xref: /freebsd/contrib/file/doc/magic.man (revision 3e11bd9e2a2b1cbd4283c87c93e3cc75e3f2dacb)
1.\" $File: magic.man,v 1.84 2014/06/03 19:01:34 christos Exp $
2.Dd June 3, 2014
3.Dt MAGIC __FSECTION__
4.Os
5.\" install as magic.4 on USG, magic.5 on V7, Berkeley and Linux systems.
6.Sh NAME
7.Nm magic
8.Nd file command's magic pattern file
9.Sh DESCRIPTION
10This manual page documents the format of the magic file as
11used by the
12.Xr file __CSECTION__
13command, version __VERSION__.
14The
15.Xr file __CSECTION__
16command identifies the type of a file using,
17among other tests,
18a test for whether the file contains certain
19.Dq "magic patterns" .
20The file
21.Pa __MAGIC__
22specifies what patterns are to be tested for, what message or
23MIME type to print if a particular pattern is found,
24and additional information to extract from the file.
25.Pp
26Each line of the file specifies a test to be performed.
27A test compares the data starting at a particular offset
28in the file with a byte value, a string or a numeric value.
29If the test succeeds, a message is printed.
30The line consists of the following fields:
31.Bl -tag -width ".Dv message"
32.It Dv offset
33A number specifying the offset, in bytes, into the file of the data
34which is to be tested.
35.It Dv type
36The type of the data to be tested.
37The possible values are:
38.Bl -tag -width ".Dv lestring16"
39.It Dv byte
40A one-byte value.
41.It Dv short
42A two-byte value in this machine's native byte order.
43.It Dv long
44A four-byte value in this machine's native byte order.
45.It Dv quad
46An eight-byte value in this machine's native byte order.
47.It Dv float
48A 32-bit single precision IEEE floating point number in this machine's native byte order.
49.It Dv double
50A 64-bit double precision IEEE floating point number in this machine's native byte order.
51.It Dv string
52A string of bytes.
53The string type specification can be optionally followed
54by /[WwcCtbT]*.
55The
56.Dq W
57flag compacts whitespace in the target, which must
58contain at least one whitespace character.
59If the magic has
60.Dv n
61consecutive blanks, the target needs at least
62.Dv n
63consecutive blanks to match.
64The
65.Dq w
66flag treats every blank in the magic as an optional blank.
67The
68.Dq c
69flag specifies case insensitive matching: lower case
70characters in the magic match both lower and upper case characters in the
71target, whereas upper case characters in the magic only match upper case
72characters in the target.
73The
74.Dq C
75flag specifies case insensitive matching: upper case
76characters in the magic match both lower and upper case characters in the
77target, whereas lower case characters in the magic only match upper case
78characters in the target.
79To do a complete case insensitive match, specify both
80.Dq c
81and
82.Dq C .
83The
84.Dq t
85flag forces the test to be done for text files, while the
86.Dq b
87flag forces the test to be done for binary files.
88The
89.Dq T
90flag causes the string to be trimmed, i.e. leading and trailing whitespace
91is deleted before the string is printed.
92.It Dv pstring
93A Pascal-style string where the first byte/short/int is interpreted as the
94unsigned length.
95The length defaults to byte and can be specified as a modifier.
96The following modifiers are supported:
97.Bl -tag -compact -width B
98.It B
99A byte length (default).
100.It H
101A 2 byte big endian length.
102.It h
103A 2 byte big little length.
104.It L
105A 4 byte big endian length.
106.It l
107A 4 byte big little length.
108.It J
109The length includes itself in its count.
110.El
111The string is not NUL terminated.
112.Dq J
113is used rather than the more
114valuable
115.Dq I
116because this type of length is a feature of the JPEG
117format.
118.It Dv date
119A four-byte value interpreted as a UNIX date.
120.It Dv qdate
121A eight-byte value interpreted as a UNIX date.
122.It Dv ldate
123A four-byte value interpreted as a UNIX-style date, but interpreted as
124local time rather than UTC.
