use sed to in-place replace a line in a file with

Question

I want to replace a line in a file with multiple lines. I know I can use \n in the sed replace, but that is rather ugly. I was hoping to HEARDOCs.

So I can do this to replace the line with multiple lines:

$ cat sedtest
DINGO=bingo

$ sed -i -e "s/^DINGO.*$/# added by $(whoami) on $(date)\nDINGO=howdy/" sedtest

$ cat sedtest
# added by user on Sun Feb  3 08:55:44 EST 2019
DINGO=howdy

In the command I want to put the replacement in new lines so it's easier to read/understand. So far I have been using HEREDOCs when I want to add new lines to a file:

CAT << EOF | sudo tee -a file1 file2 file3
line one
line two
line three
EOF

And this has worked well for appending/adding. Is it possible to do something similar but instead use the output as the replacement in sed or is there some other way to do what I'm looking for?

Answer 1

This might work for you (GNU sed):

cat <<! | sed -i -e '/^DINGO/r /dev/stdin' -e '//d' file
# added by $(whoami) on $(date)
DINGO=howdy
!

This replaces lines starting DINGO with the here-document which is piped to stdin as file within the sed command.

An alternative:

cat <<! | sed -i -e '/^DINGO/e cat /dev/stdin' -e 's/bingo/howdy' file
# added by $(whoami) on $(date)
!

N.B. In the alternative solution the here-doc will only be read once!

Answer 2

Is this what you're trying to do?

$ awk 'NR==FNR{new=(NR>1?new ORS:"") $0;next} $0=="DINGO=bingo"{$0=new} 1' - file <<!
# added by $(whoami) on $(date)
DINGO=howdy
!
# added by user on Sun, Feb  3, 2019  8:50:41 AM
DINGO=howdy

Note that the above is using literal string operations so it'll work for any characters in the old or new strings unlike your sed script which would fail given /s or any ERE regexp character or capture groups or backreferences or... in the input (see Is it possible to escape regex metacharacters reliably with sed for details).