The /p print flag prints the replaced line twice on the terminal.

If a line does not have the search pattern and is not replaced, then the /p prints that line only once.

sed 's/unix/linux/p' filename

Use the -n option along with the /p print flag to display only the replaced lines.

Here the -n option suppresses the duplicate rows generated by the /p flag and prints the replaced lines only one time.

sed -n 's/unix/linux/p' filename