So I have discovered I can do the following with scp via stdin:
Directory creation
scp -tr .
stdin -> D0755 0 <directory_name>
stdin -> \x00
File writing
scp -tr .
stdin -> C<filemode, eg. 0744> <file_size_in_bytes> <filename>
stdin -> actual file bytes
stdin -> \x00
In the man pages I can't find any mention of this, nor have I had luck with googling. Where do I find the spec for these various commands: file creation, directory creation? What else can I do? I'm curious where this is defined. I'm struggling to find where I even found this code initially. Why is there no mention of the -t flag in the scp man page?
scp transfers files by opening an SSH connection to a remote server and invoking another copy of scp on the remote system. The two scp instances communicate through the ssh channel using a simple protocol. One instance sends commands and file data; the other instance acts on the commands to store the files on its local system.
The -t option tells scp that it was invoked by another scp instance and that it'll be receiving files. There is another option -f which tells scp that it was invoked by another instance and should send files.
You'd have to ask the OpenSSH developers why the options aren't documented. One might presume that it's because they're not intended for use by humans and so not really part of the user interface.
The best online descriptions of the SCP protocol that I know of are: