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?