install

Specify rules to run at install time.

Synopsis

install(TARGETS <target>... [...])
install({FILES | PROGRAMS} <file>... [...])
install(DIRECTORY <dir>... [...])
install(SCRIPT <file> [...])
install(CODE <code> [...])
install(EXPORT <export-name> [...])

Introduction

This command generates installation rules for a project. Rules specified by calls to this command within a source directory are executed in order during installation. The order across directories is not defined.

There are multiple signatures for this command. Some of them define installation options for files and targets. Options common to multiple signatures are covered here but they are valid only for signatures that specify them. The common options are:

DESTINATION

Specify the directory on disk to which a file will be installed. If a full path (with a leading slash or drive letter) is given it is used directly. If a relative path is given it is interpreted relative to the value of the CMAKE_INSTALL_PREFIX variable. The prefix can be relocated at install time using the DESTDIR mechanism explained in the CMAKE_INSTALL_PREFIX variable documentation.

PERMISSIONS

Specify permissions for installed files. Valid permissions are OWNER_READ, OWNER_WRITE, OWNER_EXECUTE, GROUP_READ, GROUP_WRITE, GROUP_EXECUTE, WORLD_READ, WORLD_WRITE, WORLD_EXECUTE, SETUID, and SETGID. Permissions that do not make sense on certain platforms are ignored on those platforms.

CONFIGURATIONS

Specify a list of build configurations for which the install rule applies (Debug, Release, etc.). Note that the values specified for this option only apply to options listed AFTER the CONFIGURATIONS option. For example, to set separate install paths for the Debug and Release configurations, do the following:

install(TARGETS target
        CONFIGURATIONS Debug
        RUNTIME DESTINATION Debug/bin)
install(TARGETS target
        CONFIGURATIONS Release
        RUNTIME DESTINATION Release/bin)

Note that CONFIGURATIONS appears BEFORE RUNTIME DESTINATION.

COMPONENT

Specify an installation component name with which the install rule is associated, such as “runtime” or “development”. During component-specific installation only install rules associated with the given component name will be executed. During a full installation all components are installed unless marked with EXCLUDE_FROM_ALL. If COMPONENT is not provided a default component “Unspecified” is created. The default component name may be controlled with the CMAKE_INSTALL_DEFAULT_COMPONENT_NAME variable.

EXCLUDE_FROM_ALL

Specify that the file is excluded from a full installation and only installed as part of a component-specific installation

RENAME

Specify a name for an installed file that may be different from the original file. Renaming is allowed only when a single file is installed by the command.

OPTIONAL

Specify that it is not an error if the file to be installed does not exist.

Command signatures that install files may print messages during installation. Use the CMAKE_INSTALL_MESSAGE variable to control which messages are printed.

Many of the install() variants implicitly create the directories containing the installed files. If CMAKE_INSTALL_DEFAULT_DIRECTORY_PERMISSIONS is set, these directories will be created with the permissions specified. Otherwise, they will be created according to the uname rules on Unix-like platforms. Windows platforms are unaffected.

Installing Targets

install(TARGETS targets... [EXPORT <export-name>]
        [[ARCHIVE|LIBRARY|RUNTIME|OBJECTS|FRAMEWORK|BUNDLE|
          PRIVATE_HEADER|PUBLIC_HEADER|RESOURCE]
         [DESTINATION <dir>]
         [PERMISSIONS permissions...]
         [CONFIGURATIONS [Debug|Release|...]]
         [COMPONENT <component>]
         [NAMELINK_COMPONENT <component>]
         [OPTIONAL] [EXCLUDE_FROM_ALL]
         [NAMELINK_ONLY|NAMELINK_SKIP]
        ] [...]
        [INCLUDES DESTINATION [<dir> ...]]
        )

The TARGETS form specifies rules for installing targets from a project. There are several kinds of target files that may be installed:

ARCHIVE

Static libraries are treated as ARCHIVE targets, except those marked with the FRAMEWORK property on macOS (see FRAMEWORK below.) For DLL platforms (all Windows-based systems including Cygwin), the DLL import library is treated as an ARCHIVE target.

LIBRARY

Module libraries are always treated as LIBRARY targets. For non- DLL platforms shared libraries are treated as LIBRARY targets, except those marked with the FRAMEWORK property on macOS (see FRAMEWORK below.)

