Outputs the contents of the file specified to stdout beginning at the line after the last line it saw.

lastline.sh

#!/usr/bin/env bash

if [ "$#" -ne 3 ]; then
  echo "Usage: `basename $0` file filtercmd outputcmd"
  exit 85
fi

FILE=$1
FILTERCMD=$2
OUTPUTCMD=$3
LAST_LINE_FILE="`dirname $FILE`/.lastline.`basename $FILE`"
CURRENT_LINE=`wc -l < $FILE`
if [ -f "$LAST_LINE_FILE" ]; then
  LAST_LINE=$(cat $LAST_LINE_FILE)
  if [ "$LAST_LINE" -gt "$CURRENT_LINE" ]; then
    LAST_LINE="0"
  else
    LAST_LINE=$(($LAST_LINE+1))
  fi
else
  LAST_LINE="0"
fi

OUTPUT=$(tail -n+$LAST_LINE "$FILE")
echo $CURRENT_LINE > $LAST_LINE_FILE

if [ -n "$OUTPUT" ]; then
  FILTEREDOUTPUT=$(echo "$OUTPUT" | eval $FILTERCMD)
  if [ -n "$FILTEREDOUTPUT" ]; then
    echo "$FILTEREDOUTPUT" | eval $OUTPUTCMD
    exit $?
  fi
fi