Shell Editing

Change hello to goodbye

This will create a duplicate file called greetings.txt.old so you can compare.

sed -i '.old' 's/Hello/Goodbye/' greetings.txt

Undo if mistakes

mv greetings.txt.old greetings.txt
y

Change without creating a duplicate

The empty string says to override the current file.

sed -i '' 's/Hello/Goodbye/' greetings.txt