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

For adding a newline after a pattern, you can also say:

sed '/pattern/{G;}' filename 

Quoting GNU sed manual:

G     Append a newline to the contents of the pattern space, and then append the contents of the hold space to that of the pattern space. 

EDIT:

Incidentally, this happens to be covered in sed one liners:

 # insert a blank line below every line which matches "regex"  sed '/regex/G' 

Answer 2

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