Agent Procedure append log file

I have an agent procedure that is checking a few directories. If the directory do not exist, it will create them and then I would like it to write to the log file and append it. Currently it overwrites the log.txt file everytime. Here is my procedure

executeShellCommand echo %date% %time% %programfiles%\program already exists>>c:\log.txt

Any ideas? When I run this same command in the command prompt, it appends the log file.