Centos

First try running :

yum -y update bash

If this succeeds then remember to run the following command to test that the update actually fixed the problem.
env x='() { :;}; echo vulnerable' bash -c "echo this is a test"

If that fails then yum is not installed and you will need to run the commands below.

If this succeeds then remember to run the following command to test that the update actually fixed the problem.
env x='() { :;}; echo vulnerable' bash -c "echo this is a test"

Ubuntu

Run the following command :

apt-get update && apt-get install bash

If this succeeds then remember to run the following command to test that the update actually fixed the problem.
env x='() { :;}; echo vulnerable' bash -c "echo this is a test"

Debian 5 (Lenny) is no longer supported by Debian and so you are required to upgrade to a later version

Debian 6 (Squeeze)

The following command line instructions need to be run :

echo "deb http://http.debian.net/debian squeeze-lts main contrib non-free" >> /etc/apt/sources.list
apt-get update
apt-get install bash

If you get an NO_PUBKEY warning error then run the following :

apt-get install debian-archive-keyring
apt-get update
apt-get install bash

If this succeeds then remember to run the following command to test that the update actually fixed the problem.
env x='() { :;}; echo vulnerable' bash -c "echo this is a test"

Debian 7 (Wheezy)

apt-get update && apt-get install bash

env x='() { :;}; echo vulnerable' bash -c "echo this is a test"

Windows using Cygwin

The easiest method to update Cygwin is to download a fresh copy of the setup program and use that to update.
If you wish to update from within Cygwin via the command line see the FAQ for apt-cyg here : https://code.google.com/p/apt-cyg/

FreeBSD

Updating FreeBSD depends on how you have installed the Bash package.

If you have installed the "pkg" program and are using binary packages then running
pkg upgrade bash
may be all you need to do.

If you are using the more traditional ports system, in our opinion (opinions do vary on this), the best way to update ports is via portsnap

1. To download a compressed snapshot of the Ports Collection into /var/db/portsnap:
# portsnap fetch

2. When running Portsnap for the first time, extract the snapshot into /usr/ports:
# portsnap extract

3. After the first use of Portsnap has been completed as shown above, /usr/ports can be updated as needed by running:
# portsnap fetch
# portsnap update

Once the ports have been updated to actually update the ports, the portmaster tool is preferred :

If you have not installed this already you can do so via :
# cd /usr/ports/ports-mgmt/portmaster
# make install clean

Once portmaster is installed, to upgrade bash run the following command :
# portmaster shells/bash

Your Macbooks are unlikely to be affected if other shellshock attack methods have been resolved via any of the steps above.
Apple are working on a fix for their software so the instructions below should only be attempted if you have an OsX server that is internet facing and you have good backups. This information comes from an internet forum (http://apple.stackexchange.com/questions/146849/how-do-i-recompile-bash-to-avoid-shellshock-the-remote-exploit-cve-2014-6271-an) and while the steps are bona-fide, they should not be undertaken lightly.

From an OsX command line run the following via copy-and-pasting the code block below, going into Terminal and then running pbpaste | cut -c 2- | sh

$ mkdir bash-fix
$ cd bash-fix
$ curl https://opensource.apple.com/tarballs/bash/bash-92.tar.gz | tar zxf -
$ cd bash-92/bash-3.2
$ curl https://ftp.gnu.org/pub/gnu/bash/bash-3.2-patches/bash32-052 | patch -p0
$ curl https://ftp.gnu.org/pub/gnu/bash/bash-3.2-patches/bash32-053 | patch -p0
$ # See note above about ADD_IMPORT_FUNCTIONS_PATCH
$ [ "$ADD_IMPORT_FUNCTIONS_PATCH" == "YES" ] && curl http://alblue.bandlem.com/import_functions.patch | patch -p0
$ [ "$ADD_IMPORT_FUNCTIONS_PATCH" == "YES" ] || curl https://ftp.gnu.org/pub/gnu/bash/bash-3.2-patches/bash32-054 | patch -p0
$ cd ..
$ xcodebuild
$ build/Release/bash --version # GNU bash, version 3.2.54(1)-release
$ build/Release/sh --version   # GNU bash, version 3.2.54(1)-release
$ sudo cp /bin/bash /bin/bash.old
$ sudo cp /bin/sh /bin/sh.old
$ sudo cp build/Release/bash /bin
$ sudo cp build/Release/sh /bin

After this, the Bash version should be v3.2.54:

$ bash --version
GNU bash, version 3.2.54(1)-release (x86_64-apple-darwin13)
Copyright (C) 2007 Free Software Foundation, Inc.

For security, and after testing, it is recommended that you chmod -x the old versions to ensure they aren't re-used, or move them to a backup site.
$ sudo chmod a-x /bin/bash.old /bin/sh.old