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Amiga Format AFCD22 (Jan 1998, Issue 106).iso
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magic.man
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MAGIC(5) AMIGA (Public Domain) MAGIC(5)
NAME
magic - file command's magic number file
DESCRIPTION
The file(1) command identifies the type of a file using,
among other tests, a test for whether the file begins with a
certain magic number. The file /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.
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:
offset A number specifying the offset, in bytes, into the
file of the data which is to be tested.
type The type of the data to be tested. The possible
values are:
byte A one-byte value.
short A two-byte value (on most systems) in this
machine's native byte order.
long A four-byte value (on most systems) in
this machine's native byte order.
string A string of bytes.
date A four-byte value interpreted as a unix
date.
beshort A two-byte value (on most systems) in
big-endian byte order.
belong A four-byte value (on most systems) in
big-endian byte order.
bedate A four-byte value (on most systems) in
big-endian byte order, interpreted as a
unix date.
leshort A two-byte value (on most systems) in
little-endian byte order.
lelong A four-byte value (on most systems) in
little-endian byte order.
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MAGIC(5) AMIGA (Public Domain) MAGIC(5)
ledate A four-byte value (on most systems) in
little-endian byte order, interpreted as a
unix date.
The numeric types may optionally be followed by & and a
numeric value, to specify that the value is to be AND'ed
with the numeric value before any comparisons are done.
Prepending a u to the type indicates that ordered
comparisons should be unsigned.
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. \n for new-
line).
Numeric values may be preceded by a character
indicating the operation to be performed. It may be =,
to specify that the value from the file must equal the
specified value, <, to specify that the value from the
file must be less than the specified value, >, to
specify that the value from the file must be greater
than the specified value, &, to specify that the value
from the file must have set all of the bits that are
set in the specified value, or ^, to specify that the
value from the file must have clear any of the bits
that are set in the specified value.
Numeric values are specified in C form; e.g. 13 is
decimal, 013 is octal, and 0x13 is hexadecimal. to
specify that any value will match. If the character is
omitted, it is assumed to be =.
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. This means that a line can match any
string, and then presumably print that string, by doing
>\0 (because all strings are greater than the null
string).
message
The message to be printed if the comparison succeeds.
If the string contains a printf(3S) format
specification, the value from the file (with any
specified masking performed) is printed using the
message as the format string.
Some file formats contain additional information which is to
be printed along with the file type. A line which begins
with the character > indicates additional tests and messages
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MAGIC(5) AMIGA (Public Domain) MAGIC(5)
to be printed. The number of > on the line indicates the
level of the test; a line with no > at the beginning is
considered to be at level 0. Each line at level n+1 is
under the control of the line at level n most closely
preceding it in the magic file. If the test on a line at
level n succeeds, the tests specified in all the subsequent
lines at level n+1 are performed, and the messages printed
if the tests succeed. The next line at level n terminates
this. If the first character following the last > is a (
then the string after the parenthesis is interpreted as an
indirect offset. That means that the number after the
parenthesis is used as a offset in the file. The value at
that offset is read, and is used again as an offset in the
file. Indirect offsets are of the form: ((x[.[bsl]][+-
][y]). The value of x is used as an offset in the file. A
byte, short or long is read at that offset depending on the
[bsl] type specifier. To that number the value of y is added
and the result is used as an offset in the file. The default
type if one is not specified is long.
BUGS
The formats long, belong, lelong, short, beshort, leshort,
date, bedate, and ledate 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.
There is (currently) no support for specified-endian data to
be used in indirect offsets.
SEE ALSO
file(1) - the command that reads this file.
Page 3 (printed 1/23/95)