Removing Lines Adjacent to Patterns

To remove the line immediately preceding and following a specific pattern, combine address ranges with flow control commands.

sed -i -e '/ERROR/{n;d}' -e '$!N;/\n.*ERROR/!P;D' system.log

Deleting the Preceding Line

When only the line before a match is required to be removed, utilize the hold space logic to buffer lines.

sed -i -e '$!N;/\n.*ERROR/!P;D' system.log

Deleting the Subsequent Line

Removing the line immediately after a match is straightforward using the next command.

sed -i '/ERROR/{n;d}' system.log

Utilizing Shell Variables

Dynamic patterns can be passed into sed using shell variable expansion. Ensure proper quoting to handle special characters.

SEARCH_TERM="CRITICAL"
sed -i -e '/'"$SEARCH_TERM"'$/{n;d}' -e '$!N;/\n.*'"$SEARCH_TERM"'$/!P;D' audit.log

Inserting Text Around Matches

The i and a commands allow insertion before or after a matched line, respectively. Escape sequences are often required for newlines.

Inserting Before a Match

To add a configuration directive before a specific domain entry:

sed -i '/deny untrusted.net/i\allow trusted_partner.org' access.conf

Inserting After a Match

To append a rule immediately following a domain restriction:

sed -i '/deny untrusted.net/a\allow trusted_partner.org' access.conf

Adding Configuration Directives

Specific server settings can be injected relative to existing parameters.

sed -i '/port/a\ port 443;' server.cfg

Multi-line Insertion

Complex blocks, such as cron jobs or multiple config lines, can be inserted using escaped newline characters (\n).

sed -i '/backup_script.sh/a\# Maintenance Window\n*/10 * * * * /usr/bin/php /var/www/cron/cleanup.php\n*/10 * * * * /usr/bin/php /var/www/cron/report.php' crontab.txt

Inserting Before Specific Parameters

Similar to appending, insertion before a parameter uses the i flag.

sed -i '/port/i\ port 443;' server.cfg

Advanced Deletion Scenarios

Removing the Previous Line via Loop

An alternative method for deleting the line prior to a match involves labeling and branching.

sed -i -e :a -e '$!N;s/.*\n\(.*port 443\)/\1/;ta' -e 'P;D' server.cfg

Deleting Range Blocks

Content between two specific markers can be removed entire.

sed -i '/<VirtualHost>/,/<\/VirtualHost>/d' $config_file

Deleting Match and Following Lines

To remove a matching line plus a fixed number of subsequent lines, use the offset syntax.

sed -i '/TempStorage/,+2d' cache.txt

Configuration Replacement

Common tasks involve toggling comments or changing socket paths in web server configurations.

sed -i 's/#php_handler/php_handler/g;s/php_handler unix:\/var\/run\/php.sock/#php_handler/' nginx.conf