#!/usr/bin/awk -f
# HP 2013-07-29
# HP 2019-07-06 using $0, ignoring 1st line
# HP 2019-07-06 print FIRST & LAST, options for position of counter
# usage:
#   ls img_????.jpg | chkcntpos
#   ls Track_*.gpx | chkcntpos sta=7 len=3
BEGIN {
  sta = 5
  len = 4
}
{
  prv = cur
  cur = $0
  if (NR == 1) {
    print "FIRST: " cur
  }
  else if (substr(prv,sta,len)+1 != substr(cur,sta,len)+0) {
    print " JUMP: " prv " -> " cur
  }
}
END {
  print "LAST:  " cur
}