BBLayers.conf specify path using environment variable

I am trying to configure a Yocto build and would like to be able to specify the path to a layer in bblayers.conf via an environment variable. This will allow engineers to checkout a configuration and build without needing to manually modify bblayers.conf so to specify the absolute path to their checkout of the kernel module.

I have tried exporting a variable in my .zshrc file, e.g.

export TRIALS=~/TRIALS

Unfortunately, when I attempt to access it in bblayers.conf the value is not set:

# POKY_BBLAYERS_CONF_VERSION is increased each time build/conf/bblayers.conf
# changes incompatibly
POKY_BBLAYERS_CONF_VERSION = "2"

BBPATH = "${TOPDIR}"
BBFILES ?= ""
TRIALS_DIR = "${@os.environ['TRIALS']}"

BBLAYERS ?= " \
  /home/ahk/poky/meta \
  /home/ahk/poky/meta-poky \
  /home/ahk/poky/meta-yocto-bsp \
  /home/ahk/poky/meta-amd/meta-amd-bsp \
  /home/ahk/poky/meta-amd/meta-amd-distro \
  /home/ahk/poky/meta-congatec-amd \
  /home/ahk/poky/meta-dpdk \
  /home/ahk/poky/meta-openembedded/meta-oe \
  /home/ahk/poky/meta-openembedded/meta-networking \
  /home/ahk/poky/meta-openembedded/meta-python \
  ${TRIALS_DIR}/hello-layer \
  "

The error I get is

bb.BBHandledException
ERROR: Failure expanding variable TRIALS_DIR, expression was 
    ${@os.environ['TRIALS']} which triggered exception KeyError: 'TRIALS'

How do I set an environment variable in my shell that can be accessed from belayers.conf

Solution

Bitbake tightly controls the build environment to prevent unwanted contamination. This can be overridden by including any variables that you need to access to the BB_ENV_WHITELIST.

For my example above I added the $TRIALS variable to BB_ENV_WHITELIST via

export BB_ENV_WHITELIST="$BB_ENV_WHITELIST TRIALS"

After setting BB_ENV_WHITELIST it was necessary to re-initialise bitbake via:

cd ~/poky
source oe-init-build-env

The $TRIALS environment variable could then be accessed directly in bblayers.conf:

# POKY_BBLAYERS_CONF_VERSION is increased each time build/conf/bblayers.conf
# changes incompatibly
POKY_BBLAYERS_CONF_VERSION = "2"

BBPATH = "${TOPDIR}"
BBFILES ?= ""

BBLAYERS ?= " \
  /home/ahk/poky/meta \
  /home/ahk/poky/meta-poky \
  /home/ahk/poky/meta-yocto-bsp \
  /home/ahk/poky/meta-amd/meta-amd-bsp \
  /home/ahk/poky/meta-amd/meta-amd-distro \
  /home/ahk/poky/meta-congatec-amd \
  /home/ahk/poky/meta-dpdk \
  /home/ahk/poky/meta-openembedded/meta-oe \
  /home/ahk/poky/meta-openembedded/meta-networking \
  /home/ahk/poky/meta-openembedded/meta-python \
  ${TRIALS}/hello-layer \
  "