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NAME | DESCRIPTION | SEE ALSO | BUGS | COLOPHON |
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MAGIC(4) Kernel Interfaces Manual MAGIC(4)
magic — file command's magic pattern file
This manual page documents the format of magic files as used by
the file(1) command, version 5.45. 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
database of these “magic patterns” is usually located in a binary
file in /usr/local/share/misc/magic.mgc or a directory of source
text magic pattern fragment files in /usr/local/share/misc/magic.
The database 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.
The format of the source fragment files that are used to build
this database is as follows: Each line of a fragment 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. This offset can be a
negative number if it is:
• The first direct offset of the magic entry (at
continuation level 0), in which case it is
interpreted an offset from end end of the file going
backwards. This works only when a file descriptor
to the file is available and it is a regular file.
• A continuation offset relative to the end of the
last up-level field (&).
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
a /<width> option and optionally followed by
a set of flags /[bCcftTtWw]*. The width
limits the number of characters to be
copied. Zero means all characters. The
following flags are supported:
b Force binary file test.
C Use upper 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.
c Use lower 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. To do a complete case
insensitive match, specify both “c”
and “C”.
f Require that the matched string is a
full word, not a partial word match.
T Trim the string, i.e. leading and
trailing whitespace
t Force text file test.
W Compact 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.
w Treat every blank in the magic as an
optional blank. 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 little endian length.
L A 4 byte big endian length.
l A 4 byte little endian 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 An 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. The offset of the
indirect magic is by default absolute in the
file, but one can specify /r to indicate
that the offset is relative from the
beginning of the entry.
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. ^ 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 search
expression must contain the range in the
form /number, 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 order
of modifier and number is not relevant.
default This is intended to be used with the test x
(which is always true) and it has no type.
It matches when no other test at that
continuation level has matched before.
Clearing that matched tests for a
continuation level, can be done using the
clear test.
clear This test is always true and clears the
match flag for that continuation level. It
is intended to be used with the default
test.
der Parse the file as a DER Certificate file.
The test field is used as a der type that
needs to be matched. The DER types are:
eoc, bool, int, bit_str, octet_str, null,
obj_id, obj_desc, ext, real, enum, embed,
utf8_str, rel_oid, time, res2, seq, set,
num_str, prt_str, t61_str, vid_str, ia5_str,
utc_time, gen_time, gr_str, vis_str,
gen_str, univ_str, char_str, bmp_str, date,
tod, datetime, duration, oid-iri,
rel-oid-iri. These types can be followed by
an optional numeric size, which indicates
the field width in bytes.
guid A Globally Unique Identifier, parsed and
printed as XXXXXXXX-XXXX-XXXX-XXXX-
XXXXXXXXXXXX. It's format is a string.
offset This is a quad value indicating the current
offset of the file. It can be used to
determine the size of the file or the magic
buffer. For example the magic entries:
-0 offset x this file is %lld bytes
-0 offset <=100 must be more than 100 \
bytes and is only %lld
octal A string representing an octal number.
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).
Dates are treated as numerical values in the respective
internal representation.
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 slash-separated list of commonly found filename extensions can
be specified as:
!:ext ext[/ext...]
i.e. the literal string “!:ext” followed by a slash-separated
list of commonly found extensions; for example for JPEG images:
!:ext jpeg/jpg/jpe/jfif
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 uleshort <0x40 MS-DOS executable
>0x18 uleshort >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
[[.,][bBcCeEfFgGhHiIlmosSqQ]][+-][ 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 [bBcCeEfFgGhHiIlLmsSqQ] type
specifier. The value is treated as signed if “,” is specified or
unsigned if “.” is specified. 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. The following types are recognized:
Type Sy Mnemonic Sy Endian Sy Size
bcBC Byte/Char N/A 1
efg Double Little 8
EFG Double Big 8
hs Half/Short Little 2
HS Half/Short Big 2
i ID3 Little 4
I ID3 Big 4
l Long Little 4
L Long Big 4
m Middle Middle 4
o Octal Textual Variable
q Quad Little 8
Q Quad Big 8
That way variable length structures can be examined:
# MS Windows executables are also valid MS-DOS executables
0 string MZ
>0x18 uleshort <0x40 MZ executable (MS-DOS)
# skip the whole block below if it is not an extended executable
>0x18 uleshort >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 (such as 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 uleshort <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 uleshort >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 0x8664 for x86-64
>>>&0 leshort 0x184 for DEC Alpha
Indirect and relative offsets can be combined:
0 string MZ
>0x18 uleshort <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 uleshort >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 uleshort >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
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 uleshort >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
If you have a list of known values at a particular continuation
level, and you want to provide a switch-like default case:
# clear that continuation level match
>18 clear x
>18 lelong 1 one
>18 lelong 2 two
>18 default x
# print default match
>>18 lelong x unmatched 0x%x
file(1) - the command that reads this file.
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.
This page is part of the file (a file type guesser) project.
Information about the project can be found at
http://www.darwinsys.com/file/. If you have a bug report for
this manual page, see ⟨http://bugs.gw.com/my_view_page.php⟩.
This page was obtained from the project's upstream Git read-only
mirror of the CVS repository ⟨https://github.com/glensc/file⟩ on
2024-06-14. (At that time, the date of the most recent commit
that was found in the repository was 2024-06-10.) If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original
manual page), send a mail to [email protected]
GNU March 31, 2024 MAGIC(4)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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