RUNTIME

Executables are treated as RUNTIME objects, except those marked with the MACOSX_BUNDLE property on macOS (see BUNDLE below.) For DLL platforms (all Windows-based systems including Cygwin), the DLL part of a shared library is treated as a RUNTIME target.

OBJECTS

Object libraries (a simple group of object files) are always treated as OBJECTS targets.

FRAMEWORK

Both static and shared libraries marked with the FRAMEWORK property are treated as FRAMEWORK targets on macOS.

BUNDLE

Executables marked with the MACOSX_BUNDLE property are treated as BUNDLE targets on macOS.

PUBLIC_HEADER

Any PUBLIC_HEADER files associated with a library are installed in the destination specified by the PUBLIC_HEADER argument on non-Apple platforms. Rules defined by this argument are ignored for FRAMEWORK libraries on Apple platforms because the associated files are installed into the appropriate locations inside the framework folder. See PUBLIC_HEADER for details.

PRIVATE_HEADER

Similar to PUBLIC_HEADER, but for PRIVATE_HEADER files. See PRIVATE_HEADER for details.

RESOURCE

Similar to PUBLIC_HEADER and PRIVATE_HEADER, but for RESOURCE files. See RESOURCE for details.

For each of these arguments given, the arguments following them only apply to the target or file type specified in the argument. If none is given, the installation properties apply to all target types. If only one is given then only targets of that type will be installed (which can be used to install just a DLL or just an import library.)

For regular executables, static libraries and shared libraries, the DESTINATION argument is not required. For these target types, when DESTINATION is omitted, a default destination will be taken from the appropriate variable from GNUInstallDirs, or set to a built-in default value if that variable is not defined. The same is true for the public and private headers associated with the installed targets through the PUBLIC_HEADER and PRIVATE_HEADER target properties. A destination must always be provided for module libraries, Apple bundles and frameworks. A destination can be omitted for interface and object libraries, but they are handled differently (see the discussion of this topic toward the end of this section).

The following table shows the target types with their associated variables and built-in defaults that apply when no destination is given:

Target Type

GNUInstallDirs Variable

Built-In Default

RUNTIME

${CMAKE_INSTALL_BINDIR}

bin

LIBRARY

${CMAKE_INSTALL_LIBDIR}

lib

ARCHIVE

${CMAKE_INSTALL_LIBDIR}

lib

PRIVATE_HEADER

${CMAKE_INSTALL_INCLUDEDIR}

include

PUBLIC_HEADER

${CMAKE_INSTALL_INCLUDEDIR}

include

Projects wishing to follow the common practice of installing headers into a project-specific subdirectory will need to provide a destination rather than rely on the above.

To make packages compliant with distribution filesystem layout policies, if projects must specify a DESTINATION, it is recommended that they use a path that begins with the appropriate GNUInstallDirs variable. This allows package maintainers to control the install destination by setting the appropriate cache variables. The following example shows a static library being installed to the default destination provided by GNUInstallDirs, but with its headers installed to a project-specific subdirectory that follows the above recommendation:

add_library(mylib STATIC ...)
set_target_properties(mylib PROPERTIES PUBLIC_HEADER mylib.h)
include(GNUInstallDirs)
install(TARGETS mylib
        PUBLIC_HEADER
          DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/myproj
)

In addition to the common options listed above, each target can accept the following additional arguments:

NAMELINK_COMPONENT

On some platforms a versioned shared library has a symbolic link such as:

lib<name>.so -> lib<name>.so.1

where lib<name>.so.1 is the soname of the library and lib<name>.so is a “namelink” allowing linkers to find the library when given -l<name>. The NAMELINK_COMPONENT option is similar to the COMPONENT option, but it changes the installation component of a shared library namelink if one is generated. If not specified, this defaults to the value of COMPONENT. It is an error to use this parameter outside of a LIBRARY block.

Consider the following example:

install(TARGETS mylib
        LIBRARY
          COMPONENT Libraries
          NAMELINK_COMPONENT Development
        PUBLIC_HEADER
          COMPONENT Development
       )

In this scenario, if you choose to install only the Development component, both the headers and namelink will be installed without the library. (If you don’t also install the Libraries component, the namelink will be a dangling symlink, and projects that link to the library will have build errors.) If you install only the Libraries component, only the library will be installed, without the headers and namelink.

