sed sed
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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 QMAILQUEUE 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