Makefile Preprocessing Operators
Makefile preprocessing expressions can use operators that act on constant values, exit codes from commands, strings, macros, and filesystem paths. To evaluate the expression, the preprocessor first expands macros, and then executes commands, and then performs the operations. Operations are evaluated in the order of explicit grouping in parentheses, and then in the order of operator precedence. The result is a constant value.
The DEFINED operator is a logical operator that acts on a macro name. The expression DEFINED(macroname) is true if macroname is defined, even if it does not have an assigned value. DEFINED in combination with !IF or !ELSE IF is equivalent to !IFDEF or !ELSE IFDEF. However, unlike these directives, DEFINED can be used in complex expressions.
The EXIST operator is a logical operator that acts on a filesystem path. EXIST(path) is true if path exists. The result from EXIST can be used in binary expressions. If path contains spaces, enclose it in double quotation marks.
To compare two strings, use the equality (==) operator or the inequality (!=) operator. Enclose strings in double quotation marks.
Integer constants can use the unary operators for numerical negation (–), one's complement (~), and logical negation (!).
Expressions can use the following operators. The operators of equal precedence are grouped together, and the groups are listed in decreasing order of precedence. Unary operators associate with the operand to the right. Binary operators of equal precedence associate operands from left to right.
Operator 
Description 

DEFINED( macroname ) 
Produces a logical value for the current definition state of macroname. 
EXIST( path ) 
Produces a logical value for the existence of a file at path. 


! 
Unary logical NOT. 
~ 
Unary one’s complement. 
 
Unary negation. 


* 
Multiplication. 
/ 
Division. 
% 
Modulus (remainder). 


+ 
Addition. 
 
Subtraction. 


<< 
Bitwise shift left. 
>> 
Bitwise shift right. 


<= 
Less than or equal. 
>= 
Greater than or equal. 
< 
Less than. 
> 
Greater than. 


== 
Equality. 
!= 
Inequality. 


& 
Bitwise AND. 
^ 
Bitwise XOR. 
 
Bitwise OR. 


&& 
Logical AND. 


 
Logical OR. 
Note 

The bitwise XOR operator (^) is the same as the escape character, and must be escaped (as ^^) when it is used in an expression. 