How can the AOSP build number be customized?

I am building AOSP, v4.4.2. I want to specify a part of the "Build number" string (as per Settings -> About tablet).

I know that this can be done for the Kernel by using the CONFIG_LOCALVERSION defconfig value. But I want to change "Build number", not "Kernel version" (which I was able to do successfully).

Currently, the pertinent parts of my AOSP build are like this:

# Source build variables
. build/envsetup.sh

# Specify the build target:
# * user -> limited access; suited for production (no ADB)
# * userdebug -> like "user" but with root access and debuggability; preferred for debugging
# * eng -> development configuration with additional debugging tools (with ADB)
lunch mydevice-eng

# Build it!
time m 2>&1 | tee build.out

What should I change to specify the build number?

The Makefile is what defines how the build number is created (concatenated) for a build.

---

user builds

For user builds (build target, as specified with lunch), the build number will simply be "$(BUILD_ID) $(BUILD_KEYS)", except if the DISPLAY_BUILD_NUMBER parameter is set to “true”.

eng/userdebug builds

For other builds (i.e. eng/userdebug), much more info is included:

build_desc := $(TARGET_PRODUCT)-$(TARGET_BUILD_VARIANT) $(PLATFORM_VERSION) $(BUILD_ID) $(BUILD_NUMBER) $(BUILD_VERSION_TAGS)

The Makefile source is available here: https://android.googlesource.com/platform/build/+/android-4.4.2_r1/core/Makefile#106

---

Setting build parameters in a make file

As mentioned by @eldarerathis, the BUILD_ID value in build/core/build_id.mk is where part of the build string is defined, however this might be overridden in another make (*.mk) file.

When running lunch, the value of the BUILD_ID will be printed for verification. If this value is different to that found in the build_id.mk file, then search for where it’s being set, and re-configure it. For example, if as part of lunch, the output includes “4.4.2_1.0.0-ga”:

============================================
PLATFORM_VERSION_CODENAME=REL
PLATFORM_VERSION=4.4.2
...
HOST_BUILD_TYPE=release
BUILD_ID=4.4.2_1.0.0-ga
OUT_DIR=out
============================================

...then search for “4.4.2_1.0.0-ga” to find it:

me@mybox:~/AOSP$find . -name "*.mk" | xargs grep  "4.4.2_1.0.0-ga"

Then, update the .mk file that contains the BUILD_ID. Set other build parameters accordingly.

BUILD_NUMBER, PLATFORM_VERSION and BUILD_ID are located in: build/core/version_defaults.mk. The values are only set if the build is initiated without them set.

Setting build parameters as a parameter at build time

Alternatively (and in my view preferably), these parameters can be set as part of the build command line like this:

me@mybox:~/AOSP$ time m BUILD_ID="MyBuildv1.2" BUILD_NUMBER=12345   2>&1 | tee build.out