AWK: Add a primary key column

# Input

200-579-1|64-18-6|formic acid|CH2O2
200-827-9|74-98-6|propane|C3H8
201-172-1|79-05-0|propionamide|C3H7NO
201-333-6|81-20-9|2-nitro-m-xylene|C8H9NO2

awk '{ print NR "|" $0 }' input.txt > output.txt

# Output

1|200-579-1|64-18-6|formic acid|CH2O2
2|200-827-9|74-98-6|propane|C3H8
3|201-172-1|79-05-0|propionamide|C3H7NO
4|201-333-6|81-20-9|2-nitro-m-xylene|C8H9NO2