I would like to be able to add lines of code to files from the command line. For example, if I choose to deny a host in my firewall script I would want to add "iptables -A INPUT -s X.X.X.X -j DROP" from the command line. I tried adding the line of code with "cat [code] >> firewall.script" but got errors. How can I do this?

- Thank you

Well unless you've got the command in a file then you'd probably be better off using echo instead of cat.
Ex:

echo "iptables newcommand" >> firewallscript


But with that you are only adding the command to the firewall script and not enabling the rule. There are two things you can do. Either run the code and add it to your firewallscript. Or you can just run the code to add the new rule. And then there is a command in iptables to create a script file of all the current rules being used. You can use this as your new firewall script. Don't remember the command by memory though but it shouldn't be too hard to find in the docs.

Couldn't you use sed and awk to achieve this... ?? At least you could use sed at the most...