Set a normal, cache, or environment variable to a given value. See the cmake-language(7) variables documentation for the scopes and interaction of normal variables and cache entries.

Signatures of this command that specify a <value>... placeholder expect zero or more arguments. Multiple arguments will be joined as a semicolon-separated list to form the actual variable value to be set.

Set Normal Variable

set(<variable> <value>... [PARENT_SCOPE])

Set or unset <variable> in the current function or directory scope:

  • If at least one <value>... is given, set the variable to that value.

  • If no value is given, unset the variable. This is equivalent to unset(<variable>).

If the PARENT_SCOPE option is given the variable will be set in the scope above the current scope. Each new directory or function() command creates a new scope. A scope can also be created with the block() command. set(PARENT_SCOPE) will set the value of a variable into the parent directory, calling function, or encompassing scope (whichever is applicable to the case at hand). The previous state of the variable's value stays the same in the current scope (e.g., if it was undefined before, it is still undefined and if it had a value, it is still that value).

The block(PROPAGATE) and return(PROPAGATE) commands can be used as an alternate method to the set(PARENT_SCOPE) and unset(PARENT_SCOPE) commands to update the parent scope.


When evaluating Variable References of the form ${VAR}, CMake first searches for a normal variable with that name. If no such normal variable exists, CMake will then search for a cache entry with that name. Because of this, unsetting a normal variable can expose a cache variable that was previously hidden. To force a variable reference of the form ${VAR} to return an empty string, use set(<variable> ""), which clears the normal variable but leaves it defined.

Set Cache Entry

set(<variable> <value>... CACHE <type> <docstring> [FORCE])

Sets the given cache <variable> (cache entry). Since cache entries are meant to provide user-settable values this does not overwrite existing cache entries by default. Use the FORCE option to overwrite existing entries.

The <type> must be specified as one of:


Boolean ON/OFF value. cmake-gui(1) offers a checkbox.


Path to a file on disk. cmake-gui(1) offers a file dialog.


Path to a directory on disk. cmake-gui(1) offers a file dialog.


A line of text. cmake-gui(1) offers a text field or a drop-down selection if the STRINGS cache entry property is set.


A line of text. cmake-gui(1) does not show internal entries. They may be used to store variables persistently across runs. Use of this type implies FORCE.

The <docstring> must be specified as a line of text providing a quick summary of the option for presentation to cmake-gui(1) users.

If the cache entry does not exist prior to the call or the FORCE option is given then the cache entry will be set to the given value.


The content of the cache variable will not be directly accessible if a normal variable of the same name already exists (see rules of variable evaluation). If policy CMP0126 is set to OLD, any normal variable binding in the current scope will be removed.

It is possible for the cache entry to exist prior to the call but have no type set if it was created on the cmake(1) command line by a user through the -D<var>=<value> option without specifying a type. In this case the set command will add the type. Furthermore, if the <type> is PATH or FILEPATH and the <value> provided on the command line is a relative path, then the set command will treat the path as relative to the current working directory and convert it to an absolute path.

Set Environment Variable

set(ENV{<variable>} [<value>])

Sets an Environment Variable to the given value. Subsequent calls of $ENV{<variable>} will return this new value.

This command affects only the current CMake process, not the process from which CMake was called, nor the system environment at large, nor the environment of subsequent build or test processes.

If no argument is given after ENV{<variable>} or if <value> is an empty string, then this command will clear any existing value of the environment variable.

Arguments after <value> are ignored. If extra arguments are found, then an author warning is issued.

See Also