Style check shell scripts

If the command "shellcheck" exists, then find all shell scripts and
run shellcheck on them.
* Use 'gcc' format with shellcheck.
* Exclude zfs-script-config.sh (which isn't really a script).

Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3428

Makefile.am

@@ -40,10 +40,22 @@ dist-hook:
 	sed -i 's/Release:[[:print:]]*/Release:      $(RELEASE)/' \
 		$(distdir)/META
 
-checkstyle:
+checkstyle: cstyle shellcheck
+
+cstyle:
 	@find ${top_srcdir} -name '*.[hc]' ! -name 'zfs_config.*' \
 		! -name '*.mod.c' -type f -exec scripts/cstyle.pl {} \+
 
+shellcheck:
+	@if type shellcheck > /dev/null 2>&1; then \
+		(find ${top_srcdir} -type f -name '*.sh.in' -o -type f \
+		 -name '*.sh'; find etc/init.d/zfs*.in -type f) | \
+		 grep -v 'zfs-script-config' | \
+		 while read file; do \
+			shellcheck --format gcc "$$file"; \
+		 done; \
+	 fi
+
 ctags:
 	$(RM) $(top_srcdir)/tags
 	find $(top_srcdir) -name .git -prune -o -name '*.[hc]' | xargs ctags