How do I check if a package is installed on my server?

Are you looking to know about all dpkg commands with options? Have a read from the below link.

15 dpkg commands to Manage Debian based Linux Servers

To List all Installed Packages

Here less is a simple text reader used to scroll through the list of packages in a new buffer that opens in the existing terminal window. The list will not be mixed with other terminal commands and output. Hit q to return to the terminal prompt. See man less for more info.

dpkg -l | less

To check whether a package is installed or not:

dpkg -l {package_name}
dpkg -l vlc

To check if the package is installed or not (for example, vlc). If installed, launch the package:

dpkg -l | grep vlc

Show the location where the package is installed. The -S (capital S) stands for "search"

sudo dpkg -S {package_name}
sudo dpkg -S skype

To use Grep to search:

dpkg -l | grep {keywords}
dpkg -l | grep pdf

apt -qq list PACKAGE can also be used for checking whether the PACKAGE is installed.

If installed it'll print something like (with [installed] at the end of the line):

$ apt -qq list awscli
awscli/stable,now 1.4.2-1 all [installed]

If not installed the output will be:

$ apt -qq list awscli
awscli/stable 1.4.2-1 all

Use dpkg

This command is the Debian package manager.

You can list all the installed packages with

dpkg -l

You can see details for a specific package with

dpkg -p packagename

And to learn if it is installed, use

dpkg -s packagename | grep Status

You can learn which package contains the software you want with

apt-cache search name*  

In your case you should use this command to search the package name you want

apt-cache search virtual machine host 

I've found a way to check for package installation with just dpkg-query command and no pipes.

I searched for such solution while writing a task for ansible playbook.

You can do something like this:

/usr/bin/dpkg-query --show --showformat='${db:Status-Status}\n' '<your package name>'

This is just one another way to do the thing.

Simpler solution:

There is now an apt list command that lists available packages, and the --installed command will show only installed packages. You can also search for a specific package with

apt list <package>

Or to see only the matching installed packages

apt list <package> --installed

There are also the --upgradeable and --all-versions flags.

If you do not know the exact package name, you can use apt search followed by part of it to see a list of related packages. This will allow you to use a partial name or regex term(s) to filter a list of available packages, but it does not have the nifty flags for filtering that apt list has.

Also see man apt for more information.

I am writing a new answer as Tasksel 'Task names' DO NOT show up in dpkg or apt queries. Meaning the other answers are incomplete (or wrong).

"I can't remember If I have checked the 'virtual machine host' checkbox.."

Check if there is an asterisk by the name after re-running sudo tasksel (or an ' i ' if running tasksel --list-tasks).

If you do not see this, then you probably do not have the 'tasksel meta-package' installed.

"..How do I check if my server has the corresponding packages?"

If you want to find out what is installed under the Virtual Machine Host tasksel meta-package:

tasksel --list-tasks

(in your case tasksel --list-tasks | grep virt is enough)

This gives the proper task name and an indication as to whether it is installed.

In your case this tasksel name is virt-host.

"What packages come with the virtual machine host?"

From the last command, find the tasksel task name. This is needed for the next command:

tasksel --task-packages virt-host

This lists the individual Tasksel meta-package (denoted by appended '^') that make up the name. In your case it's virt-host^.

Once you find proper meta-name you can use apt:

 apt show virt-host^ | grep "Package:"

(or apt-cache show ... if apt show doesn't work.)

This will show all packages making up the given meta package.

Then:

 dpkg -l {each individual pkg name listed above}

This will tell you if the individual package is installed/uninstalled (noted by 2nd letter: 'i' status means installed). Do this for all packages listed under the task/meta package.

This should answer the original questions.

Side note: /usr/share/tasksel/descs/ubuntu-tasks.desc also holds descriptions of Tasksel meta-packages. If you: grep "virt" /usr/share/tasksel/descs/ubuntu-tasks.desc it will show the proper APT meta-package name. (In your case this is ubuntu-virt-server). You can also use apt commands with this meta-name.

If you are wanting to remove tasksel packages, this gets into the weeds a bit. Please consult this Ask Ubuntu answer or any number of other answers out there.

Please note that "meta" apt packages do not match Tasksel package names. Please consult the Tasksel Community Page for more info.

Use tasksel (sudo apt-get install tasksel if you don't already have it) and run tasksel --list-tasks.

tasksel will print out a list of the tasks, and there will be an i next to each task that is installed, and a u next to each task that is not installed.

If you wish to install the task, use tasksel install <task>. The name of the task you are looking for is virt-host.

The software selection items are called "tasks".

I created a little script with which you can automatically install from a list of packages or applications or simply check if they are already installed:

#!/bin/bash
apt -qq list YOUR_LIST_GOES_HERE | grep -v "installed" | awk -F/ '{print $1}' > /root/list.txt
packages=$(cat /root/list.txt)
grep -q '[^[:space:]]' < /root/list.txt
CHECK_LIST=$?
if [[ $CHECK_LIST -eq 1 ]]; then
echo "Your packages are already installed"
else
echo "Installing packages"
apt-get  install -y $packages
fi

Hope it helps someone.

Check if some packages are installed and install missing ones otherwise

pkgs='hello certbot'
install=false
for pkg in $pkgs; do
  status="$(dpkg-query -W --showformat='${db:Status-Status}' "$pkg" 2>&1)"
  if [ ! $? = 0 ] || [ ! "$status" = installed ]; then
    install=true
    break
  fi
done
if "$install"; then
  sudo apt install $pkgs
fi

More info at: https://stackoverflow.com/questions/1298066/check-if-an-apt-get-package-is-installed-and-then-install-it-if-its-not-on-linu/54239534#54239534

Similar technique previously mentioned at: https://askubuntu.com/a/1102572/52975

Another (not necessarily better) way: see whether the field APT-Manual-Installed appears in apt show <pkgname>. If it is absent then the package isn't installed. Unfortunately it's not shown in apt-cache show <pkgname>.

As always, the apt is for interactive use and not recommended for scripting.