125.It Dv qldate
126An eight-byte value interpreted as a UNIX-style date, but interpreted as
127local time rather than UTC.
128.It Dv qwdate
129An eight-byte value interpreted as a Windows-style date.
130.It Dv beid3
131A 32-bit ID3 length in big-endian byte order.
132.It Dv beshort
133A two-byte value in big-endian byte order.
134.It Dv belong
135A four-byte value in big-endian byte order.
136.It Dv bequad
137An eight-byte value in big-endian byte order.
138.It Dv befloat
139A 32-bit single precision IEEE floating point number in big-endian byte order.
140.It Dv bedouble
141A 64-bit double precision IEEE floating point number in big-endian byte order.
142.It Dv bedate
143A four-byte value in big-endian byte order,
144interpreted as a Unix date.
145.It Dv beqdate
146An eight-byte value in big-endian byte order,
147interpreted as a Unix date.
148.It Dv beldate
149A four-byte value in big-endian byte order,
150interpreted as a UNIX-style date, but interpreted as local time rather
151than UTC.
152.It Dv beqldate
153An eight-byte value in big-endian byte order,
154interpreted as a UNIX-style date, but interpreted as local time rather
155than UTC.
156.It Dv beqwdate
157An eight-byte value in big-endian byte order,
158interpreted as a Windows-style date.
159.It Dv bestring16
160A two-byte unicode (UCS16) string in big-endian byte order.
161.It Dv leid3
162A 32-bit ID3 length in little-endian byte order.
163.It Dv leshort
164A two-byte value in little-endian byte order.
165.It Dv lelong
166A four-byte value in little-endian byte order.
167.It Dv lequad
168An eight-byte value in little-endian byte order.
169.It Dv lefloat
170A 32-bit single precision IEEE floating point number in little-endian byte order.
171.It Dv ledouble
172A 64-bit double precision IEEE floating point number in little-endian byte order.
173.It Dv ledate
174A four-byte value in little-endian byte order,
175interpreted as a UNIX date.
176.It Dv leqdate
177An eight-byte value in little-endian byte order,
178interpreted as a UNIX date.
179.It Dv leldate
180A four-byte value in little-endian byte order,
181interpreted as a UNIX-style date, but interpreted as local time rather
182than UTC.
183.It Dv leqldate
184An eight-byte value in little-endian byte order,
185interpreted as a UNIX-style date, but interpreted as local time rather
186than UTC.
187.It Dv leqwdate
188An eight-byte value in little-endian byte order,
189interpreted as a Windows-style date.
190.It Dv lestring16
191A two-byte unicode (UCS16) string in little-endian byte order.
192.It Dv melong
193A four-byte value in middle-endian (PDP-11) byte order.
194.It Dv medate
195A four-byte value in middle-endian (PDP-11) byte order,
196interpreted as a UNIX date.
197.It Dv meldate
198A four-byte value in middle-endian (PDP-11) byte order,
199interpreted as a UNIX-style date, but interpreted as local time rather
200than UTC.
201.It Dv indirect
202Starting at the given offset, consult the magic database again.
203.It Dv name
204Define a
205.Dq named
206magic instance that can be called from another
207.Dv use
208magic entry, like a subroutine call.
209Named instance direct magic offsets are relative to the offset of the
210previous matched entry, but indirect offsets are relative to the beginning
211of the file as usual.
212Named magic entries always match.
213.It Dv use
214Recursively call the named magic starting from the current offset.
215If the name of the referenced begins with a
216.Dv ^
217then the endianness of the magic is switched; if the magic mentioned
218.Dv leshort
219for example,
220it is treated as
221.Dv beshort
222and vice versa.
223This is useful to avoid duplicating the rules for different endianness.
224.It Dv regex
225A regular expression match in extended POSIX regular expression syntax
226(like egrep).
227Regular expressions can take exponential time to process, and their
228performance is hard to predict, so their use is discouraged.
229When used in production environments, their performance
230should be carefully checked.
231The size of the string to search should also be limited by specifying
232.Dv /<length> ,
233to avoid performance issues scanning long files.
