A four-line YAML file (port: 8080, support_email, transport: tcp, report_dir: /var/reports) and the rename most people type first: sed -i 's/port/listen_port/g' app.yml. GNU sed 4.9 exited 0 and printed nothing, because -i never does. Afterwards the file held suplisten_port_email, translisten_port and /var/relisten_ports. One line was meant to change. All four did.
sed matches characters, not words. The letters p-o-r-t sit inside support, transport and report, and g replaced every occurrence. GNU sed's \b word boundary fixes the match, and one pipe shows the damage before anything is written:
sed 's/\bport\b/listen_port/g' app.yml | diff app.yml -
Without -i the file on disk stays intact, and diff prints the lines that would change. It exits 1 when it finds differences, so the same check works inside a script. Only after the diff reads right does -i go back on.
Two caveats before trusting \b. Underscore counts as a word character, so db_port stays untouched. And \b is a GNU extension: BSD sed on macOS spells word boundaries [[:<:]] and [[:>:]], and its -i requires a suffix argument (sed -i '') where GNU's is optional.
The replacement side bites too. An unescaped & means the whole match, so s/CEO/Smith & Sons/ produces Smith CEO Sons.
The bulk-replace script, with a grep -rl file list, per-file diff and an --apply gate that fails toward reading, is on the sed find-and-replace snippet page.