Im very tempted to reply by giving clues and where to look since you are very new to UNIX
(how much is very new? - Why did you not post something in dummies then?)
1) Learning the hard way! (but then Im sure you will have learned alot by the time you finished...)
- a - Start by reading the script and look at all words that seem to be unix commands, the look what tha manuals say : man <command> e.g. man df
- b- Uncomment (remove the #) the line set -x and place it after the first line (To undestand this part, its man sh or ksh...)
- c - Run the script, and see what it does...
- d - Modify the script to see what happens (e.g. remove grep -v Free|... and again something else etc...)
- e - If you did not make a copy before, you are now doomed... (ask your collegues for the script again...) Now you shou have an alomst clear understanding of what is going on, if not post here in dummies what you executed, the result and what you dont understand...
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- f - Try to figure out how you will have to modify your script to achieve the requested task and do it.
- G - Come back here with your freshly written script explaining what you wanted to do, the result you got, and explain your issue.
We will gladly comment your work, your progress in understanding, and hopefully assist you through the debugging of now your own script
So courage!
..
Waiting to see your first question...

---------- Post updated at 18:18 ---------- Previous update was at 18:17 ----------

Should I bother for
2) ?

---------- Post updated at 18:40 ---------- Previous update was at 18:18 ----------

2) You could start by comparing the output of df -m versus your script then figure out what is missing...(in both cases in regard of the appointed given task...)