diff --git a/lib/http_tools.sh b/lib/http_tools.sh
index e031486..ba073b0 100755
--- a/lib/http_tools.sh
+++ b/lib/http_tools.sh
@@ -25,7 +25,7 @@
 # response body. If the server responds with 304, return the cached 
 # resource instead.
 #
-# Usage: conditional_get [-v] [-F | -C] -d CACHE_DIR -T TMP_DIR HTTP_LOCATION
+# Usage: conditional_get [-v] [-F | -C] -d CACHE_DIR -T TMP_DIR [-L LOG_FILE] HTTP_LOCATION
 #
 # This function requires two option arguments (CACHE_DIR and TMP_DIR)
 # and a command-line argument (HTTP_LOCATION). The rest of the command
@@ -37,6 +37,7 @@
 #   -C   check the cache only
 #   -d   the cache directory (REQUIRED)
 #   -T   a temporary directory (REQUIRED)
+#   -L   a log file
 #
 # Use option -F or -C to alter the default behavior of the function.
 #
@@ -83,12 +84,18 @@
 
 conditional_get () {
 
-	if [ "$COMMAND_PATHS" != true ]; then
-		echo "ERROR: $FUNCNAME: global command paths not found" >&2
+	if [ "$_COMPATIBILITY_MODE" != true ]; then
+		echo "ERROR: $FUNCNAME: compatibility mode not enabled" >&2
 		return 2
 	fi
 	
-	local script_version="0.7"
+	# external dependency
+	if [ "$(type -t print_log_message)" != function ]; then
+		echo "ERROR: $FUNCNAME: function print_log_message not found" >&2
+		return 2
+	fi
+
+	local script_version="0.8"
 	local user_agent_string="HTTP Conditional GET client $script_version"
 	
 	local hash
@@ -113,13 +120,13 @@ conditional_get () {
 	local cache_only_mode=false
 	local cache_dir
 	local tmp_dir
-	local tmp_log_file
+	local log_file
 	local location
 	
 	local opt
 	local OPTARG
 	local OPTIND
-	while getopts ":vFCd:T:" opt; do
+	while getopts ":vFCd:T:L:" opt; do
 		case $opt in
 			v)
 				verbose_mode=true
@@ -138,6 +145,9 @@ conditional_get () {
 			T)
 				tmp_dir="$OPTARG"
 				;;
+			L)
+				log_file="$OPTARG"
+				;;
 			\?)
 				echo "ERROR: $FUNCNAME: Unrecognized option: -$OPTARG" >&2
 				return 2
@@ -149,6 +159,16 @@ conditional_get () {
 		esac
 	done
 	
+	# a log file is optional
+	if [ -z "$log_file" ]; then
+		log_file=/dev/null
+	else
+		if [ ! -f "$log_file" ]; then
+			echo "ERROR: $FUNCNAME: log file does not exist: $log_file" >&2
+			return 2
+		fi
+	fi
+
 	# a temporary directory is required
 	if [ -z "$tmp_dir" ]; then
 		echo "ERROR: $FUNCNAME: no temporary directory specified" >&2
@@ -158,8 +178,7 @@ conditional_get () {
 		echo "ERROR: $FUNCNAME: directory does not exist: $tmp_dir" >&2
 		return 2
 	fi
-	tmp_log_file="$tmp_dir/${FUNCNAME}_log"
-	$verbose_mode && echo "$FUNCNAME using temporary directory $tmp_dir" > "$tmp_log_file"
+	$verbose_mode && print_log_message "$FUNCNAME using temporary directory $tmp_dir" "$log_file"
 
 	# a cache directory is required
 	if [ -z "$cache_dir" ]; then
@@ -170,7 +189,7 @@ conditional_get () {
 		echo "ERROR: $FUNCNAME: directory does not exist: $cache_dir" >&2
 		return 2
 	fi
-	$verbose_mode && echo "$FUNCNAME using cache directory $cache_dir" >> "$tmp_log_file"
+	$verbose_mode && print_log_message "$FUNCNAME using cache directory $cache_dir" "$log_file"
 
 	# determine the URL location
 	shift $(( OPTIND - 1 ))
@@ -183,7 +202,7 @@ conditional_get () {
 		echo "ERROR: $FUNCNAME: empty URL argument" >&2
 		return 2
 	fi
-	$verbose_mode && echo "$FUNCNAME using location $location" >> "$tmp_log_file"
+	$verbose_mode && print_log_message "$FUNCNAME using location $location" "$log_file"
 	
 	#######################################################################
 	#
@@ -209,15 +228,16 @@ conditional_get () {
 	cached_content_file="$cache_dir/${hash}_content"
 
 	if $verbose_mode; then
-		echo "$FUNCNAME using cached header file: ${cached_header_file}" >> "$tmp_log_file"
-		echo "$FUNCNAME using cached content file: ${cached_content_file}" >> "$tmp_log_file"
+		print_log_message "$FUNCNAME using cached header file: ${cached_header_file}" "$log_file"
+		print_log_message "$FUNCNAME using cached content file: ${cached_content_file}" "$log_file"
 	fi
 
 	# check if the resource is cached
 	if [ -f "$cached_header_file" ] && [ -f "$cached_content_file" ]; then
 		if $cache_only_mode; then
+			print_log_message "$FUNCNAME reading cached content file: ${cached_content_file}" "$log_file"
 			/bin/cat "$cached_content_file"
-			return 0
+			return
 		fi
 		
 		conditional_get_mode=true
@@ -226,7 +246,10 @@ conditional_get () {
 		/bin/rm -f "$cached_header_file" "$cached_content_file" >&2
 		
