Write a program to number the lines in a file?

You can number the lines in a file using a simple Python program. Here's an example:

<code class="language-python">with open('input.txt', 'r') as infile, open('output.txt', 'w') as outfile:
for line_number, line in enumerate(infile, start=1): outfile.write(f"{line_number}: {line}")
</code>

This code reads from input.txt, numbers each line, and writes the numbered lines to output.txt. The enumerate function is used to keep track of the line numbers, starting from 1.