magic.4

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.TH MAGIC FILES "Public Domain"
.\" install as magic.4 on USG, magic.5 on V7 or Berkeley systems.
.SH NAME
magic \- file command's magic number file
.SH DESCRIPTION
The
.IR file (1)
command identifies the type of a file using,
among other tests,
a test for whether the file begins with a certain
.IR "magic number" .
The file
.B /etc/magic
specifies what magic numbers are to be tested for,
what message to print if a particular magic number is found,
and additional information to extract from the file.
.PP
Each line of the file specifies a test to be performed.
A test compares the data starting at a particular offset
in the file with a 1-byte, 2-byte, or 4-byte numeric value or
a string.  If the test succeeds, a message is printed.
The line consists of the following fields:
.IP offset \w'message'u+2n
A number specifying the offset, in bytes, into the file of the data
which is to be tested.
.IP type
The type of the data to be tested.  The possible values are:
.RS
.IP byte \w'message'u+2n
A one-byte value.
.IP short
A two-byte value (on most systems).
.IP long
A four-byte value (on most systems).
.IP string
A string of bytes.
.RE
.IP test
The value to be compared with the value from the file.  If the type is
numeric, this value
is specified in C form; if it is a string, it is specified as a C string
with the usual escapes permitted (e.g. \en for new-line).
.IP
Numeric values
may be preceded by a character indicating the operation to be performed.
It may be
.BR = ,
to specify that the value from the file must equal the specified value,
.BR < ,
to specify that the value from the file must be less than the specified
value,
.BR > ,
to specify that the value from the file must be greater than the specified
value,
or
.BR & ,
to specify that the value is to be AND'ed with the
numeric value before any comparisons are done.
Numeric values are specified in C form; e.g.
.B 13
is decimal,
.B 013
is octal, and
.B 0x13
is hexadecimal.
to specify that any value will match.  If the character
is omitted, it is assumed to be
.BR = .
.IP
For string values, the byte string from the
file must match the specified byte string. 
The operators =, < and > (but not &) can be applied to strings.
The length used for matching is that of the string argument
in the magic file.
.IP message
The message to be printed if the comparison succeeds.  If the string
contains a
.IR printf (3S)
format specification, the value from the file (with any specified masking
performed) is printed using the message as the format string.
.PP
Some file formats contain additional information which is to be printed
along with the file type.  A line which begins with the character
.B >
indicates additional tests and messages to be printed.  If the test on the
line preceding the first line with a
.B >
succeeds, the tests specified in all the subsequent lines beginning with
.B >
are performed, and the messages printed if the tests succeed.  The next
line which does not begin with a
.B >
terminates this.
.SH BUGS
The formats 
.I long
and
.I short
are system-dependant; perhaps they should be specified as a number
of bytes (2B, 4B, etc), 
since the files being recognized typically come from
a system on which the lengths are invariant.
.PP
There should be more than one level of subtests,
with the level possibly indicated by
the number of
.B >
at the beginning of the line.
.SH SEE ALSO
.IR file (1)
\- the command that reads this file.
.\"
.\" From: [e-mail] (Guy Harris)
.\" Newsgroups: net.bugs.usg
.\" Subject: /etc/magic's format isn't well documented
.\" Message-ID: <[e-mail]>
.\" Date: 3 Sep 85 08:19:07 GMT
.\" Organization: Sun Microsystems, Inc.
.\" Lines: 136
.\" 
.\" Here's a manual page for the format accepted by the "file" made by adding
.\" the changes I posted to the S5R2 version.
.\"
.\" Modified for Ian Darwin's version of the file command.
.\" @(#)/projects/agile/cvs/harvest/src/gatherer/essence/file/magic.4,v 1.1.1.1 1994/04/26 19:07:47 hardy Exp