234The type specification can also be optionally followed by
235.Dv /[c][s][l] .
236The
237.Dq c
238flag makes the match case insensitive, while the
239.Dq s
240flag update the offset to the start offset of the match, rather than the end.
241The
242.Dq l
243modifier, changes the limit of length to mean number of lines instead of a
244byte count.
245Lines are delimited by the platforms native line delimiter.
246When a line count is specified, an implicit byte count also computed assuming
247each line is 80 characters long.
248If neither a byte or line count is specified, the search is limited automatically
249to 8KiB.
250.Dv ^
251and
252.Dv $
253match the beginning and end of individual lines, respectively,
254not beginning and end of file.
255.It Dv search
256A literal string search starting at the given offset.
257The same modifier flags can be used as for string patterns.
258The search expression must contain the range in the form
259.Dv /number,
260that is the number of positions at which the match will be
261attempted, starting from the start offset.
262This is suitable for
263searching larger binary expressions with variable offsets, using
264.Dv \e
265escapes for special characters.
266The order of modifier and number is not relevant.
267.It Dv default
268This is intended to be used with the test
269.Em x
270(which is always true) and it has no type.
271It matches when no other test at that continuation level has matched before.
272Clearing that matched tests for a continuation level, can be done using the
273.Dv clear
274test.
275.It Dv clear
276This test is always true and clears the match flag for that continuation level.
277It is intended to be used with the
278.Dv default
279test.
280.El
281.Pp
282For compatibility with the Single
283.Ux
284Standard, the type specifiers
285.Dv dC
286and
287.Dv d1
288are equivalent to
289.Dv byte ,
290the type specifiers
291.Dv uC
292and
293.Dv u1
294are equivalent to
295.Dv ubyte ,
296the type specifiers
297.Dv dS
298and
299.Dv d2
300are equivalent to
301.Dv short ,
302the type specifiers
303.Dv uS
304and
305.Dv u2
306are equivalent to
307.Dv ushort ,
308the type specifiers
309.Dv dI ,
310.Dv dL ,
311and
312.Dv d4
313are equivalent to
314.Dv long ,
315the type specifiers
316.Dv uI ,
317.Dv uL ,
318and
319.Dv u4
320are equivalent to
321.Dv ulong ,
322the type specifier
323.Dv d8
324is equivalent to
325.Dv quad ,
326the type specifier
327.Dv u8
328is equivalent to
329.Dv uquad ,
330and the type specifier
331.Dv s
332is equivalent to
333.Dv string .
334In addition, the type specifier
335.Dv dQ
336is equivalent to
337.Dv quad
338and the type specifier
339.Dv uQ
340is equivalent to
341.Dv uquad .
342.Pp
343Each top-level magic pattern (see below for an explanation of levels)
344is classified as text or binary according to the types used.
345Types
346.Dq regex
347and
348.Dq search
349are classified as text tests, unless non-printable characters are used
350in the pattern.
351All other tests are classified as binary.
352A top-level
353pattern is considered to be a test text when all its patterns are text
354patterns; otherwise, it is considered to be a binary pattern.
355When
356matching a file, binary patterns are tried first; if no match is
357found, and the file looks like text, then its encoding is determined
358and the text patterns are tried.
359.Pp
360The numeric types may optionally be followed by
361.Dv \*[Am]
362and a numeric value,
363to specify that the value is to be AND'ed with the
364numeric value before any comparisons are done.
365Prepending a
366.Dv u
367to the type indicates that ordered comparisons should be unsigned.
368.It Dv test
369The value to be compared with the value from the file.
370If the type is
371numeric, this value
372is specified in C form; if it is a string, it is specified as a C string
373with the usual escapes permitted (e.g. \en for new-line).
374.Pp
375Numeric values
376may be preceded by a character indicating the operation to be performed.
