How do I resolve unmet dependencies after adding a PPA?

APT is a package management system for Debian and other Linux distributions based on it, such as Ubuntu. For the most part, APT is easy to use for installing, removing, and updating packages. In rare instances, often when you are mixing in third-party dependencies, there is a chance that apt-get may end up giving you an error telling you that a package installation could not be completed.

Solutions:

It is always a good idea to back up configuration files like /etc/apt/sources.list, so you can revert the changes if needed.

If the error shows something like this:

<some-package>: Depends: <other-package> (= version) but this-version is to be installed

Then make sure that the restricted and universe repositories are enabled. Hit Alt+F2, type software-properties-gtk and hit Enter.

Under Ubuntu Software tab, enable all the repositories.
One possible cause of unmet dependencies could be corrupted package database, and/or some packages weren’t installed properly. To fix this problem, hit Alt+Ctrl+T to open terminal and try to run one of the following commands:
```
sudo apt-get clean
```
or,
```
sudo apt-get autoclean
```
apt-get clean clears out the local repository of retrieved package files (the .deb files). It removes everything but the lock file from /var/cache/apt/archives/ and /var/cache/apt/archives/partial/. apt-get autoclean clears out the local repository of retrieved package files, but unlike apt-get clean, it only removes package files that can no longer be downloaded, and are largely useless.
One of the most basic fixes to resolve dependencies problems is to run:
```
sudo apt-get -f install
```
The -f here stands for “fix broken”. Apt will attempt to correct broken dependencies. If you manually installed a package that had unmet dependencies, apt-get will install those dependencies, if possible, otherwise it may simply remove the package that you installed in order to resolve the problem.

Then run:
```
sudo dpkg --configure -a
```
Then run this again:
```
sudo apt-get -f install
```
If the output is:

0 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.

That means it failed.
Next solution is to run:
```
sudo apt-get -u dist-upgrade
```
If it shows any held packages, it is best to eliminate them. Packages are held because of dependency conflicts that apt cannot resolve. Try this command to find and repair the conflicts:
```
sudo apt-get -o Debug::pkgProblemResolver=yes dist-upgrade
```
If it cannot fix the conflicts, it will exit with:

0 upgraded, 0 newly installed, 0 to remove and 6 not upgraded.

Delete the held packages one by one, running dist-upgrade each time, until there are no more held packages. Then reinstall any needed packages. Be sure to use the --dry-run option, so that you are fully informed of consequences:
```
sudo apt-get remove --dry-run package-name
```
Since removing the package you are trying to install may not be ideal, you might also try finding a repository that has the packages you need to satisfy the dependencies.

Finally, if all else fails, you can attempt to satisfy the dependencies yourself, either by finding and installing the necessary packages, or by installing them from source and then creating “deb” packages for them.

Disable/Remove/Purge PPAs:

PPAs (Personal Package Archive) are repositories hosted on Launchpad. You can use PPAs to install or upgrade packages that are not available in the official Ubuntu repositories.

One of the most common causes of unmet dependencies are PPAs, especially when used to upgrade the existing package in Ubuntu repositories. To solve the problem you have three options: disable, purge (revert back to original package in Ubuntu repositories) or remove PPA.

Disable:

Disabling a PPA means no more updates for the packages installed from that PPA. To disable a PPA: Open Software Center > Edit > Software Sources Or,

Hit Alt+F2 and run software-properties-gtk.

Click on Other Software tab, you'll see that each PPA have two lines here, one for the compiled packages and one for the source, Uncheck both lines to disable a PPA. Here you can also add and remove PPAs.
Purge:

Purging a PPA means, downgrading the packages in the selected PPA to the version in the official Ubuntu repositories and disabling that PPA. PPA Purge does exactly that. To install PPA Purge run the following command:
```
sudo apt-get install ppa-purge
```

But, considering the question apt is broken, the above command will fail. So use this command

mkdir ppa-purge && cd ppa-purge && wget http://mirror.pnl.gov/ubuntu/pool/universe/p/ppa-purge/ppa-purge_0.2.8+bzr56_all.deb && wget http://mirror.pnl.gov/ubuntu//pool/main/a/aptitude/aptitude_0.6.6-1ubuntu1_i386.deb && sudo dpkg -i ./*.deb

To use PPA Purge:

sudo ppa-purge ppa:someppa/ppa

If 'ppa-purge' command fails for some reason, you can't run 'ppa-purge' again unless you re-enable the PPA (To enable the PPA follow the same steps as disabling a PPA and Check the both lines of a particular PPA to enable it).

