2 files changed, 31 insertions, 0 deletions
diff --git a/deps.mk b/deps.mk
index fbae84d..b88fd94 100644
--- a/deps.mk
+++ b/deps.mk
@@ -1,4 +1,5 @@
 sources.sh = \
+	src/assert-arg \
 	src/now \
 	src/timestamp \
 	src/uuid \
diff --git a/src/assert-arg b/src/assert-arg
new file mode 100755
index 0000000..8eaaea8
--- /dev/null
+++ b/src/assert-arg
@@ -0,0 +1,30 @@
+#!/bin/sh
+set -eu
+
+usage() {
+	cat <<-'EOF'
+		Usage:
+		  assert-arg [--] STRING MESSAGE
+	EOF
+}
+
+
+if [ "${1:-}" = '--' ]; then
+	shift
+fi
+
+if [ $# -ne 2 ]; then
+	usage >&2
+	exit 2
+fi
+
+STRING="$1"
+MESSAGE="$2"
+
+if [ -z "$STRING" ]; then
+	printf 'Missing %s.\n\n' "$MESSAGE" >&2
+	cat <<-'EOF'
+		usage >&2
+		exit 2
+	EOF
+fi