems) in big-endian
byte order, interpreted as a Unix date.
beldate A four-byte value (on most systems) 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 (on most systems) in little-endian
byte order.
lelong A four-byte value (on most systems) in little-endian
byte order.
ledate A four-byte value (on most systems) in little-endian
byte order, interpreted as a UNIX date.
leldate A four-byte value (on most systems) 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 (on most systems) in middle-endian
(PDP-11) byte order.
medate A four-byte value (on most systems) in middle-endian
(PDP-11) byte order, interpreted as a UNIX date.
meldate A four-byte value (on most systems) 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 (much like egrep). The type speci
fication can be optionally followed by /c for case-
insensitive matches. The regular expression is
always tested against the first N lines, where N is
the given offset, thus it is only useful for (single-
byte encoded) text. ^ and $ will match the beginning
and end of individual lines, respectively, not begin
ning and end of file.
search A literal string search starting at the given offset.
It must be followed by / which specifies how
many matches shall be attempted (the range). This is
suitable for searching larger binary expressions with
variable offsets, using \ escapes for special charac
ters.
The numeric types may optionally be followed by & and a numeric value,
to specify that the value is to be ANDed 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 spec
ify 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 =. For all tests except string and regex,
operation ! 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 byte string from the file must match the
specified byte 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 string, and then presumably print that string, by
doing >\0 (because all strings are greater than the null
string).
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.
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 one 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 as-is, there are simple calcula
tions possible: appending [+-*/%&|^] inside parentheses allows
one to modify the value read from the file before it is used as an off
set:
# MS Windows executables are also valid MS-DOS executables
0 string MZ
# sometimes, the value at 0x18 is less that 0x40 but theres 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
uplevel 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 its 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 uplevel 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 parenthesed expression can be taken from the
file itself, using another set of parentheses. Note that this addi
tional 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, 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.
SEE ALSO
file(1) - the command that reads this file.
Public Domain MAGIC(5)
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