sed sed

find part of a line, substitute, keep rest of time intact

 sed -i -e "s/^#DatabaseDirectory \(.*\)/DatabaseDirectory \\1/" myfile.txt

find pattern in a line, insert new line before it

This finds line that begins with exit(ignore leading spaces), then insert 'authdarmond start' before it.

 sed -i -e "/^\\s*exit/i/authdaemond start" /etc/rc.local;

fine pattern and append line

Note how replace pattern spans multiple lines

sed -i -e '/#!\/bin\/sh/a\
QMAILQUEUE="/var/qmail/bin/qmail-scanner-queue.pl" ; export QMAILQUEUE' /var/qmail/supervise/qmail-smtpd/run