Answer by Rui F Ribeiro for Write a program that read from a file and print the line with line number

Using awk it is enough to do:

awk ' { print NR". " $1 } ' < file.txt

$1 is the string of text in the input, and NR an internal awk variable that tracks the current line number (e.g. Number of Records).

8 Powerful Awk Built-in Variables – FS, OFS, RS, ORS, NR, NF, FILENAME, FNR