MAGIC(4) Misc. Reference Manual Page MAGIC(4)
NAME
magic - file command's magic number file
DESCRIPTION
This manual page documents the format of the magic file as
used by the file(1) command, version 3.22.
The file command identifies the type of a file using, among
other tests, a test for whether the file begins with a cer-
tain magic number. The file 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
MAGIC(4) Misc. Reference Manual Page MAGIC(4)
little-endian byte order.
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 comparis-
ons 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 indicat-
ing 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, ^, to specify that the
value from the file must have clear any of the bits
that are set in the specified value, or x, to specify
that any value will match. If the character is omitted,
it is assumed to be =.
Numeric values are specified in C form; e.g. 13 is
decimal, 013 is octal, and 0x13 is hexadecimal.
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 specifica-
tion, the value from the file (with any specified mask-
ing performed) is printed using the message as the for-
mat string.
MAGIC(4) Misc. Reference Manual Page MAGIC(4)
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. The number of > on the line indicates the
level of the test; a line with no > at the beginning is con-
sidered 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 an 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.
Sometimes you do not know the exact offset as this depends
on the length of preceding fields. You can specify an offset
relative to the end of the last uplevel field (of course
this may only be done for sublevel tests, i.e. test begin-
ning with > ). Such a relative offset is specified using &
as a prefix to the offset.
BUGS
The formats long, belong, lelong, short, beshort, leshort,
date, bedate, and ledate are system-dependent; perhaps they
should be specified as a number of bytes (2B, 4B, etc),
since the files being recognized typically come from a sys-
tem 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.