I'm using sed in a bash script to search and replace and the correct syntax is alluding me.
Specifically I need the script to locate a line in a text file and replace the match. Below is my sample including the file to edit (source) and what I'd like the resulting file to look like. So far, I've not been able to expand ARG before writing it to the file.

thanks in advance - SeeBee

#!/bin/bash
ARGS='--channelfile /etc/mail/SA/sare-sa-update-channels.txt --gpgkey 856AA88A'

# Below line is the problem line.
sed -i 's/SAUPDATEARGS=""/SAUPDATEARGS="${ARGS}"/' update_sare

# eof

Source file: update_sare
#
# spamassassin settings
#
SAUPDATEARGS=""


Desired output: update_sare
#
# spamassassin settings
#
SAUPDATEARGS="--channelfile /etc/mail/SA/sare-sa-update-channels.txt --gpgkey 856AA88A"

Bash doesn't perform variable substitution in single quotes. Also, double quotes aren't special inside single quotes.

Notice the single quotes around the double-quoted variable. The first single quote ends the single-quoted string, and the second one starts it again. And in bash, strings that are stuck together with no space in between are concatenated.

Almost there. Judging from the OP the replacement should also be enclosed in double-quotes.
Of course you can also use the single-quote approach and have the double-quotes within the single-quotes, thus eliminating the need to escape them:

I would strongly recommend this, since in a real script, I would never put a regular expression in anything but single quotes.

thanks for the responses and the explanations. I tried options and kept at it. I finally figured out that the / in the ARGS was the issue and substituted the ^ symbol.

Works great now. thank you for the tips, they helped me get the solution!

ARGS="--channelfile /etc/mail/SA/sare-sa-update-channels.txt --gpgkey 856AA88A"

sed -i 's^SAUPDATEARGS=""^SAUPDATEARGS="'"${ARGS}"'"^' /tmp/update_sare

Please mark as SOLVED once you have a solution.