YETUS-817. upgrade to shellcheck 0.6.0

Signed-off-by: Sean Busbey <busbey@apache.org>
diff --git a/precommit/src/main/shell/test-patch-docker/Dockerfile b/precommit/src/main/shell/test-patch-docker/Dockerfile
index acac18b..138aa03 100644
--- a/precommit/src/main/shell/test-patch-docker/Dockerfile
+++ b/precommit/src/main/shell/test-patch-docker/Dockerfile
@@ -199,10 +199,12 @@
 ####
 # Install shellcheck (shell script lint)
 ####
-RUN add-apt-repository -y ppa:jonathonf/ghc-8.0.2
-RUN apt-get -q update && apt-get -q install --no-install-recommends -y shellcheck \
+RUN apt-get -q update && apt-get -q install --no-install-recommends -y cabal-install \
     && apt-get clean \
     && rm -rf /var/lib/apt/lists/*
+RUN cabal update
+RUN cabal install ShellCheck \
+    && cp -p /root/.cabal/bin/shellcheck /usr/local/bin/shellcheck
 
 ###
 # Install hadolint
diff --git a/precommit/src/main/shell/test-patch.d/maven.sh b/precommit/src/main/shell/test-patch.d/maven.sh
index 09c7064..64fbcc2 100755
--- a/precommit/src/main/shell/test-patch.d/maven.sh
+++ b/precommit/src/main/shell/test-patch.d/maven.sh
@@ -538,7 +538,7 @@
 {
   declare filtered
 
-  if [[ ! -z "${UNIT_TEST_FILTER_FILE}" ]]; then
+  if [[ -n "${UNIT_TEST_FILTER_FILE}" ]]; then
     while read -r line || [[ -n "${line}" ]]; do
       if [[ -z $line ]]; then
         continue
diff --git a/precommit/src/test/shell/run-bats.sh b/precommit/src/test/shell/run-bats.sh
index 566f47a..24ea79e 100755
--- a/precommit/src/test/shell/run-bats.sh
+++ b/precommit/src/test/shell/run-bats.sh
@@ -17,7 +17,7 @@
 targetdir=../../../target
 mkdir -p ${targetdir}/surefire-reports ${targetdir}/tap
 
-batsexe=$(which bats) 2>/dev/null
+batsexe=$(command -v bats) 2>/dev/null
 
 if [[ -z ${batsexe} ]]; then
   echo "not ok - no bats executable found" >  "${targetdir}/tap/shelltest.tap"