creating intrusion detection system with shell script - shell

Question

i have UNI work where i have to create a shell script that monitors a directory with a few text files that can inform you (can be manual check to be informed) if they have been modified. honestly im not sure where to start and im bad at linux and cant find anything to help. i can only use standard tools in ubuntu. any help would be great. thankyou
update - this is what i have so far and i need a way to verify that the values printed are the same after altering a file (if they are not the same then print whats files have been changed)
also sorry first time using the site trying to learn..
#!/bin/sh
echo "press 1 to check - press 2 to exit"
while :
do
read INPUT_STR
case $INPUT_STR in
1)
echo "checking sums"
md5sum Target/bob
md5sum Target/bec
md5sum Target/john
md5sum Target/mary
md5sum Target/mike
;;
2)
break
;;
*)
echo "incorrect input"
esac
done
echo "thankyou for using IDS"

Answer 1

You mean something like that?
#!/bin/bash
WORKDIR=/home
TARGETS=(
"$WORKDIR/bob"
"$WORKDIR/bec"
"$WORKDIR/john"
"$WORKDIR/mary"
"$WORKDIR/mike"
)
for target in "${TARGETS[#]}"; do
md5file="${target##*/}.md5"
if [[ -e "$md5file" ]]; then
md5sum --quiet -c "$md5file"
else
echo "create md5sum referenz file $md5file"
md5sum "$target"/* > "$md5file"
fi
done
At first run it creates for every directory a reference file. On the second run the directory will be compare with reference file. Modifications will be shown. When you delete one reference file, it will be created again on the next run.
explanation
# loop over the array with the targets
for target in "${TARGETS[#]}"; do
# remove all directories except the last, append .md5
# and assign it to the variable md5file
md5file="${target##*/}.md5"
# if md5file exists, use it as reference and check if
# modifications where happend
if [[ -e "$md5file" ]]; then
# use the md5file as reference and only show
# modified files, ignore ok messages
md5sum --quiet -c "$md5file"
else
# when no md5file exists
echo "create md5sum referenz file $md5file"
# use target, append /* to it and run md5sum
# redirect output to md5file (reference file for next run)
md5sum "$target"/* > "$md5file"
fi
done