To search text in files with grep, the core command is grep 'pattern' file, and grep -r 'pattern' . searches every file under the current folder. That covers most of what you reach for: finding where a function is defined, which config sets a value, or what is filling up a log. A few flags do the heavy lifting: -i ignores case, -n adds line numbers, -l prints only the filenames that matched, and -v flips it to show the lines that do not match. It is on every Linux and macOS box already. Here's the everyday command, the flags worth committing to memory, and how to grep a whole project without drowning in noise.
The short answer
grep 'pattern' file searches one file, grep -r 'pattern' . searches
everything below the current folder. Add -i to ignore case, -n for line
numbers, -l for filenames only, -v to invert the match.
Search one file
grep "error" app.log Every line containing error prints. Quote the pattern when it has spaces or
shell characters in it.
Search a whole project
grep -rin "todo" src/ -r recurses into subfolders, -i ignores case (so TODO and todo both
hit), -n prints the line number next to each match. This is the one I type
most.
Just the filenames
When you only care which files match, not the lines:
grep -rl "API_KEY" . 
The flags worth knowing
A short list carries you a long way: -w for whole words, -v to show lines
that do not match, -c to count hits instead of printing them, -C 3 for
three lines of context around each match, and -F to search a literal string
when the pattern has regex characters you want taken at face value.
Frequently asked questions
How do I search every file in a folder and its subfolders?
Add -r (recursive): "grep -r pattern .". The dot means start from here. Add -n for line numbers and -i to ignore case, so "grep -rin pattern ." is the combination most people settle on.
How do I show only the names of files that match?
Use -l: "grep -rl pattern .". Instead of every matching line, you get one filename per file that contains the pattern, which is what you want when you are about to open or edit them.
How do I search for a whole word, not a substring?
Use -w. Searching for cat normally also matches category and concatenate; "grep -w cat" matches only the standalone word. Pair it with -i when case does not matter.
grep treats my search term like a pattern and matches odd things. Why?
By default grep reads the term as a basic regular expression, so characters like dot, star and brackets have special meaning. To search for a literal string, add -F (fixed): it finds the exact text and treats those characters as plain.