magic.man 18 KB

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  1. .\" $File: magic.man,v 1.69 2011/05/13 22:11:44 christos Exp $
  2. .Dd April 20, 2011
  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
  10. This manual page documents the format of the magic file as
  11. used by the
  12. .Xr file __CSECTION__
  13. command, version __VERSION__.
  14. The
  15. .Xr file __CSECTION__
  16. command identifies the type of a file using,
  17. among other tests,
  18. a test for whether the file contains certain
  19. .Dq "magic patterns" .
  20. The file
  21. .Pa __MAGIC__
  22. specifies what patterns are to be tested for, what message or
  23. MIME type to print if a particular pattern is found,
  24. and additional information to extract from the file.
  25. .Pp
  26. Each line of the file specifies a test to be performed.
  27. A test compares the data starting at a particular offset
  28. in the file with a byte value, a string or a numeric value.
  29. If the test succeeds, a message is printed.
  30. The line consists of the following fields:
  31. .Bl -tag -width ".Dv message"
  32. .It Dv offset
  33. A number specifying the offset, in bytes, into the file of the data
  34. which is to be tested.
  35. .It Dv type
  36. The type of the data to be tested.
  37. The possible values are:
  38. .Bl -tag -width ".Dv lestring16"
  39. .It Dv byte
  40. A one-byte value.
  41. .It Dv short
  42. A two-byte value in this machine's native byte order.
  43. .It Dv long
  44. A four-byte value in this machine's native byte order.
  45. .It Dv quad
  46. An eight-byte value in this machine's native byte order.
  47. .It Dv float
  48. A 32-bit single precision IEEE floating point number in this machine's native byte order.
  49. .It Dv double
  50. A 64-bit double precision IEEE floating point number in this machine's native byte order.
  51. .It Dv string
  52. A string of bytes.
  53. The string type specification can be optionally followed
  54. by /[WwcCtb]*.
  55. The
  56. .Dq W
  57. flag compacts whitespace in the target, which must
  58. contain at least one whitespace character.
  59. If the magic has
  60. .Dv n
  61. consecutive blanks, the target needs at least
  62. .Dv n
  63. consecutive blanks to match.
  64. The
  65. .Dq w
  66. flag treats every blank in the target as an optional blank.
  67. The
  68. .Dq c
  69. flag, specifies case insensitive matching: lower case
  70. characters in the magic match both lower and upper case characters in the
  71. target, whereas upper case characters in the magic only match upper case
  72. characters in the target.
  73. The
  74. .Dq C
  75. flag, specifies case insensitive matching: upper case
  76. characters in the magic match both lower and upper case characters in the
  77. target, whereas lower case characters in the magic only match upper case
  78. characters in the target.
  79. To do a complete case insensitive match, specify both
  80. .Dq c
  81. and
  82. .Dq C .
  83. The
  84. .Dq t
  85. flag, forces the test to be done for text files, while the
  86. .Dq b
  87. flag, forces the test to be done for binary files.
  88. .It Dv pstring
  89. A Pascal-style string where the first byte/short/int is interpreted as the an
  90. unsigned length.
  91. The length defaults to byte and can be specified as a modifier.
  92. The following modifiers are supported:
  93. .Bl -tag -compact -width B
  94. .It B
  95. A byte length (default).
  96. .It H
  97. A 2 byte big endian length.
  98. .It h
  99. A 2 byte big little length.
  100. .It L
  101. A 4 byte big endian length.
  102. .It l
  103. A 4 byte big little length.
  104. .It J
  105. The length includes itself in its count.
  106. .El
  107. The string is not NUL terminated.
  108. .Dq J
  109. is used rather than the more
  110. valuable
  111. .Dq I
  112. because this type of length is a feature of the JPEG
  113. format.
  114. .It Dv date
  115. A four-byte value interpreted as a UNIX date.
  116. .It Dv qdate
  117. A eight-byte value interpreted as a UNIX date.
  118. .It Dv ldate
  119. A four-byte value interpreted as a UNIX-style date, but interpreted as
  120. local time rather than UTC.
  121. .It Dv qldate
  122. An eight-byte value interpreted as a UNIX-style date, but interpreted as
  123. local time rather than UTC.
