How do I insert a newline/linebreak after a line using sed

Question

It took me a while to figure out how to do this, so posting in case anyone else is looking for the same.

Answer 1

This sed command:

sed -i '' '/pid = run/ a\
\
' file.txt

Finds the line with: pid = run

file.txt before

; Note: the default prefix is /usr/local/var
; Default Value: none
;pid = run/php-fpm.pid

; Error log file

and adds a linebreak after that line inside file.txt

file.txt after

; Note: the default prefix is /usr/local/var
; Default Value: none
;pid = run/php-fpm.pid


; Error log file

Or if you want to add text and a linebreak:

sed -i '/pid = run/ a\
new line of text\
' file.txt

file.txt after

; Note: the default prefix is /usr/local/var
; Default Value: none
;pid = run/php-fpm.pid
new line of text

; Error log file