SUM() {
    echo "The sum is $(($1+$2+$3+$4+$5+$6+$7+$8+$9))"
}

SUM 3 8
bash: 3+8+++++++: syntax error: operand expected (error token is "+")

sum() {
[[ -n $2 ]] && echo $(( $(tr ' ' '+' <<<"$@") ))
}

$ sum 1 2 3
6

sum() {
[[ -n $2 ]] && (IFS=+; echo $(( $* )))
}

SUM() {
 echo "The sum is $(($1+$2+${3:-0}+${4:-0}+${5:-0}+${6:-0}+${7:-0}+${8:-0}+${9:-0}))" || false
}

SUM() {
 echo "The sum is $(($1+$2+${3:=0}+${4:=0}+${5:=0}+${6:=0}+${7:=0}+${8:=0}+${9:=0}))" || false
}

sum () {
    local total=0;
    while [ $# -gt 0 ]; do
        total=$(($total + $1))
        shift
    done
    echo $total
}

sum () {
    test $1 && echo $(( $1 + $(shift; sum $@) )) || echo 0
}

SUM () {
 [ $# -lt "2" ] && return 1
 for par in $@; do
   local sum=`expr $sum + $par`
 done
 echo $sum
 return 0
}

SUM 3 4 5
SUM 3 4 5 1 1 1 1 2 3 4 5

sum(){
    t=0;
    for i in "$@"; do t=$((t + i )); done
    echo $t;
}

sum(){
 echo "$@" | perl -lane '$s+=$_ for @F; print $s'
}

sum(){
 echo "$@" | awk '{for(i=1; i<=NF; i++){k+=$i} print k}'
}

SUM() {
    echo "The sum is $(($1+$2+$[$3]+$[$4]+$[$5]+$[$6]+$[$7]+$[$8]+$[$9]))"
 }

$ SUM 4 6 5
The sum is 15

SUM() {
    echo "The sum is $((${1:-0}+${2:-0}+${3:-0}+${4:-0}+${5:-0}+${6:-0}+${7:-0}+${8:-0}+${9:-0}))"
}

${parameter:-word}
    Use Default Values. If parameter is unset or null, the expansion
    of word is substituted. Otherwise, the value of parameter is
    substituted.

$ SUM

$ SUM 1 2

$ SUM 1 1 1 1 1 1 1 1 1

SUM() {
  echo -e ${@/%/\n} | awk '{s+=$1} END {print "The sum is " s}'
}