Insert a line after match using sed

Brian Z's comment is a good suggestion. Indeed, using the a command in sed will append a string after the match.

However, if you directly use the string in your question as the matching string, you will find that nothing will happen. The problem is the * character inside your string, which should be escaped, since sed uses this character for regex match. In sed you can escape a character by using a backslash () in front of it. You can find some more info about the characters that should be escaped in this U&L question.

So, the sed command you need to run in order to append a string newstring in a file called file.txt after matching your string is (notice the escaped *):

sed -i -e '/set_config_object("m_lin_1_pad_agent*","io_agent_config",m_lin_1_pad_agent_cfg,0);/ a newstring' file.txt

In the above command -i is used to change the file in place. If you want to preview the changes first and ensure that the command works as it should, run the command without the -i flag.