  124. .It Dv beid3
  125. A 32-bit ID3 length in big-endian byte order.
  126. .It Dv beshort
  127. A two-byte value in big-endian byte order.
  128. .It Dv belong
  129. A four-byte value in big-endian byte order.
  130. .It Dv bequad
  131. An eight-byte value in big-endian byte order.
  132. .It Dv befloat
  133. A 32-bit single precision IEEE floating point number in big-endian byte order.
  134. .It Dv bedouble
  135. A 64-bit double precision IEEE floating point number in big-endian byte order.
  136. .It Dv bedate
  137. A four-byte value in big-endian byte order,
  138. interpreted as a Unix date.
  139. .It Dv beqdate
  140. An eight-byte value in big-endian byte order,
  141. interpreted as a Unix date.
  142. .It Dv beldate
  143. A four-byte value in big-endian byte order,
  144. interpreted as a UNIX-style date, but interpreted as local time rather
  145. than UTC.
  146. .It Dv beqldate
  147. An eight-byte value in big-endian byte order,
  148. interpreted as a UNIX-style date, but interpreted as local time rather
  149. than UTC.
  150. .It Dv bestring16
  151. A two-byte unicode (UCS16) string in big-endian byte order.
  152. .It Dv leid3
  153. A 32-bit ID3 length in little-endian byte order.
  154. .It Dv leshort
  155. A two-byte value in little-endian byte order.
  156. .It Dv lelong
  157. A four-byte value in little-endian byte order.
  158. .It Dv lequad
  159. An eight-byte value in little-endian byte order.
  160. .It Dv lefloat
  161. A 32-bit single precision IEEE floating point number in little-endian byte order.
  162. .It Dv ledouble
  163. A 64-bit double precision IEEE floating point number in little-endian byte order.
  164. .It Dv ledate
  165. A four-byte value in little-endian byte order,
  166. interpreted as a UNIX date.
  167. .It Dv leqdate
  168. An eight-byte value in little-endian byte order,
  169. interpreted as a UNIX date.
  170. .It Dv leldate
  171. A four-byte value in little-endian byte order,
  172. interpreted as a UNIX-style date, but interpreted as local time rather
  173. than UTC.
  174. .It Dv leqldate
  175. An eight-byte value in little-endian byte order,
  176. interpreted as a UNIX-style date, but interpreted as local time rather
  177. than UTC.
  178. .It Dv lestring16
  179. A two-byte unicode (UCS16) string in little-endian byte order.
  180. .It Dv melong
  181. A four-byte value in middle-endian (PDP-11) byte order.
  182. .It Dv medate
  183. A four-byte value in middle-endian (PDP-11) byte order,
  184. interpreted as a UNIX date.
  185. .It Dv meldate
  186. A four-byte value in middle-endian (PDP-11) byte order,
  187. interpreted as a UNIX-style date, but interpreted as local time rather
  188. than UTC.
  189. .It Dv indirect
  190. Starting at the given offset, consult the magic database again.
  191. .It Dv regex
  192. A regular expression match in extended POSIX regular expression syntax
  193. (like egrep).
  194. Regular expressions can take exponential time to process, and their
  195. performance is hard to predict, so their use is discouraged.
  196. When used in production environments, their performance
  197. should be carefully checked.
  198. The type specification can be optionally followed by
  199. .Dv /[c][s] .
  200. The
  201. .Dq c
  202. flag makes the match case insensitive, while the
  203. .Dq s
  204. flag update the offset to the start offset of the match, rather than the end.
  205. The regular expression is tested against line
  206. .Dv N + 1
  207. onwards, where
  208. .Dv N
  209. is the given offset.
  210. Line endings are assumed to be in the machine's native format.
  211. .Dv ^
  212. and
  213. .Dv $
  214. match the beginning and end of individual lines, respectively,
  215. not beginning and end of file.
  216. .It Dv search
  217. A literal string search starting at the given offset.
  218. The same modifier flags can be used as for string patterns.
  219. The modifier flags (if any) must be followed by
  220. .Dv /number
  221. the range, that is, the number of positions at which the match will be
  222. attempted, starting from the start offset.
  223. This is suitable for
  224. searching larger binary expressions with variable offsets, using
  225. .Dv \e
  226. escapes for special characters.
  227. The offset works as for regex.
  228. .It Dv default
  229. This is intended to be used with the test
  230. .Em x
  231. (which is always true) and a message that is to be used if there are
  232. no other matches.
  233. .El
  234. .Pp
  235. Each top-level magic pattern (see below for an explanation of levels)
  236. is classified as text or binary according to the types used.
  237. Types
  238. .Dq regex
  239. and
  240. .Dq search
  241. are classified as text tests, unless non-printable characters are used
  242. in the pattern.
  243. All other tests are classified as binary.
  244. A top-level
  245. pattern is considered to be a test text when all its patterns are text
  246. patterns; otherwise, it is considered to be a binary pattern.
  247. When
  248. matching a file, binary patterns are tried first; if no match is
  249. found, and the file looks like text, then its encoding is determined
  250. and the text patterns are tried.
  251. .Pp
  252. The numeric types may optionally be followed by
  253. .Dv \*[Am]
  254. and a numeric value,
  255. to specify that the value is to be AND'ed with the
  256. numeric value before any comparisons are done.
  257. Prepending a
  258. .Dv u
  259. to the type indicates that ordered comparisons should be unsigned.
  260. .It Dv test
  261. The value to be compared with the value from the file.
  262. If the type is
  263. numeric, this value
  264. is specified in C form; if it is a string, it is specified as a C string
  265. with the usual escapes permitted (e.g. \en for new-line).
  266. .Pp
  267. Numeric values
  268. may be preceded by a character indicating the operation to be performed.
  269. It may be
  270. .Dv = ,
  271. to specify that the value from the file must equal the specified value,
  272. .Dv \*[Lt] ,
  273. to specify that the value from the file must be less than the specified
  274. value,
  275. .Dv \*[Gt] ,
  276. to specify that the value from the file must be greater than the specified
  277. value,
  278. .Dv \*[Am] ,
  279. to specify that the value from the file must have set all of the bits
  280. that are set in the specified value,
  281. .Dv ^ ,
  282. to specify that the value from the file must have clear any of the bits
  283. that are set in the specified value, or
  284. .Dv ~ ,
  285. the value specified after is negated before tested.
  286. .Dv x ,
  287. to specify that any value will match.
  288. If the character is omitted, it is assumed to be
  289. .Dv = .
  290. Operators
  291. .Dv \*[Am] ,
  292. .Dv ^ ,
  293. and
  294. .Dv ~
  295. don't work with floats and doubles.
  296. The operator
  297. .Dv !\&
  298. specifies that the line matches if the test does
  299. .Em not
  300. succeed.
  301. .Pp
  302. Numeric values are specified in C form; e.g.
  303. .Dv 13
  304. is decimal,
  305. .Dv 013
  306. is octal, and
  307. .Dv 0x13
  308. is hexadecimal.
  309. .Pp
  310. For string values, the string from the
  311. file must match the specified string.
  312. The operators
  313. .Dv = ,
  314. .Dv \*[Lt]
  315. and
  316. .Dv \*[Gt]
  317. (but not
  318. .Dv \*[Am] )
  319. can be applied to strings.
  320. The length used for matching is that of the string argument
  321. in the magic file.
  322. This means that a line can match any non-empty string (usually used to
  323. then print the string), with
  324. .Em \*[Gt]\e0
  325. (because all non-empty strings are greater than the empty string).
  326. .Pp
  327. The special test
  328. .Em x
  329. always evaluates to true.
  330. .It Dv message
  331. The message to be printed if the comparison succeeds.
  332. If the string contains a
  333. .Xr printf 3
  334. format specification, the value from the file (with any specified masking
  335. performed) is printed using the message as the format string.
  336. If the string begins with
  337. .Dq \eb ,
  338. the message printed is the remainder of the string with no whitespace
  339. added before it: multiple matches are normally separated by a single
  340. space.
  341. .El
  342. .Pp
  343. An APPLE 4+4 character APPLE creator and type can be specified as:
  344. .Bd -literal -offset indent
  345. !:apple CREATYPE
  346. .Ed
  347. .Pp
  348. A MIME type is given on a separate line, which must be the next
  349. non-blank or comment line after the magic line that identifies the
  350. file type, and has the following format:
  351. .Bd -literal -offset indent
  352. !:mime MIMETYPE
  353. .Ed
  354. .Pp
  355. i.e. the literal string
  356. .Dq !:mime
  357. followed by the MIME type.
  358. .Pp
  359. An optional strength can be supplied on a separate line which refers to
  360. the current magic description using the following format:
  361. .Bd -literal -offset indent
  362. !:strength OP VALUE
  363. .Ed
  364. .Pp
  365. The operand
  366. .Dv OP
  367. can be:
  368. .Dv + ,
  369. .Dv - ,
  370. .Dv * ,
  371. or
  372. .Dv /
  373. and
  374. .Dv VALUE
  375. is a constant between 0 and 255.
  376. This constant is applied using the specified operand
  377. to the currently computed default magic strength.
  378. .Pp
  379. Some file formats contain additional information which is to be printed
  380. along with the file type or need additional tests to determine the true
  381. file type.
