A bash IRCd 🐣 💬

This is an IRC server written in bash. It is nearly "pure" bash, because it does not use any external commands. But it does cheat slightly by using some loadable builtins.

Credit to YSAP for bash-web-server. Inspiration from example.fi's IRCd service, which unfortunately hasn't published the code behind it.

Running

./ircd.sh

Connect to localhost:6667 with an IRC client. If using Irssi you need to use /connect -nocap localhost.

Try it

Try it now by connecting to irc.st:6697 channel #bash (the TLS is done outside of bash by a proxy).

Example of raw IRC session...

$ telnet irc.st 6667
Connected to irc.st.
Escape character is '^]'.
NICK test
USER test test test test
:irc.st 001 test :Welcome to IRC, test!
:irc.st 002 test :Your host is irc.st on bash-ircd v0.0.1, bash 5.3.3(1)-release
:irc.st 004 test irc.st 0.0.1 i o o
:irc.st 375 test :- irc.st Message of the Day
:irc.st 372 test :- Welcome to a pure Bash IRCd!
:irc.st 372 test :-
:irc.st 372 test :- For more details see https://dgl.cx/bash-ircd
:irc.st 372 test :-
:irc.st 372 test :- Be excellent to each other 😇
:irc.st 376 test :End of /MOTD command.
JOIN #bash
:test!user@host JOIN #bash
:irc.st 353 test = #bash :dgl dg test
:irc.st 366 test #bash :End of /NAMES list
:irc.st 329 test #bash 1761878952
:irc.st 332 test #bash :bash?
:irc.st 333 test #bash dg 0

Architecture 🏗️

The accept loadable bash builtin makes it possible to listen on a socket. See the very end of the script for that.

Then "process-client" runs as another process with stdin/stdout set to the client (this means echo to send to the client just works). Nicknames are kept by a user-${name} FIFO on the filesystem. Once the client has registered another process ("watcher") that reads from that FIFO and writes to the client is started, this means other clients can directly write to the FIFO, which in turn directly writes to the user's connection.

Channels are plain text files with a nickname per line and are simply expanded by process-client running as the sending user when needing to send to them.

Unlike the original IRC server software, which used non-blocking I/O so it could efficiently support many clients in a single process, this architecture results in at least 2 processes per connection, so is hardly scalable. However, because it uses FIFOs parts of it can be upgraded without disconnecting other clients. If you ^C ircd.sh (and keep your shell/session running) clients will stay connected, but new clients cannot connect, until you run ircd.sh again.

As each user is a FIFO, you can use this to send messages to users from outside the IRCd on the server machine:

msg-user() {
  local to="$1"
  local msg="$2"
  echo ':'"${USER}"'!user@host PRIVMSG '"${to} :${msg}" >> "user-${to}"
}

Bugs 🐛

This is full of them. Not recommended for production use, in particular the use of FIFOs means there are various cases where a misbehaving client can block sending to them, which could slowly block all messages being sent to a channel. I don't think that is fixable in pure Bash.

Security 🔐 🚨

I suspect this has some hilarious security holes. You could put stunnel in front of it to make it do TLS, but that's like putting lipstick on a pig.

Contributing 🧑‍💻

PRs welcome. This so far has been written without any AI, please disclose any usage.