Alternative symlink-creating mode for file(INSTALL ...)

An new environment variable 'CMAKE_INSTALL_MODE' is introduced,
which can be used to ask CMake to create symbolic links
instead of copying files during a file(INSTALL ...).

The operation is at the file level only, directory trees are
still created using actual directories, not links.

Signed-off-by: Felix Lelchuk <felix.lelchuk@gmx.de>

1 files changed, 37 insertions, 0 deletions

diff --git a/Help/envvar/CMAKE_INSTALL_MODE.rst b/Help/envvar/CMAKE_INSTALL_MODE.rst
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+CMAKE_INSTALL_MODE
+------------------
+
+.. versionadded:: 3.22
+
+.. include:: ENV_VAR.txt
+
+The ``CMAKE_INSTALL_MODE`` environment variable allows users to operate
+CMake in an alternate mode of :command:`file(INSTALL)` and :command:`install()`.
+
+The default behavior for an installation is to copy a source file from a
+source directory into a destination directory. This environment variable
+however allows the user to override this behavior, causing CMake to create
+symbolic links instead.
+
+.. note::
+  A symbolic link consists of a reference file path rather than contents of its own,
+  hence there are two ways to express the relation, either by a relative or an absolute path.
+
+The following values are allowed for ``CMAKE_INSTALL_MODE``:
+
+* empty, unset or ``COPY``: default behavior, duplicate the file at its destination
+* ``ABS_SYMLINK``: create an *absolute* symbolic link to the source file at the destination *or fail*
+* ``ABS_SYMLINK_OR_COPY``: like ``ABS_SYMLINK`` but silently copy on error
+* ``REL_SYMLINK``: create an *relative* symbolic link to the source file at the destination *or fail*
+* ``REL_SYMLINK_OR_COPY``: like ``REL_SYMLINK`` but silently copy on error
+* ``SYMLINK``: try as if through ``REL_SYMLINK`` and fall back to ``ABS_SYMLINK`` if the referenced
+  file cannot be expressed using a relative path. Fail on error.
+* ``SYMLINK_OR_COPY``: like ``SYMLINK`` but silently copy on error
+
+Installing symbolic links rather than copying files can help conserve storage space because files do
+not have to be duplicated on disk. However, modifications applied to the source immediately affects
+the symbolic link and vice versa. *Use with caution*.
+
+.. note:: ``CMAKE_INSTALL_MODE`` only affects files, *not* directories.
+
+.. note:: Symbolic links are not available on all platforms.