Replacing first occurance of a match with sed

Question

I am doing a find and replace with sed, replacing the BASH variable $a (when at the start of a new line) with the BASH variable $b:

Answer 1

This might work for you:

 sed -i 'x;/^./{x;b};x;/^'"$a"'/{s//'"$b"'/;h}' file

Or:

 sed -i ':a;$!{N;ba};s/^'"$a/$b"'/m' file