socat bidirectional communication with user-defined bash function

Question

So i want socat to persistently listen for connections, get the first x lines and reply back with a message. Ideally i want to use a user defined function to handle that logic but i couldn't find a way to achieve that.

My scenario:

client 1:

cat <(printf "line1\nline2\n") -|nc socat_server socat_port

client 2:

cat <(printf "line3\nline4\n") -|nc socat_server socat_port

I want to get: line1 and line3 and the clients a message (e.g. EXIT)

I've tried with:

exec {fd}> >(my_custom_function)
socat tcp-listen:10000,reuseaddr,fork system:"head -n1>&$fd;echo EXIT;exit"

but i get "Bad fd number". Any ways around this?

Answer 1

With {fd}> file, the value of $fd will be greater than 9.

With system:shell-code, socat invokes sh to interpret that shell code.

sh implementations are not required to support fds above 9 in their redirection operators. Implementations such as dash or mksh don't. Also note that ksh93 (one of the three shells with zsh and bash that supports that syntax) marks the fd obtained with exec {fd}> file with the close-on-exec flag, so won't be inherited by socat there.

So, here, you'd want to use a fd below 10:

exec 4> >(my_custom_function)
socat tcp-listen:10000,reuseaddr,fork system:"head -n1>&4;echo EXIT;exit"

Or invoke a shell that you know supports fds above 9 like zsh:

exec {fd}> >(my_custom_function)
socat tcp-listen:10000,reuseaddr,fork "exec:'zsh -c \"head -n1>&$fd;echo EXIT;exit\"'"

(not from ksh93).