getopts is not capturing arguments (file rename script)

Question

Based on Google's Style Guide, I made the following script. The script should take files like a.txt and b.txt and change them to PREFIX_1_prefix_a.txt and PREFIX_1_prefix_b.txt. However, when I enter:

$ sh rename_enum.sh -n 100 -P X -p Y

I expect X_100_Y_a.txt and X_101_Y_b.txt, but I get _1_a.txt and _2_b.txt. Can anyone tell me what I'm doing wrong?

#!/bin/bash

#verbose='false'
nflag=1
Pflag=''
pflag='_'
files='*.txt'

while getopts 'nPpf' flag; do
    case "${flag}" in
        n) nflag="${OPTARG}" ;;
        P) Pflag="${OPTARG}_" ;;
        p) pflag="_${OPTARG}_" ;;
        f) files="${OPTARG}" ;;
        #v) verbose='true' ;;
        *) error "Unexpected option ${flag}" ;;
    esac
done

for i in ${files}
do
    mv "$i" "${Pflag}${nflag}${pflag}$i"
    nflag=$((nflag + 1))
done

Answer 1

I found this man page which explains that if a flag has an argument, then it should be followed by a colon. All I had to do was change the line...

while getopts 'nPpf' flag; do

... to ...

while getopts 'n:P:p:f:' flag; do

... and I got the desired result. Thanks for the comments, though!