4 .\" install as magic.4 on USG, magic.5 on V7 or Berkeley systems.
11 .Nd file command's magic number file
13 This manual page documents the format of the magic file as
16 command, version 3.22. The
18 command identifies the type of a file using,
20 a test for whether the file begins with a certain
23 .Pa /usr/share/misc/magic
24 specifies what magic numbers are to be tested for,
25 what message to print if a particular magic number is found,
26 and additional information to extract from the file.
28 Each line of the file specifies a test to be performed.
29 A test compares the data starting at a particular offset
30 in the file with a 1-byte, 2-byte, or 4-byte numeric value or
31 a string. If the test succeeds, a message is printed.
32 The line consists of the following fields:
33 .Bl -tag -width indent
35 A number specifying the offset, in bytes, into the file of the data
36 which is to be tested.
38 The type of the data to be tested. The possible values are:
39 .Bl -tag -width indent
43 A two-byte value (on most systems) in this machine's native byte order.
45 A four-byte value (on most systems) in this machine's native byte order.
49 A four-byte value interpreted as a unix date.
51 A two-byte value (on most systems) in big-endian byte order.
53 A four-byte value (on most systems) in big-endian byte order.
55 A four-byte value (on most systems) in big-endian byte order,
56 interpreted as a unix date.
58 A two-byte value (on most systems) in little-endian byte order.
60 A four-byte value (on most systems) in little-endian byte order.
62 A four-byte value (on most systems) in little-endian byte order,
63 interpreted as a unix date.
67 The numeric types may optionally be followed by
70 to specify that the value is to be AND'ed with the
71 numeric value before any comparisons are done. Prepending a
73 to the type indicates that ordered comparisons should be unsigned.
74 .Bl -tag -width indent
76 The value to be compared with the value from the file. If the type is
78 is specified in C form; if it is a string, it is specified as a C string
79 with the usual escapes permitted (e.g. \en for new-line).
82 may be preceded by a character indicating the operation to be performed.
85 to specify that the value from the file must equal the specified value,
87 to specify that the value from the file must be less than the specified
90 to specify that the value from the file must be greater than the specified
93 to specify that the value from the file must have set all of the bits
94 that are set in the specified value,
96 to specify that the value from the file must have clear any of the bits
97 that are set in the specified value, or
99 to specify that any value will match. If the character is omitted,
103 Numeric values are specified in C form; e.g.
111 For string values, the byte string from the
112 file must match the specified byte string.
120 can be applied to strings.
121 The length used for matching is that of the string argument
122 in the magic file. This means that a line can match any string, and
123 then presumably print that string, by doing
125 (because all strings are greater than the null string).
127 The message to be printed if the comparison succeeds. If the string
130 format specification, the value from the file (with any specified masking
131 performed) is printed using the message as the format string.
134 Some file formats contain additional information which is to be printed
135 along with the file type. A line which begins with the character
137 indicates additional tests and messages to be printed. The number of
139 on the line indicates the level of the test; a line with no
141 at the beginning is considered to be at level 0.
144 is under the control of the line at level
146 most closely preceding it in the magic file.
147 If the test on a line at level
149 succeeds, the tests specified in all the subsequent lines at level
151 are performed, and the messages printed if the tests succeed. The next
155 If the first character following the last
159 then the string after the parenthesis is interpreted as an indirect offset.
160 That means that the number after the parenthesis is used as an offset in
161 the file. The value at that offset is read, and is used again as an offset
162 in the file. Indirect offsets are of the form:
163 .Em (x[.[bsl]][+-][y]) .
166 is used as an offset in the file. A byte, short or long is read at that offset
169 type specifier. To that number the value of
171 is added and the result is used as an offset in the file. The default type
172 if one is not specified is long.
174 Sometimes you do not know the exact offset as this depends on the length of
175 preceding fields. You can specify an offset relative to the end of the
176 last uplevel field (of course this may only be done for sublevel tests, i.e.
179 Such a relative offset is specified using
181 as a prefix to the offset.
194 are system-dependent; perhaps they should be specified as a number
195 of bytes (2B, 4B, etc),
196 since the files being recognized typically come from
197 a system on which the lengths are invariant.
199 There is (currently) no support for specified-endian data to be used in
204 .\" From: guy@sun.uucp (Guy Harris)
205 .\" Newsgroups: net.bugs.usg
206 .\" Subject: /etc/magic's format isn't well documented
207 .\" Message-ID: <2752@sun.uucp>
208 .\" Date: 3 Sep 85 08:19:07 GMT
209 .\" Organization: Sun Microsystems, Inc.
212 .\" Here's a manual page for the format accepted by the "file" made by adding
213 .\" the changes I posted to the S5R2 version.
215 .\" Modified for Ian Darwin's version of the file command.