Fix typo and clean up stale packages

install.sh

@@ -6,22 +6,43 @@ if [ "$VERSION" == "latest" ]; then
 fi
 MAJOR_VERSION=${VERSION%%.*}
 
+# Function to run apt-get if needed
+apt_get_update_if_needed()
+{
+  if [ ! -d "/var/lib/apt/lists" ] || [ "$(ls /var/lib/apt/lists/ | wc -l)" = "0" ]; then
+    echo "Running apt-get update..."
+    apt-get update
+  else
+    echo "Skipping apt-get update."
+  fi
+}
+
 # Checks if packages are installed and installs them if not
 check_packages() {
-    if ! dpkg -s "$@" > /dev/null 2>&1; then
-        apt_get_update
-        apt-get -y install --no-install-recommends "$@"
-    fi
+  if ! dpkg -s "$@" > /dev/null 2>&1; then
+    apt_get_update_if_needed
+    apt-get -y install --no-install-recommends "$@"
+  fi
 }
 
 check_packages lsb-release wget software-properties-common gnupg
 
+# Remove any previous LLVM that may be in the base image
+# LLVM packages packaged by Ubuntu may get picked over us and
+# cause problems later.
+if dpkg -s llvm > /dev/null 2>&1; then
+  apt-get purge -y llvm && apt-get autoremove -y
+fi
+
 cd /tmp
 wget https://apt.llvm.org/llvm.sh
 chmod +x llvm.sh
 ./llvm.sh $VERSION all
 rm llvm.sh
 
+# Remove downloads to keep Docker layer small
+apt-get clean -y && rm -rf /var/lib/apt/lists/
+
 ln -sf /usr/bin/clang-${MAJOR_VERSION} /usr/bin/clang
 ln -sf /usr/bin/clang++-${MAJOR_VERSION} /usr/bin/clang++
 ln -sf /usr/bin/lld-${MAJOR_VERSION} /usr/bin/lld