445 lines
13 KiB
Groff
445 lines
13 KiB
Groff
.\"
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.\" $FreeBSD$
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.\"
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.\" install as magic.4 on USG, magic.5 on V7 or Berkeley systems.
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.\"
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.Dd February 19, 2006
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.Dt MAGIC 5 "Public Domain"
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.Os
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.Sh NAME
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.Nm magic
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.Nd file command's magic number file
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.Sh DESCRIPTION
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This manual page documents the format of the magic file as
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used by the
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.Nm
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command, version 4.21.
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The
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.Nm file
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command identifies the type of a file using,
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among other tests,
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a test for whether the file begins with a certain
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.Em "magic number" .
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The file
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.Pa /usr/share/misc/magic
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specifies what magic numbers are to be tested for,
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what message to print if a particular magic number is found,
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and additional information to extract from the file.
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.Pp
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Each line of the file specifies a test to be performed.
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A test compares the data starting at a particular offset
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in the file with a 1-byte, 2-byte, or 4-byte numeric value or
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a string.
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If the test succeeds, a message is printed.
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The line consists of the following fields:
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.Bl -tag -width indent
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.It offset
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A number specifying the offset, in bytes, into the file of the data
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which is to be tested.
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.It type
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The type of the data to be tested.
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The possible values are:
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.Bl -tag -width indent
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.It byte
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A one-byte value.
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.It short
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A two-byte value (on most systems) in this machine's native byte order.
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.It long
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A four-byte value (on most systems) in this machine's native byte order.
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.It string
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A string of bytes.
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The string type specification can be optionally followed
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by /[Bbc]*.
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The
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.Dq B
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flag compacts whitespace in the target, which must contain
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at least one whitespace character.
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If the magic has
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.Ar n
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consecutive blanks, the target needs at least
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.Ar n
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consecutive blanks to match.
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The
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.Dq b
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flag treats every blank in the target as an optional blank.
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Finally the
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.Dq c
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flag, specifies case insensitive matching: lowercase characters
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in the magic match both lower and upper case characters in the
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targer, whereas upper case characters in the magic, only much
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uppercase characters in the target.
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.It pstring
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A pascal style string where the first byte is interpreted as the an
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unsigned length.
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The string is not
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.Dv NUL
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terminated.
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.It date
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A four-byte value interpreted as a
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.Ux
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date.
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.It ldate
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A four-byte value interpreted as a
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.Ux Ns -style
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date, but interpreted as
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local time rather than UTC.
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.It beshort
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A two-byte value (on most systems) in big-endian byte order.
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.It belong
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A four-byte value (on most systems) in big-endian byte order.
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.It bedate
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A four-byte value (on most systems) in big-endian byte order,
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interpreted as a
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.Ux
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date.
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.It beldate
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A four-byte value (on most systems) in big-endian byte order,
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interpreted as a
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.Ux Ns -style
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date, but interpreted as local time rather
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than UTC.
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.It bestring16
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A two-byte unicode (UCS16) string in big-endian byte order.
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.It leshort
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A two-byte value (on most systems) in little-endian byte order.
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.It lelong
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A four-byte value (on most systems) in little-endian byte order.
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.It ledate
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A four-byte value (on most systems) in little-endian byte order,
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interpreted as a
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.Ux
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date.
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.It leldate
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A four-byte value (on most systems) in little-endian byte order,
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interpreted as a
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.Ux Ns -style
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date, but interpreted as local time rather
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than UTC.
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.It lestring16
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A two-byte unicode (UCS16) string in little-endian byte order.
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.It melong
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A four-byte value (on most systems) in middle-endian (PDP-11) byte order.
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.It medate
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A four-byte value (on most systems) in middle-endian (PDP-11) byte order,
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interpreted as a
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.Ux
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date.
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.It meldate
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A four-byte value (on most systems) in middle-endian (PDP-11) byte order,
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interpreted as a
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.Ux Ns -style
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date, but interpreted as local time rather
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than UTC.
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.It regex
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A regular expression match in extended
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.Tn POSIX
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regular expression syntax
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(much like egrep).
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The type specification can be optionally followed by
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.Ql /c
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for case-insensitive matches.
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The regular expression is always
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tested against the first
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.Ar N
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lines, where
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.Ar N
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is the given offset, thus it
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is only useful for (single-byte encoded) text.
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.Ql ^
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and
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.Ql $
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will match the beginning and end of individual lines, respectively,
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not beginning and end of file.
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.It search
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A literal string search starting at the given offset.
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It must be followed by
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.Li / Ns Aq Ar number
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which specifies how many matches shall be attempted (the range).
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This is suitable for searching larger binary expressions with variable
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offsets, using
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.Ql \e
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escapes for special characters.
