# Define a pre-build command without creating a new target

**URL:** <https://discourse.cmake.org/t/define-a-pre-build-command-without-creating-a-new-target/1623>\
**Category:** Code\
**Tags:** os:linux, gen:makefiles\
**Created:** [July 25, 2020, 6:49pm UTC](https://discourse.cmake.org/t/define-a-pre-build-command-without-creating-a-new-target/1623 "2020-07-25T18:49:27Z")\
**Posts on this page:** 1\
**Showing post:** 3

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**Author:** ![Artemis](https://discourse.cmake.org/user_avatar/discourse.cmake.org/artemis/32/616_2.png) [@Artemis](https://discourse.cmake.org/u/Artemis)\
**Post date:** [July 27, 2020, 9:05am UTC](https://discourse.cmake.org/t/define-a-pre-build-command-without-creating-a-new-target/1623/3 "2020-07-27T09:05:46Z")

</div>

I’ve made a minimal reproducible example: [dummy.zip](https://discourse.cmake.org/uploads/short-url/nwCcp234sVSHw0JFCO55xz6s4Dq.zip) (1.0 KB)  
The three behaviours I get are:

#### with `add_custom_command(OUTPUT "${dummy_BINARY_DIR}/date.h" "foo"`

> **output**
>
> ```plaintext
> ====================[Build | dummy | Debug]===================================
> /usr/bin/cmake --build /mnt/d/Users/Nemo/Desktop/dummy/cmake-build-debug --target dummy -- -j 12
> Scanning dependencies of target dummy
> [33%] Building C object CMakeFiles/dummy.dir/main.c.o
> /mnt/d/Users/Nemo/Desktop/dummy/main.c:2:10: fatal error: date.h: No such file or directory
> #include "date.h"
> ^ ~~~~~~~
> compilation terminated.
> CMakeFiles/dummy.dir/build.make:88: recipe for target 'CMakeFiles/dummy.dir/main.c.o' failed
> make[3]: *** [CMakeFiles/dummy.dir/main.c.o] Error 1
> CMakeFiles/Makefile2:94: recipe for target 'CMakeFiles/dummy.dir/all' failed
> make[2]: *** [CMakeFiles/dummy.dir/all] Error 2
> CMakeFiles/Makefile2:101: recipe for target 'CMakeFiles/dummy.dir/rule' failed
> make[1]: *** [CMakeFiles/dummy.dir/rule] Error 2
> Makefile:137: recipe for target 'dummy' failed
> make: *** [dummy] Error 2
> 
> ```

#### with `add_custom_command(OUTPUT "foo" "${dummy_BINARY_DIR}/date.h"`

- case 1: ~~the header change doesn’t trigger a recompilation~~ fixed by clearing the CMake cache

> **output**
>
> ```plaintext
> ====================[Build | dummy | Debug]===================================
> /usr/bin/cmake --build /mnt/d/Users/Nemo/Desktop/dummy/cmake-build-debug --target dummy -- -j 12
> [33%] generating build date header
> [100%] Built target dummy
> 
> Build finished
> 
> ```

- case 2: it works, but two “generating” messages are print, which is suspicious

> **output**
>
> ```plaintext
> ====================[Build | dummy | Debug]===================================
> /usr/bin/cmake --build /mnt/d/Users/Nemo/Desktop/dummy/cmake-build-debug --target dummy -- -j 12
> [33%] generating build date header
> Scanning dependencies of target dummy
> [33%] generating build date header
> [66%] Building C object CMakeFiles/dummy.dir/main.c.o
> [100%] Linking C executable dummy
> [100%] Built target dummy
> 
> Build finished
> 
> ```

* * *

> [@hex](#):
>
> Before moving on with your question - have you had a look at chapter 19.3 Source Control Commits? It implements the same concept, but differently.

It implements the same concept, but he runs it at configure time, with `execute_process()`, so he doesn’t really have to handle dependencies.

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_[View the full topic](https://discourse.cmake.org/t/define-a-pre-build-command-without-creating-a-new-target/1623)._