rename file in bourne shell - scripting

Question

I am trying to write a bourne-shell script that takes a directory as a parameter and look for images named ixxx.a and rename them to ixxx_a.img where "xxx means the extension number for exemple image files would be named i001.a , i002.a ,i003.a ...)
here what I tried
mv $1/f[0-9][0-9][0-9].a $1/f[0-9][0-9][0-9]_a.img
but it says that the DEST is not a directory.
Any help would be much appreciated. Thanks.

Answer 1

for i in $1/f[0-9][0-9][0-9].a; do
mv $i ${i%.a}_a.img
done
However, this does not consider blank spaces in the file/folder names. In this case you'd have to use while so you get one file name per line (see below for bonus). There are probably dozens of other ways, including rename.
find $1 -maxdepth 1 -type f -name "f[0-9][0-9][0-9].a"|while read i; do
mv "$i" "${i%.a}_a.img"
done
Edit: Perhaps I should explain what I did there. It's called string substitution and the main use cases are these for a variable var:
# Get first two characters
${var:0:2}
# Remove shortest rear-anchored pattern - this one would give the directory name of a file, for example
${var%/*}
# Remove longest rear-anchored pattern
${var%%/*}
# Remove shortest front-anchored pattern - this in particular removes a leading slash
${var#/}
# Remove longest front-anchored pattern - this would remove all but the base name of a file given its path
# Replace a by b
${var//a/b}
${var##*/}
For more see the man page.