magic(4) magic(4)NAME magic - magic number file for file command DESCRIPTION The file(1) command identifies the type of a file by using, among other tests, a test to ascertain whether the file be- gins with a certain magic number. The file /etc/magic specifies the magic numbers are to be tested for, what mes- sage to print if a particular magic number is found, and what additional information is to be 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. A 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. long A four-byte value. string 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 example, 13 is de- cimal, 013 is octal, and 0x13 is hexadecimal. 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 the type is a string, it is speci- fied as a C string with the usual escapes permitted (for example, \n for newline). Numeric values may be preceded by a character indi- cating the operation to be performed. The character may be an =, to specify that the value from the file must equal the specified value, a <, to specify that the value from the file must be less than the speci- fied value, a >, to specify that the value from the file must be greater than the specified value, or an x to specify that any value will match. If the char- April, 1990 1
magic(4) magic(4)acter 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. messageThe message to be printed if the comparison succeeds. If the string contains a printf(3S) format specifica- tion, 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 that tests and messages are to be printed. If the test on the line preced- ing the first line with a > succeeds, the tests specified in all the subsequent lines beginning with > are performed, and the messages are printed if the tests succeed. The next line which does not begin with a > terminates this command. FILES /etc/magic SEE ALSO file(1). BUGS There should be more than one level of subtests, with the level indicated by the number of > at the beginning of the line. 2 April, 1990