Consider the file shown below whose name is `DRC.txt`.
The contents of the file are:

 #############Actions#################
 set a [project $p get_action ICV_DRC]
 action $a set_parameter "ruledeck"
 action $a set_parameter "options"
 
Now I have one variable called `ruledeck = /praj/test/drc_tsn/test1`. I need to append the value of this variable which is the path at the end of third line in the DRC.txt. I was trying to do it using `sed`, but the path is not added only the variable is. How can I solve the above problem? 

 sed -i -e '3s/$/"$ruledeck"/' DRC.txt

This command gives output as shown below:

 #############Actions#################
 set a [project $p get_action ICV_DRC]
 action $a set_parameter "ruledeck" $ruledeck
 action $a set_parameter "options"
 
I want output like this instead:

 #############Actions#################
 set a [project $p get_action ICV_DRC]
 action $a set_parameter "ruledeck" /praj/test/drc_tsn/test1
 action $a set_parameter "options"