title: VScode: Defining own variables in tasks.json
body: <p>I've set up a tasks.json file for building a project on multiple platforms. All platforms see the same content of the project repository. This is done either via disk sharing, because of running another platform in a VM, or via sync with the Git repository.
  So far so good, they all see the same task.json. However some command lines are rather long and those long lines are identical for most part.
  for example:</p>
  
  <p><code>"rm -rf build; mkdir build; cd build; ../configure --with-bash-malloc=no CFLAGS=\"-O3 -fno-builtin-malloc -fno-builtin-calloc -fno-builtin-realloc -fno-builtin-free\" LDFLAGS=-L/usr/local/lib LIBS=\"-ltcmalloc -lcurl\" CC=clang</code></p>
  
  <p>Similar lines are there for the different platforms. 
  The configure part is always the same for the different platforms, so it would be nice to factor out this common part. Thus the question is if it is possible to define your own variables, so you can use them similar to <code>${workspaceRoot}</code>.</p>
  
  <p>Thus define somewhere</p>
  
  <pre><code>"win_dir": "build_windows",
  "linux_dir": "build",
  "osx_dir": "build_osx",
  "configure": "../configure --with-bash-malloc=no CFLAGS=\"-O3 -fno-builtin-malloc -fno-builtin-calloc -fno-builtin-realloc -fno-builtin-free\" LDFLAGS=-L/usr/local/lib LIBS=\"-ltcmalloc -lcurl\" CC=clang"
  </code></pre>
  
  <p>And then write </p>
  
  <pre><code>"tasks": [
      {
          "taskName": "configure",
          "command": "bash",
          "windows": {
              "args": ["-c", "rm -rf ${win_dir}; mkdir ${win_dir}; cd ${win_dir}; ${configure}"]
          },
          "linux": {
              "args": ["-c", "rm -rf ${linux_dir}; mkdir ${linux_dir}; cd ${linux_dir}; ${configure}"]
          },
          "osx": {
              "args": ["-c", "rm -rf ${osx_dir}; mkdir ${osx_dir}; cd ${osx_dir}; ${configure}"]
          },
          "isBuildCommand": true,
          "problemMatcher": "$make-compile"
      },
      ... others tasks using the variables
  </code></pre>
  
  <p>When making changes to the build directory or arguments passed to configure etc, then the tasks.json file needs only editing at one place, instead of many.</p>
  
  <p>Perhaps it is already possible but I'm unable to find out how. I tried to do something with the declares block, but that seems to be hard tied to problemMatcher. You can find some examples, but I could not find clear documentation of of the elements of the tasks.json file and how they interact.</p>
  
  <p>Perhaps I'm missing something, please educate me!</p>