recursive_eval.sh

trap "exit 1" TERM
export TOP_PID=$$

function error() {
   echo "Error: $1"
   kill -s TERM $TOP_PID
} >&2

HDIR='${HOME}'
HHDIR='${HDIR}'

recursive_eval () {
    local expr="$1"
    local loop_guard=0
    while [[ $expr =~ \$(\{[A-Za-z0-9_]+\}|[A-Za-z0-9_]+) ]]; do
        ((loop_guard++))
        if [[ "${loop_guard}" -gt 2 ]]; then
            echo $(error "Possible infinite loop, aborting.")
            exit 1
        fi
        expr=$(eval echo "${expr}")
    done
    echo "${expr}"
}

echo $(recursive_eval 'abc')
echo $(recursive_eval 'My home is: ${HOME}')
echo $(recursive_eval 'My home is: $HDIR')
echo $(recursive_eval 'My home is: ${HHDIR}')

echo "Success!"