feat: Add shellcheck linter

Makefile

@@ -13,6 +13,7 @@ help:
 	@echo
 	@echo 'help	print this help message'
 	@echo 'tests	run tests from $(tests_dir)'
+	@echo 'lint	run shellcheck'
 	@echo 'docs	build documentation from $(docs_dir)'
 	@echo
 	@echo See README.rst for more info.
@@ -21,6 +22,10 @@ tests:
 	# See bats(1), https://bats-core.readthedocs.io/en/latest/index.html
 	for test in $(tests_dir)/*.bats; do bats --verbose-run --print-output-on-failure "$$test"; done
 
+lint:
+	shellcheck $(src_dir)/baf || true
+	shellcheck $(src_dir)/lib/* || true
+
 docs: builddir
 	# See rst2man(1), rst2html(1), https://docutils.sourceforge.io/docs/index.html
 	rst2man $(docs_dir)/baf.1.rst | gzip -9 > $(docs_build_dir)/baf.1