if
--
Conditionally execute a group of commands.
Synopsis
^^^^^^^^
.. code-block:: cmake
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 :command:`else` and :command:`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`:
Condition Syntax
^^^^^^^^^^^^^^^^
The following syntax applies to the ``condition`` argument of
the ``if``, ``elseif`` and :command:`while` clauses.
Compound conditions are evaluated in the following order of precedence:
1. Parentheses.
2. Unary tests such as `EXISTS`_, `COMMAND`_, and `DEFINED`_.
3. 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`_,
`PATH_EQUAL`_, and `MATCHES`_.
4. Unary logical operator `NOT`_.
5. Binary logical operators `AND`_ and `OR`_, from left to right,
without any short-circuit.
Basic Expressions
"""""""""""""""""
``if(<constant>)``
True if the constant is ``1``, ``ON``, ``YES``, ``TRUE``, ``Y``,
or a non-zero number (including floating point numbers).
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 (see `Variable Expansion`_
further below) and one of the following two forms applies.
``if(<variable>)``
True if given a variable that is defined to a value that is not a false
constant. False otherwise, including if the variable is undefined.
Note that macro arguments are not variables.
:ref:`Environment Variables <CMake Language Environment Variables>` also
cannot be tested this way, e.g. ``if(ENV{some_var})`` will always evaluate
to false.
``if(<string>)``
A quoted string always evaluates to false unless:
* The string's value is one of the true constants, or
* Policy :policy:`CMP0054` is not set to ``NEW`` and the string's value
happens to be a variable name that is affected by :policy:`CMP0054`'s
behavior.
Logic Operators
"""""""""""""""
.. _NOT:
``if(NOT <condition>)``
True if the condition is not true.
.. _AND:
``if(<cond1> AND <cond2>)``
True if both conditions would be considered true individually.
.. _OR:
``if(<cond1> OR <cond2>)``
True if either condition would be considered true individually.
``if((condition) AND (condition OR (condition)))``
The conditions inside the parenthesis are evaluated first and then
the remaining condition is evaluated as in the other examples.
Where there are nested parenthesis the innermost are evaluated as part
of evaluating the condition that contains them.
Existence Checks
""""""""""""""""
.. _COMMAND:
``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 :command:`add_executable`, :command:`add_library`,
or :command:`add_custom_target` command that has already been invoked
(in any directory).
``if(TEST test-name)``
.. versionadded:: 3.3
True if the given name is an existing test name created by the
:command:`add_test` command.
.. _DEFINED:
``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 the following caveats:
* Macro arguments are not variables.
* It is not possible to test directly whether a `<name>` is a non-cache
variable. The expression ``if(DEFINED someName)`` will evaluate to true
if either a cache or non-cache variable ``someName`` exists. In
comparison, the expression ``if(DEFINED CACHE{someName})`` will only
evaluate to true if a cache variable ``someName`` exists. Both expressions
need to be tested if you need to know whether a non-cache variable exists:
``if(DEFINED someName AND NOT DEFINED CACHE{someName})``.
.. versionadded:: 3.14
Added support for ``CACHE{<name>}`` variables.
``if(<variable|string> IN_LIST <variable>)``
.. versionadded:: 3.3
True if the given element is contained in the named list variable.
File Operations
"""""""""""""""
.. _EXISTS:
``if(EXISTS path-to-file-or-directory)``
True if the named file or directory exists. Behavior is well-defined
only for explicit full paths (a leading ``~/`` is not expanded as
a home directory and is considered a relative path).
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. Note the following special
cases:
* An empty ``path`` evaluates to false.
* On Windows hosts, any ``path`` that begins with a drive letter and colon
(e.g. ``C:``), a forward slash or a backslash will evaluate to true.
This means a path like ``C:no\base\dir`` will evaluate to true, even
though the non-drive part of the path is relative.
* On non-Windows hosts, any ``path`` that begins with a tilde (``~``)
evaluates to true.
