UNIX for Dummies Questions & Answers

So, I am so new that I can't even call myself a unix user. I have an aptitude for programing, but I don't know the language. Anyway, here is my problem:

I have to rename a batch of files that look like:
2001_0001.asc
2001_0002.asc
.
.
.
2006_0548.asc

The names are a date code, but they need to be altered such that 2001_0001.asc becomes 2000_183.asc, 2001_0184 becomes 2001_001, and 2006_0548.asc becomes 2006_365.asc.

so, I think it should start like:

for i in `ls*`
do

then things get a bit hazy
I will now convert to semi-programing speak

split aaaa_bbbb.asc into aaaa and bbbb
interpret strings aaaa and bbbb as numbers x and y respectively
if y - 183 < 1,
then x' = x - 1 and y' = y - 183 + 365,
else x' = x and y' = y - 183.
convert numbers x and y to strings aaaa' and bbb' (note the three digits
in bbb such that y' = 1 will convert to bbb' = 001)
rename file i to aaaa'_bbb'.asc

any help would be appreciated
thanks,
-TT

Code:

for i in *.asc
 
 do 
   
   a=${i%_*}; b=${i#*_}; c=${b%.*}
 
    if [ $(expr $c - 183) -lt 1 ]
      then
         x=$(expr $a - 1); y=$(printf "%03s\n" $(expr $c - 183 + 365))
         mv $i $x"_"$y.asc
      else 
         x=$a; y=$(printf "%03s" $(expr $c - 183))
         mv $i $x"_"$y.asc
    fi
 
done

Try it first with echo, then do a rename with mv.