#!/bin/ksh
IAm=${0##*/}			# basename of this script
TempDir="/tmp/$IAm.$$"		# script working directory
ThumbDir='/var/media/ftp'	# Flashdrive mount point

BlackList='alertlist.txt'	# List of names on the blacklist
LookScript='look.sh'		# Script to look for nearby WiFi users
Log="$TempDir/log"		# Log of issues to report on this loop iteration
OldFile="$TempDir/old"		# LookScript output from previous loop iteration
NewFile="$TempDir/new"		# LookScript outupt from current loop iteration

# Remove script working directory on script termination.
trap 'rm -rf "$TempDir"' EXIT

# Verify that needed files are present and copy them from flashdrive to work
# area.
if ! cd "$ThumbDir"
then	printf '%s: Thumb Drive not mounted at "%s".  Aborting.\n' \
	    "$IAm" "$ThumbDir" >&2
	exit 1
fi
if [ ! -f "$BlackList" ] || [ ! -x "$LookScript" ]
then	ls -l "$BlackList" "$LookScript"
	printf '%s: "%s" or "%s" not found in "$ThumbDir".  Aborting.\n' \
	    "$IAm" "$BlackList" "$LookScript" "$ThumbDIr" >&2
	exit 2
fi

# Create a temporary work area (not on the flashdrive)...
if ! mkdir -p -m 0700 "$TempDir"
then	printf '%s: Could not create work area ("%s").  Aborting.\n' \
	    "$IAm" "$TempDir" >&2
	exit 3
fi
if ! cp "$BlackList" "$LookScript" "$TempDir"
then	printf '%s: Copying files to work araa failed.  Aborting.\n' "$IAm" >&2
	exit 4
fi

# And note the hour when the above files were copied.
hr=$(date +%H)

# Note also that each of the following executions of look.sh, grep, mv, and
# mailx shouldbe checked for successful completion, but doing that is left as
# an exercise for the reader...

# Note that whenever $Log is a non-empty file, a mail message will be sent...
# so the following will send a message the first time we go through the
# following loop even if no blacklisted users are nearby when this script
# starts.
date +"$IAm started on %x at %X" > "$Log"

# Get initial list of nearby blacklisted users.  (Note that this list is going
# to $OldFile instead of $NewFile so it can serve as the comparison point for
# the 1st time through the following loop.)
"$TempDir/$LookScript" active | grep -Fix -f "$TempDir/$BlackList" > "$OldFile"

# And, if it is not empty, log them as newly near...
[ -s "$OldFile" ] && while read -r name
do	printf '%s joined\n' "$name"
done < "$OldFile" >> "$Log"

# Now run the body of our script every four seconds.  Note that it would be
# more efficient to change directory to $TempDir, but staying on the flashdrive
# reduces accesses to the flashdrive.
while sleep 4
do	# Check to see if we need to update the blacklist at the top of every
	# hour...
	now=$(date +%H)
	if [ $now != $hr ]
	then	# We have just started a new hour, check to see if the blacklist
		# has changed.
		[[ "$BlackList" -nt "$TempDir/$BlackList" ]] &&
		    cp "$BlackList" "$TempDir/$BlackList" &&
		    echo  "$BlackList update installed." >> "$Log"
		# If your shell does not accept the syntax of the above test
		# command list, # comment it out and uncomment the following
		# slower find command that will make more accesses to the
		# flashdrive than the above AND list.
		# find . -name "$BlackList" -newer "$TempDir/$BlackList" \
		#     -exec cp {} "$TempDir/$BlackList" \; \
		#     -exec echo "$BlackList update installed." \; >> "$Log"
		hr=$now
	fi

	# Find blacklisted users currently on nearby WiFi.
	"$TempDir/$LookScript" active | grep -Fix -f "$TempDir/$BlackList" \
	    > "$NewFile"

	if [ -s "$OldFile" ] && [ -s "$NewFile" ]
	then	# Gather list of blacklisted users who are no longer nearby.
		grep -Fivx -f "$NewFile" "$OldFile" | while read -r name
		do	printf '%s left\n' "$name"
		done >> "$Log"

		# Gather list of blacklisted users who were not seen last time.
		grep -Fivx -f "$OldFile" "$NewFile" | while read -r name
		do	printf '%s joined\n' "$name"
		done >> "$Log"
	elif [ -s "$OldFile" ]
	then	# Gather list of blacklisted users who are no longer nearby.
		while read -r name
		do	printf '%s left\n' "$name"
		done < "$OldFile" >> "$Log"
	elif [ -s "$NewFile" ]
	then	# Gather list of blacklisted users who were not seen last time.
		while read -r name
		do	printf '%s joined\n' "$name"
		done < "$NewFile" >> "$Log"
	fi

	# If anything is different on this iteration, report our findings.
	if [ -s "$Log" ]
	then	mailx -s "Nearby blacklisted users" user@company.com < "$Log"
		# Clear the log file and move the current user list to be the
		# previous user list for the next iteration of the loop.
		> "$Log"
		mv "$NewFile" "$OldFile"
	fi
done