How to automatically connect to open wireless network

You can add the network in /etc/network/interfaces but this will mean that network manager and apps like that will not be able to manipulate the device. You can use the interfaces file to configure the interface and it will be configured at bootup even before the users log on.

actionparsnip: That will not do. The users must be able to use network manager to change network as they please after logging in.

So far, I have come up with this zsh shellscript as a solution. Must be called with wireless network interface name as argument:

#!/bin/zsh
function randomline() {
    RANDOM=$(date +%N)
    local num_lines=0 line picked
    while read line; do
        (( (RANDOM % ++num_lines) == 0 )) && picked=$line
    done
    echo $picked
}

# Select a random open wireless network
ESSID=$(iwlist wlan0 scan |grep key:off -A2 |grep ESSID |cut -d':' -f2 | randomline)
IFACE=$1
iwconfig $IFACE mode Managed
iwconfig $IFACE essid $ESSID
dhclient $IFACE

Not pretty, but it kinda works.
If someone can suggest a more elegant solution, I am all ears.