Anyone got a logfile script? - Help a n00b

[Daniel Stuart Kelly]

What I'm looking for is a little script when run will dig into a log file and 
look for a variable.

When it finds the variable I want it to print that line and the 20 or so right 
above it to either screen or a plaintext file. 

I know it sounds simple, but I've only started learning this and I've no idea 
where to begin.. Any tips or pointers would also be helpful...

Thanks
/Daniel