Cannot echo the changes made into the files - linux

Question

I am checking if the files have been modified and I need to echo what new strings have been added. When I try this script for a single file it works, but when I iterate through multiple files in a directory it does not work as it should. Any suggestion?
#! /bin/bash
GAP=5
while :
do
FILES=/home/Desktop/*
for f in $FILES
do
len=`wc -l $f | awk '{ print $1 }'`
if [ -N $f ]; then
echo "`date`: New entries in $f:"
newlen=`wc -l $f | awk '{ print $1 }'`
newlines=`expr $newlen - $len`
tail -$newlines $f
len=$newlen
fi
sleep $GAP
done
done

Answer 1

Continuing from the comments, here is the original solution I envisioned using inotifywait (from the inotify-tools package) and an associative array. The benefit here is inotifywait will block and will not waste resources endlessly checking the line count of each file on each loop iteration. I'll work on a solution using a temporary file, but when you go that route you open yourself up to a change occurring in between loop iterations. Here is the first solution:
#!/bin/bash
watchdir="${1:-$PWD}"
events="-e modify -e attrib -e close_write -e create -e delete -e move"
declare -A lines
for i in "$watchdir"/*; do
[ -f "$i" ] && lines[$i]=$(wc -l <"$i")
done
while :; do ## watch for changes in chosen dir
fname="${watchdir}/$(inotifywait -q $events --format '%f' "$watchdir")"
newlc=$(wc -l <"$fname") ## get line count for changed file
if [ "${lines[$fname]}" -ne "$newlc" ]; then ## if changed, print
printf " lines chanaged : %s -> %s (%s)\n" \
"${lines[$fname]}" "$newlc" "$fname"
lines[$fname]=$newlc ## update saved line count for file
fi
done
Original testfile.txt
$ cat dat/tmp/testfile.txt
1 1.2
2 2.2
Example Use/Output
Script saved in watchdir.sh. Start watchdir.sh so inotifywait is watching the dat/tmp directory
$ ./watchdir.sh dat/tmp
Using a second terminal, modify file in the dat/tmp directory
$ echo "newline" >> ~/scr/tmp/stack/dat/tmp/testfile.txt
$ echo "newline" >> ~/scr/tmp/stack/dat/tmp/testfile.txt
Output of watchdir.sh running in separate terminal (or background)
$ ./watchdir.sh dat/tmp
lines chanaged : 2 -> 3 (dat/tmp/testfile.txt)
lines chanaged : 3 -> 4 (dat/tmp/testfile.txt)
Resulting testfile.txt
$ cat dat/tmp/testfile.txt
1 1.2
2 2.2
newline
newline
Second Solution Using [ -N file ]
Here is a second solution a bit closer to your first attempt. It is a less robust way to approach the solution (it will miss multiple changes between tests, etc.). Look it over and let me know if you have questions
#!/bin/bash
watchdir="${1:-$PWD}"
gap=5
tmpfile="$TMPDIR/watchtmp" ## temp file in system $TMPDIR (/tmp)
:>"$tmpfile"
trap 'rm $tmpfile' SIGTERM EXIT ## remove tmpfile on exit
for i in "$watchdir"/*; do ## populate tmpfile with line counts
[ -f "$i" ] && echo "$i,$(wc -l <"$i")" >> "$tmpfile"
done
while :; do ## loop every $gap seconds
for i in "$watchdir"/*; do ## for each file
if [ -N "$i" ]; then ## check changed
cnt=$(wc -l <"$i") ## get new line count
oldcnt=$(grep "$i" "$tmpfile") ## get old count
oldcnt=${oldcnt##*,}
if [ "$cnt" -ne "$oldcnt" ]; then ## if not equal, print
printf " lines chanaged : %s -> %s (%s)\n" \
"$oldcnt" "$cnt" "$i"
## update tmpfile with new count
sed -i "s|^${i}[,][0-9][0-9]*.*$|${i},$cnt|" "$tmpfile"
fi
fi
done
sleep $gap
done
Use/Output
Start watchdir.sh
$ ./watchdir2.sh dat/tmp
In second terminal modify file
$ echo "newline" >> ~/scr/tmp/stack/dat/tmp/testfile.txt
wait for $gap to expire (if changed twice - it will not register)
$ echo "newline" >> ~/scr/tmp/stack/dat/tmp/testfile.txt
Results
$ ./watchdir2.sh dat/tmp
lines chanaged : 10 -> 11 (dat/tmp/testfile.txt)
lines chanaged : 11 -> 12 (dat/tmp/testfile.txt)