illumos: manual page: magic.5

MAGIC(5) Standards, Environments, and Macros MAGIC(5)

NAME

magic - file command's magic number file

SYNOPSIS

/etc/magic

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 /etc/magic file, or a file specified as an option-argument to the -m
or -M options of file(1), 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 position-sensitive test to perform. A
test compares the data starting at a particular offset in the file with a
1-byte, 2-byte, 4-byte, or 8-byte numeric value or string. If the test
succeeds, a message is printed. The line consists of the following fields
(separated by tabs): offset type value message

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, d1, dC
A one-byte signed value.

short, d2, dS
A 2-byte signed value.

long, d4, dI, dL, d
A 4-byte signed value.

llong, d8
An 8-byte signed value

ubyte, u1, uC
A one-byte unsigned value.

ushort, u2, uS
A 2-byte unsigned value.

ulong, u4, uI, uL, u
A 4-byte unsigned value.

ullong, u8
An 8-byte unsigned value.

string, s
A string of bytes.

All type specifiers, except for string and s, may be followed
by a mask specifier of the form &number. If a mask specifier
is given, the value is AND'ed with the number before any
comparisons are done. The number is specified in C form. For
instance, 13 is decimal, 013 is octal, and 0x13 is
hexadecimal.

value
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 (for instance, \n for NEWLINE).

Numeric values may be preceded by a character indicating the
operation to be performed, as follows:

=
The value from the file must equal the specified value.

<
The value from the file must be less than the specified
value.

>
The value from the file must be greater than the
specified value.

&
All the bits in the specified value must be set in the
value from the file.

^
At least one of the bits in the specified value must not
be set in the value from the file.

x
Any value will match.

If the character is omitted, it is assumed to be "=".

For comparison of numeric values, the sign and size of both
the value in the file and the value from the value field of
the magic entry will match that of the corresponding type
field. If there is a non-zero mask (&) in the type field, the
comparison will be unsigned.

For string values, the byte string from the file must match
the specified byte string. The byte string from the file which
is matched is the same length as the specified byte string. If
the value is a string, it can contain the following sequences:

\character
The backslash-escape sequences \\, \a, \b, \f,
\n, \r, \t, \v.

\octal
Octal sequences that can be used to represent
characters with specific coded values. An octal
sequence consists of a backslash followed by
the longest sequence of one, two, or three
octal-digit characters (01234567).

message
The message to be printed if the comparison succeeds. If the
string contains a printf(3C) 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 to be printed. If the test on the
line preceding the first line with a ">" succeeds, the tests specified in
all the subsequent lines beginning with ">" are performed, and the
messages are printed if the tests succeed. The next line which does not
begin with a ">" terminates this.

FILES

/etc/magic

NOTES

In Solaris 9 and prior releases, the file utility may have performed
unsigned comparisons for types byte, short, and long. Old user-defined
magic files, which were specified with the -m option, will need
modification of byte, short, and long entries to their corresponding
unsigned types (ubyte, ushort, or ulong) for those entries for which all
of the following are true:

o The entry uses the "<" or the ">" operator.

o The type field does not contain a non-zero mask.

o The intention of the entry is to test unsigned values.

For example, if the following entry is expected to match any non-zero,
one-byte value from the file, including values for which the sign bit is
on:

#offset type value message
0 byte >0 this matches any non-zero value

then that entry should be changed to:

0 ubyte >0 this matches any non-zero value

In Solaris 7 through Solaris 9, when applying tests for magic file
entries whose type field is the numeric type "short" or "long", the file
utility in the x86 environment would switch the byte order of the numeric
values read. Starting in Solaris 10, the byte order will not be switched
on x86. A test for a numeric value whose byte order is identical in both
little- and big-endian architectures may require two magic file entries,
to ensure that the test correctly identifies files in both environments.
For example, a magic file entry that will match on a big-endian system
may look like this:

0 long 0xf00000ff extended accounting file

Its corresponding magic file entry that will match the same value on a
little-endian system would look like this:

0 long 0xff0000f0 extended accounting file

BUGS