Shell Programming and Scripting

Code:

## NAME: topntail
## USAGE: topntail [-b N] [-e N] [FILE ...]

b=1  ## No. of lines to remove from beginning
e=1  ## No. of lines to remove from end

## Parse command-line options
while getopts b:e: opt
do
  case $opt in
      b) b=$OPTARG ;;  ## Number of lines to remove from beginning
      e) e=$OPTARG ;;  ## Number of lines to remove from end
  esac
done
shift $(( $OPTIND - 1 ))

case $b$e in   ## check for non-numeric characters in $b and $e
    *[!0-9]*) exit 5 ;;
esac

if [ $e -eq 0 ]
then
  sed "1,${b}d" "$@"  ## just remove from the top
else
  ## The buf[] array is a rotating buffer which contains the last N lines
  ## where N is the number of lines to be removed from the bottom of the file
  ## Printing starts when the line number is equal to the sum of the number
  ## of lines to be removed from top and bottom, and continues to the end
  ## of the file; the earliest line in the buffer is printed.
  ## The last N lines will not be printed.
  awk 'NR > b + e { print buf[ NR % e ] }
                  { buf[ NR % e ] = $0 }' b=$b e=$e "$@"
fi