diff --git a/README.md b/README.md
index bcf9550..f9557d3 100644
--- a/README.md
+++ b/README.md
@@ -8,12 +8,18 @@ Checks a given folder for certain file extensions recursively. All found files w
 
 The [check_folder_for_extension test folder](cffe_test) contains of 13 files, 12 of them images, which should be found by the test.
 
-function call:
+Function call:
 ```
-check_folder_for_extension ./cffe_test "*.jpg *.JPG"
+check_folder $my_folder $extension_list
+```
+
+To test the function call:
+```
+check_folder_for_extension.sh ./cffe_test "*.jpg *.JPG"
 ```
 For multiple extensions (or other parts of the file name to match) space separate the list, like shown above.
 
+
 ## Clean up
 
 Cleaning up in a script and exit with a given exit code.
@@ -27,3 +33,39 @@ For testing the function call the script with:
 ```
 clean_up.sh test
 ```
+
+## Logger
+
+[Logger](logger.sh) to write messages with individual levels to a file. 
+
+In level *debug* all logged messages are printed to stdout, too.
+
+Log levels are 
+
+5 LLDebug,     *for exceptionally verbose messages, also on std out*
+
+4 LLInfo,      *informative messages*
+
+3 LLWarning,   *warning messages*
+
+2 LLError,     *error messages*
+
+1 LLMdtry and  *mandatory messages to get minimal info in a log file*
+
+0 LLMute       *really nothing gets written*
+
+The higher the level set in `$Log_Level` to the logger, the more verbose it gets.
+
+```
+logger $LLWARNING "This is my message";
+```
+produces the following message in the log file:
+
+```
+logger 20231122_22:43:19 Warning: This is my message!
+```
+
+For testing the function call the script with just:
+```
+logger.sh
+```
diff --git a/logger.sh b/logger.sh
new file mode 100755
index 0000000..2d0b928
--- /dev/null
+++ b/logger.sh
@@ -0,0 +1,112 @@
+# Log levels for messages
+LLMute=0
+LLMdtry=1  # mandatory: messages that are always written to the log (except mute)
+LLError=2
+LLWarning=3
+LLInfo=4
+LLDebug=5
+## Actual log level of the logger used
+Log_Level=$LLInfo
+
+## base name of the logging script.
+Log_Name="${0##*/}";
+
+## Write log of the script.
+## $1 Loglevel to log the message with.
+## $2 message to log.
+function logger() {
+	local _logging_level=$1;
+	local _log_message=$2;
+	if [[ $Log_Level -lt $_logging_level ]]; then
+		return;
+	fi
+	local _now=$($DATE +%Y%m%d_%H:%M:%S);
+	if [[ $_logging_level -eq $LLInfo ]]; then
+		_type=" Info:";
+	elif [[ $_logging_level -eq $LLError ]]; then
+		_type=" ERROR!";
+	elif [[ $_logging_level -eq $LLWarning ]]; then
+		_type=" Warning:";
+	elif [[ $_logging_level -eq $LLDebug ]]; then
+		_type=" DEBUG:";
+	else
+		_type=":";
+	fi
+	printf "$Log_Name ${_now}${_type} ${_log_message}\n" >> $Log_File;
+	if [[ $Log_Level -eq $LLDebug ]]; then
+		printf "${_now}${_type} ${_log_message}\n";
+	fi
+}
+
+function test() {
+	printf "Running tests for logger().\n";
+	local _exit_val=0;
+	# Test log level mute: no logfile should be created!
+	Log_Level=$LLMute;
+	logger $LLMdtry "In level mute nothing should be logged!";
+	if [ -e "$Log_File" ]; then  # the logfile should not exist!
+		((_exit_val+=1))
+		printf "logger() test on log level Mute: FAILED!\n";
+		rm $Log_File;
+	else
+		printf "logger() test on log level Mute: OK.\n";
+	fi
+
+	# Test log level Mdtry, but log with level error: still no logfile should be created!
+	Log_Level=$LLMdtry;
+	logger $LLError "In level Mdtry an error should not be logged!";
+	if [ -e "$Log_File" ]; then  # the logfile should not exist!
+		((_exit_val+=1))
+		printf "logger() test on log level Mandatory; logging an error happend, so test FAILED!\n";
+		rm $Log_File;
+	else
+		printf "logger() test on log level Mandatory: when logging an error failed: test OK.\n";
+	fi
+
+	# Test log level Info, but log with level error: logfile should be created!
+	Log_Level=$LLInfo;
+	logger $LLError "In level Info an error should be logged!";
+	if [ -e "$Log_File" ]; then  # the logfile should now exist.
+		printf "logger() test on log level Info: logging an error succeeded, test OK.\n";
+		rm $Log_File;
+	else
+		((_exit_val+=1))
+		printf "logger() test on log level Info: logging an error FAILED!\n";
+	fi
+
+	# Test log level Info, but log with level debug: logfile should not be created!
+	Log_Level=$LLInfo;
+	logger $LLDebug "In level Info a debug message should not be logged!";
+	if [ -e "$Log_File" ]; then  # the logfile should not exist.
+		((_exit_val+=1))
+		printf "logger() test on log level Info: logging a debug message succeeded, so test FAILED!\n";
+		rm $Log_File;
+	else
+		printf "logger() test on log level Info: logging a debug message failed: test OK.\n";
+	fi
+
+	# Test log level Debug, but log with level debug: logfile should be created!
+	Log_Level=$LLDebug;
+	logger $LLDebug "In level Debug a debug message should be logged. ALso the message should appaer on std out.";
+	if [ -e "$Log_File" ]; then  # the logfile should not exist.
+		printf "logger() test on log level Debug: logging a debug message succeeded, so test OK.\n";
+		rm $Log_File;
+	else
+		((_exit_val+=1))
+		printf "logger() test on log level Debug: logging a debug message failed: test FAILED!\n";
+	fi
+
+	return $_exit_val;
+}
+
+DATE=$(which date);
+
+Run_Date=$($DATE +%Y_%m_%d)
+Log_File="${Run_Date}_${Log_Name}.log";
+
+test
+if [ $? -eq 0 ]; then
+	printf "All test succeeded.\n";
+else
+	printf "$? of 5 tests FAILED!\n";
+fi