apt-get use a proxy only if available. Shamelessly lifted from here: http://askubuntu.com/questions/53443/how-do-i-ignore-a-proxy-if-not-available

30detectproxy

# /etc/apt/apt.conf.d/30detectproxy:

# Fail immediately if a file could not be retrieved. Comment if you have a bad
# Internet connection
Acquire::Retries 0;

# undocumented feature which was found in the source. It should be an absolute
# path to the program, no arguments are allowed. stdout contains the proxy
# server, stderr is shown (in stderr) but ignored by APT
Acquire::http::ProxyAutoDetect "/etc/apt/detect-http-proxy";

# Override the default proxy, DIRECT causes a direct connection to be used
Acquire::http::Proxy {
    deb.opera.com DIRECT;
    dl.google.com DIRECT;
};

detect-http-proxy.sh

#!/bin/bash
#  - Returns a HTTP proxy which is available for use

# Author: Lekensteyn <[email protected]>

# Supported since APT 0.7.25.3ubuntu1 (Lucid) and 0.7.26~exp1 (Debian Squeeze)
# Unsupported: Ubuntu Karmic and before, Debian Lenny and before

# Put this file in /usr/local/bin/detect-http-proxy.shy and create and add the below
# configuration in /etc/apt/apt.conf.d/30detectproxy
#    Acquire::http::ProxyAutoDetect "/etc/apt/detect-http-proxy";

# APT calls this script for each host that should be connected to. Therefore
# you may see the proxy messages multiple times (LP 814130). If you find this
# annoying and wish to disable these messages, set show_proxy_messages to 0
show_proxy_messages=1

# on or more proxies can be specified. Note that each will introduce a routing
# delay and therefore its recommended to put the proxy which is most likely to
# be available on the top. If no proxy is available, a direct connection will
# be used
try_proxies=(
10.0.0.1:8000
10.0.0.2:8000
)

print_msg() {
    # \x0d clears the line so [Working] is hidden
    [ "$show_proxy_messages" = 1 ] && printf '\x0d%s\n' "$1" >&2
}

for proxy in "${try_proxies[@]}"; do
    # if the host machine / proxy is reachable...
    if nc -z ${proxy/:/ }; then
        proxy=http://$proxy
        print_msg "Proxy that will be used: $proxy"
        echo "$proxy"
        exit
    fi
done
print_msg "No proxy will be used"

# Workaround for Launchpad bug 654393 so it works with Debian Squeeze (<0.8.11)
echo DIRECT

Note: In some cases (I noticed this on bionic) if the first entry in try_proxies does not exist, the script will hang, ProxyAutoDetect will (probably) time out and give a E: Sub-process ProxyAutoDetect returned an error code (100).
My solution was to add a -w timeout to nc, so the command does not hang forever. The only con of this is, if your proxy is really slow for some reason, it will be treated as non-existing, but probably if it cannot respond in for example 3 secs to nc it does not worth using it...

Thanks for tip I was trying figure out how to set the timeout, thanks again and thanks for the creator, nice script.