Shell commands using Cmake add_custom_command on Linux

Question

I can't figure out the right syntax to run a shell command in a post-build step in Cmake on Linux. I can make a simple echo work, but when I want to e.g. iterate over all files and echo those, I'm getting an error.

The following works:

add_custom_command(TARGET ${MY_LIBRARY_NAME}
                   POST_BUILD
                   COMMAND echo Hello world!
                   USES_TERMINAL)

This correctly prints Hello world!.

But now I would like to iterate over all .obj files and print those. I thought I should do:

add_custom_command(TARGET ${MY_LIBRARY_NAME} 
                   POST_BUILD
                   COMMAND for file in *.obj; do echo @file ; done 
                   VERBATIM
                   USES_TERMINAL)

But that gives the following error:

/bin/sh: 1: Syntax error: end of file unexpected

I've tried all sorts of combinations with quotation marks or starting with sh, but none of that seems to work. How can I do this correctly?

Answer 1

add_custom_command (and all the other CMake functions that execute a COMMAND) don't run shell scripts, but only allow the execution of a single command. The USES_TERMINAL doesn't cause the command to be run in an actual terminal or allow the use of shell builtins like the for loop.

From the documentation:

To run a full script, use the configure_file() command or the file(GENERATE) command to create it, and then specify a COMMAND to launch it.

Or, alternatively, for very simple scripts you can do what @lojza suggested in the comment to your question and run the bash command with the actual script content as an argument.

add_custom_command(
  TARGET ${MY_LIBRARY_NAME}
  POST_BUILD 
  COMMAND bash -c [[for file in *.obj; do echo ${file}; done]]
  VERBATIM
)

Note that I deliberately used a CMake raw string literal here so that ${file} is not expanded as a CMake variable. You could also use bash -c "for file in *obj; do echo $file; done" with a regular string literal, in which case $file also isn't expanded due to the lack of curly braces. Having copied and pasted bash code from other sources into CMake before I know how difficult it is to track down bugs caused by an unexpected expansions of CMake variables in such scripts, though, so I'd always recommend to use [[ and ]] unless you actually want to expand a CMake variable.

However, for your concrete example of doing something with all files which match a pattern there is an even simpler alternative: Use the find command instead of a bash loop:

add_custom_command(
  TARGET ${MY_LIBRARY_NAME}
  POST_BUILD 
  COMMAND find
    -maxdepth 1 
    -name *.obj 
    -printf "%P\\n"
  VERBATIM
)

Answer 2

Sometimes, proper scripting of the COMMAND could be performed using debugging. Below the error message there is a point to the line which causes it. Something like

/bin/sh: 1: Syntax error: end of file unexpected
make[2]: *** [CMakeFiles/my_target.dir/build.make:57: CMakeFiles/my_target] Error 2

So, you can look into the line 57 of the file CMakeFiles/my_target.dir/build.make and find out the command which is actually placed into the Makefile:

CMakeFiles/my_target:
    for file in "*.obj" do echo @file done

As you can see, CMake drops all semicolons (;): this is because this symbol is a separator for CMake.

Quoting semicolons doesn't help in VERBATIM mode:

The commmand

COMMAND for file in *.obj ";" do echo @file ";" done

is transformed

CMakeFiles/my_target:
    for file in "*.obj" ";" do echo @file ";" done

But shell doesn't treat quoted semicolons (";") as commands separator!

Omitting VERBATIM gives better transformation:

CMakeFiles/my_target:
    for file in *.obj ; do echo @file ; done

The next step is to properly refer to the file variable in the script (@ is definitely a wrong way). A script should see either $file or ${file}, but both CMake and Make have a special way for process a dollar ($). (VERBATIM could automatically escape things for the Make, but we cannot use it because of semicolons.)

Resulted command could be either

COMMAND for file in *.obj ";" do echo $$file ";" done

or

COMMAND for file in "CMake*" ";" do echo $\${file} ";" done

---

As you can see, even VERBATIM doesn't handle all situations correctly.