How do I include a literal double quote in a custom CMake command?

You need three backslashes. I needed this recently to get a preprocessor define from PkgConfig and apply it to my C++ flags:

pkg_get_variable(SHADERDIR movit shaderdir)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DSHADERDIR=\\\"${SHADERDIR}\\\"")

The obvious choice - often recommended when hitting the boundaries of COMMAND especially with older versions of CMake - is to use an external script.

I just wanted to add some simple COMMAND only variations that do work and won't need a shell, but are - I have to admit - still partly platform dependent.

  • One example would be to put only the quoted part into a variable:

    set(vars_as_string "-Ldir -Ldir2")
    add_custom_target(
        QuotedEnvVar
        COMMAND env LD_FLAGS=${vars_as_string} | grep LD_FLAGS
    )
    

    Which actually does escape the space and not the quotes.

  • Another example would be to add it with escaped quotes as a "launcher" rule:

    add_custom_target(
        LauncherEnvVar
        COMMAND env | grep LD_FLAGS
    )
    set_target_properties(
        LauncherEnvVar 
        PROPERTIES RULE_LAUNCH_CUSTOM "env LD_FLAGS=\"-Ldir -Ldir2\""
    )
    

Edit: Added examples for multiple quoted arguments without the need of escaping quotes

  • Another example would be to "hide some of the complexity" in a function and - if you want to add this to all your custom command calls - use the global/directory RULE_LAUNCH_CUSTOM property:

    function(set_env)
        get_property(_env GLOBAL PROPERTY RULE_LAUNCH_CUSTOM)
        if (NOT _env)
            set_property(GLOBAL PROPERTY RULE_LAUNCH_CUSTOM "env")
        endif()
        foreach(_arg IN LISTS ARGN)
            set_property(GLOBAL APPEND_STRING PROPERTY RULE_LAUNCH_CUSTOM " ${_arg}")
        endforeach()
    endfunction(set_env)
    
    set_env(LDFLAGS="-Ldir1 -Ldir2" CFLAGS="-Idira -Idirb")
    
    add_custom_target(
        MultipleEnvVar
        COMMAND env | grep -E 'LDFLAGS|CFLAGS'
    )
    

Alternative (for CMake >= 3.0)

  • I think what we actually are looking for here (besides the cmake -E env ...) is named Bracket Argument and does allow any character without the need of adding backslashes:

    set_property(
        GLOBAL PROPERTY 
            RULE_LAUNCH_CUSTOM [=[env LDFLAGS="-Ldir1 -Ldir2" CFLAGS="-Idira -Idirb"]=]
    )
    add_custom_target(
        MultipleEnvVarNew
        COMMAND env | grep -E 'LDFLAGS|CFLAGS'
    )
    

References

  • 0005145: Set environment variables for ADD_CUSTOM_COMMAND/ADD_CUSTOM_TARGET
  • How to modify environment variables passed to custom CMake target?
  • [CMake] How to set environment variable for custom command
  • cmake: when to quote variables?