135 lines
4.2 KiB
Groff
135 lines
4.2 KiB
Groff
.\"
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.\" PATCHES MAGIC LEVEL PATCH THAT GOT US HERE
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.\" -------------------- ----- ----------------------
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.\" CURRENT PATCH LEVEL: 1 00103
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.\" -------------------- ----- ----------------------
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.\"
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.\" 16 Feb 93 Rodney W. Grimes Fixed all the commands that had two
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.\" dots in them.
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.\"
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.TH MAGIC FILES "Public Domain"
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.\" install as magic.4 on USG, magic.5 on V7 or Berkeley systems.
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.SH NAME
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magic \- file command's magic number file
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.SH DESCRIPTION
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The
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.IR file (1)
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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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.IR "magic number" .
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The file
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.B /etc/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. If the test succeeds, a message is printed.
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The line consists of the following fields:
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.IP offset \w'message'u+2n
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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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.IP type
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The type of the data to be tested. The possible values are:
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.RS
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.IP byte \w'message'u+2n
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A one-byte value.
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.IP short
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A two-byte value (on most systems).
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.IP long
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A four-byte value (on most systems).
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.IP string
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A string of bytes.
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.RE
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.IP test
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The value to be compared with the value from the file. 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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.IP
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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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.BR = ,
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to specify that the value from the file must equal the specified value,
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.BR < ,
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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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.BR > ,
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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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or
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.BR & ,
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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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Numeric values are specified in C form; e.g.
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.B 13
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is decimal,
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.B 013
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is octal, and
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.B 0x13
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is hexadecimal.
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to specify that any value will match. If the character
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is omitted, it is assumed to be
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.BR = .
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.IP
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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 =, < and > (but not &) 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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.IP message
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The message to be printed if the comparison succeeds. If the string
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contains a
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.IR printf (3S)
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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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.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. A line which begins with the character
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.B >
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indicates additional tests and messages to be printed. If the test on the
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line preceding the first line with a
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.B >
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succeeds, the tests specified in all the subsequent lines beginning with
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.B >
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are performed, and the messages printed if the tests succeed. The next
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line which does not begin with a
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.B >
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terminates this.
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.SH BUGS
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The formats
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.I long
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and
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.I short
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are system-dependant; 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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There should be more than one level of subtests,
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with the level possibly indicated by
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the number of
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.B >
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at the beginning of the line.
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.SH SEE ALSO
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.IR file (1)
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\- the command that reads this file.
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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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.\" @(#)$Header: /cvsroot/src/usr.bin/file/Attic/magic.5,v 1.2 1993/03/21 18:04:42 cgd Exp $
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