Comparisons
"""""""""""
.. _MATCHES:
``if(<variable|string> MATCHES regex)``
True if the given string or variable's value matches the given regular
expression. See :ref:`Regex Specification` for regex format.
.. versionadded:: 3.9
``()`` groups are captured in :variable:`CMAKE_MATCH_<n>` variables.
.. _LESS:
``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.
.. _GREATER:
``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.
.. _EQUAL:
``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.
.. _LESS_EQUAL:
``if(<variable|string> LESS_EQUAL <variable|string>)``
.. versionadded:: 3.7
True if the given string or variable's value is a valid number and less
than or equal to that on the right.
.. _GREATER_EQUAL:
``if(<variable|string> GREATER_EQUAL <variable|string>)``
.. versionadded:: 3.7
True if the given string or variable's value is a valid number and greater
than or equal to that on the right.
.. _STRLESS:
``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.
.. _STRGREATER:
``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.
.. _STREQUAL:
``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.
.. _STRLESS_EQUAL:
``if(<variable|string> STRLESS_EQUAL <variable|string>)``
.. versionadded:: 3.7
True if the given string or variable's value is lexicographically less
than or equal to the string or variable on the right.
.. _STRGREATER_EQUAL:
``if(<variable|string> STRGREATER_EQUAL <variable|string>)``
.. versionadded:: 3.7
True if the given string or variable's value is lexicographically greater
than or equal to the string or variable on the right.
Version Comparisons
"""""""""""""""""""
.. _VERSION_LESS:
``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.
.. _VERSION_GREATER:
``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.
.. _VERSION_EQUAL:
``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.
.. _VERSION_LESS_EQUAL:
``if(<variable|string> VERSION_LESS_EQUAL <variable|string>)``
.. versionadded:: 3.7
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.
.. _VERSION_GREATER_EQUAL:
``if(<variable|string> VERSION_GREATER_EQUAL <variable|string>)``
.. versionadded:: 3.7
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.
Path Comparisons
""""""""""""""""
.. _PATH_EQUAL:
``if(<variable|string> PATH_EQUAL <variable|string>)``
.. versionadded:: 3.24
Compares the two paths component-by-component. Only if every component of
both paths match will the two paths compare equal. Multiple path separators
are effectively collapsed into a single separator, but note that backslashes
are not converted to forward slashes. No other
:ref:`path normalization <Normalization>` is performed.
Component-wise comparison is superior to string-based comparison due to the
handling of multiple path separators. In the following example, the
expression evaluates to true using ``PATH_EQUAL``, but false with
``STREQUAL``:
.. code-block:: cmake
# comparison is TRUE
if ("/a//b/c" PATH_EQUAL "/a/b/c")
...
endif()
# comparison is FALSE
if ("/a//b/c" STREQUAL "/a/b/c")
...
endif()
See :ref:`cmake_path(COMPARE) <Path COMPARE>` for more details.
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
.. code-block:: cmake
set(var1 OFF)
set(var2 "var1")
if(${var2})
appears to the if command as
.. code-block:: cmake
if(var1)
and is evaluated according to the ``if(<variable>)`` case documented
above. The result is ``OFF`` which is false. However, if we remove the
``${}`` from the example then the command sees
.. code-block:: cmake
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.
.. versionchanged:: 3.1
To prevent ambiguity, potential variable or keyword names can be
specified in a :ref:`Quoted Argument` or a :ref:`Bracket Argument`.
A quoted or bracketed variable or keyword will be interpreted as a
string and not dereferenced or interpreted.
See policy :policy:`CMP0054`.
There is no automatic evaluation for environment or cache
:ref:`Variable References`. Their values must be referenced as
``$ENV{<name>}`` or ``$CACHE{<name>}`` wherever the above-documented
condition syntax accepts ``<variable|string>``.
See also
^^^^^^^^
* :command:`else`
* :command:`elseif`
* :command:`endif`
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