Tribblix: manual page: magic.5

MAGIC(5) File Formats and Configurations 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