The first desktop file will create a new directory /tmp/testdir if it does not already exist.
Code:
knoppix@Knoppix720cd:~$ cat /home/knoppix/.config/autostart/mkTestdir.desktop 
[Desktop Entry]
Name=mkTestdir
Exec=/bin/mkdir -p /tmp/testdir
Terminal=false
Type=Application
The second desktop file will create an empty file /tmp/testfile if it does not already exist.
Code:
knoppix@Knoppix720cd:~$ cat /home/knoppix/.config/autostart/mkTestfile.desktop 
[Desktop Entry]
Name=mkTestfile
Exec=/usr/bin/touch /tmp/testfile
Terminal=false
Type=Application