From: http://hacktux.com/bash/socket

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You can connect to a socket using Bash by using exec and redirecting to and from the pseudo-path /dev/tcp/<hostname>/<port> or /dev/udp/<hostname>/<port>. For instance, to connect to your localhost SSH port using TCP:

exec 3<>/dev/tcp/localhost/22

Then, use cat and echo to read or write to the socket. Here is an example read:

cat <&3
SSH-2.0-OpenSSH_5.6

Notice that there is no such file as /dev/tcp or /dev/udp. Bash interprets the pseudo-path.

ls -l /dev/tcp
ls: cannot access /dev/tcp: No such file or directory

ls -l /dev/udp
ls: cannot access /dev/udp: No such file or directory

As another example, maybe you want to download a webpage:

exec 3<>/dev/tcp/www.fedora.org/80
echo -e "GET /\n" >&3
cat <&3

Finally, let’s say you wanted to connect to an IRC server. Here is an example:

#!/bin/bash

##########################################################
# Config

NICK="mynick"
SERVER="irc.freenode.net"
PORT=6667
CHANNEL="#bashirc"

##########################################################
# Main

exec 3<>/dev/tcp/${SERVER}/${PORT}
echo "NICK ${NICK}" >&3
echo "USER ${NICK} 8 * : ${NICK}" >&3
echo "JOIN ${CHANNEL}" >&3
cat <&3

exit $?

Sources

tldp.org: Advanced Bash-Scripting Guide – Chapter 29
thesmithfam.org: Bash socket programming with /dev/tcp

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