diff --git a/lib/core_lib.sh b/lib/core_lib.sh
index 3c2dcc7..d470c97 100755
--- a/lib/core_lib.sh
+++ b/lib/core_lib.sh
@@ -174,71 +174,99 @@ make_temp_file () {
 # Prints a message to a log file.
 #
 # Usage:
-#   $ print_log_message [-DWIE] LOG_MESSAGE LOG_FILE
+#   $ print_log_message [-TDIWEF] LOG_MESSAGE
 #
-# Computes a timestamp and prepends the timestamp to the message
-# before appending the message to the log file.
+# Computes and prepends a timestamp to the LOG_MESSAGE
+# before appending the message to a log file.
 #
 # This script is compatible with Mac OS and GNU/Linux.
 #######################################################################
 
 print_log_message () {
 
-	# local vars
-	local prefix
+	# determine the log file
+	if [ -z "$LOG_FILE" ]; then
+		echo "FATAL: $FUNCNAME requires env var LOG_FILE" >&2
+		exit 2
+	fi
+	if [ ! -f "$LOG_FILE" ]; then
+		echo "FATAL: $FUNCNAME: file does not exist: $LOG_FILE" >&2
+		exit 2
+	fi
+
+	local error_message
 	local log_message
-	local log_file
 	local tstamp
+	
+	# compute timestamp
+	tstamp=$( /bin/date -u +%Y-%m-%dT%TZ )
+	if [ $? -ne 0 ]; then 
+		tstamp=0000-00-00T00:00:00Z
+		error_message="$FUNCNAME: unable to compute timestamp"
+		printf "%s %s %s\n" "$tstamp" ERROR "$error_message" >> "$LOG_FILE"
+	fi
+	
+	# default log level
+	local prefix=INFO
+	local log_level=3
 
+	# process command-line options and arguments
 	local opt
 	local OPTARG
 	local OPTIND
-	while getopts ":DWIE" opt; do
+	while getopts ":TDIWEF" opt; do
 		case $opt in
-			D)
-				prefix=DEBUG
+			T)
+				prefix=TRACE; log_level=5
 				;;
-			W)
-				prefix=WARN
+			D)
+				prefix=DEBUG; log_level=4
 				;;
 			I)
-				prefix=INFO
+				prefix=INFO; log_level=3
+				;;
+			W)
+				prefix=WARN; log_level=2
 				;;
 			E)
-				prefix=ERROR
+				prefix=ERROR; log_level=1
+				;;
+			F)
+				prefix=FATAL; log_level=0
 				;;
 			\?)
-				echo "ERROR: $FUNCNAME: Unrecognized option: -$OPTARG" >&2
-				return 2
+				error_message="$FUNCNAME: Unrecognized option: -$OPTARG"
+				printf "%s %s %s\n" "$tstamp" ERROR "$error_message" >> "$LOG_FILE"
 				;;
 			:)
-				echo "ERROR: $FUNCNAME: Option -$OPTARG requires an argument" >&2
-				return 2
+				error_message="$FUNCNAME: Option -$OPTARG requires an argument"
+				printf "%s %s %s\n" "$tstamp" ERROR "$error_message" >> "$LOG_FILE"
 				;;
 		esac
 	done
 	
-	# default to INFO
-	if [ -z "$prefix" ]; then
-		prefix=INFO
-	fi
-	
-	# make sure there are two command-line arguments
+	# make sure there is one command-line argument
 	shift $(( OPTIND - 1 ))
-	if [ $# -ne 2 ]; then
-		echo "ERROR: $FUNCNAME: incorrect number of arguments: $# (2 required)" >&2
-		return 2
+	if [ $# -lt 1 ]; then
+		error_message="$FUNCNAME: no arguments (1 required)"
+		printf "%s %s %s\n" "$tstamp" ERROR "$error_message" >> "$LOG_FILE"
+		log_message=NULL
+	elif [ $# -gt 1 ]; then
+		error_message="$FUNCNAME: too many arguments: $# (1 required)"
+		printf "%s %s %s\n" "$tstamp" ERROR "$error_message" >> "$LOG_FILE"
+		log_message="$1"
+	else
+		log_message="$1"
 	fi
 	
-	# get the log message
-	log_message="$1"
-	log_file="$2"
-	
-	# compute timestamp
-	tstamp=$( /bin/date -u +%Y-%m-%dT%TZ )
+	# if insufficient log level, short-circuit the log operation
+	if [ -z "$LOG_LEVEL" ]; then
+		if [ "$log_level" -gt 3 ]; then return 0; fi
+	else
+		if [ "$log_level" -gt "$LOG_LEVEL" ]; then return 0; fi
+	fi
 	
-	# prepend timestamp to the log message
-	printf "%s %s %s\n" "$tstamp" "$prefix" "$log_message" >> "$log_file"
+	printf "%s %s %s\n" "$tstamp" "$prefix" "$log_message" >> "$LOG_FILE"
 }
 
 #######################################################################