Shells can perform variable expansion, this is really useful for default argument variables in shell functions.
Here is a simple example of assigning a variable from an argument or defaulting to running a different shell command:
clog() {
log_date=${1-`date +%Y-%m-%d`}
git log --after="$log_date 00:00" --before="$log_date 23:59"
}
This allows me to write
clog to default to today’s date or
clog 2018-10-02 to look at the logs from yesterday
The date in the example clog 2018-10-02 would come through as $1 in the function, so ${1-`date`} means run this shell command if there is no $1.
*references:
https://www.tldp.org/LDP/abs/html/parameter-substitution.html#USAGEMESSAGE