This option is typically used for package managers that have separate runtime and development packages. For example, on Debian systems, the library is expected to be in the runtime package, and the headers and namelink are expected to be in the development package.

See the VERSION and SOVERSION target properties for details on creating versioned shared libraries.

NAMELINK_ONLY

This option causes the installation of only the namelink when a library target is installed. On platforms where versioned shared libraries do not have namelinks or when a library is not versioned, the NAMELINK_ONLY option installs nothing. It is an error to use this parameter outside of a LIBRARY block.

When NAMELINK_ONLY is given, either NAMELINK_COMPONENT or COMPONENT may be used to specify the installation component of the namelink, but COMPONENT should generally be preferred.

NAMELINK_SKIP

Similar to NAMELINK_ONLY, but it has the opposite effect: it causes the installation of library files other than the namelink when a library target is installed. When neither NAMELINK_ONLY or NAMELINK_SKIP are given, both portions are installed. On platforms where versioned shared libraries do not have symlinks or when a library is not versioned, NAMELINK_SKIP installs the library. It is an error to use this parameter outside of a LIBRARY block.

If NAMELINK_SKIP is specified, NAMELINK_COMPONENT has no effect. It is not recommended to use NAMELINK_SKIP in conjunction with NAMELINK_COMPONENT.

The install(TARGETS) command can also accept the following options at the top level:

EXPORT

This option associates the installed target files with an export called <export-name>. It must appear before any target options. To actually install the export file itself, call install(EXPORT), documented below. See documentation of the EXPORT_NAME target property to change the name of the exported target.

INCLUDES DESTINATION

This option specifies a list of directories which will be added to the INTERFACE_INCLUDE_DIRECTORIES target property of the <targets> when exported by the install(EXPORT) command. If a relative path is specified, it is treated as relative to the $<INSTALL_PREFIX>.

One or more groups of properties may be specified in a single call to the TARGETS form of this command. A target may be installed more than once to different locations. Consider hypothetical targets myExe, mySharedLib, and myStaticLib. The code:

install(TARGETS myExe mySharedLib myStaticLib
        RUNTIME DESTINATION bin
        LIBRARY DESTINATION lib
        ARCHIVE DESTINATION lib/static)
install(TARGETS mySharedLib DESTINATION /some/full/path)

will install myExe to <prefix>/bin and myStaticLib to <prefix>/lib/static. On non-DLL platforms mySharedLib will be installed to <prefix>/lib and /some/full/path. On DLL platforms the mySharedLib DLL will be installed to <prefix>/bin and /some/full/path and its import library will be installed to <prefix>/lib/static and /some/full/path.

Interface Libraries may be listed among the targets to install. They install no artifacts but will be included in an associated EXPORT. If Object Libraries are listed but given no destination for their object files, they will be exported as Interface Libraries. This is sufficient to satisfy transitive usage requirements of other targets that link to the object libraries in their implementation.

Installing a target with the EXCLUDE_FROM_ALL target property set to TRUE has undefined behavior.

install(TARGETS) can install targets that were created in other directories. When using such cross-directory install rules, running make install (or similar) from a subdirectory will not guarantee that targets from other directories are up-to-date. You can use target_link_libraries() or add_dependencies() to ensure that such out-of-directory targets are built before the subdirectory-specific install rules are run.

An install destination given as a DESTINATION argument may use “generator expressions” with the syntax $<...>. See the cmake-generator-expressions(7) manual for available expressions.

Installing Files

install(<FILES|PROGRAMS> files...
        TYPE <type> | DESTINATION <dir>
        [PERMISSIONS permissions...]
        [CONFIGURATIONS [Debug|Release|...]]
        [COMPONENT <component>]
        [RENAME <name>] [OPTIONAL] [EXCLUDE_FROM_ALL])

The FILES form specifies rules for installing files for a project. File names given as relative paths are interpreted with respect to the current source directory. Files installed by this form are by default given permissions OWNER_WRITE, OWNER_READ, GROUP_READ, and WORLD_READ if no PERMISSIONS argument is given.

The PROGRAMS form is identical to the FILES form except that the default permissions for the installed file also include OWNER_EXECUTE, GROUP_EXECUTE, and WORLD_EXECUTE. This form is intended to install programs that are not targets, such as shell scripts. Use the TARGETS form to install targets built within the project.

