use current date(1) if $1 is missing

aoc2021/getday.sh

@@ -5,9 +5,18 @@
 if [ -z "$session" ]; then
   printf "missing session cookie\n"
 fi
-  
-curl -s https://adventofcode.com/2021/day/"$1"/input \
-  --cookie "session=$session" \
-  -o "$(printf "input/day%02d.in" "$1")"
 
-sed "s/XX/$1/g" DayXX.cs.txt > "$(printf "Day%02d.cs" "$1")"
+if [ -z "$1" ]; then
+  day=$(date +"%e" | xargs)
+else
+  day="$1"
+fi
+  
+curl -s https://adventofcode.com/2021/day/"$day"/input \
+  --cookie "session=$session" \
+  -o "$(printf "input/day%02d.in" "$day")"
+
+class=$(printf "Day%02d.cs" "$day")
+if [ ! -f "$class" ]; then
+  sed "s/XX/$1/g" DayXX.cs.txt > "$class"
+fi