Shell Programming and Scripting

#!/usr/bin/perl -w
unless(scalar(@ARGV)==1){
   print "error: incorrect number of arguments",
   "\n",
   "usage: linenum.pl [filename]",
   "\n";
      exit 1;
}
 
# Open function used with  filehandle and input file.
open(MY_FILE, "$ARGV[0]") or die
   "error: argument must be a file\n",
   "usage: linenum.pl [filename]\n$!\n";
 
# Check if file is a directory
if (!-f "$ARGV[0]"){
   print "error: argument must be a file",
   "\n",
   "usage: linenum.pl [filename]\n";
   exit 1;
}
 
$COUNTER=1;  # Used for printing line numbers
 
# Loop and write lines from
while($LINE <= <MY_FILE>){
   # Adds leading zeros for numbers 1 digit long
   if ($COUNTER<10){
      print "000";
   }
   # Adds leading zeros for numbers 2 digits long
   if (($COUNTER>9) && ($COUNTER<100)){
      print "00";
   }
   # Adds leading zeros for numbers 3 digits long
   if (($COUNTER>99) && ($COUNTER<1000)){
   print "0";
   }
   # Prints line number and line
   print "$COUNTER: $LINE";
   $COUNTER+=1;
}
 
exit 0;