NUMBER=$1
# One argument must be provided, otherwise don't execute
if [ "$#" != 1 ] # $# gives the nb of arguments. There MUST be space after [ and before ]
then
   echo “error: program must be executed with 1 argument.”
   exit
fi # if You already exited, ther's no need to put any else statement
while [ $NUMBER -gt 0 ]
do
   echo -n $NUMBER
   if [ $NUMBER -gt 1 ] # Take care of spaces
   then
      echo -n ", "
   fi
   NUMBER=$(($NUMBER - 1)) # ((NUMBER++)) should work
done
echo

[ "$#" != 1 ] && { echo “error: program must be executed with 1 argument.”; exit; }
for ((i=$1; i>0; i--))
do
   echo -n $i
   ((i>1)) && echo -n ", "
done
echo