I’m running a Debian Squeeze web server. I’ve installed memcached on it, and configured memcached to listen on a Unix domain socket (at /tmp/memcached.sock), as it only needs to receive messages from the website, which lives on the same server.

It seems to be working fine, but I’d also like to communicate with memcached via the shell, to check that it’s doing what I think it’s doing.

memcached accepts messages via a simple ASCII protocol (if I understand correctly). If it was listening on TCP/IP, I could send messages to it via e.g. nc:

$ echo "stats settings" | nc localhost 11211

But I can’t figure out how to send that text to the domain socket instead.

On my laptop (which runs OS X Lion), both nc and telnet have options (-U and -u respectively) to use domain sockets. However, on my Debian Squeeze web server, these options aren’t present.


With netcat-openbsd, there is a -U option. If you don't have it, you probably have netcat-traditional installed instead; I'd suggest switching.

Example command: nc -U /var/run/socket

  • so netcat will create a new file at /var/run/socket? Is there a way to reuse an existing file? – Alexander Mills Jun 4 '19 at 19:22
  • 1
    @AlexanderMills If you tell it to listen (-l), it'll create the file. Otherwise, it expects the socket to already exist (and already have something listening on it). So the example command in the answer does not create the file. – derobert Jun 5 '19 at 18:51

With socat (a 'bidirectional data relay between two data channels') you can connect to the unix domain socket like this:

$ socat - UNIX-CONNECT:/tmp/memcached.sock

netcat-openbsd supports connecting to UNIX-domain sockets. Using this you can connect to either a UNIX-domain stream socket or a UNIX-domain datagram socket, and therefore you have to tell the socket's type to netcat.

for example, /dev/log file in Linux is a UNIX-domain datagram socket socket, thus nc -U /dev/log won't work. Instead use nc -uU /dev/log. Using -u along with -U tells netcat that it is a UNIX-domain datagram socket.

nc -U /tmp/socket  #Connect to UNIX-domain stream socket
nc -uU /tmp/socket #Connect to UNIX-domain datagram socket

Similarly, while using socat, use UNIX-CLIENT option. Using this option you can connect to both UNIX-domain stream and UNIX-domain datagram sockets. From its man page (man socat), "It first tries to connect and, if that fails, assumes it is a datagram socket, thus supporting both types".

socat - UNIX-CLIENT:/dev/socket #connect to UNIX-domain socket, irrespective of its type

You can use socat on Debian. To install it:

# apt-get install socat

If you happen to be on a system that only has busybox installed, it may look like it doesn't support unix domain sockets (the -U option doesn't exist) but I found the following works:

$ echo "stats slabs" | nc local:/tmp/memcached.sock
STAT active_slabs 0
STAT total_malloced 0

This can be really helpful for creating health checks for alpine containers.

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