oleg@macbook:~ $ trap 'echo "VARIABLE-TRACE> \$variable = \"$variable\""' DEBUG
VARIABLE-TRACE> $variable = ""
oleg@macbook:~ $ variable=10
VARIABLE-TRACE> $variable = ""
VARIABLE-TRACE> $variable = "10"
oleg@macbook:~ $ variable=$((variable+variable)) && variable=0
VARIABLE-TRACE> $variable = "10"
VARIABLE-TRACE> $variable = "20"
VARIABLE-TRACE> $variable = "0"