Taking Control of the Matrix

The normal way to set up your matrix of Travis build configurations is to specify each of the possible axes (os, compiler, env) as a list, so that Travis builds every possible combination of those options. However, once you start adding custom builds controlled by env values, such as the sanitizer builds described in the previous entry, not all of the possible combinations make sense – for example, you might choose to just run an ASAN build on Linux + Clang, or only generate code coverage output from a single build.

So when your valid build combinations are sparse in the matrix, it can be easier to explicitly include the combinations that are valid, like so:

    matrix:
        include:
            - os: linux
              compiler: gcc
              env: BUILD_TYPE=normal
            - os: linux
              compiler: clang
              env: BUILD_TYPE=normal
            - os: linux
              compiler: gcc
              env: BUILD_TYPE=coverage
            - os: linux
              compiler: clang
              env: BUILD_TYPE=ubsan
            - os: linux
              compiler: clang
              env: BUILD_TYPE=asan
            - os: osx
              compiler: gcc
              env: BUILD_TYPE=normal
            - os: osx
              compiler: clang
              env: BUILD_TYPE=normal
  

Note that it's still possible to use the env.global key to specify an environment variable that should apply across all of the builds (rather than adding a new combination option).

    env:
      global:
        - CPPFLAGS = -Wall
  

One more trick to help with complex build matrices – YAML anchors and aliases can help to cut down repetition in your Travis config. Roughly speaking, adding &name after a key allows later config lines to use *name to repeat the value of the earlier key, either as-is or as the base for other additions:

    matrix:
      include:
        - os: linux
          compiler: clang
          addons:
            apt:
              sources: &common_sources
                - ubuntu-toolchain-r-test
                - llvm-toolchain-precise-3.6
              packages: &common_packages
                - autoconf
                - automake
        - os: linux
          compiler: clang
          env: REAL_CC=clang-3.6 REAL_CXX=clang++-3.6
          addons:
            apt:
              sources: *common_sources
              packages:
              - *common_packages
              - clang-3.6
    …
  

(Thanks to Al for suggesting this tip, and check out the Certificate Transparency Travis config for example of this in action.)