Filter the failures

Filters keep what matters

A filter is a tool that reads text, transforms it, and writes text back out. The one you'll reach for most is grep PATTERN: it keeps only the lines that match the pattern and drops the rest.

grep ERROR access.log

grep reads access.log line by line and prints just the lines containing ERROR. Two details worth knowing:

• It's case-sensitive: ERROR and error are different patterns (grep -i ignores case).
• It matches anywhere in the line, so ERROR finds 500 ERROR as well as a line that merely mentions an error.

In access.log, the failed requests are the ones marked ERROR.