How would you read a log file and print only the lines that contain error messages?

You can read the file line by line and filter for the keyword 'ERROR' (or any pattern that identifies an error). For example, in Python:

with open('app.log', 'r') as f:

for line in f:
    if 'ERROR' in line.upper():
        print(line.rstrip())

This approach streams the file, uses minimal memory, and outputs only the error lines. Adjust the keyword to match your log’s error indicator.