How to call a cmake method from shell script? - shell

Question

I have a parameterised function in mytestprogram.cmake written like below:
function(get_output_from_input output input)
set(${output} "test" PARENT_SCOPE)
endfunction()
Question:
How do I call the cmake method get_output_from_input from a shell script?
I learnt that there is -P <source.cmake> flag, that puts CMake into script processing mode and we can execute arbitrary CMake code with it using something like below.
execute_process(COMMAND ${CMAKE_PROGRAM} -P yourscript.cmake)
So, in this case I believe the way to call get_output_from_input from a shell script would be something like below?
input="some_input"
output=""
execute_process(get_output_from_input(output, input) ${CMAKE_PROGRAM} -P mytestprogram.cmake)
echo $output
But above trick does not work. Is the way I am running execute_process correct?
Trying to figure out whats wrong, it seems echo $CMAKE_PROGRAM returns empty? Could that be the reason? What am I missing here to call get_output_from_input?
Environment:
cmake version 3.18.2
macOS Catalina

Answer 1

cmake -P executes an entire script, not a single function defined in a CMake file, so need to add an additional script file that you can then execute via cmake -P. Arguments to a script executed via cmake -P need to be passed as CMake variables, e.g. cmake -DINPUT=some_input -P myscript.cmake .
Since you need the output of the CMake script in an bash variable, the CMake script "mytestscript.cmake" would look something like this:
# make `get_output_from_input` available in current script
# assuming the script file is located next to `mytestprogram.cmake`
include(${CMAKE_CURRENT_LIST_DIR}/mytestprogram.cmake)
get_output_from_input(RESULT "${INPUT}")
message("${RESULT}")
and the shell script contains
#!/bin/bash
input="some_input"
# assumes `mytestscript.cmake` is located in the current working directory
output=$(cmake -DINPUT="${input}" -P mytestscript.cmake 2>&1)
# Below statement should print the value of output.
echo $output
Regarding execute_process: That is meant to be used if you need to execute another process from within a CMake script, not to execute a CMake script from a shell process. Depending on what you plan to do with the output of the CMake script you could possibly use execute_process instead of additional bash.

Answer 2

What I think you're asking for—setting a Bash variable directly from CMake's script mode—is impossible. The script mode (-P) just lets you run CMake code without a project / build directory. And execute_process is a CMake command, not a shell command, so calling it from sh/bash is bound to fail.
The CMake function you wrote is wrong, too. All it does is write to a local variable which is never read and immediately gets destroyed. You probably wanted set(VAR "VALUE" PARENT_SCOPE). The return() is pointless and should be removed.
If you explained a bit more about what you were trying to do, maybe this question would be answerable.
Here's a basic demonstration of CMake's script mode:
# ./script.cmake
# Input variable: file
# Output: md5sum of file
cmake_minimum_required(VERSION 3.19)
file(MD5 "${file}" hash)
message("${hash}")
Running this from the terminal (equiv. a shell script):
$ ls
my_file.txt script.cmake
$ cat my_file.txt
Hello, world!
$ cmake -Dfile=my_file.txt -P script.cmake
746308829575e17c3331bbcb00c0898b