if

Conditionally execute a group of commands.

if(expression)  # then section.  COMMAND1(ARGS ...)  COMMAND2(ARGS ...)  ...elseif(expression2)  # elseif section.  COMMAND1(ARGS ...)  COMMAND2(ARGS ...)  ...else(expression)  # else section.  COMMAND1(ARGS ...)  COMMAND2(ARGS ...)  ...endif(expression)

Evaluates the given expression. If the result is true, the commandsin the THEN section are invoked. Otherwise, the commands in the elsesection are invoked. The elseif and else sections are optional. Youmay have multiple elseif clauses. Note that the expression in theelse and endif clause is optional. Long expressions can be used andthere is a traditional order of precedence. Parenthetical expressionsare evaluated first followed by unary tests such as EXISTS,COMMAND, and DEFINED. Then any binary tests such asEQUAL, LESS, GREATER, STRLESS, STRGREATER,STREQUAL, and MATCHES will be evaluated. Then boolean NOToperators and finally boolean AND and then OR operators willbe evaluated.

Possible expressions are:

if(<constant>)
True if the constant is 1, ON, YES, TRUE, Y,or a non-zero number. False if the constant is 0, OFF,NO, FALSE, N, IGNORE, NOTFOUND, the empty string,or ends in the suffix -NOTFOUND. Named boolean constants arecase-insensitive. If the argument is not one of these constants, itis treated as a variable.
if(<variable>)
True if the variable is defined to a value that is not a falseconstant. False otherwise. (Note macro arguments are not variables.)
if(NOT <expression>)
True if the expression is not true.
if(<expr1> AND <expr2>)
True if both expressions would be considered true individually.
if(<expr1> OR <expr2>)
True if either expression would be considered true individually.
if(COMMAND command-name)
True if the given name is a command, macro or function that can beinvoked.
if(POLICY policy-id)
True if the given name is an existing policy (of the form CMP<NNNN>).
if(TARGET target-name)
True if the given name is an existing logical target name such as thosecreated by the add_executable(), add_library(), oradd_custom_target() commands.
if(EXISTS path-to-file-or-directory)
True if the named file or directory exists. Behavior is well-definedonly for full paths.
if(file1 IS_NEWER_THAN file2)
True if file1 is newer than file2 or if one of the two files doesn’texist. Behavior is well-defined only for full paths. If the filetime stamps are exactly the same, an IS_NEWER_THAN comparison returnstrue, so that any dependent build operations will occur in the eventof a tie. This includes the case of passing the same file name forboth file1 and file2.
if(IS_DIRECTORY path-to-directory)
True if the given name is a directory. Behavior is well-defined onlyfor full paths.
if(IS_SYMLINK file-name)
True if the given name is a symbolic link. Behavior is well-definedonly for full paths.
if(IS_ABSOLUTE path)
True if the given path is an absolute path.
if(<variable|string> MATCHES regex)
True if the given string or variable’s value matches the given regularexpression.
if(<variable|string> LESS <variable|string>)
True if the given string or variable’s value is a valid number and lessthan that on the right.
if(<variable|string> GREATER <variable|string>)
True if the given string or variable’s value is a valid number and greaterthan that on the right.
if(<variable|string> EQUAL <variable|string>)
True if the given string or variable’s value is a valid number and equalto that on the right.
if(<variable|string> STRLESS <variable|string>)
True if the given string or variable’s value is lexicographically lessthan the string or variable on the right.
if(<variable|string> STRGREATER <variable|string>)
True if the given string or variable’s value is lexicographically greaterthan the string or variable on the right.
if(<variable|string> STREQUAL <variable|string>)
True if the given string or variable’s value is lexicographically equalto the string or variable on the right.
if(<variable|string> VERSION_LESS <variable|string>)
Component-wise integer version number comparison (version format ismajor[.minor[.patch[.tweak]]]).
if(<variable|string> VERSION_EQUAL <variable|string>)
Component-wise integer version number comparison (version format ismajor[.minor[.patch[.tweak]]]).
if(<variable|string> VERSION_GREATER <variable|string>)
Component-wise integer version number comparison (version format ismajor[.minor[.patch[.tweak]]]).
if(DEFINED <variable>)
True if the given variable is defined. It does not matter if thevariable is true or false just if it has been set. (Note macroarguments are not variables.)
if((expression) AND (expression OR (expression)))
The expressions inside the parenthesis are evaluated first and thenthe remaining expression is evaluated as in the previous examples.Where there are nested parenthesis the innermost are evaluated as partof evaluating the expression that contains them.

The if command was written very early in CMake’s history, predatingthe ${} variable evaluation syntax, and for convenience evaluatesvariables named by its arguments as shown in the above signatures.Note that normal variable evaluation with ${} applies before the ifcommand even receives the arguments. Therefore code like:

set(var1 OFF)set(var2 "var1")if(${var2})

appears to the if command as:

if(var1)

and is evaluated according to the if(<variable>) case documentedabove. The result is OFF which is false. However, if we remove the${} from the example then the command sees:

if(var2)

which is true because var2 is defined to “var1” which is not a falseconstant.

Automatic evaluation applies in the other cases whenever theabove-documented signature accepts <variable|string>:

  • The left hand argument to MATCHES is first checked to see if it isa defined variable, if so the variable’s value is used, otherwise theoriginal value is used.
  • If the left hand argument to MATCHES is missing it returns falsewithout error
  • Both left and right hand arguments to LESS, GREATER, andEQUAL are independently tested to see if they are definedvariables, if so their defined values are used otherwise the originalvalue is used.
  • Both left and right hand arguments to STRLESS, STREQUAL, andSTRGREATER are independently tested to see if they are definedvariables, if so their defined values are used otherwise the originalvalue is used.
  • Both left and right hand arguments to VERSION_LESS,VERSION_EQUAL, and VERSION_GREATER are independently testedto see if they are defined variables, if so their defined values areused otherwise the original value is used.
  • The right hand argument to NOT is tested to see if it is a booleanconstant, if so the value is used, otherwise it is assumed to be avariable and it is dereferenced.
  • The left and right hand arguments to AND and OR are independentlytested to see if they are boolean constants, if so they are used assuch, otherwise they are assumed to be variables and are dereferenced.