# configure\_file in add\_custom\_command?

**URL:** <https://discourse.cmake.org/t/configure-file-in-add-custom-command/15207>\
**Category:** Code\
**Created:** [October 1, 2025, 12:46pm UTC](https://discourse.cmake.org/t/configure-file-in-add-custom-command/15207 "2025-10-01T12:46:48Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![sdbbs](https://discourse.cmake.org/user_avatar/discourse.cmake.org/sdbbs/32/5380_2.png) [@sdbbs](https://discourse.cmake.org/u/sdbbs)\
**Post date:** [October 1, 2025, 12:46pm UTC](https://discourse.cmake.org/t/configure-file-in-add-custom-command/15207/1 "2025-10-01T12:46:49Z")

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In a CMakeLists.txt of a project of mine, I had a `configure_file` that converted `MyHeader.h.in` to `MyHeader.h` while doing transformations of CMake variables - and that worked great.

My problem now is that I ended up generating `MyHeader.h.in`itself by an `add_custom_command`, so at build time - while `configure_file` runs at configure time. So, I need to convert the `configure_file` to something else that can transform CMake variables in the source file at build time.

I found this: [Implementing build time configure\_file functionality (#25534) · Issue · cmake/cmake](https://gitlab.kitware.com/cmake/cmake/-/issues/25534) :

> In the `COMMAND` for the `add_custom_command()` call, you can use one of the various `cmake -E copy...` commands to do a simple file copy if you don’t need to do any variable substitution as part of the copying

Great - but what do I do, if I _do_ need to do variable substitution as part of the copying? Can I use `configure_file` directly as an argument of the `cmake` command? Or maybe should I save the `configure_file` call to a separate .cmake script file, and then call `cmake` on this script from `add_custom_command`?

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**Author:** ![Angew](https://discourse.cmake.org/user_avatar/discourse.cmake.org/angew/32/229_2.png) [@Angew](https://discourse.cmake.org/u/Angew)\
**Post date:** [October 3, 2025, 11:35am UTC](https://discourse.cmake.org/t/configure-file-in-add-custom-command/15207/2 "2025-10-03T11:35:57Z")

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Are you in control of the build-time process generating the header? If so, the easiest solution would be to inject the CMake variables you need into that process, and generate the header file directly with the variables already substituted (i.e. generate directly `MyHeader.h` instead of generating `MyHeader.h.in`).

If that is not an option, you will have to move the `configure_file` process to build time, as you indicated in your last sentence. However, that has one major thing you need to solve: you need to make sure that the CMake which you will launch at build time actually has the variables you wish to configure. You may want to look at the [`load_cache()`](https://cmake.org/cmake/help/latest/command/load_cache.html) command, or you may end up having to `configure_file()` the script to get the variable values from your configure time into the build-time script.