How to assign default values to variables in Bash.
#!/usr/bin/env bash
# If $VARIABLE is not set or null, set $FOO with "defaultValue"
FOO=${VARIABLE:-defaultValue}
# If $VARIABLE is not set or null, set both $FOO and $VARIABLE with "defaultValue"
FOO=${VARIABLE:=defaultValue}
# If the first command line argument "$1" is not set or null, set $FOO with "defaultValue"
FOO=${1:-defaultValue}