 		# quiet failure mode
-		$cache_only_mode && return 1
+		if $cache_only_mode; then
+			echo "ERROR: $FUNCNAME: resource not cached: $location" >&2
+			return 1
+		fi
 		
 		conditional_get_mode=false
 	fi
@@ -242,9 +265,9 @@ conditional_get () {
 	tmp_stderr_file="$tmp_dir/${FUNCNAME}_curl_stderr"
 
 	if $verbose_mode; then
-		echo "$FUNCNAME using temp header file: ${tmp_header_file}" >> "$tmp_log_file"
-		echo "$FUNCNAME using temp content file: ${tmp_content_file}" >> "$tmp_log_file"
-		echo "$FUNCNAME using temp stderr file: ${tmp_stderr_file}" >> "$tmp_log_file"
+		print_log_message "$FUNCNAME using temp header file: ${tmp_header_file}" "$log_file"
+		print_log_message "$FUNCNAME using temp content file: ${tmp_content_file}" "$log_file"
+		print_log_message "$FUNCNAME using temp stderr file: ${tmp_stderr_file}" "$log_file"
 	fi
 
 	#######################################################################
@@ -298,7 +321,7 @@ conditional_get () {
 
 	# invoke curl
 	cmd="/usr/bin/curl $curl_opts $location"
-	$verbose_mode && printf "$FUNCNAME issuing curl command: %s\n" "$cmd" >> "$tmp_log_file"
+	$verbose_mode && print_log_message "$FUNCNAME issuing curl command: $cmd" "$log_file"
 	eval $cmd
 	exit_code=$?
 	if [ $exit_code -ne 0 ]; then
@@ -327,7 +350,7 @@ conditional_get () {
 		echo "ERROR: $FUNCNAME: get_response_code failed (return code: $return_code)" >&2
 		return 7
 	fi
-	$verbose_mode && printf "$FUNCNAME received response code: %d\n" "$response_code" >> "$tmp_log_file"
+	print_log_message "$FUNCNAME received response code: $response_code" "$log_file"
 
 	if [ "$response_code" = "200" ]; then
 
@@ -352,11 +375,11 @@ conditional_get () {
 		fi
 
 		if $verbose_mode; then
-			echo "$FUNCNAME downloaded ${actual_content_length} bytes" >> "$tmp_log_file"
+			print_log_message "$FUNCNAME downloaded ${actual_content_length} bytes" "$log_file"
 			if $do_conditional_get; then
-				echo "$FUNCNAME refreshing cache files" >> "$tmp_log_file"
+				print_log_message "$FUNCNAME refreshing cache files" "$log_file"
 			else
-				echo "$FUNCNAME initializing cache files" >> "$tmp_log_file"
+				print_log_message "$FUNCNAME initializing cache files" "$log_file"
 			fi
 		fi
 
@@ -368,6 +391,7 @@ conditional_get () {
 			echo "ERROR: $FUNCNAME failed copy to file $cached_header_file (exit code: $exit_code)" >&2
 			return 8
 		fi
+		print_log_message "$FUNCNAME writing cached content file: ${cached_content_file}" "$log_file"
 		/bin/cp -f "$tmp_content_file" "$cached_content_file" >&2
 		exit_code=$?
 		if [ $exit_code -ne 0 ]; then
@@ -378,9 +402,12 @@ conditional_get () {
 
 	elif [ "$response_code" = "304" ]; then
 		# quiet failure mode
-		$force_output_mode && return 1
+		if $force_output_mode; then
+			echo "ERROR: $FUNCNAME: resource not modified: $location" >&2
+			return 1
+		fi
 		
-		$verbose_mode && echo "$FUNCNAME downloaded 0 bytes (cache is up-to-date)" >> "$tmp_log_file"
+		$verbose_mode && print_log_message "$FUNCNAME downloaded 0 bytes (cache is up-to-date)" "$log_file"
 	else
 		echo "ERROR: $FUNCNAME failed with HTTP response code $response_code" >&2
 		return 9
@@ -392,8 +419,9 @@ conditional_get () {
 	#
 	#######################################################################
 	
+	print_log_message "$FUNCNAME reading cached content file: ${cached_content_file}" "$log_file"
 	/bin/cat "$cached_content_file"
-	return 0
+	return
 }
 
 #######################################################################
@@ -413,8 +441,8 @@ conditional_get () {
 
 get_response_code () {
 
-	if [ "$COMMAND_PATHS" != true ]; then
-		echo "ERROR: $FUNCNAME: global command paths not found" >&2
+	if [ "$_COMPATIBILITY_MODE" != true ]; then
+		echo "ERROR: $FUNCNAME: compatibility mode not enabled" >&2
 		return 2
 	fi
 	
@@ -455,8 +483,8 @@ get_response_code () {
 
 get_header_value () {
 
-	if [ "$COMMAND_PATHS" != true ]; then
-		echo "ERROR: $FUNCNAME: global command paths not found" >&2
+	if [ "$_COMPATIBILITY_MODE" != true ]; then
+		echo "ERROR: $FUNCNAME: compatibility mode not enabled" >&2
 		return 2
 	fi