SHELL

assign default value in shell script

FOO=${VARIABLE:-default}  # If variable not set or null, use default.

Setting default variable value in bash

FOO="${VARIABLE:=default}"  # If variable not set or null, set it to default.

default parameter value in bash

VAR=${1:-DEFAULTVALUE}

Default parameter value in bash

function server() {
    local port="${1:-8000}"
    python -m SimpleHTTPServer "$port"
}