377It may be
378.Dv = ,
379to specify that the value from the file must equal the specified value,
380.Dv \*[Lt] ,
381to specify that the value from the file must be less than the specified
382value,
383.Dv \*[Gt] ,
384to specify that the value from the file must be greater than the specified
385value,
386.Dv \*[Am] ,
387to specify that the value from the file must have set all of the bits
388that are set in the specified value,
389.Dv ^ ,
390to specify that the value from the file must have clear any of the bits
391that are set in the specified value, or
392.Dv ~ ,
393the value specified after is negated before tested.
394.Dv x ,
395to specify that any value will match.
396If the character is omitted, it is assumed to be
397.Dv = .
398Operators
399.Dv \*[Am] ,
400.Dv ^ ,
401and
402.Dv ~
403don't work with floats and doubles.
404The operator
405.Dv !\&
406specifies that the line matches if the test does
407.Em not
408succeed.
409.Pp
410Numeric values are specified in C form; e.g.
411.Dv 13
412is decimal,
413.Dv 013
414is octal, and
415.Dv 0x13
416is hexadecimal.
417.Pp
418Numeric operations are not performed on date types, instead the numeric
419value is interpreted as an offset.
420.Pp
421For string values, the string from the
422file must match the specified string.
423The operators
424.Dv = ,
425.Dv \*[Lt]
426and
427.Dv \*[Gt]
428(but not
429.Dv \*[Am] )
430can be applied to strings.
431The length used for matching is that of the string argument
432in the magic file.
433This means that a line can match any non-empty string (usually used to
434then print the string), with
435.Em \*[Gt]\e0
436(because all non-empty strings are greater than the empty string).
437.Pp
438Dates are treated as numerical values in the respective internal
439representation.
440.Pp
441The special test
442.Em x
443always evaluates to true.
444.It Dv message
445The message to be printed if the comparison succeeds.
446If the string contains a
447.Xr printf 3
448format specification, the value from the file (with any specified masking
449performed) is printed using the message as the format string.
450If the string begins with
451.Dq \eb ,
452the message printed is the remainder of the string with no whitespace
453added before it: multiple matches are normally separated by a single
454space.
455.El
456.Pp
457An APPLE 4+4 character APPLE creator and type can be specified as:
458.Bd -literal -offset indent
459!:apple	CREATYPE
460.Ed
461.Pp
462A MIME type is given on a separate line, which must be the next
463non-blank or comment line after the magic line that identifies the
464file type, and has the following format:
465.Bd -literal -offset indent
466!:mime	MIMETYPE
467.Ed
468.Pp
469i.e. the literal string
470.Dq !:mime
471followed by the MIME type.
472.Pp
473An optional strength can be supplied on a separate line which refers to
474the current magic description using the following format:
475.Bd -literal -offset indent
476!:strength OP VALUE
477.Ed
478.Pp
479The operand
480.Dv OP
481can be:
482.Dv + ,
483.Dv - ,
484.Dv * ,
485or
486.Dv /
487and
488.Dv VALUE
489is a constant between 0 and 255.
490This constant is applied using the specified operand
491to the currently computed default magic strength.
492.Pp
493Some file formats contain additional information which is to be printed
494along with the file type or need additional tests to determine the true
495file type.
496These additional tests are introduced by one or more
497.Em \*[Gt]
498characters preceding the offset.
499The number of
500.Em \*[Gt]
501on the line indicates the level of the test; a line with no
502.Em \*[Gt]
503at the beginning is considered to be at level 0.
504Tests are arranged in a tree-like hierarchy:
505if the test on a line at level
506.Em n
507succeeds, all following tests at level
508.Em n+1
509are performed, and the messages printed if the tests succeed, until a line
510with level
511.Em n
512(or less) appears.
513For more complex files, one can use empty messages to get just the
514"if/then" effect, in the following way:
515.Bd -literal -offset indent
5160      string   MZ
517\*[Gt]0x18  leshort  \*[Lt]0x40   MS-DOS executable
518\*[Gt]0x18  leshort  \*[Gt]0x3f   extended PC executable (e.g., MS Windows)
519.Ed
520.Pp
521Offsets do not need to be constant, but can also be read from the file
522being examined.
523If the first character following the last
524.Em \*[Gt]
525is a
526.Em \&(
527then the string after the parenthesis is interpreted as an indirect offset.
