if

Conditionally execute a group of commands.

Synopsis

if(<condition>)
  <commands>
elseif(<condition>) # optional block, can be repeated
  <commands>
else()              # optional block
  <commands>
endif()

Evaluates the condition argument of the if clause according to the Condition syntax described below. If the result is true, then the commands in the if block are executed. Otherwise, optional elseif blocks are processed in the same way. Finally, if no condition is true, commands in the optional else block are executed.

Per legacy, the else() and endif() commands admit an optional <condition> argument. If used, it must be a verbatim repeat of the argument of the opening if command.

Condition Syntax

The following syntax applies to the condition argument of the if, elseif and while() clauses.

Compound conditions are evaluated in the following order of precedence: Innermost parentheses are evaluated first. Next come unary tests such as EXISTS, COMMAND, and DEFINED. Then binary tests such as EQUAL, LESS, LESS_EQUAL, GREATER, GREATER_EQUAL, STREQUAL, STRLESS, STRLESS_EQUAL, STRGREATER, STRGREATER_EQUAL, VERSION_EQUAL, VERSION_LESS, VERSION_LESS_EQUAL, VERSION_GREATER, VERSION_GREATER_EQUAL, and MATCHES. Then the boolean operators in the order NOT, AND, and finally OR.

Possible conditions 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 are case-insensitive. If the argument is not one of these specific constants, it is treated as a variable or string and the following signature is used.
if(<variable|string>)
True if given a variable that is defined to a value that is not a false constant. False otherwise. (Note macro arguments are not variables.)
if(NOT <condition>)
True if the condition is not true.
if(<cond1> AND <cond2>)
True if both conditions would be considered true individually.
if(<cond1> OR <cond2>)
True if either condition would be considered true individually.
if(COMMAND command-name)
True if the given name is a command, macro or function that can be invoked.
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 created by a call to the add_executable(), add_library(), or add_custom_target() command that has already been invoked (in any directory).
if(TEST test-name)
True if the given name is an existing test name created by the add_test() command.
if(EXISTS path-to-file-or-directory)
True if the named file or directory exists. Behavior is well-defined only for full paths. Resolves symbolic links, i.e. if the named file or directory is a symbolic link, returns true if the target of the symbolic link exists.
if(file1 IS_NEWER_THAN file2)
True if file1 is newer than file2 or if one of the two files doesn’t exist. Behavior is well-defined only for full paths. If the file time stamps are exactly the same, an IS_NEWER_THAN comparison returns true, so that any dependent build operations will occur in the event of a tie. This includes the case of passing the same file name for both file1 and file2.
if(IS_DIRECTORY path-to-directory)
True if the given name is a directory. Behavior is well-defined only for full paths.
if(IS_SYMLINK file-name)
True if the given name is a symbolic link. Behavior is well-defined only 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 regular condition. See Regex Specification for regex format. () groups are captured in CMAKE_MATCH_<n> variables.
if(<variable|string> LESS <variable|string>)
True if the given string or variable’s value is a valid number and less than that on the right.
if(<variable|string> GREATER <variable|string>)
True if the given string or variable’s value is a valid number and greater than that on the right.
if(<variable|string> EQUAL <variable|string>)
True if the given string or variable’s value is a valid number and equal to that on the right.
if(<variable|string> LESS_EQUAL <variable|string>)
True if the given string or variable’s value is a valid number and less than or equal to that on the right.
if(<variable|string> GREATER_EQUAL <variable|string>)
True if the given string or variable’s value is a valid number and greater than or equal to that on the right.
if(<variable|string> STRLESS <variable|string>)
True if the given string or variable’s value is lexicographically less than the string or variable on the right.
if(<variable|string> STRGREATER <variable|string>)
True if the given string or variable’s value is lexicographically greater than the string or variable on the right.
if(<variable|string> STREQUAL <variable|string>)
True if the given string or variable’s value is lexicographically equal to the string or variable on the right.
if(<variable|string> STRLESS_EQUAL <variable|string>)
True if the given string or variable’s value is lexicographically less than or equal to the string or variable on the right.
if(<variable|string> STRGREATER_EQUAL <variable|string>)
True if the given string or variable’s value is lexicographically greater than or equal to the string or variable on the right.
if(<variable|string> VERSION_LESS <variable|string>)
Component-wise integer version number comparison (version format is major[.minor[.patch[.tweak]]], omitted components are treated as zero). Any non-integer version component or non-integer trailing part of a version component effectively truncates the string at that point.
if(<variable|string> VERSION_GREATER <variable|string>)
Component-wise integer version number comparison (version format is major[.minor[.patch[.tweak]]], omitted components are treated as zero). Any non-integer version component or non-integer trailing part of a version component effectively truncates the string at that point.
if(<variable|string> VERSION_EQUAL <variable|string>)
Component-wise integer version number comparison (version format is major[.minor[.patch[.tweak]]], omitted components are treated as zero). Any non-integer version component or non-integer trailing part of a version component effectively truncates the string at that point.
if(<variable|string> VERSION_LESS_EQUAL <variable|string>)
Component-wise integer version number comparison (version format is major[.minor[.patch[.tweak]]], omitted components are treated as zero). Any non-integer version component or non-integer trailing part of a version component effectively truncates the string at that point.
if(<variable|string> VERSION_GREATER_EQUAL <variable|string>)
Component-wise integer version number comparison (version format is major[.minor[.patch[.tweak]]], omitted components are treated as zero). Any non-integer version component or non-integer trailing part of a version component effectively truncates the string at that point.
if(<variable|string> IN_LIST <variable>)
True if the given element is contained in the named list variable.
if(DEFINED <name>|CACHE{<name>}|ENV{<name>})
True if a variable, cache variable or environment variable with given <name> is defined. The value of the variable does not matter. Note that macro arguments are not variables.
if((condition) AND (condition OR (condition)))
The conditions inside the parenthesis are evaluated first and then the remaining condition is evaluated as in the previous examples. Where there are nested parenthesis the innermost are evaluated as part of evaluating the condition that contains them.

