Handling file transfers with scp

Question

I'm trying to understand how scp works "under the hood".

The only reference I found that seem to explain how things work is a blog post on the Oracle website, through web archive.

Well, there, we can find a few examples, for instance:

{ echo D0755 0 testdir; echo C0644 6 test 123;
    printf "hello\\n"; echo E; } | scp -rqt /tmp

While it does create /tmp/testdir/test 123 with the right content, it actually exit 1. We can get more info by removing the -q flag:

$ { echo D0755 0 testdir; echo C0644 6 test 123;
     printf "hello\\n"; echo E; } | scp -tr /tmp
test 123         0%    0     0.0KB/s   --:-- ETA

$ echo $?
1

So, we can see, it exits 0 and the progress is never updated.

My question is: why?

At first I thought it was because the terminal is not a SSH session, but then I actually tried to do it using golang.org/x/crypto/ssh, and I get the same results.

My guess is that both the terminal and my implementation are missing something, I don't know what it is, though.

Any pointers?

Answer 1

OK, I figured it out!

Need to send a null byte after the file:

$ { 
  echo D0755 0 testdir;
  echo C0644 6 test 123;
  printf "hello\\n";
  printf "\x00";
  echo E;  
} | scp -tr /tmp
test 123         100%    6   102.4KB/s   00:00

$ echo $?
0