528That means that the number after the parenthesis is used as an offset in
529the file.
530The value at that offset is read, and is used again as an offset
531in the file.
532Indirect offsets are of the form:
533.Em (( x [.[bislBISL]][+\-][ y ]) .
534The value of
535.Em x
536is used as an offset in the file.
537A byte, id3 length, short or long is read at that offset depending on the
538.Em [bislBISLm]
539type specifier.
540The capitalized types interpret the number as a big endian
541value, whereas the small letter versions interpret the number as a little
542endian value;
543the
544.Em m
545type interprets the number as a middle endian (PDP-11) value.
546To that number the value of
547.Em y
548is added and the result is used as an offset in the file.
549The default type if one is not specified is long.
550.Pp
551That way variable length structures can be examined:
552.Bd -literal -offset indent
553# MS Windows executables are also valid MS-DOS executables
5540           string  MZ
555\*[Gt]0x18       leshort \*[Lt]0x40   MZ executable (MS-DOS)
556# skip the whole block below if it is not an extended executable
557\*[Gt]0x18       leshort \*[Gt]0x3f
558\*[Gt]\*[Gt](0x3c.l)  string  PE\e0\e0  PE executable (MS-Windows)
559\*[Gt]\*[Gt](0x3c.l)  string  LX\e0\e0  LX executable (OS/2)
560.Ed
561.Pp
562This strategy of examining has a drawback: You must make sure that
563you eventually print something, or users may get empty output (like, when
564there is neither PE\e0\e0 nor LE\e0\e0 in the above example)
565.Pp
566If this indirect offset cannot be used directly, simple calculations are
567possible: appending
568.Em [+-*/%\*[Am]|^]number
569inside parentheses allows one to modify
570the value read from the file before it is used as an offset:
571.Bd -literal -offset indent
572# MS Windows executables are also valid MS-DOS executables
5730           string  MZ
574# sometimes, the value at 0x18 is less that 0x40 but there's still an
575# extended executable, simply appended to the file
576\*[Gt]0x18       leshort \*[Lt]0x40
577\*[Gt]\*[Gt](4.s*512) leshort 0x014c  COFF executable (MS-DOS, DJGPP)
578\*[Gt]\*[Gt](4.s*512) leshort !0x014c MZ executable (MS-DOS)
579.Ed
580.Pp
581Sometimes you do not know the exact offset as this depends on the length or
582position (when indirection was used before) of preceding fields.
583You can specify an offset relative to the end of the last up-level
584field using
585.Sq \*[Am]
586as a prefix to the offset:
587.Bd -literal -offset indent
5880           string  MZ
589\*[Gt]0x18       leshort \*[Gt]0x3f
590\*[Gt]\*[Gt](0x3c.l)  string  PE\e0\e0    PE executable (MS-Windows)
591# immediately following the PE signature is the CPU type
592\*[Gt]\*[Gt]\*[Gt]\*[Am]0       leshort 0x14c     for Intel 80386
593\*[Gt]\*[Gt]\*[Gt]\*[Am]0       leshort 0x184     for DEC Alpha
594.Ed
595.Pp
596Indirect and relative offsets can be combined:
597.Bd -literal -offset indent
5980             string  MZ
599\*[Gt]0x18         leshort \*[Lt]0x40
600\*[Gt]\*[Gt](4.s*512)   leshort !0x014c MZ executable (MS-DOS)
601# if it's not COFF, go back 512 bytes and add the offset taken
602# from byte 2/3, which is yet another way of finding the start
603# of the extended executable
604\*[Gt]\*[Gt]\*[Gt]\*[Am](2.s-514) string  LE      LE executable (MS Windows VxD driver)
605.Ed
606.Pp
607Or the other way around:
608.Bd -literal -offset indent
6090                 string  MZ
610\*[Gt]0x18             leshort \*[Gt]0x3f
611\*[Gt]\*[Gt](0x3c.l)        string  LE\e0\e0  LE executable (MS-Windows)
612# at offset 0x80 (-4, since relative offsets start at the end
613# of the up-level match) inside the LE header, we find the absolute
614# offset to the code area, where we look for a specific signature
615\*[Gt]\*[Gt]\*[Gt](\*[Am]0x7c.l+0x26) string  UPX     \eb, UPX compressed
616.Ed
617.Pp
618Or even both!
