I recently had a client who needed to change the phone number across the site. I didn't build the site, but I quickly found out the previous developer didn't use any variables or partials. The phone number was scattered throughout several hundred static files across the site.
Luckily I had shell access to the server, so I just needed a script to find and replace all the files.
I had already grep'ed the files which told me the phone number was littered throughout the site files:
grep -r 555-555-5555 ./
Now I just needed a way to replace that number with the new one. Sed to the rescue.
grep -rl 555-555-5555 | xargs sed -i 's/555\-555\-5555/444\-444\-4444/g'
There you go, several hundred files changed at once.