magic - file command's magic number file
The file(1) command identifies the type of a file using a number of tests, including a test for whether the file begins with a certain 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 one-byte, two-byte, or four-byte numeric value or a string. If the test succeeds, a message is printed. The line consists of the following fields:
offset type test message
The fields are separated by white space; the message filed is the remainder of the line.
The numeric types can 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.
Numeric values can be preceded by a character indicating the operation to be performed.
| Character | Description |
|---|---|
| = | The test succeeds if the value from the file equals the specified value. |
| < | The test succeeds if the value from the file is less than the specified value. |
| > | The test succeeds if the value from the file is greater than the specified value. |
| & | The test succeeds if the value from the file has set all of the bits that are set in the specified value. |
| ^ | The test succeeds if the value from the file has clear any of the bits that are set in the specified value. |
| x | The test succeeds if there is any value in the file. |
Numeric values are specified in C form; that is, 13 is decimal, 013 is octal, and 0x13 is hexadecimal. 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 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).
Some file formats contain additional information that is to be printed along with the file type. A line that 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 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 messages printed if the tests succeed. The next line at level n terminates this. If the first character following the last > is a (, 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 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.
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, and so on), because 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.
file(1)