$ ls -1
10abc
1abc
2cde
3vfr
4bgt
5nhy
6muj
7gyu
8asf
9ikhj

$ ls -1
01_abc
02_cde
03_vfr
04_bgt
05_nhy
06_muj
07_gyu
08_asf
09_ikhj
10_abc

#!/bin/bash

# Pads numbers in file names if found.
# See help message for more.

# Set the environment
shopt -s extglob
IFS=""

BCYAN="${BCYAN:-\e[1;36m}"    #Define some color codes, for prettified output.
#BGREEN="${BGREEN:-\e[1;32m}"  #Use environment defaults if they exist.
#BRED="${BRED:-\e[1;31m}"
#BBLUE="${BBLUE:-\e[1;34m}"
#BMAGENTA="${BMAGENTA:-\e[1;35m}"
RESET="${RESET:-\e[0m}"

#set default padding level
pad=2


# Set up the help dialog
help(){
	local -a help
	help+=( '' 																				)
	help+=( '\tA quick script to zero-pad files that contain numbers.' 									)
	help+=( '\tIt will only pad the first number string it finds in the name, and ignores files without numbers.'	)
	help+=( '' 																				)
	help+=( "\tUsage: \t${BCYAN}${0##*/} [-n <num>] <files>${RESET}"										)
	help+=( "\t\t${BCYAN}${0##*/} -h${RESET}" 														)
	help+=( '' 																				)
	help+=( '\tUse -n to specify the number of digits to pad, from 1-9.  Defaults to '"$pad"' if not used.'		)
	help+=( '\tIf no files are given, it processes the current directory.' 								)
	help+=( '' 																				)
	help+=( '\t-h prints this help message'															)
	help+=( '' 																				)

	local IFS=$'\n'
	echo -e "${help[*]}"
}


# Process input options
# If "-h" print help & exit.
# If "-n" test for valid inputs and set "pad" variable
# Ignore anything else
while getopts ":hn:" opt; do

     case "$opt" in

          h) help >&2
             exit "2"
          ;;

          n)	if [[ "$OPTARG" =~ [^[:digit:]] ]] || (( "10#$OPTARG" < 1 )) || (( "10#$OPTARG" > 9 )); then
				echo
				echo -e "${BCYAN}invalid option: [$OPTARG].${RESET}" >&2
				echo -e "${BCYAN}-n must be an integer from 1 to 9${RESET}" >&2
				echo -e "${BCYAN}Falling back to the default of $pad${RESET}"
				echo
			else
				pad="$(( 10#$OPTARG ))"
			fi
          ;;

          \?) echo -e "${BCYAN}Invalid option: [-$OPTARG].  Ignoring.${RESET}" >&2
          ;;
     esac
done

shift $(( OPTIND - 1 )) ; OPTIND=1

# Now test for files.
# If nothing given, set input parameters to files in current directory.
if [[ -z "$*" ]]; then
	set -- ./*
fi

# Process files in input parameters
re='([^[0-9]*)([0-9]+)(.*)'

for file ; do

	# Ignore files without digits
	[[ "$file" != *[0-9]* ]] && continue

	# Split filename into prefix-digits-suffix
	[[ "$file" =~ $re ]]

	# Pad digits to desired width
	printf -v numpad "%0*d" "$pad" "${BASH_REMATCH[2]##*(0)}"

	# Add old and new filenames to arrays for final processing
	oldfile+=( "$file" )
	newfile+=( "${BASH_REMATCH[1]}${numpad}${BASH_REMATCH[3]}" )

done

# If there are any files to rename, ask to confirm the operation.
# And rename if confirmed.
if [[ -n "${oldfile[*]}" ]]; then

	echo
	echo -e "${BCYAN}Rename the following files?${RESET}"
	echo
	for i in "${!oldfile[@]}" ; do
		echo -e "${oldfile[i]/#$PWD/.}\t-->\t${newfile[i]/#$PWD/.}"
	done
	echo

	read -p "(y/n): "

	echo

	case "${REPLY,,}" in

		y*)  	for i in "${!oldfile[@]}" ; do
					mv -n "${oldfile[i]}" "${newfile[i]}"
				done
				echo
				;;

		*)		echo -e "${BCYAN}Aborting.${RESET}"
				echo
				exit 1
				;;
	esac

# Otherwise just exit.
else
	echo
	echo -e "${BCYAN}No files to rename.${RESET}" >&2
	echo -e "${BCYAN}Exiting.${RESET}" >&2
	echo
	exit 1

fi

exit 0