Sed: Stream Editing Safely

What you’ll practice

• Use sed substitution
• Address lines
• Use backup files for in-place edits

The mental model

• sed reads text, applies editing rules, and prints results.
• Test without -i first.
• If editing in place, prefer -i.bak while learning.

Try it in your terminal

cd ~/linux-practice
echo -e 'Hello\nWorld\nHello World' > text.txt
sed 's/Hello/Hi/' text.txt
sed 's/Hello/Hi/g' text.txt

sed -n '2p' text.txt
sed -n '1,2p' text.txt

sed -i.bak 's/Hello/Hi/g' text.txt
cat text.txt
cat text.txt.bak

Put it into practice

1. Replace apple with orange in a practice file.
2. Verify the .bak backup exists.

Common mistakes

• You run `sed -i 's/foo/bar/' file.txt` and realize too late that your pattern was wrong. The file is now changed. No backup, no undo. Always run the command without `-i` first. See the output. Then add `-i.bak` — it keeps a backup file so you can recover if things go sideways.
• You type `sed 's/Hello/Hi/' file.txt` and notice only the first 'Hello' on each line changed. That's the default behavior — sed replaces only the first match per line. Add the `g` flag at the end for 'global': `sed 's/Hello/Hi/g'`. The `g` stands for global, and it catches every match on the line.

Check your understanding

Explain these in your own words. If you’re unsure, revisit the examples before moving on.

1. What does the g flag do?
2. Why is -i.bak safer than -i?