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.El
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.El
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.Pp
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The numeric types may optionally be followed by
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.Em &
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and a numeric value,
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to specify that the value is to be AND'ed with the
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numeric value before any comparisons are done.
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Prepending a
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.Em u
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to the type indicates that ordered comparisons should be unsigned.
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.Bl -tag -width indent
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.It test
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The value to be compared with the value from the file.
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If the type is
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numeric, this value
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is specified in C form; if it is a string, it is specified as a C string
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with the usual escapes permitted (e.g.\& \en for new-line).
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.It ""
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Numeric values
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may be preceded by a character indicating the operation to be performed.
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It may be
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.Em = ,
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to specify that the value from the file must equal the specified value,
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.Em < ,
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to specify that the value from the file must be less than the specified
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value,
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.Em > ,
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to specify that the value from the file must be greater than the specified
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value,
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.Em & ,
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to specify that the value from the file must have set all of the bits
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that are set in the specified value,
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.Em ^ ,
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to specify that the value from the file must have clear any of the bits
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that are set in the specified value, or
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.Em ~ ,
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the value specified after is negated before tested, or
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.Em x ,
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to specify that any value will match.
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If the character is omitted,
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it is assumed to be
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.Em = .
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For all tests except
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.Dq string
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and
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.Dq regex ,
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operation
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.Em !\&
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specifies that the line matches if the test does
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.Em not
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succeed.
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.It ""
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Numeric values are specified in C form; e.g.\&
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.Em 13
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is decimal,
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.Em 013
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is octal, and
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.Em 0x13
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is hexadecimal.
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.It ""
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For string values, the byte string from the
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file must match the specified byte string.
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The operators
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.Em = ,
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.Em <
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and
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.Em >
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(but not
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.Em & )
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can be applied to strings.
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The length used for matching is that of the string argument
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in the magic file.
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This means that a line can match any string, and
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then presumably print that string, by doing
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.Em >\e0
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(because all strings are greater than the null string).
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.It message
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The message to be printed if the comparison succeeds.
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If the string
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contains a
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.Xr printf 3
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format specification, the value from the file (with any specified masking
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performed) is printed using the message as the format string.
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.El
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.Pp
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Some file formats contain additional information which is to be printed
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along with the file type or need additional tests to determine the true
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file type.
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These additional tests are introduced by one or more
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.Em >
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characters preceding the offset.
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The number of
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.Em >
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on the line indicates the level of the test; a line with no
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.Em >
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at the beginning is considered to be at level 0.
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Tests are arranged in a tree-like hierarchy:
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If a the test on a line at level
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.Em n
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succeeds, all following tests at level
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.Em n+1
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are performed, and the messages printed if the tests succeed, until a line
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with level
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.Em n
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(or less) appears.
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For more complex files, one can use empty messages to get just the
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"if/then" effect, in the following way:
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.Bd -literal -offset indent
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0 string MZ
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>0x18 leshort <0x40 MS-DOS executable
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>0x18 leshort >0x3f extended PC executable (e.g., MS Windows)
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.Ed
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.Pp
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Offsets do not need to be constant, but can also be read from the file
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being examined.
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If the first character following the last
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.Em >
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is a
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.Em \&(
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then the string after the parenthesis is interpreted as an indirect offset.
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That means that the number after the parenthesis is used as an offset in
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the file.
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The value at that offset is read, and is used again as an offset
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in the file.
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Indirect offsets are of the form:
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.Em (x[.[bslBSL]][+\-][y]) .
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The value of
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.Em x
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is used as an offset in the file.
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A byte, short or long is read at that offset
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depending on the
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.Em [bslBSLm]
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type specifier.
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The capitalized types interpret the number as a big endian value, whereas
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a small letter versions interpret the number as a little endian value;
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the
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.Em m
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type interprets the number as a middle endian (PDP-11) value.
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To that number the value of
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.Em y
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is added and the result is used as an offset in the file.
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The default type
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if one is not specified is long.
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.Pp
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That way variable length structures can be examined:
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.Bd -literal -offset indent
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# MS Windows executables are also valid MS-DOS executables
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0 string MZ
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>0x18 leshort <0x40 MZ executable (MS-DOS)
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# skip the whole block below if it is not an extended executable
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>0x18 leshort >0x3f
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>>(0x3c.l) string PE\e0\e0 PE executable (MS-Windows)
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>>(0x3c.l) string LX\e0\e0 LX executable (OS/2)
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.Ed
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.Pp
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This strategy of examining has one drawback: You must make sure that
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you eventually print something, or users may get empty output (like, when
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there is neither PE\e0\e0 nor LE\e0\e0 in the above example).