The list of files... given to FILES or PROGRAMS may use “generator expressions” with the syntax $<...>. See the cmake-generator-expressions(7) manual for available expressions. However, if any item begins in a generator expression it must evaluate to a full path.

Either a TYPE or a DESTINATION must be provided, but not both. A TYPE argument specifies the generic file type of the files being installed. A destination will then be set automatically by taking the corresponding variable from GNUInstallDirs, or by using a built-in default if that variable is not defined. See the table below for the supported file types and their corresponding variables and built-in defaults. Projects can provide a DESTINATION argument instead of a file type if they wish to explicitly define the install destination.

TYPE Argument

GNUInstallDirs Variable

Built-In Default

BIN

${CMAKE_INSTALL_BINDIR}

bin

SBIN

${CMAKE_INSTALL_SBINDIR}

sbin

LIB

${CMAKE_INSTALL_LIBDIR}

lib

INCLUDE

${CMAKE_INSTALL_INCLUDEDIR}

include

SYSCONF

${CMAKE_INSTALL_SYSCONFDIR}

etc

SHAREDSTATE

${CMAKE_INSTALL_SHARESTATEDIR}

com

LOCALSTATE

${CMAKE_INSTALL_LOCALSTATEDIR}

var

RUNSTATE

${CMAKE_INSTALL_RUNSTATEDIR}

<LOCALSTATE dir>/run

DATA

${CMAKE_INSTALL_DATADIR}

<DATAROOT dir>

INFO

${CMAKE_INSTALL_INFODIR}

<DATAROOT dir>/info

LOCALE

${CMAKE_INSTALL_LOCALEDIR}

<DATAROOT dir>/locale

MAN

${CMAKE_INSTALL_MANDIR}

<DATAROOT dir>/man

DOC

${CMAKE_INSTALL_DOCDIR}

<DATAROOT dir>/doc

Projects wishing to follow the common practice of installing headers into a project-specific subdirectory will need to provide a destination rather than rely on the above.

Note that some of the types’ built-in defaults use the DATAROOT directory as a prefix. The DATAROOT prefix is calculated similarly to the types, with CMAKE_INSTALL_DATAROOTDIR as the variable and share as the built-in default. You cannot use DATAROOT as a TYPE parameter; please use DATA instead.

To make packages compliant with distribution filesystem layout policies, if projects must specify a DESTINATION, it is recommended that they use a path that begins with the appropriate GNUInstallDirs variable. This allows package maintainers to control the install destination by setting the appropriate cache variables. The following example shows how to follow this advice while installing headers to a project-specific subdirectory:

include(GNUInstallDirs)
install(FILES mylib.h
        DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/myproj
)

An install destination given as a DESTINATION argument may use “generator expressions” with the syntax $<...>. See the cmake-generator-expressions(7) manual for available expressions.

Installing Directories

install(DIRECTORY dirs...
        TYPE <type> | DESTINATION <dir>
        [FILE_PERMISSIONS permissions...]
        [DIRECTORY_PERMISSIONS permissions...]
        [USE_SOURCE_PERMISSIONS] [OPTIONAL] [MESSAGE_NEVER]
        [CONFIGURATIONS [Debug|Release|...]]
        [COMPONENT <component>] [EXCLUDE_FROM_ALL]
        [FILES_MATCHING]
        [[PATTERN <pattern> | REGEX <regex>]
         [EXCLUDE] [PERMISSIONS permissions...]] [...])

The DIRECTORY form installs contents of one or more directories to a given destination. The directory structure is copied verbatim to the destination. The last component of each directory name is appended to the destination directory but a trailing slash may be used to avoid this because it leaves the last component empty. Directory names given as relative paths are interpreted with respect to the current source directory. If no input directory names are given the destination directory will be created but nothing will be installed into it. The FILE_PERMISSIONS and DIRECTORY_PERMISSIONS options specify permissions given to files and directories in the destination. If USE_SOURCE_PERMISSIONS is specified and FILE_PERMISSIONS is not, file permissions will be copied from the source directory structure. If no permissions are specified files will be given the default permissions specified in the FILES form of the command, and the directories will be given the default permissions specified in the PROGRAMS form of the command.

The MESSAGE_NEVER option disables file installation status output.

