Conditionally add or append to a file in linux script

Question

I need to modify a file via a script.
I need to do the following:
IF a specific string does NOT exist then append it.

So I created the following script:

#!/bin/bash  
if grep -q "SomeParameter A" "./theFile"; then  
echo exist  
else  
   echo doesNOTexist  
   echo "# Adding parameter" >> ./theFile    
   echo "SomeParameter A" >> ./theFile    
fi

This works but I need to make some improvements.
I think it would be better if I checked if "SomeParameter" exists and then see if it is followed by "A" or "B". If it is "B" then make it "A".
Otherwise append the string (like I do) BUT BEFORE the start of the last block of comments.
How could I do this?
I am not good in scripting.
Thanks!

Answer 1

First, change any SomeParameter lines if they already exist. This should work with lines like SomeParameter or SomeParameter B, with any number of extra spaces:

sed -i -e 's/^ *SomeParameter\( \+B\)\? *$/SomeParameter A/' "./theFile"

Then add the line if it doesn't exist:

if ! grep -qe "^SomeParameter A$" "./theFile"; then
    echo "# Adding parameter" >> ./theFile    
    echo "SomeParameter A" >> ./theFile    
fi