linux - Indent lines matching a word, then print them with a replacement -

i trying replace occurrences of open not open. want print original text modified lines indented , preserved in original form followed modified form.

this command not enough:

sed 's/open/not open/' file.txt 

sample input:

this first line of text text open , next line of text  

sample output:

this first line of text     text open     text not open  , next line of text  

this might work (gnu sed):

 sed '/\<open\>/!b;s/^/\t/p;s/\<open\>/not &/g' file 

any line not contain word open print normal. otherwise, insert tab @ start of line , print original line, replace open not open , print.