Monitors the file you pass in, printing the number of lines. You pass in the optional file as the first parameter (defaults to most recently updated file) and optional sleep time in seconds as the second argument.

monitor.sh

#!/bin/sh
#
# Monitors the file you pass in, printing the number of lines.  You
# pass in the optional file as the first parameter (defaults to most
# recently updated file) and optional sleep time in seconds as the
# second argument.
#
# Examples:
#
# % ./monitor                  # -- use the most recently updated file
#
# % ./monitor some-file.txt 1  # -- updates every 1 second
# 
# % ./monitor some-file.txt    # -- updates every 5 seconds
# some-file.txt : 1496
# some-file.txt : 1501 [1 line(s)/sec]
# some-file.txt : 1519 [3 line(s)/sec]
#
#

file=$1
shift
SLEEP=${1-5}
shift

if [ "$file" == "" ]; then
    file=$(ls -l | awk '{print $(NF-1) " " $NF}' | \
	sort -r | grep -v "^total" | head -1 | awk '{print $2}')
    echo "Using $file"
fi

last=-1
while [ 1 ]; do
    num=$(wc -l $file | awk '{print $1}')
    if [ "$last" != "-1" ]; then
	lines_per_sec=$(( ($num-$last)/$SLEEP ))
	msg="$num [${lines_per_sec} line(s)/sec]"
    else
	msg=$num
    fi
    last=$num
    echo "$file : $msg"
    sleep $SLEEP
done