Hello
Under Windows the piping Jo mentioned above is quite easy.
You just have to attach a > (or >>) and the name of the logfile behind the tgdcli command
'>' means the file will be created, or overwritten if it already exists
'>>' means the file will be created, or if already exists, the output will be attached
e.g. tgdcli -p d:\myproject.tgd -hide -exit -r -f 1 > d:\myproject.log
This renders frame 1 of myproject without GUI and creates/overwrites d:\myproject.log
Attached you can may find an example how the output could look like. (had to use txt extension instead of log

to be able to post it here)
Regards,
Michael