magic
MAGIC(5) MAGIC(5)
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 4.10. The file 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 /usr/share/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 com-
pares 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 suc-
ceeds, 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. The string type specification
can be optionally followed by /[Bbc]*. The ‘‘B’’
flag compacts whitespace in the target, which must
contain at least one whitespace character. If the
magic has n consecutive blanks, the target needs at
least n consecutive blanks to match. The ‘‘b’’ flag
treats every blank in the target as an optional
blank. Finally the ‘‘c’’ flag, specifies case
insensitive matching: lowercase characters in the
magic match both lower and upper case characters in
the targer, whereas upper case characters in the
magic, only much uppercase characters in the target.
date A four-byte value interpreted as a UNIX date.
ldate A four-byte value interpreted as a UNIX-style date,
but interpreted as local time rather than UTC.
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.
ledate A four-byte value (on most systems) in little-endian
byte order, interpreted as a UNIX date.
leldate A four-byte value (on most systems) in little-endian
byte order, interpreted as a UNIX-style date, but
interpreted as local time rather than UTC.
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 spec-
ify that the value from the file must be less than the speci-
fied 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(3) 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 char-
acter > 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 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 suc-
ceeds, 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 follow-
ing the last > is a ( then the string after the parenthesis is inter-
preted as an indirect offset. That means that the number after the
parenthesis is used as an offset in the file. The value at that off-
set is read, and is used again as an offset in the file. Indirect
offsets are of the form: ((x[.[bslBSL]][+-][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 [bslBSL] type specifier. The capitalized
types interpret the number as a big endian value, whereas the small
letter versions interpret the number as a little endian value. 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 beginning 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 speci-
fied as a number of bytes (2B, 4B, etc), since the files being recog-
nized 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.
Public Domain MAGIC(5)
