[Eclug] INI-style configuration file manipulation

I wanted to ask if there are any ready-made, free, or built-in bash 
tools or utilities for scripting manipulation of configuration file 
settings -- you know, the kind of file that is ubiquitous in Linux 
whereby "key = value" pairs are specified. Did a brief Google search and 
didn't readily find anything suitable. Maybe I didn't use the right 
search keywords.

The files typically may contain comment lines, denoted by leading "#" 
characters, and there may be logical division of related groups by means 
of white space, neither of which I want to disturb or lose.

So I'm thinking of a command-line utility which takes three arguments, 
say a command appearing something like

inisetvalue filename keyname value

for example it might be run with

[berend at nova ~]$ sudo inisetvalue /etc/profile TMOUT 300

which searches in file "/etc/profile" for a key named "TMOUT" and sets 
the RHS value to "300". If the key is not found, then append that 
key-value pair to the file.

I'm sure it's pretty easy to do with perl or even awk, but I'd be hard 
pressed to believe that no one else has already produced such a utility 
that handles the various permutations like for when existing key-value 
pairs may or may not have white space surrounding the "=" sign, or the 
key name possibly including white space, or the keyname appearing in a 
comment line or other context besides the value setting per se, etc.

Regards,
Berend Tober