Mass File Renamer: By Number Sequence

#!/bin/bash
## Change file names into a sequence of numbers.
NO_ARGS=0 
E_OPTERROR=65
DEBUG=0
FNAME="f"
maxdepth=1
directory=`pwd`
set -o noglob

#---- Functions ------------#

usage() {
 printf "Usage: `basename $0` [-D directory] [-M depth]  [-s startseq] [-e endseq] [-p pattern] [-d] [-X] [filename(s)]\n"
 cat << EOF
        -D : starting directory. Default=current directory.
        -M : max depth to recurse subdirectories (default=1), eg 1,2
        -s : starting sequence, eg 1,01,001,0001
        -e : ending sequence, eg 7, 12,1001. No effect with leading zeroes. Can be used with a tag
             Eg -s 001 -e image20 -p "file" ==>  files will be named image001 to image020, replacing the word "file" in the filename
        -p : pattern to be substituted with the sequence. Can be simple shell patterns. eg [0-9], [a-z]
             Use ".*" to specify ALL files to be changed.
        -d : Debugging mode. Used to list all files to be changed, without changes taking effect
        -X : Only check directory names, not file names
        filename(s) : file names eg *.txt. If omitted, default to all files
        Example: 
            1) -s 1 -e 12 -p "file" -d *.txt ===> change all name of text files 
                 with pattern "file" with sequence starting from 1 till 12
            2) -s 001 -e 12 -p ".*"  ==> change ALL files to sequence 001 to 012
            3) -s 00001 -e "file.30" -p "file" "*.txt" =====> change all text files with word "file" to file.00001 to file.00030
            4) -s 01 -e "40.images" -p "file" "*.jpg" ====> change all jpg files with word "file" to "01.images" till "40.images"
        
EOF

}
#----------------------------#

if [ $# -eq "$NO_ARGS" ]  # Script invoked with no command-line args?
then
   usage
   exit $E_OPTERROR        # Exit and explain usage, if no argument(s) given.
fi  


while getopts ":D:s:e:p:M:Xd" Option
do
  case $Option in
    D     ) directory=$OPTARG
            ;;
    s     ) startseq=$OPTARG;;
    e     ) endseq=$OPTARG;;
    p     ) 
            pattern=$OPTARG
            [ -z "${pattern}" ] && pattern=".*"    
            ;;
    M     ) 
            maxdepth=$OPTARG            
            case ${maxdepth} in
                0 | *[a-z]*| "") maxdepth=1;; 
            esac    
            ;;
    d     ) DEBUG=1 ;;
    X     ) FNAME="d";;
    *     ) echo "Unimplemented option chosen."
            exit;;
  esac
done
shift $(($OPTIND - 1))

# get last argument

argument=$#
if [ $argument -eq 0 ];then
    ext="*"
else    
    ext=$(eval echo \"\${${argument}}\")
fi

find "${directory}" -maxdepth "${maxdepth}" -type "${FNAME}" -name "$ext" -printf "%f:%h:%p\n" | \
awk -F":" -v startseq="$startseq" -v endseq="$endseq" -v pattern="$pattern" -v debug="$DEBUG" 'BEGIN{
    q="\042"    
    match(endseq,"[0-9]") # get starting index of number 
    index_num=RSTART
    match(endseq,"[^0-9]") #get starting index of not number
    index_alpha=RSTART
    numendseq=endseq
    strendseq = endseq
    gsub(/[^0-9]+/,"",numendseq) #get number
    gsub(/[0-9]+/,"",strendseq) #get besides number
    endseq=numendseq
    lenseq = length(endseq)
    lenstseq = length(startseq)    
    # generate sequence
    for ( i = startseq+0 ; i<=endseq+0 ; i++ ){
        _="0"lenstseq 
        num[++d]=sprintf("%0*d" , _ ,i)
    }
}

$1 ~ pattern{    
    ++e 
    if ( index_alpha > index_num ) {        
        s = gsub (pattern, num[e] strendseq , $1 )    
    }else {
        s = gsub (pattern, strendseq num[e] , $1 )
    }
    if ( debug) {
        print "mv -u " q $3 q " "q $2 "/" $1 q        
    }else {
        cmd = "mv -u " q $3 q " "q $2 "/" $1 q
        system(cmd)
    }
    if ( d==e ) { exit }
    
}'