Gradle: Save current Git hash in JAR manifest

build.gradle.kts

@@ -167,6 +167,22 @@ tasks {
     }
     jar {
         archiveVersion.set(rootProject.version.toString())
+
+        manifest.attributes["Implementation-Build"] = run { // retrieve the git commit hash
+            val gitFolder = "$projectDir/.git/"
+            val digit = 7
+            /*  '.git/HEAD' contains either
+             *      in case of detached head: the currently checked out commit hash
+             *      otherwise: a reference to a file containing the current commit hash     */
+            val head = file(gitFolder + "HEAD").readText().split(":") // .git/HEAD
+            val isCommit = head.size == 1 // e5a7c79edabbf7dd39888442df081b1c9d8e88fd
+            // def isRef = head.length > 1     // ref: refs/heads/main
+            when {
+                isCommit -> head[0] // e5a7c79edabb
+                else -> file(gitFolder + head[1].trim()) // .git/refs/heads/main
+                    .readText()
+            }.trim().take(digit)
+        }
         manifest.attributes["Implementation-Version"] = project.version
     }