Automatically answer 'Yes' when using apt-get install

The problem with:

apt-get --yes install $something

is that it will ask for a manual confirmation if the package signature owner's public-key is not in the keyring, or some other conditions. to be sure it does not ask a confirmation just do this:

apt-get --yes --force-yes install $something

If you want to have these settings permanent, create a file in /etc/apt/apt.conf.d/, like /etc/apt/apt.conf.d/90forceyes with the following content:

APT::Get::Assume-Yes "true";
APT::Get::force-yes "true";

Note that if you also want to automatically go by the default answers when an interactive prompt appears, you can use DEBIAN_FRONTEND=noninteractive

Single install:

sudo DEBIAN_FRONTEND=noninteractive apt-get -y install [packagename]

E.g.:

sudo DEBIAN_FRONTEND=noninteractive apt-get -y install postfix

All updates:

sudo DEBIAN_FRONTEND=noninteractive apt-get -y update 

You can set up finer options with -o Dpkg::Options::="--force-confdef" and -o Dpkg::Options::="--force-confold".

Examples:

apt-get update
sudo DEBIAN_FRONTEND=noninteractive apt-get upgrade -y -o Dpkg::Options::="--force-confdef" -o Dpkg::Options::="--force-confold"

or

apt-get -o Dpkg::Options::="--force-confdef" -o Dpkg::Options::="--force-confold" dist-upgrade

Example of interactive prompt:

Interesting read: Perform an unattended installation of a Debian package

APT::Get::Assume-Yes "true";

APT::Get::force-yes "true";

This should at least be in /etc/apt/apt.conf and commented out. I worry Ubuntu is taking the Microsoft tack of always asking for permission.

"Are you sure?", of course I am sure, I am not a trained monkey simply typing away at the keyboard, going click happy.

Next the door will ask, "Are you sure you want to go outside?"

The oven will ask, "Are you sure you want to cook?"

The automobile will ask, "Are you sure you want to apply brakes?"

The fire extinguisher will ask, "Are you sure you want to put out the fire?"

I am sorry Dave, I can't let you do that.

HAL9000 could use a contraction but Data could not, or couldn't.

From the apt-get HOWTO

Use the -y switch: apt-get -y install packagename

apt-get -y update
apt-get -y install [package]

generally the options from the manual should work well

apt-get -y --force-yes install package

if it does not succeed you can try to use the yes command.

yes | apt-get -y --force-yes install package

did use this with my vagrant shell provisioning script

PS: in case you want non-interactive but with generally stating no then you can try this:

yes no | apt-get install package

I was looking for a way to select a non-default in a script, specifically when installing wireshark, and ended up using tmux to interact with a shell, as follows:

# Start a detached root session
sudo tmux new-session -d
# Send the command
sudo tmux send-keys "DEBIAN_FRONTEND=readline apt-get -qq install wireshark-common; exit" enter
# Wait for the tmux session to get to the interactive stage
sleep 5
# Answer the question
sudo tmux send-keys "yes" enter
# Now attach to the session so we wait for command completion
sudo tmux attach

The new (well) apt alias takes the -y (--yes) switch too:

sudo apt -y upgrade