The following script make me able on sco unix openserver to sort big files. The problem was that sort command is limited to 1 MegaByte memory and file I had to sort have 6 MegaByte size.

# BigSort
#
# sort big file
#
# $1 input file
# $2 output file
#
# equ   sort -t";" -k 1,1 $1 > $2

BigSort()
{
if [ -s $1 ]; then
  rm $1.split.* > /dev/null 2>&1
  split -l 2500 -a 5 $1 $1.split.
  rm $1.sort > /dev/null 2>&1
  touch $1.sort1
  for FILE in `ls $1.split.*`
  do
    echo "sort $FILE"
    sort -t";" -k 1,1 $FILE > $FILE.sort
    sort -m -t";" -k 1,1 $1.sort1 $FILE.sort > $1.sort2
    mv $1.sort2 $1.sort1
  done
  mv $1.sort1 $2
  rm $1.split.* > /dev/null 2>&1
else
  # work for empty file !
  cp $1 $2
fi
}