Added tests for helper functions (#51)

Makefile

@@ -1,15 +1,21 @@
+# Distros to test install.sh on
+
 unsupported := alpine voidlinux/voidlinux-musl ubuntu_16.04 debian_9 linuxmintd/mint18-amd64 fedora_26 opensuse/leap_42.3
 supported := ubuntu_18.04 debian_10 linuxmintd/mint19-amd64 fedora_27 fedora_41 opensuse/leap_15 opensuse/tumbleweed rockylinux_9 manjarolinux/base
 distros := $(unsupported) $(supported)
 
-test: $(distros)
+test: shellcheck ut $(distros)
+
+# Output per target, use bash in recipes, fail on errors, be quiet by default
 
 MAKEFLAGS := -rRO
 SHELL := $(shell command -v bash)
 .SHELLFLAGS := -eEo pipefail -c
 .ONESHELL:
 $(V).SILENT:
-.PHONY: clean shellcheck test $(distros) $(distros:%=%_clean)
+.PHONY: clean shellcheck test ut $(distros) $(distros:%=%_clean)
+
+# Test install.sh on different distributions via docker
 
 $(distros): distro = $(subst _,:,$@)
 $(distros) $(distros:%=%_clean): log = $(subst /,_,$(subst _,:,$(@:%_clean=%))).log
@@ -35,10 +41,32 @@ $(supported):
 	    printf "Failed\n\n" && tail -v "$(log)" && false
 	fi
 
-shellcheck:
-	shellcheck -e SC2086 install.sh
-
 clean: $(distros:%=%_clean)
 
 $(distros:%=%_clean):
 	rm -f "$(log)"
+
+# Test helper functions from install.sh in an alpine container
+
+uts := ut_available ut_first_of ut_show ut_newer ut_supported
+.PHONY: $(uts)
+ut: $(uts)
+
+ut_available: test = available ls && ! available foo
+ut_first_of: test = [ $$(first_of ls foo) = ls ] && [ $$(first_of foo bar ls) = ls ]
+ut_show: test = show ls 2>&1 >/dev/null|grep -qFx "+ ls"
+ut_newer: test = newer 1.12 1.9 && newer 0.1.1 0.0.2 && ! newer "" non-empty
+ut_supported: test = supported foo 1.12 1.9
+
+$(uts): ut_%:
+	printf "Testing $*()... "
+	docker run --rm -v "$$PWD/install.sh:/install.sh" alpine \
+	    sh -$(if $(V),x,)ec 'source <(grep -x "\w\w*() {.*}" /install.sh) && $(test)'
+	echo OK
+
+# Analyze install.sh with shellcheck
+
+shellcheck:
+	printf "Analyzing install.sh with shellcheck... "
+	shellcheck -e SC2086 install.sh
+	echo OK