Installation of directories may be controlled with fine granularity using the PATTERN or REGEX options. These “match” options specify a globbing pattern or regular expression to match directories or files encountered within input directories. They may be used to apply certain options (see below) to a subset of the files and directories encountered. The full path to each input file or directory (with forward slashes) is matched against the expression. A PATTERN will match only complete file names: the portion of the full path matching the pattern must occur at the end of the file name and be preceded by a slash. A REGEX will match any portion of the full path but it may use / and $ to simulate the PATTERN behavior. By default all files and directories are installed whether or not they are matched. The FILES_MATCHING option may be given before the first match option to disable installation of files (but not directories) not matched by any expression. For example, the code

install(DIRECTORY src/ DESTINATION include/myproj
        FILES_MATCHING PATTERN "*.h")

will extract and install header files from a source tree.

Some options may follow a PATTERN or REGEX expression and are applied only to files or directories matching them. The EXCLUDE option will skip the matched file or directory. The PERMISSIONS option overrides the permissions setting for the matched file or directory. For example the code

install(DIRECTORY icons scripts/ DESTINATION share/myproj
        PATTERN "CVS" EXCLUDE
        PATTERN "scripts/*"
        PERMISSIONS OWNER_EXECUTE OWNER_WRITE OWNER_READ
                    GROUP_EXECUTE GROUP_READ)

will install the icons directory to share/myproj/icons and the scripts directory to share/myproj. The icons will get default file permissions, the scripts will be given specific permissions, and any CVS directories will be excluded.

Either a TYPE or a DESTINATION must be provided, but not both. A TYPE argument specifies the generic file type of the files within the listed directories being installed. A destination will then be set automatically by taking the corresponding variable from GNUInstallDirs, or by using a built-in default if that variable is not defined. See the table below for the supported file types and their corresponding variables and built-in defaults. Projects can provide a DESTINATION argument instead of a file type if they wish to explicitly define the install destination.

TYPE Argument

GNUInstallDirs Variable

Built-In Default

BIN

${CMAKE_INSTALL_BINDIR}

bin

SBIN

${CMAKE_INSTALL_SBINDIR}

sbin

LIB

${CMAKE_INSTALL_LIBDIR}

lib

INCLUDE

${CMAKE_INSTALL_INCLUDEDIR}

include

SYSCONF

${CMAKE_INSTALL_SYSCONFDIR}

etc

SHAREDSTATE

${CMAKE_INSTALL_SHARESTATEDIR}

com

LOCALSTATE

${CMAKE_INSTALL_LOCALSTATEDIR}

var

RUNSTATE

${CMAKE_INSTALL_RUNSTATEDIR}

<LOCALSTATE dir>/run

DATA

${CMAKE_INSTALL_DATADIR}

<DATAROOT dir>

INFO

${CMAKE_INSTALL_INFODIR}

<DATAROOT dir>/info

LOCALE

${CMAKE_INSTALL_LOCALEDIR}

<DATAROOT dir>/locale

MAN

${CMAKE_INSTALL_MANDIR}

<DATAROOT dir>/man

DOC

${CMAKE_INSTALL_DOCDIR}

<DATAROOT dir>/doc

Note that some of the types’ built-in defaults use the DATAROOT directory as a prefix. The DATAROOT prefix is calculated similarly to the types, with CMAKE_INSTALL_DATAROOTDIR as the variable and share as the built-in default. You cannot use DATAROOT as a TYPE parameter; please use DATA instead.

To make packages compliant with distribution filesystem layout policies, if projects must specify a DESTINATION, it is recommended that they use a path that begins with the appropriate GNUInstallDirs variable. This allows package maintainers to control the install destination by setting the appropriate cache variables.

The list of dirs... given to DIRECTORY and an install destination given as a DESTINATION argument may use “generator expressions” with the syntax $<...>. See the cmake-generator-expressions(7) manual for available expressions.

Custom Installation Logic

install([[SCRIPT <file>] [CODE <code>]]
        [COMPONENT <component>] [EXCLUDE_FROM_ALL] [...])

The SCRIPT form will invoke the given CMake script files during installation. If the script file name is a relative path it will be interpreted with respect to the current source directory. The CODE form will invoke the given CMake code during installation. Code is specified as a single argument inside a double-quoted string. For example, the code

install(CODE "MESSAGE(\"Sample install message.\")")

will print a message during installation.

