Replace string in another file with sed from bash script

I'm trying to replace the value of an environment variable definition, if it exists, from a bash script. I know I can use sed to do this, I'm not sure, however, how to replace the value of the environment var开发者_运维百科iable?

Here's what I'd like to do:

Given a file with this line (found with grep):

export MY_ENV=SOME_VALUE

I'd like to replace SOME_VALUE with something else. How do I do that with sed?

This searches for the line beginning with export MY_ENV= and substitutes the rest of the line with NEW_VALUE:

sed 's/^\(export MY_ENV=\).*$/\1NEW_VALUE/'

sed -e "s/SOME_VALUE/$MY_ENV/"

This locates the string SOME_VALUE and substitutes the value of $MY_ENV. The double quotes are crucial; don't use single quotes unless you want $, M, Y, ... in the replacement text.