diff --git a/Jenkinsfile.update b/Jenkinsfile.update
index 5d255e0..a3ab6c7 100644
--- a/Jenkinsfile.update
+++ b/Jenkinsfile.update
@@ -50,23 +50,25 @@ node {
 			stage('Sources') {
 				sh '''
 					# we only need to regenerate "sources.json" if ".doi", ".scripts", or "subset.txt" have changed since we last generated it
-					needsUpdate=
-					if ! git diff --exit-code .doi .scripts; then
-						# if .doi or .scripts updated in this job ("git submodule update" above)
-						needsUpdate=1
-					else
-						# if subset.txt, .doi, or .scripts updated outside this job (merged PR, etc)
-						sourcesCommit="$(git log -1 --format='format:%H' -- sources.json)"
-						for f in subset.txt .doi .scripts; do
-							commit="$(git log -1 --format='format:%H' -- "$f")"
-							if ! git merge-base --is-ancestor "$commit" "$sourcesCommit"; then
-								needsUpdate=1
-								break
-							fi
-						done
+
+					needsBuild=
+					if [ ! -s commits.json ] || [ ! -s sources.json ]; then
+						needsBuild=1
+					fi
+
+					doi="$(git -C .doi log -1 --format='format:%H')"
+					scripts="$(git -C .scripts log -1 --format='format:%H')"
+					subset="$(sha256sum subset.txt | cut -d' ' -f1)"
+					export doi scripts subset
+					jq -n '{ doi: env.doi, scripts: env.scripts, subset: env.subset }' | tee commits.json
+					if [ -z "$needsBuild" ] && ! git diff --exit-code commits.json; then
+						needsBuild=1
 					fi
-					if [ -n "$needsUpdate" ]; then
-						.scripts/sources.sh $(cat subset.txt) > sources.json
+
+					if [ -n "$needsBuild" ]; then
+						images="$(cat subset.txt)"
+						[ -n "$images" ]
+						.scripts/sources.sh $images > sources.json
 					fi
 				'''
 			}
@@ -76,9 +78,8 @@ node {
 		}
 		stage('Commit') {
 			sh '''
-				# only commit submodule updates if our JSON has changed (see https://github.com/docker-library/meta-scripts/pull/8)
-				if ! git diff --exit-code sources.json builds.json; then
-					git add -A .
+				git add -A .
+				if ! git diff --staged --exit-code; then # commit fails if there's nothing to commit
 					git commit -m 'Update and regenerate'
 				fi
 			'''