  382. These additional tests are introduced by one or more
  383. .Em \*[Gt]
  384. characters preceding the offset.
  385. The number of
  386. .Em \*[Gt]
  387. on the line indicates the level of the test; a line with no
  388. .Em \*[Gt]
  389. at the beginning is considered to be at level 0.
  390. Tests are arranged in a tree-like hierarchy:
  391. if the test on a line at level
  392. .Em n
  393. succeeds, all following tests at level
  394. .Em n+1
  395. are performed, and the messages printed if the tests succeed, until a line
  396. with level
  397. .Em n
  398. (or less) appears.
  399. For more complex files, one can use empty messages to get just the
  400. "if/then" effect, in the following way:
  401. .Bd -literal -offset indent
  402. 0 string MZ
  403. \*[Gt]0x18 leshort \*[Lt]0x40 MS-DOS executable
  404. \*[Gt]0x18 leshort \*[Gt]0x3f extended PC executable (e.g., MS Windows)
  405. .Ed
  406. .Pp
  407. Offsets do not need to be constant, but can also be read from the file
  408. being examined.
  409. If the first character following the last
  410. .Em \*[Gt]
  411. is a
  412. .Em \&(
  413. then the string after the parenthesis is interpreted as an indirect offset.
  414. That means that the number after the parenthesis is used as an offset in
  415. the file.
  416. The value at that offset is read, and is used again as an offset
  417. in the file.
  418. Indirect offsets are of the form:
  419. .Em (( x [.[bislBISL]][+\-][ y ]) .
  420. The value of
  421. .Em x
  422. is used as an offset in the file.
  423. A byte, id3 length, short or long is read at that offset depending on the
  424. .Em [bislBISLm]
  425. type specifier.
  426. The capitalized types interpret the number as a big endian
  427. value, whereas the small letter versions interpret the number as a little
  428. endian value;
  429. the
  430. .Em m
  431. type interprets the number as a middle endian (PDP-11) value.
  432. To that number the value of
  433. .Em y
  434. is added and the result is used as an offset in the file.
  435. The default type if one is not specified is long.
  436. .Pp
  437. That way variable length structures can be examined:
  438. .Bd -literal -offset indent
  439. # MS Windows executables are also valid MS-DOS executables
  440. 0 string MZ
  441. \*[Gt]0x18 leshort \*[Lt]0x40 MZ executable (MS-DOS)
  442. # skip the whole block below if it is not an extended executable
  443. \*[Gt]0x18 leshort \*[Gt]0x3f
  444. \*[Gt]\*[Gt](0x3c.l) string PE\e0\e0 PE executable (MS-Windows)
  445. \*[Gt]\*[Gt](0x3c.l) string LX\e0\e0 LX executable (OS/2)
  446. .Ed
  447. .Pp
  448. This strategy of examining has a drawback: You must make sure that
  449. you eventually print something, or users may get empty output (like, when
  450. there is neither PE\e0\e0 nor LE\e0\e0 in the above example)
  451. .Pp
  452. If this indirect offset cannot be used directly, simple calculations are
  453. possible: appending
  454. .Em [+-*/%\*[Am]|^]number
  455. inside parentheses allows one to modify
  456. the value read from the file before it is used as an offset:
  457. .Bd -literal -offset indent
  458. # MS Windows executables are also valid MS-DOS executables
  459. 0 string MZ
  460. # sometimes, the value at 0x18 is less that 0x40 but there's still an
  461. # extended executable, simply appended to the file
  462. \*[Gt]0x18 leshort \*[Lt]0x40
  463. \*[Gt]\*[Gt](4.s*512) leshort 0x014c COFF executable (MS-DOS, DJGPP)
  464. \*[Gt]\*[Gt](4.s*512) leshort !0x014c MZ executable (MS-DOS)
  465. .Ed
  466. .Pp
  467. Sometimes you do not know the exact offset as this depends on the length or
  468. position (when indirection was used before) of preceding fields.
