diff --git a/README.md b/README.md
index b104763..b0d90a8 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1,4 @@
 # git-fixup
 
+Link git-fixup.sh somewhere in your `PATH`.
+Omit the `.sh` file extension to use `git fixup <commit-ref>` as a shorthand.
diff --git a/git-fixup.sh b/git-fixup.sh
new file mode 100755
index 0000000..35791f4
--- /dev/null
+++ b/git-fixup.sh
@@ -0,0 +1,21 @@
+#!/bin/bash -e
+
+if [[ $# -lt 1 ]]; then
+	(
+		echo "Usage:"
+		echo "	$0 <commit-id>"
+	) >&2
+	exit 2
+fi
+
+COMMIT_ID="$1"
+shift
+
+COMMIT_ID="$(git rev-parse "$COMMIT_ID")"
+exitcode=$?
+if [[ $exitcode -ne 0 ]]; then
+	echo >&2 "Invalid ref \"$COMMIT_ID\""
+	exit 1
+fi
+
+exec git commit -m "fixup! $COMMIT_ID" "$@"