#!/usr/bin/env bash
 
target="id"   # the field you want to sum
target_idx=   # the column number of that field
sum=0         # the sum that wal found so far
 
{
  # first, just read the header...
  IFS=, read -r -a header
  for idx in "${!header[@]}"; do  # and look for the id field in it
    [[ ${header[$idx]} = $target ]] && { target_idx=$idx; break; }
  done
  [[ $target_idx ]] || { echo "ERROR: No $target field found" >&2; exit 1; }
 
  # then, iterate over other lines
  while IFS=, read -r -a line; do
    sum=$(( sum + ${line[$target_idx]} ))
  done
}
 
echo "$sum"
 

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

aWQsbmFtZQoxMCxhYmMKMjAseHl6CjMwLGRlZgo=

id,name
10,abc
20,xyz
30,def