Problem: Default Travis build variables not defined.

Solution: simplify Travis config file by defining default build options
DRAFT and ZMQ_VERSION in ci_build.sh script

.travis.yml

@@ -16,7 +16,7 @@ matrix:
     - os: osx
       osx_image: xcode9.1
       compiler: clang
-      env: ZMQ_VERSION=4.2.5 DRAFT=1
+      env: DRAFT=1
 
     ##########################################################
     # GCC on Linux
@@ -27,11 +27,10 @@ matrix:
       sudo: true
       env: ZMQ_VERSION=4.2.0 BUILD_TYPE=pkgconfig
 
-    # GCC default, draft disabled, latest libzmq
+    # GCC default, draft disabled, default libzmq (defined in ci_build.sh)
     - os: linux
-      env: ZMQ_VERSION=4.2.5
 
-    # GCC 7, draft enabled, latest libzmq
+    # GCC 7, draft enabled (default), latest libzmq (default)
     - os: linux
       addons:
         apt:
@@ -40,7 +39,7 @@ matrix:
           packages:
             - g++-7
       env:
-        - MATRIX_EVAL="CC=gcc-7 && CXX=g++-7" ZMQ_VERSION=4.2.5 DRAFT=1
+        - MATRIX_EVAL="CC=gcc-7 && CXX=g++-7" DRAFT=1
   # - env: BUILD_TYPE=cmake DO_CLANG_FORMAT_CHECK=1 CLANG_FORMAT=/usr/local/clang-5.0.0/bin/clang-format
     # os: linux
     # addons:

ci_build.sh

@@ -6,10 +6,12 @@ set -e
 BUILD_TYPE=${BUILD_TYPE:-cmake}
 LIBZMQ=${PWD}/libzmq-build
 CPPZMQ=${PWD}/cppzmq-build
+ZMQ_VERSION="4.2.5"
+DRAFT=${DRAFT:-0}
 # Travis machines have 2 cores
 JOBS=2
 
-if [ "$DRAFT" = "1" ] ; then
+if [ "${DRAFT}" = "1" ] ; then
     # if we enable drafts during the libzmq cmake build, the pkgconfig
     # data should set ZMQ_BUILD_DRAFT_API in dependent builds, but this
     # does not appear to work (TODO)
@@ -17,16 +19,16 @@ if [ "$DRAFT" = "1" ] ; then
 fi
 
 libzmq_install() {
-    curl -L https://github.com/zeromq/libzmq/archive/v${ZMQ_VERSION}.tar.gz \
+    curl -L https://github.com/zeromq/libzmq/archive/v"${ZMQ_VERSION}".tar.gz \
       >zeromq.tar.gz
     tar -xvzf zeromq.tar.gz
-    if [ "$BUILD_TYPE" = "cmake" ] ; then
+    if [ "${BUILD_TYPE}" = "cmake" ] ; then
         cmake -Hlibzmq-${ZMQ_VERSION} -B${LIBZMQ} -DWITH_PERF_TOOL=OFF \
                                                   -DZMQ_BUILD_TESTS=OFF \
                                                   -DCMAKE_BUILD_TYPE=Release \
                                                   ${ZEROMQ_CMAKE_FLAGS}
         cmake --build ${LIBZMQ} -- -j${JOBS}
-    elif [ "$BUILD_TYPE" = "pkgconfig" ] ; then
+    elif [ "${BUILD_TYPE}" = "pkgconfig" ] ; then
         pushd .
         cd libzmq-${ZMQ_VERSION}
         ./autogen.sh &&
@@ -41,12 +43,12 @@ libzmq_install() {
 # build zeromq first
 cppzmq_build() {
     pushd .
-    if [ "$BUILD_TYPE" = "cmake" ] ; then
+    if [ "${BUILD_TYPE}" = "cmake" ] ; then
         export ZeroMQ_DIR=${LIBZMQ}
     fi
     cmake -H. -B${CPPZMQ} ${ZEROMQ_CMAKE_FLAGS}
     cmake --build ${CPPZMQ} -- -j${JOBS}
-    if [ "$BUILD_TYPE" = "pkgconfig" ] ; then
+    if [ "${BUILD_TYPE}" = "pkgconfig" ] ; then
         cd ${CPPZMQ}
         sudo make install
     fi
@@ -62,7 +64,7 @@ cppzmq_tests() {
 
 cppzmq_demo() {
     pushd .
-    if [ "$BUILD_TYPE" = "cmake" ] ; then
+    if [ "${BUILD_TYPE}" = "cmake" ] ; then
         export ZeroMQ_DIR=${LIBZMQ}
         export cppzmq_DIR=${CPPZMQ}
     fi