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

Question

I'm trying to create a custom command that runs with some environment variables, such as LDFLAGS, whose value needs to be quoted if it contains spaces:

LDFLAGS="-Lmydir -Lmyotherdir"

I cannot find a way to include this argument in a CMake custom command, due to CMake's escaping rules. Here's what I've tried so far:

COMMAND LDFLAGS="-Ldir -Ldir2" echo blah VERBATIM)

yields "LDFLAGS=\"-Ldir -Ldir2\"" echo blah

COMMAND LDFLAGS=\"-Ldir -Ldir2\" echo blah VERBATIM)

yields LDFLAGS=\"-Ldir -Ldir2\" echo blah

It seems I either get the whole string quoted, or the escaped quotes don't resolve when used as part of the command.

I would appreciate either a way to include the literal double-quote or as an alternative a better way to set environment variables for a command. Please note that I'm still on CMake 2.8, so I don't have the new "env" command available in 3.2.

Note that this is not a duplicate of When to quote variables? as none of those quoting methods work for this particular case.

Answer 1

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}\\\"")

Answer 2

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?

Answer 3

Florian's answer is wrong on several counts:

• Putting the quoted part in a variable makes no difference.
• You should definitely use VERBATIM. It fixes platform-specific quoting bugs.
• You definitely shouldn't use RULE_LAUNCH_CUSTOM for this. It isn't intended for this and only works with some generators.
• You shouldn't use env as the command. It isn't available on Windows.

It turns out the real reason OPs code doesn't work is that CMake always fully quotes the first word after COMMAND because it's supposed to be the name of an executable. You simply shouldn't put environment variables first.

For example:

add_custom_command(
    OUTPUT q1.txt
    COMMAND ENV_VAR="a b" echo "hello" > q1.txt
    VERBATIM
)

add_custom_target(q1 ALL DEPENDS q1.txt)

$ VERBOSE=1 make
...
"ENV_VAR=\"a b\"" echo hello > q1.txt
/bin/sh: ENV_VAR="a b": command not found

So how do you pass an environment variable with spaces? Simple.

add_custom_command(
    OUTPUT q1.txt
    COMMAND ${CMAKE_COMMAND} -E env ENV_VAR="a b" echo "hello" > q1.txt
    VERBATIM
)

Answer 4

Ok, I removed my original answer as the one proposed by @Florian is better. There is one additional tweak needed for multiple quoted args. Consider a list of environment variables as such:

set(my_env_vars LDFLAGS="-Ldir1 -Ldir2" CFLAGS="-Idira -Idirb")

In order to produce the desired expansion, convert to string and then replace ; with a space.

set(my_env_string "${my_env_vars}") #produces LDFLAGS="...";CFLAGS="..."
string(REPLACE ";" " " my_env_string "${my_env_string}")

Then you can proceed with @Florian's brilliant answer and add the custom launch rule. If you need semicolons in your string then you'll need to convert them to something else first.

Note that in this case I didn't need to launch with env:

set_target_properties(mytarget PROPERTIES RULE_LAUNCH_CUSTOM "${my_env_string}")

This of course depends on your shell.

On second thought, my original answer is below as I also have a case where I don't have access to the target name.

set(my_env LDFLAGS=\"-Ldir -Ldir2" CFLAGS=\"-Idira -Idirb\")
add_custom_command(COMMAND sh -c "${my_env} grep LDFLAGS" VERBATIM)

This technique still requires that the semicolons from the list->string conversion be replaced.

Answer 5

Some folks suggest to use ${CMAKE_COMMAND} and pass your executable as an argument, e.g:

COMMAND ${CMAKE_COMMAND} -E env "$(WindowsSdkDir)/bin/x64/makecert.exe" ...

That worked for me.