shell讀取參數，並設置默認值

讀取參數，設置默認值，代碼示例：

continue_prompt()
# continue_prompt
# to call: continue_prompt "string to display" default_answer
{
        _STR=$1
        _DEFAULT=$2
        # check we have the right params
        if [ $# -lt 1 ]; then
                echo "continue_prompt: I need a string to display"
                return 1
        fi      
        # loop forever
        while :
        do      
                echo -n "$_STR [Y..N] [$_DEFAULT]:"
                read _ANS
                # if user hits return set the default and determine the return
                # value, that's s: then a <space> then $
                : ${_ANS:=$_DEFAULT}
                if [ "$_ANS" == "" ]; then
                        case $_ANS in
                                Y) return 0;;
                                N) return 1;;
                        esac    
                fi      
                # user has selected something
                case $_ANS in
                        y|Y|Yes|YES) return 0
                        ;;
                        n|N|No|NO) return 1
                        ;;
                        *) echo "Answer either Y or N, default is $_DEFAULT"
                        ;;
                esac    
                echo $_ANS
        done    
}