using a shell script to automatically modify a text file

Im looking to do the following with a sh shell script.


modify /etc/sysctl.conf by adding a set of records surrounded by a marker of some kind.

writing to the file is no big deal really just a series of appended writes to the bottom of the file will work with the ">" operator
or was it the ">>" which ever one does not overwrite whats already there. (Its been awhile, I need to look that up).

But the problem is the rewrite function for when changes are made.

suppose the write function adds the following to the bottom of the /etc/sysctl.conf file:

#scripted_changes_by_program - start
#timestamp
sysctl.knob.0=VALUE0
sysctl.knob.1=VALUE1
...

#scripted_changes_by_program - end


now suppose I want to edit those values, I could go through with sed/awk and look for the knob and change its value in the file.

But I want to allow for the addition of sysctl.knob=VALUE pairs over time.

so to me the best solution is to wipe out everything in the file between the boundaries created by the "scripted_changes_by_program" phrase  (including the boundaries themselves), and then just write the code block again at the new end of the file, where ever that may be.

Catch ONE, since sysctl.conf is a system file, and there may be other changes in it for other things, I dont want to alter anything not between the boundaries.

catch TWO, I'd like (for safety, if nothing else) to NOT write to the IN USE sysctl.conf file.

so I figured that I should make a temp copy of the file, work on that, and swap the temp in for the original, and delete the temp when we are finished.

I know something like this is possible, but I just cant wrap my head around how to do it correctly.

Your help appreciated.