Problem

You have a bash script that accepts parameter(s) but you want to give a default value for a missing parameter.

Solution

For example you have a script that accepts a parameter with the year and month (ie 201703), but you want to give it a default of two months ago if the parameter is missing.

#!/bin/bash

LM=$(date -d '2 month ago' +%Y%m)
YEARMONTH=${1:-${LM}}
echo $YEARMONTH