--- a/Makefile	Mon Nov 30 10:49:15 2020 +0200
+++ b/Makefile	Fri Jan 08 00:00:03 2021 +0200
@@ -2,7 +2,8 @@
 ################################################################
 # Standard GNU Makefile settings.
 
-SHELL = /bin/sh -eu
+SHELL = /bin/bash
+.SHELLFLAGS = -xeuc
 export PATH := /bin:/usr/bin:${PATH}
 
 # Disable built in pattern rules.
@@ -107,12 +108,12 @@
 .PHONY: install
 install:
 	for item in $(OVERRIDDEN_ITEMS); do \
-		if [ -f $$item ]; then \
+		if [[ -f $$item ]]; then \
 			$(INSTALL_DATA) $$item $(HOME)/$$item; \
 		fi; \
-		if [ -d $$item ]; then \
+		if [[ -d $$item ]]; then \
 			for file in `find $$item`; do \
-				if [ -d $$file ]; then \
+				if [[ -d $$file ]]; then \
 					mkdir -p $(HOME)/$$file; \
 					continue; \
 				fi; \
@@ -124,7 +125,7 @@
 	chmod a+x ~/.fvwm/FvwmKbdd.pl ~/.fvwm/xinit
 	command -v fvwm || exit 0; \
 	for f in /etc/xdg/menus/*.menu; do \
-		[ -f "$$f" ] || continue; \
+		[[ -f "$$f" ]] || continue; \
 		fname=$${f%.menu}; \
 		fname=$${fname##*/}; \
 		python .fvwm/xdg2fvwm.py Menu-$$fname $$f >~/.fvwm/$$fname.hook; \
@@ -133,16 +134,16 @@
 	done
 	mkdir -p ~/.screenshot
 	for item in $(IFNONEXIST_ITEMS); do \
-		if [ -f $$item -a ! -f $(HOME)/$$item ]; then \
+		if [[ -f $$item ]] && ! [[ -f $(HOME)/$$item ]]; then \
 			$(INSTALL_DATA) $$item $(HOME)/$$item; \
 		fi; \
-		if [ -d $$item ]; then \
+		if [[ -d $$item ]]; then \
 			for file in `find $$item`; do \
-				if [ -d $$file ]; then \
+				if [[ -d $$file ]]; then \
 					mkdir -p $(HOME)/$$file; \
 					continue; \
 				fi; \
-				if [ ! -f $(HOME)/$$file ]; then \
+				if [[ ! -f $(HOME)/$$file ]]; then \
 					$(INSTALL_DATA) $$file $(HOME)/$$file; \
 				fi; \
 			done; \