sed sed

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

find pattern and append line

This will add qmail-scanner-queue environment variable to qmail-smtpd/run. Notice how the replace pattern spans multiple lines using backslash to escape.

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

print everything between two patterns

Use ranges for this. A range can take a line number or a pattern.

sed -n "/begin_pattern/,/end_pattern/p" myfile.txt

search and replace in multiple files

This will perform a search and replace on all files in a directory tree:

find ./ -type f -exec sed -i -e "s/find_this_text/replace_with_this_text/g" '{}' \;

sed documentation

This is one of the best sed documents I found: Grymoire Sed