example: I want to find this stretch of text: “/dev/ttyU” in the /etc directory.

I do not know where exactly the file containing this text is, so I do:
# grep -r “/dev/ttyU” /etc
and the result is:
/etc/ppp/peers/cdma:/dev/ttyUSB0 # choose modem
/etc/ppp/peers/oskar:/dev/ttyU0 # device bound to T610

which tells me there are two files called “cdma” and “oskar” in the directory of /etc/ppp/peers/ that contain the string I was searching for.

I hope that is to some good!
cheers
Kolaloka