how can I load environment file (.env) when run cmake command? - cmake

Question

is there any option can allow me to load key=value pairs from a dotenv file? I knew I can do -E env key=value But I have lots of values and I want to load them from the dotenv file, since other tools can load environment variables from it. I do not want to do bash tricks because I would like it to be as simple as possible and cross-platform.

Answer 1

There's no built-in support for dotenv files, but here's a full implementation in pure CMake. For your CMakeLists.txt, here's an example:
cmake_minimum_required(VERSION 3.21)
project(env-test NONE)
add_custom_target(
example
COMMAND
${CMAKE_COMMAND} -P ${CMAKE_CURRENT_SOURCE_DIR}/env.cmake
${CMAKE_CURRENT_SOURCE_DIR}/.env # dotenv
env # command
)
Here's the contents of the .env file:
foo=bar
bears=beets
battlestar=galactica
Here's a demo of this working:
$ cmake -G Ninja -S . -B build
$ cmake --build build --target example
...
foo=bar
bears=beets
battlestar=galactica
and here's the (complicated) env.cmake script that implements this:
cmake_minimum_required(VERSION 3.21)
if (CMAKE_ARGC LESS 5)
message(FATAL_ERROR "Usage: cmake -P env.cmake <dotenv> [command...]")
endif ()
set(dotenv "${CMAKE_ARGV3}")
if (NOT EXISTS "${dotenv}")
message(FATAL_ERROR "Dot-env file not found: ${dotenv}")
endif ()
set(command "")
math(EXPR argc "${CMAKE_ARGC} - 1")
foreach (i RANGE 4 ${argc})
list(APPEND command "${CMAKE_ARGV${i}}")
endforeach ()
file(STRINGS "${dotenv}" entries)
foreach (entry IN LISTS entries)
if (entry MATCHES "^([^=]+)=(.*)$")
set(ENV{${CMAKE_MATCH_1}} "${CMAKE_MATCH_2}")
else ()
message(FATAL_ERROR "Malformed dotenv entry:\n${entry}")
endif ()
endforeach ()
execute_process(COMMAND ${command} COMMAND_ERROR_IS_FATAL ANY)