Help inserting a new line of text after matching a line of text (sed)?

Question

I need to insert a hidden HTML input tag into any form tag within a bunch of HTML files. I assume this is possible with sed, but need help forming the command.

My idea is to search for any instance of

<form

and if found, insert a line below it that contains:

<input type="hidden" name="csrf_token" value="$csrf_token" /> 

What's the best way to tackle this? I'm close with

sed -e '/<form/a\<input type="hidden" name="csrf_token" value="$csrf_token"/>'

Answer 1

Got it. Here is how it's done:

find . -name \*.html | xargs sed -i '/<form/a\<input type="hidden" name="csrf_token" value="$csrf_token" />'

Answer 2

To add something below each <form you have to add & to indicate that you want to keep the caught sequence (plus \n as you mentioned you need it) :

$ find *.html | xargs sed -ie 's/<form/&\n\<input type="hidden" name="csrf_token" value="$csrf_token"\/>/g'