619.Bd -literal -offset indent
6200                string  MZ
621\*[Gt]0x18            leshort \*[Gt]0x3f
622\*[Gt]\*[Gt](0x3c.l)       string  LE\e0\e0 LE executable (MS-Windows)
623# at offset 0x58 inside the LE header, we find the relative offset
624# to a data area where we look for a specific signature
625\*[Gt]\*[Gt]\*[Gt]\*[Am](\*[Am]0x54.l-3)  string  UNACE  \eb, ACE self-extracting archive
626.Ed
627.Pp
628If you have to deal with offset/length pairs in your file, even the
629second value in a parenthesized expression can be taken from the file itself,
630using another set of parentheses.
631Note that this additional indirect offset is always relative to the
632start of the main indirect offset.
633.Bd -literal -offset indent
6340                 string       MZ
635\*[Gt]0x18             leshort      \*[Gt]0x3f
636\*[Gt]\*[Gt](0x3c.l)        string       PE\e0\e0 PE executable (MS-Windows)
637# search for the PE section called ".idata"...
638\*[Gt]\*[Gt]\*[Gt]\*[Am]0xf4          search/0x140 .idata
639# ...and go to the end of it, calculated from start+length;
640# these are located 14 and 10 bytes after the section name
641\*[Gt]\*[Gt]\*[Gt]\*[Gt](\*[Am]0xe.l+(-4)) string       PK\e3\e4 \eb, ZIP self-extracting archive
642.Ed
643.Pp
644If you have a list of known avalues at a particular continuation level,
645and you want to provide a switch-like default case:
646.Bd -literal -offset indent
647# clear that continuation level match
648\*[Gt]18	clear
649\*[Gt]18	lelong	1	one
650\*[Gt]18	lelong	2	two
651\*[Gt]18	default	x
652# print default match
653\*[Gt]\*[Gt]18	lelong	x	unmatched 0x%x
654.Ed
655.Sh SEE ALSO
656.Xr file __CSECTION__
657\- the command that reads this file.
658.Sh BUGS
659The formats
660.Dv long ,
661.Dv belong ,
662.Dv lelong ,
663.Dv melong ,
664.Dv short ,
665.Dv beshort ,
666and
667.Dv leshort
668do not depend on the length of the C data types
669.Dv short
670and
671.Dv long
672on the platform, even though the Single
673.Ux
674Specification implies that they do.  However, as OS X Mountain Lion has
675passed the Single
676.Ux
677Specification validation suite, and supplies a version of
678.Xr file __CSECTION__
679in which they do not depend on the sizes of the C data types and that is
680built for a 64-bit environment in which
681.Dv long
682is 8 bytes rather than 4 bytes, presumably the validation suite does not
683test whether, for example
684.Dv long
685refers to an item with the same size as the C data type
686.Dv long .
687There should probably be
688.Dv type
689names
690.Dv int8 ,
691.Dv uint8 ,
692.Dv int16 ,
693.Dv uint16 ,
694.Dv int32 ,
695.Dv uint32 ,
696.Dv int64 ,
697and
698.Dv uint64 ,
699and specified-byte-order variants of them,
700to make it clearer that those types have specified widths.
701.\"
702.\" From: guy@sun.uucp (Guy Harris)
703.\" Newsgroups: net.bugs.usg
704.\" Subject: /etc/magic's format isn't well documented
705.\" Message-ID: <2752@sun.uucp>
706.\" Date: 3 Sep 85 08:19:07 GMT
707.\" Organization: Sun Microsystems, Inc.
708.\" Lines: 136
709.\"
710.\" Here's a manual page for the format accepted by the "file" made by adding
711.\" the changes I posted to the S5R2 version.
712.\"
713.\" Modified for Ian Darwin's version of the file command.
714