#!/bin/sh
 # script input should be (sh groupsum.sh /etc/passwd)

 if [ -f $1 ] ; then
     awk -F ':' '{print $4}' $1 > /tmp/numb
     A=`awk '{s+=$1} END {print s}' /tmp/numb`
     echo $A
 else
     echo "its not a file"
 fi

#!/bin/bash
[[ -f "$1" ]] || { echo "Not a file" >&2; exit 1; }
exec <"$1"
total=0
while IFS=':' read -r _ _ _ groupid _; do
  (( total += groupid ))
done
echo "$total"

$ yourscript <(getent passwd)