Half-close netcat connection

Looks to me like your core is solid -- this should be reliable, and should quit once the file has been completely sent.

If out.txt already exists, though, then this setup might behave unexpectely. If out.txt is longer than test.txt, the last part of out.txt will remain, since socat is overwriting the file byte by byte instead of making sure the file is empty. There are a few ways to fix this, depending on what you want to do:

• OPEN:out.txt,creat,trunc will delete all the bytes in out.txt before writing to it. This option mimics what you'd expect from cp, and is probably what you want.
• OPEN:out.txt,creat,excl will refuse to write out.txt if it already exists. Use this option for extra safety.
• OPEN:out.txt,creat,append will append data to out.txt.

I also like to run md5sum on the source and destination files whenever I cobble something like this together, because of these sorts of corner cases.

As @Patrick said, this problem is usually due to netcat exiting before the response has been given. You remedy that by adding -q 2 to the command line, i.e., tell netcat to hang around 2 seconds after detecting EOF on standard input. Obviously you can make it wait some other number of seconds as well.