<file> or <code> may use “generator expressions” with the syntax $<...> (in the case of <file>, this refers to their use in the file name, not the file’s contents). See the cmake-generator-expressions(7) manual for available expressions.

Installing Exports

install(EXPORT <export-name> DESTINATION <dir>
        [NAMESPACE <namespace>] [[FILE <name>.cmake]|
        [PERMISSIONS permissions...]
        [CONFIGURATIONS [Debug|Release|...]]
        [EXPORT_LINK_INTERFACE_LIBRARIES]
        [COMPONENT <component>]
        [EXCLUDE_FROM_ALL])
install(EXPORT_ANDROID_MK <export-name> DESTINATION <dir> [...])

The EXPORT form generates and installs a CMake file containing code to import targets from the installation tree into another project. Target installations are associated with the export <export-name> using the EXPORT option of the install(TARGETS) signature documented above. The NAMESPACE option will prepend <namespace> to the target names as they are written to the import file. By default the generated file will be called <export-name>.cmake but the FILE option may be used to specify a different name. The value given to the FILE option must be a file name with the .cmake extension. If a CONFIGURATIONS option is given then the file will only be installed when one of the named configurations is installed. Additionally, the generated import file will reference only the matching target configurations. The EXPORT_LINK_INTERFACE_LIBRARIES keyword, if present, causes the contents of the properties matching (IMPORTED_)?LINK_INTERFACE_LIBRARIES(_<CONFIG>)? to be exported, when policy CMP0022 is NEW.

When a COMPONENT option is given, the listed <component> implicitly depends on all components mentioned in the export set. The exported <name>.cmake file will require each of the exported components to be present in order for dependent projects to build properly. For example, a project may define components Runtime and Development, with shared libraries going into the Runtime component and static libraries and headers going into the Development component. The export set would also typically be part of the Development component, but it would export targets from both the Runtime and Development components. Therefore, the Runtime component would need to be installed if the Development component was installed, but not vice versa. If the Development component was installed without the Runtime component, dependent projects that try to link against it would have build errors. Package managers, such as APT and RPM, typically handle this by listing the Runtime component as a dependency of the Development component in the package metadata, ensuring that the library is always installed if the headers and CMake export file are present.

In addition to cmake language files, the EXPORT_ANDROID_MK mode maybe used to specify an export to the android ndk build system. This mode accepts the same options as the normal export mode. The Android NDK supports the use of prebuilt libraries, both static and shared. This allows cmake to build the libraries of a project and make them available to an ndk build system complete with transitive dependencies, include flags and defines required to use the libraries.

The EXPORT form is useful to help outside projects use targets built and installed by the current project. For example, the code

install(TARGETS myexe EXPORT myproj DESTINATION bin)
install(EXPORT myproj NAMESPACE mp_ DESTINATION lib/myproj)
install(EXPORT_ANDROID_MK myproj DESTINATION share/ndk-modules)

will install the executable myexe to <prefix>/bin and code to import it in the file <prefix>/lib/myproj/myproj.cmake and <prefix>/share/ndk-modules/Android.mk. An outside project may load this file with the include command and reference the myexe executable from the installation tree using the imported target name mp_myexe as if the target were built in its own tree.

Note

This command supercedes the install_targets() command and the PRE_INSTALL_SCRIPT and POST_INSTALL_SCRIPT target properties. It also replaces the FILES forms of the install_files() and install_programs() commands. The processing order of these install rules relative to those generated by install_targets(), install_files(), and install_programs() commands is not defined.

Generated Installation Script

The install() command generates a file, cmake_install.cmake, inside the build directory, which is used internally by the generated install target and by CPack. You can also invoke this script manually with cmake -P. This script accepts several variables:

COMPONENT

Set this variable to install only a single CPack component as opposed to all of them. For example, if you only want to install the Development component, run cmake -DCOMPONENT=Development -P cmake_install.cmake.

BUILD_TYPE

Set this variable to change the build type if you are using a multi-config generator. For example, to install with the Debug configuration, run cmake -DBUILD_TYPE=Debug -P cmake_install.cmake.

DESTDIR

This is an environment variable rather than a CMake variable. It allows you to change the installation prefix on UNIX systems. See DESTDIR for details.