magic(4) File Formats magic(4)NAMEmagic - 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 indicat‐
ing the operation to be performed, as follows:
= The value from the file must equal the speci‐
fied 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 corre‐
sponding 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 char‐
acters 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 specifica‐
tion, the value from the file (with any specified mask‐
ing performed) is printed using the message as the for‐
mat 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 speci‐
fied 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
SEE ALSOfile(1), file(1B), printf(3C)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 modifi‐
cation 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:
· The entry uses the "<" or the ">" operator.
· The type field does not contain a non-zero mask.
· 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 identi‐
cal 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
There should be more than one level of subtests, with the level indi‐
cated by the number of `>' at the beginning of the line.
SunOS 5.10 6 Feb 2004 magic(4)
