Explain the `-i` flag for sed.

The -i flag in sed stands for in-place.

By default, sed is a "stream editor," meaning it transforms text and prints the result to your terminal (standard output) without changing the original file. When you add the -i flag, sed saves the changes directly to the file.

Comparison:

Without -i (Preview Only):

sed 's/Hi/Hello/' sed_test.txt

• The modified text is printed on your screen.
• If you run cat sed_test.txt afterward, the file remains unchanged.

With -i (Save Changes):

sed -i 's/Hi/Hello/' sed_test.txt

• The terminal shows no output.
• The file sed_test.txt is permanently updated with the new text.

Pro-Tip: Making Backups

Because -i overwrites your file immediately, it can be risky if your command isn't perfect. You can provide a suffix to the -i flag to create a backup original file:

sed -i.bak 's/Hi/Hello/' sed_test.txt

• This edits sed_test.txt in-place.
• It also creates a copy of the original file named sed_test.txt.bak just in case you made a mistake!

Caution: Always double-check your sed command without -i first to make sure it does exactly what you want before applying it to the file!