abridge

#!/bin/sh
help() { cat <</help
Usage: abridge [-n] [-LINES[/BUNDLE]] [FILE...]
Show an abridged version of the given input(s)
  -n      Print line numbers with output lines
  LINES   The number of lines to show for each BUNDLE (default=10)
  BUNDLE  Number of lines per LINES (pipe default=100, file default=all lines)
/help
  version
}
version() {
  echo "abridge 0.4.20191121 copyright 2016+ by Adam Katz, GPL v2+"
  echo "https://github.com/adamhotep/misc-scripts"
  exit
}

unset RATE BUNDLE NUMBER
while getopts 0123456789/b:hl:nr:vV: arg; do
  case $arg in
    [0-9/] )	RATE="$RATE$arg" ;;
    b ) 	BUNDLE="$OPTARG" ;;
    h ) 	help ;;
    [lr] )	RATE="$OPTARG" ;;
    n ) 	ACTION='{ printf "%d:%s\n", NR, $0; }' ;;
    [Vv] )	version ;;
    * ) 	exit 2 ;;  # getopts already reported the illegal option
  esac
done
shift $((OPTIND-1))

if [ -z "$RATE" ]; then
  RATE=10
elif [ "$RATE" != "${RATE#*/*/}" ]; then
  echo "Invalid rate '$RATE' -- you cannot have more than one BUNDLE" >&2
  echo "See --help for usage" >&2
  exit 2
elif [ "$RATE" != "${RATE#*/}" ]; then
  if [ -z "$BUNDLE" ]; then
    BUNDLE="${RATE#*/}"
  else
    echo "WARNING: Ignoring given rate's bundle (${RATE#*/}) for -b $BUNDLE" >&2
  fi
  RATE="${RATE%%/*}"
fi

# what are you doing asking for zero or one line?  Shunting to head...
if [ "$RATE" -lt 2 ] && [ -z "$BUNDLE" ]; then
  exec head -n$RATE "$@"
  exit $?
fi

# ensure we get the first line, decrement and use modulo == 1 (instead of 0)
RATE=$((RATE-1))

if [ ! -t 0 ] && [ "$#" = 0 ]; then # data via pipeline
  if [ "${BUNDLE:=100}" -le "$RATE" ]; then
    cat
  else
    awk "NR % int($BUNDLE/$RATE) == 1 $ACTION"
  fi
else
  for FILE in "$@"; do
    if [ "$#" -gt 1 ]; then
      echo "==> $FILE <=="
    fi
    THIS_BUNDLE="$BUNDLE"
    if [ -z "$THIS_BUNDLE" ]; then
      THIS_BUNDLE="$(wc -l < "$FILE")"
    fi
    if [ $THIS_BUNDLE -le "$RATE" ]; then
      cat "$FILE"
    else
      awk "NR % int($THIS_BUNDLE/$RATE) == 1 $ACTION" "$FILE"
    fi
  done
fi