rules.mk: Add targets to check shell syntax

Add the target sh-syntax-check, which triggers bash syntax linting with shellcheck This commit stops short of making target all depend on it, because the fallout is impressive.
Signed-off-by: Jan Lindemann <jan@janware.com>

make/defs.mk

@@ -98,6 +98,14 @@ endif
 ifndef PRISTINE_MAKE
   PRISTINE_MAKE             := umask $(MINIMAL_UMASK) && MAKEFLAGS=$(MINIMAL_MAKEFLAGS) LD_LIBRARY_PATH=$(MINIMAL_LD_LIBRARY_PATH) PATH=$(MINIMAL_PATH) $(MAKE)
 endif
+ifndef SHELLCHECK
+  SHELLCHECK                := $(shell $(WHICH) shellcheck)
+endif
+ifdef SHELLCHECK
+  SHELLCHECK                += -f gcc
+else
+  SHELLCHECK                := bash -n
+endif
 
 # ----- Function definitions
 

make/rules.mk

@@ -16,6 +16,7 @@ distclean: clean
 	%.this \
 	allclean \
 	checkroot \
+	check-syntax \
 	collect \
 	coreclean \
 	depclean \
@@ -39,6 +40,7 @@ distclean: clean
 	rccclean \
 	remake \
 	runclean \
+	sh-check-syntax \
 	textclean \
 	tmpclean \
 	uicclean \
@@ -430,4 +432,19 @@ find:
 collect:
 	bash $(JWB_SCRIPT_DIR)/collect-installed-pkg-changes.sh -t $(TOPDIR)
 
+check: check-syntax
+check-syntax: sh-check-syntax
+sh-check-syntax:
+
+ifneq ($(strip $(EXE_SH)),)
+$(info EXE_SH=>$(EXE_SH)<)
+sh-check-syntax: sh-check-syntax-rule
+sh-check-syntax-rule: $(EXE_SH)
+	$(SHELLCHECK) $^
+	set -e; for script in $^; do \
+		echo "Checking $$script" ;\
+		$(SHELLCHECK) $$script ;\
+	done
+endif
+
 endif # ifndef JW_PKG_RULES_MK_INCLUDED