Add a shellcheck test, fix or ignore all warnings reported by shellcheck.

.travis.yml

@@ -11,6 +11,7 @@ matrix:
         apt:
           packages:
             - valgrind
+            - shellcheck
             - cppcheck
             - doxygen
             - cmake
@@ -25,6 +26,7 @@ matrix:
         apt:
           packages:
             - valgrind
+            - shellcheck
             - cppcheck
             - doxygen
             - cmake
@@ -39,6 +41,7 @@ matrix:
         apt:
           packages:
             - valgrind
+            - shellcheck
             - cppcheck
             - doxygen
             - cmake
@@ -66,6 +69,7 @@ matrix:
         apt:
           packages:
             - valgrind
+            - shellcheck
             - cppcheck
             - doxygen
             - cmake
@@ -126,7 +130,7 @@ script:
   # when using bionic, Travis seems to ignore the "addons" section, so installing the packages with apt-get...
   - if [ -n "$CHECK" ]; then
       if [ "$TRAVIS_DIST" = "bionic" ]; then
-        sudo apt-get install -y valgrind cppcheck doxygen;
+        sudo apt-get install -y valgrind shellcheck cppcheck doxygen;
       fi;
       make distcheck;
       if type cppcheck &> /dev/null ; then cppcheck --error-exitcode=1 --quiet *.h *.c tests/ ; fi;