Commands

sed

sed [-i] 's/old/new/' <file>

Risk: Careful

What it does

Changes text on its way through. With -i it edits the file itself.

When to use it

To fix one word in a settings file. Test before you restart.

Example

$ sudo sed -i 's/lisen/listen/' /etc/nginx/sites-available/default

Learn it in: The service that won't start · Official docs