How could I EOF `cat` through `netcat` without closing connection?

Question

I have run into a problem where I have a ncat server with the command ncat -l [port] -k -c "cat > foo; cat > bar". When I connect to this server through netcat, it allows me to write to the file foo, and after that, bar. The problem is that when the client connecting wants to stop writing foo by sending a ^D, it closes the client instead of moving on to writing to bar, so bar is never written to. How can I bypass this? What's an alternative to cat that allows me to quit with a command instead of an EOF?

Answer 1

If what you want to do is transfer 2 files with only 1 connection, you will have to somehow use a marker to separate the files in the data stream, as end-of-file is a read of length 0 that netcat will use to close the socket.

Control-D may be what you can type in a terminal to signify end-of-file, but it does not generate a character, it terminates the read with a length of 0. There is no Control-D in the data stream.

To transfer 2 files simply, you could use an existing packaging command like tar. Use nc -k -l 4458 -v -c 'tar xf - >&2' as your receiver and tar cf - foo bar as your sender. (Since the filenames come from the sender you must trust the sender).

Alternatively, if your files are simple text you could introduce a special line to separate the 2 files. Eg at the sender cat foo; echo bye; cat bar and at the receiver something like

nc -k -l 4458 -v -c 'awk -v file=foo '\''/^bye$/{ file="bar";next }{print >file}'\'

This GNU awk script holds the first filename in variable file, then when it sees the "bye" line it changes to the second filename.