MAGIC(5) MAGIC(5)
NAME
magic - file command's magic number file
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 file /etc/magic specifies
what magic numbers are to be tested for, what message to
print if a particular magic number is found, and addi-
tional 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
little-endian byte order.
ledate A four-byte value (on most systems) in
little-endian byte order, interpreted as
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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.
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 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, or ^, to specify that the value
from the file must have clear any of the bits
that are set in the specified value.
Numeric values are specified in C form; e.g. 13
is decimal, 013 is octal, and 0x13 is hexadeci-
mal. to specify that any value will match. If
the character is omitted, it is assumed to be =.
For string values, the byte string from the file
must match the specified byte string. The opera-
tors =, < 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 suc-
ceeds. If the string contains a printf(3S) for-
mat 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. The number of > on the line indi-
cates 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 succeeds, the tests specified in all the
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MAGIC(5) MAGIC(5)
subsequent lines at level n+1 are performed, and the mes-
sages 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 a 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 num-
ber the value of y is added and the result is used as an
offset in the file. The default type if one is not speci-
fied is long.
BUGS
The formats long, belong, lelong, short, beshort, leshort,
date, bedate, and ledate are system-dependant; perhaps
they should be specified as a number of bytes (2B, 4B,
etc), since the files being recognized 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.
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