  469. You can specify an offset relative to the end of the last up-level
  470. field using
  471. .Sq \*[Am]
  472. as a prefix to the offset:
  473. .Bd -literal -offset indent
  474. 0 string MZ
  475. \*[Gt]0x18 leshort \*[Gt]0x3f
  476. \*[Gt]\*[Gt](0x3c.l) string PE\e0\e0 PE executable (MS-Windows)
  477. # immediately following the PE signature is the CPU type
  478. \*[Gt]\*[Gt]\*[Gt]\*[Am]0 leshort 0x14c for Intel 80386
  479. \*[Gt]\*[Gt]\*[Gt]\*[Am]0 leshort 0x184 for DEC Alpha
  480. .Ed
  481. .Pp
  482. Indirect and relative offsets can be combined:
  483. .Bd -literal -offset indent
  484. 0 string MZ
  485. \*[Gt]0x18 leshort \*[Lt]0x40
  486. \*[Gt]\*[Gt](4.s*512) leshort !0x014c MZ executable (MS-DOS)
  487. # if it's not COFF, go back 512 bytes and add the offset taken
  488. # from byte 2/3, which is yet another way of finding the start
  489. # of the extended executable
  490. \*[Gt]\*[Gt]\*[Gt]\*[Am](2.s-514) string LE LE executable (MS Windows VxD driver)
  491. .Ed
  492. .Pp
  493. Or the other way around:
  494. .Bd -literal -offset indent
  495. 0 string MZ
  496. \*[Gt]0x18 leshort \*[Gt]0x3f
  497. \*[Gt]\*[Gt](0x3c.l) string LE\e0\e0 LE executable (MS-Windows)
  498. # at offset 0x80 (-4, since relative offsets start at the end
  499. # of the up-level match) inside the LE header, we find the absolute
  500. # offset to the code area, where we look for a specific signature
  501. \*[Gt]\*[Gt]\*[Gt](\*[Am]0x7c.l+0x26) string UPX \eb, UPX compressed
  502. .Ed
  503. .Pp
  504. Or even both!
  505. .Bd -literal -offset indent
  506. 0 string MZ
  507. \*[Gt]0x18 leshort \*[Gt]0x3f
  508. \*[Gt]\*[Gt](0x3c.l) string LE\e0\e0 LE executable (MS-Windows)
  509. # at offset 0x58 inside the LE header, we find the relative offset
  510. # to a data area where we look for a specific signature
  511. \*[Gt]\*[Gt]\*[Gt]\*[Am](\*[Am]0x54.l-3) string UNACE \eb, ACE self-extracting archive
  512. .Ed
  513. .Pp
  514. Finally, if you have to deal with offset/length pairs in your file, even the
  515. second value in a parenthesized expression can be taken from the file itself,
  516. using another set of parentheses.
  517. Note that this additional indirect offset is always relative to the
  518. start of the main indirect offset.
  519. .Bd -literal -offset indent
  520. 0 string MZ
  521. \*[Gt]0x18 leshort \*[Gt]0x3f
  522. \*[Gt]\*[Gt](0x3c.l) string PE\e0\e0 PE executable (MS-Windows)
  523. # search for the PE section called ".idata"...
  524. \*[Gt]\*[Gt]\*[Gt]\*[Am]0xf4 search/0x140 .idata
  525. # ...and go to the end of it, calculated from start+length;
  526. # these are located 14 and 10 bytes after the section name
  527. \*[Gt]\*[Gt]\*[Gt]\*[Gt](\*[Am]0xe.l+(-4)) string PK\e3\e4 \eb, ZIP self-extracting archive
  528. .Ed
  529. .Sh SEE ALSO
  530. .Xr file __CSECTION__
  531. \- the command that reads this file.
  532. .Sh BUGS
  533. The formats
  534. .Dv long ,
  535. .Dv belong ,
  536. .Dv lelong ,
  537. .Dv melong ,
  538. .Dv short ,
  539. .Dv beshort ,
  540. .Dv leshort ,
  541. .Dv date ,
  542. .Dv bedate ,
  543. .Dv medate ,
  544. .Dv ledate ,
  545. .Dv beldate ,
  546. .Dv leldate ,
  547. and
  548. .Dv meldate
  549. are system-dependent; perhaps they should be specified as a number
  550. of bytes (2B, 4B, etc),
  551. since the files being recognized typically come from
  552. a system on which the lengths are invariant.
  553. .\"
  554. .\" From: guy@sun.uucp (Guy Harris)
  555. .\" Newsgroups: net.bugs.usg
  556. .\" Subject: /etc/magic's format isn't well documented
  557. .\" Message-ID: <2752@sun.uucp>
  558. .\" Date: 3 Sep 85 08:19:07 GMT
  559. .\" Organization: Sun Microsystems, Inc.
  560. .\" Lines: 136
  561. .\"
  562. .\" Here's a manual page for the format accepted by the "file" made by adding
  563. .\" the changes I posted to the S5R2 version.
  564. .\"
  565. .\" Modified for Ian Darwin's version of the file command.