Insert Text Before or After a Particular Line With sed

You have a text file and you want to append multiple lines after a line containing particular string, a command can be used as below

sed -i".bkp_`date +%Y%m%d`" '/\[Login\]/a RemoveIPC=no' /etc/systemd/logind.conf

Above command adds a line below the line [Login] in /etc/systemd/logind.conf file.

-i".bkp_date +%Y%m%d" will create a backup of the file with extension suffix .bkp_<current_date> and edit the original in place.

Similarly to add a line before the pattern i command can be used.

sed -i.bkp '/\[Login\]/i #Disabled removing of user IPC \
#DO NOT COMMENT RemoveIPC line' /etc/systemd/logind.conf

NOTES:

Multiple lines can be added before or after a pattern, instead of single line by using backslash( \ ) at the end of lines.
Warning: If there are multiple lines matching the address pattern, then the given text will be appended below all the lines matching the pattern. Usually this is not what you want.
Above RemoveIPC=no addition was done to prevent systemd from automatically removing IPC shared memory on user logout. This is needed when daemon services are run under normal user id.

References:

IN-Decent