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.Pp
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If this indirect offset cannot be used as-is, there are simple calculations
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possible: appending
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.Em [+-*/%&|^]<number>
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inside parentheses allows one to modify
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the value read from the file before it is used as an offset:
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.Bd -literal -offset indent
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# MS Windows executables are also valid MS-DOS executables
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0 string MZ
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# sometimes, the value at 0x18 is less that 0x40 but there's still an
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# extended executable, simply appended to the file
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>0x18 leshort <0x40
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>>(4.s*512) leshort 0x014c COFF executable (MS-DOS, DJGPP)
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>>(4.s*512) leshort !0x014c MZ executable (MS-DOS)
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.Ed
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.Pp
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Sometimes you do not know the exact offset as this depends on the length or
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position (when indirection was used before) of preceding fields.
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You can
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specify an offset relative to the end of the last uplevel field using
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.Em &
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as a prefix to the offset:
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.Bd -literal -offset indent
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0 string MZ
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>0x18 leshort >0x3f
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>>(0x3c.l) string PE\e0\e0 PE executable (MS-Windows)
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# immediately following the PE signature is the CPU type
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>>>&0 leshort 0x14c for Intel 80386
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>>>&0 leshort 0x184 for DEC Alpha
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.Ed
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.Pp
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Indirect and relative offsets can be combined:
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.Bd -literal -offset indent
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0 string MZ
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>0x18 leshort <0x40
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>>(4.s*512) leshort !0x014c MZ executable (MS-DOS)
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# if it's not COFF, go back 512 bytes and add the offset taken
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# from byte 2/3, which is yet another way of finding the start
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# of the extended executable
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>>>&(2.s-514) string LE LE executable (MS Windows VxD driver)
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.Ed
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.Pp
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Or the other way around:
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.Bd -literal -offset indent
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0 string MZ
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>0x18 leshort >0x3f
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>>(0x3c.l) string LE\e0\e0 LE executable (MS-Windows)
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# at offset 0x80 (-4, since relative offsets start at the end
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# of the uplevel match) inside the LE header, we find the absolute
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# offset to the code area, where we look for a specific signature
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>>>(&0x7c.l+0x26) string UPX \eb, UPX compressed
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.Ed
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.Pp
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Or even both!
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.Bd -literal -offset indent
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0 string MZ
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>0x18 leshort >0x3f
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>>(0x3c.l) string LE\e0\e0 LE executable (MS-Windows)
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# at offset 0x58 inside the LE header, we find the relative offset
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# to a data area where we look for a specific signature
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>>>&(&0x54.l-3) string UNACE \eb, ACE self-extracting archive
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.Ed
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.Pp
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Finally, if you have to deal with offset/length pairs in your file, even the
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second value in a parenthesed expression can be taken from the file itself,
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using another set of parentheses.
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Note that this additional indirect offset
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is always relative to the start of the main indirect offset.
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.Bd -literal -offset indent
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0 string MZ
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>0x18 leshort >0x3f
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>>(0x3c.l) string PE\e0\e0 PE executable (MS-Windows)
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# search for the PE section called ".idata"...
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>>>&0xf4 search/0x140 .idata
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# ...and go to the end of it, calculated from start+length;
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# these are located 14 and 10 bytes after the section name
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>>>>(&0xe.l+(-4)) string PK\e3\e4 \eb, ZIP self-extracting archive
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.Ed
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.Sh BUGS
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The formats
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.Em long ,
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.Em belong ,
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.Em lelong ,
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.Em melong ,
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.Em short ,
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.Em beshort ,
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.Em leshort ,
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.Em date ,
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.Em bedate ,
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.Em medate ,
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.Em ledate ,
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.Em beldate ,
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.Em leldate ,
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and
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.Em meldate
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are system-dependent; perhaps they should be specified as a number
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of bytes (2B, 4B, etc),
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since the files being recognized typically come from
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a system on which the lengths are invariant.
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.Pp
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If
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.Pa /usr/share/misc/magic
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is newer than
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.Pa /usr/share/misc/magic.mgc
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it is not used.
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Use the command:
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.Dq Li "cd /usr/share/misc && file -C -m magic"
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to rebuild.
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.Sh SEE ALSO
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.Xr file 1
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.\"
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.\" From: guy@sun.uucp (Guy Harris)
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.\" Newsgroups: net.bugs.usg
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.\" Subject: /etc/magic's format isn't well documented
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.\" Message-ID: <2752@sun.uucp>
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.\" Date: 3 Sep 85 08:19:07 GMT
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.\" Organization: Sun Microsystems, Inc.
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.\" Lines: 136
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.\"
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.\" Here's a manual page for the format accepted by the "file" made by adding
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.\" the changes I posted to the S5R2 version.
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.\"
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.\" Modified for Ian Darwin's version of the file command.
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.\" @(#)$Id: magic.man,v 1.30 2006/02/19 18:16:03 christos Exp $
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