Shell Programming and Scripting

I typically take one of two approaches
1. capture the current line count to a variable in a script that remains in memory
2. capture the current line count to a file so you can read it in the next time you need it
The next time you read fileA use tail +$previous_line_count >> fileB

this is what i have right now:
INFILE=input.txt
OUTFILE=output.txt

LINECOUNT= wc -l < $INFILE
OUTCOUNT=$LINCOUNT
tail -f $INFILE + $OUTCOUNT >> $OUTFILE

Right now with the input file being the following:
1
2
3
4
5
6
7
8
9

with running this
I am only getting:
5
6
7
8
9
in the output.txt

Also how do i put this in while loop?

The idea is the input.txt file is going to be updated regularly.So with the original input being:
1
2
3
4
5
6
7
8
9
If the file is update to put:
10
12
13
at the end file how do i add that to the end of the output.txt file in a while loop

Here is an option that will allow the script to run until a time in the future

Code:

INFILE=input.txt
OUTFILE=output.txt
PREV_LINECOUNT=0
CURRENT_TIME=`date +%H%M`

# # time format HHMM
EXIT_TIME="$1"

# # keep gather data until you reach exit time
until (( $CURRENT_TIME >= $EXIT_TIME )); do
  LINECOUNT=`wc -l < $INFILE`
  CURRENT_TIME=`date +%H%M`
  tail +${PREV_LINECOUNT} $INFILE >> $OUTFILE
  PREV_LINECOUNT=`expr $LINECOUNT + 1`
  # # wait 1 min and get data again
  sleep 60
done

exit 0

If you want it to run until you tell it to stop then replace the until line with a while loop that will require you to use ctrl+c to exit the script

Code:

while true; do