Use shellcheck

.github/workflows/main.yml

@@ -45,12 +45,17 @@ jobs:
         echo "$(pwd)/bin" >> ${GITHUB_PATH}
     - name: Install aspell
       run: sudo apt-get install aspell
+    - name: Install shellcheck
+      run: sudo apt-get install shellcheck
     - name: Report versions
       run: |
         rustup --version
         rustc -Vv
         mdbook --version
         aspell --version
+        shellcheck --version
+    - name: Shellcheck
+      run: find . -name '*.sh' | xargs shellcheck
     - name: Spellcheck
       run: bash ci/spellcheck.sh list
     - name: Lint for local file paths