MAGIC(4)MAGIC(4)NAMEmagic ‐ 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 3.33. The file com‐
mand 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 additional informa‐
tion 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. The string type spec‐
ification can be optionally followed by
/[Bbc]*. The ‘‘B’’ flag compacts whites‐
pace in the target, which must contain
at least one whitespace character. If
the magic has "n" consecutive blanks,
the target needs at least "n" consecu‐
tive 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 charac‐
ters in the magic, only much uppercase
characters in the target.
date A four‐byte value interpreted as a unix
date.
beshort A two‐byte value (on most systems) in
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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.
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 compar‐
isons should be unsigned.
test The value to be compared with the value from the
file. If the type is numeric, this value is speci‐
fied in C form; if it is a string, it is specified
as a C string with the usual escapes permitted
(e.g. for new‐line).
Numeric values may be preceded by a character indi‐
cating 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 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
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MAGIC(4)MAGIC(4)
that a line can match any string, and then presum‐
ably 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) 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. 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
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 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[.[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 interpet the num‐
ber 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 off‐
set 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 speci‐
fied 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 system on which the lengths are invariant.
There is (currently) no support for specified‐endian data
to be used in indirect offsets.
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MAGIC(4)MAGIC(4)SEE ALSOfile(1) ‐ the command that reads this file.
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