Splits extremely large CSVs into smaller CSVs by a common date format match using awk

split-activity.awk

#!/usr/bin/awk -f

#use BEGIN sepecial character to set FS built-in variable
BEGIN {
  FS = ","
}
{
  # split DD/MM/YYY into an array of values
  date_string = substr($5, 2, 10)
  file = "exports/activity-"date_string".csv"
  print $ROW > file
}
END {
}