Searching Text with grep

Relax. We'll talk through this in plain words — no textbook voice.

What you'll walk away with

Get comfortable searching text with grep — no need to fear it
Practice running the commands yourself in the terminal
See how these commands come in handy on a real server or project

Let's think about it this way for a second

grep pattern file finds and prints the lines in file that contain pattern (grep stands for 'global regular expression print' — a confusing name, but simple to use). The -i flag makes the search case-insensitive. The -r flag searches an entire folder, including all sub-folders. The -n flag shows the line number alongside each match — so you instantly know which line an error is on inside a huge file. The -v flag does the opposite — it shows only the lines that don't contain the pattern.

Let's connect this to a real-world scenario

If you just want to see the error lines in a big log file, type grep -n "ERROR" app.log. If you want to find which files across your whole project folder still have leftover TODO comments, grep -rn "TODO" . scans every folder and prints the file name plus line number for each — this is one of the most commonly used commands during code review. Combined with a pipe (from the last chapter), grep also plays a big role in workflows like finding processes with ps aux | grep nginx.

grep pattern file = searches for and prints matching lines
-i = case-insensitive, -r = search folders recursively, -n = show line numbers
-v = shows lines that don't contain the pattern (the opposite)

Let's try it together in the terminal

bash

grep "ERROR" app.log
grep -in "error" app.log
grep -rn "TODO" .
grep -v "DEBUG" app.log > errors-only.log

You should see

Lines containing 'ERROR' will show up in the terminal along with their line numbers.

5-Minute Try-It

Write 5 lines in a notes.txt file (put TODO in one of them). Search for it with grep -n TODO notes.txt.

A Quick Word of Caution

If your grep pattern contains special characters (., *, [, etc.), they can automatically be interpreted as regex — if you want to search for a literal dot, you can use grep -F (fixed string).

Easy traps

Not quoting the grep pattern, so searching for a phrase with spaces throws an error — you need to wrap it in quotes ("phrase here")
Thinking you need -r for a single file instead of a folder (a single file doesn't need -r)

Now Try It Yourself

Write 5 lines in a notes.txt file (put TODO in one of them). Search for it with grep -n TODO notes.txt.

You'll know it worked when: Lines containing 'ERROR' will show up in the terminal along with their line numbers.