grep from zero

lesson 4-1 · ~9 min · 10/29

Quiz

Warm-up from lesson 3-1: which command prints an entire file to the terminal at once?

grep: find lines that match

Here is a week-one engineering task: a service misbehaves, the log file is 80,000 lines, and the one line you need contains the word "error". Scrolling is not a strategy. This is the problem grep was built for, and why it's one of the most-used commands in existence.

grep pattern file reads the file and prints only the lines containing the pattern. That's the whole idea.

grep milk shopping.txt

The flags you'll actually use:

-i: ignore case, so milk also matches Milk and MILK.
-n: show line numbers next to each match.
-v: invert, printing lines that do NOT match.
-c: just count matching lines instead of printing them.

grep is case-sensitive by default. Beginners lose ten minutes to this at least once, so watch it happen safely in the example below.

Code exercise · bash

Run it. Plain grep misses "Milk shake" (capital M). Adding -i catches it.

Code exercise · bash

The other flags on a numbers file: -n shows where matches are, -c counts them, -v flips the filter. (The | head part trims -v's long output to 3 lines. The | pipe is unit 5's topic.)

Quiz

You want every line of config.txt that does NOT contain the word "disabled". Which command?

Code exercise · bash

Your turn. The starter builds a 4-line log. Print every error line regardless of case, then print how many there are. Expected output: ``` error: disk full ERROR: no network 2 ```