grep -i, -n, -v & -c

Search inside files

Lesson 9 of 20 · 9 min · Module 3: Text superpowers

Logs can be thousands of lines long. grep finds the lines that matter in a fraction of a second.

Free. No installation: you write and run the code in your browser, and it is checked automatically.

By the end, you’ll be able to…

Find matching lines
Control matching with options
Quote a pattern that contains spaces

The idea

grep word file prints every line containing word. Useful options: -i ignores upper/lower case, -n adds line numbers, -v shows lines that do not match, and -c counts matching lines instead of printing them.

grep ERROR logs/app.log
grep -c INFO logs/app.log

01grep matches text, not words

grep ERROR finds any line that contains those letters, in that exact case. Add -i to also find error and Error.

grep -i error logs/app.log | head -n 1

Output

2026-10-18 08:07:03 ERROR Payment service timeout

02Quote patterns with spaces

Without quotes, grep signed in app.log looks for signed inside files named in and app.log. With quotes, "signed in" is one pattern.

03Count, number, or invert

-c answers "how many?", -n answers "where?", and -v answers "what else?". grep -v INFO quickly hides routine messages so the problems stand out.

Your turn

Show every line containing ERROR in logs/app.log, with line numbers. Then count how many lines contain signed in.

What the checks look for

Shows ERROR lines with line numbers
Counts 3 sign-ins

Try it in the editor →

Take this with you

grep finds the needle; options decide how it is reported.

Go a little further

Combine options: grep -in error logs/app.log finds every case of error and shows where.