Change text in files with sed

What you'll practice

1. Preview a change. sed 's/old/new/' file prints the file with the change. The file itself stays as it was, so this is a safe preview.
2. Change the file. -i means "in place": it writes the change to the file, with no confirmation and no undo.
3. Keep a backup. Add a suffix to -i, as in -i.bak, and sed keeps the original under that name. It's a good habit on a real server.
4. Every match on a line. By default, sed only changes the first match on each line. g at the end means all of them.
5. Print some lines. -n turns off normal printing, and p prints what you ask for. It's handy for reading exact lines of a big file, such as the line number in an error message.
6. Remove comments. d deletes matching lines from the output. ^ means the start of a line, so /^#/d removes comment lines. It's a quick way to see what a long config really sets.
7. Paths have slashes. A path full of / clashes with the slashes in the command. sed lets you use another separator, such as #, so you don't have to escape each one.

See also

• Regex Tester
• find, grep, sed and awk Cheat Sheet
• Nginx Config Generator
• All lessons