More filtering

Here's an easy way to turn a two argument bash script into a filter. It needs to be invoked like io_assignment IN $1 $2. The DEFAULT option is for setting what the default behavior is with only one argument. Sticking this into a library is probably recommended (hrm, this may be harder than it sounds as bash doesn't appear to respect variables set in a function in an included file.)

io_assignment () {
    DEFAULT="$1"
    shift
    case "$#" in
      0) IN_FILE="/dev/stdin"
         OUT_FILE="/dev/stdout"
         ;;
      1) case DEFAULT in
             IN) IN_FILE="$1"
                 OUT_FILE="/dev/stdout"
                 ;;
             OUT) IN_FILE="/dev/stdin"
                  OUT_FILE="$1"
                 ;;
         esac
         ;;
      2) IN_FILE="$1"
         OUT_FILE="$2"
         ;;
    esac
}

Also shell seems to be capable of allowing you to write code that needs a million error checks which I have not done.