That is true... but learning 'sed' takes quite some time!
In this case I use the famous 's' command to substitute. '/' here separates the three parts of the command (but it could be any character): 1. what to substitute (the string "GRUB_DISABLE_RECOVERY" but that could be any regular expression); 2. how to substitute (#&, where & repeats what is to be substituted) and 3. a flag (here nothing, what means 1, i.e., substitute the first match per line).
Also, I use option "-i" for an "inplace" modification (the default is to not modify the input and write the output in the terminal).