Variable Expansion

The if command was written very early in CMake’s history, predating the ${} variable evaluation syntax, and for convenience evaluates variables named by its arguments as shown in the above signatures. Note that normal variable evaluation with ${} applies before the if command 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 documented above. 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 false constant.

Automatic evaluation applies in the other cases whenever the above-documented condition syntax accepts <variable|string>:

  • The left hand argument to MATCHES is first checked to see if it is a defined variable, if so the variable’s value is used, otherwise the original value is used.
  • If the left hand argument to MATCHES is missing it returns false without error
  • Both left and right hand arguments to LESS, GREATER, EQUAL, LESS_EQUAL, and GREATER_EQUAL, are independently tested to see if they are defined variables, if so their defined values are used otherwise the original value is used.
  • Both left and right hand arguments to STRLESS, STRGREATER, STREQUAL, STRLESS_EQUAL, and STRGREATER_EQUAL are independently tested to see if they are defined variables, if so their defined values are used otherwise the original value is used.
  • Both left and right hand arguments to VERSION_LESS, VERSION_GREATER, VERSION_EQUAL, VERSION_LESS_EQUAL, and VERSION_GREATER_EQUAL are independently tested to see if they are defined variables, if so their defined values are used otherwise the original value is used.
  • The right hand argument to NOT is tested to see if it is a boolean constant, if so the value is used, otherwise it is assumed to be a variable and it is dereferenced.
  • The left and right hand arguments to AND and OR are independently tested to see if they are boolean constants, if so they are used as such, otherwise they are assumed to be variables and are dereferenced.

To prevent ambiguity, potential variable or keyword names can be specified in a Quoted Argument or a Bracket Argument. A quoted or bracketed variable or keyword will be interpreted as a string and not dereferenced or interpreted. See policy CMP0054.

There is no automatic evaluation for environment or cache Variable References. Their values must be referenced as $ENV{<name>} or $CACHE{<name>} wherever the above-documented condition syntax accepts <variable|string>.