diff --git a/ui/build/build.go b/ui/build/build.go
index 14d23a7..9d5c330 100644
--- a/ui/build/build.go
+++ b/ui/build/build.go
@@ -15,11 +15,13 @@
 package build
 
 import (
+	"fmt"
 	"io/ioutil"
 	"os"
 	"path/filepath"
 	"sync"
 	"text/template"
+	"time"
 
 	"android/soong/ui/metrics"
 )
@@ -29,6 +31,7 @@
 func SetupOutDir(ctx Context, config Config) {
 	ensureEmptyFileExists(ctx, filepath.Join(config.OutDir(), "Android.mk"))
 	ensureEmptyFileExists(ctx, filepath.Join(config.OutDir(), "CleanSpec.mk"))
+	ensureEmptyDirectoriesExist(ctx, config.TempDir())
 
 	// Potentially write a marker file for whether kati is enabled. This is used by soong_build to
 	// potentially run the AndroidMk singleton and postinstall commands.
@@ -56,6 +59,31 @@
 	} else {
 		ctx.Fatalln("Missing BUILD_DATETIME_FILE")
 	}
+
+	// BUILD_NUMBER should be set to the source control value that
+	// represents the current state of the source code.  E.g., a
+	// perforce changelist number or a git hash.  Can be an arbitrary string
+	// (to allow for source control that uses something other than numbers),
+	// but must be a single word and a valid file name.
+	//
+	// If no BUILD_NUMBER is set, create a useful "I am an engineering build
+	// from this date/time" value.  Make it start with a non-digit so that
+	// anyone trying to parse it as an integer will probably get "0".
+	buildNumber, ok := config.environ.Get("BUILD_NUMBER")
+	if ok {
+		writeValueIfChanged(ctx, config, config.OutDir(), "file_name_tag.txt", buildNumber)
+	} else {
+		var username string
+		if username, ok = config.environ.Get("BUILD_USERNAME"); !ok {
+			ctx.Fatalln("Missing BUILD_USERNAME")
+		}
+		buildNumber = fmt.Sprintf("eng.%.6s.%s", username, time.Now().Format("20060102.150405" /* YYYYMMDD.HHMMSS */))
+		writeValueIfChanged(ctx, config, config.OutDir(), "file_name_tag.txt", username)
+	}
+	// Write the build number to a file so it can be read back in
+	// without changing the command line every time.  Avoids rebuilds
+	// when using ninja.
+	writeValueIfChanged(ctx, config, config.SoongOutDir(), "build_number.txt", buildNumber)
 }
 
 var combinedBuildNinjaTemplate = template.Must(template.New("combined").Parse(`
@@ -246,8 +274,6 @@
 	// checkCaseSensitivity issues a warning if a case-insensitive file system is being used.
 	checkCaseSensitivity(ctx, config)
 
-	ensureEmptyDirectoriesExist(ctx, config.TempDir())
-
 	SetupPath(ctx, config)
 
 	what := evaluateWhatToRun(config, ctx.Verboseln)