magic - file command's magic number file
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 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 perform. 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
(separated by tabs):
offset type value message
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
byte A one-byte value.
short A two-byte value.
long A four-byte value.
A string of bytes.
The types byte, short, and long may optionally 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 hexa-
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
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 all the bits in the specified
value must be set in the value from the file, `^', to
specify that at least one of the bits in the specified
value must not be set in the value from the file, or x
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 byte string from
the file which is matched is the same length as the
specified byte string.
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
masking performed) is printed using the message as the
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 mes-
sages to be printed. If the test on the line preceding the
first line with a `>' succeeds, the tests specified in all
the subsequent lines beginning with `>' are performed, and
the messages printed if the tests succeed. The next line
which does not begin with a `>' terminates this.
file(1), file(1B), printf(3C)
There should be more than one level of subtests, with the
level indicated by the number of `>' at the beginning of the
Man(1) output converted with