Note: PPA Purge doesn't remove PPAs (may be in future), So you'll have to manually remove the PPA.

Remove:

If the package installed from a PPA doesn't exist in official Ubuntu repositories then using PPA Purge is not recommended, because there's nothing to be downgraded and PPA Purge wouldn't delete it either. To remove a PPA and installed packages run the following commands: (Ignore the first command if you don't want to remove the installed packages)
```
sudo apt-get autoremove --purge package-name
sudo add-apt-repository --remove ppa:someppa/ppa
sudo apt-get autoclean
```

Alternatively you can use Y PPA Manager to disable/remove/purge PPA's (Details at the end of answer).

Preventive Measures:

So how can we avoid this from happening in the first place?

Keep Ubuntu Up to date. Ubuntu automatically notifies when updates are available, you can also check for available updates by clicking on Session Indicator in Unity panel:

Or, Hit Alt+Ctrl+T to open terminal and run following commands:
```
sudo apt-get update
sudo apt-get upgrade
```
Update: Synchronizes your list of available packages with the servers in source repositories. Upgrade: Downloads & installs any newer versions of your installed packages.
If you decide to add other repositories to sources.list, make sure that the repository is meant to work (and known to work) with Ubuntu. Repositories that are not designed to work with your version of Ubuntu can introduce inconsistencies in your system and might force you to re-install. Also, make sure that you really need to add external repositories as the software package(s) you are looking for may already have been introduced into the official repositories! Source
Remove duplicate PPAs. Y PPA Manager (installation guide at the end of answer) can easily scan and remove duplicate PPAs.
- Hit Alt+F2 and run y-ppa-manager to open Y PPA Manager.
- Once open, double click or hit Enter on Advanced.
- In resulting window select Scan and remove duplicate PPAs and click OK.

enter image description here

Getting Help:

When asking for help on Askubuntu or any other help forum, you should include the output of the following commands in your question:

Output of actual error:
```
sudo apt-get install package-name
```
This will show your sources.list:
```
cat /etc/apt/sources.list
```
This will show the list of PPAs (If any):
```
cat /etc/apt/sources.list.d/*
```

Additional Sources:

'apt' Ubuntu Manpage
'apt-get' Ubuntu Manpage
'apt-get/how-to' Community Ubuntu documentation
'Repositories/Ubuntu' Community Ubuntu documentation
'Repositories/CommandLine' Community Ubuntu documentation

Y PPA Manager: Y PPA Manager is an one stop shop for all of your PPA needs.

y ppa manager

Here are some of its features:

search packages in all Launchpad PPAs
list and download packages in a PPA
add / remove / purge a PPA
backup and restore PPA sources
remove duplicate PPA sources

To install Y PPA Manager, open terminal by hitting Alt+Ctrl+T and run following commands:

sudo add-apt-repository ppa:webupd8team/y-ppa-manager
sudo apt-get update
sudo apt-get install y-ppa-manager

Considering the question, apt is broken, so use these commands instead

sudo su

and

32 Bit:

mkdir y-ppa-manager && cd y-ppa-manager && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/launchpad-getkeys_0.3.2-1~webupd8~oneiric_all.deb && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/y-ppa-manager_0.0.8.6-1~webupd8~precise_all.deb && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/yad_0.17.1.1-1~webupd8~precise_i386.deb && dpkg -i ./*.deb

64 Bit:

mkdir y-ppa-manager && cd y-ppa-manager && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/launchpad-getkeys_0.3.2-1~webupd8~oneiric_all.deb && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/y-ppa-manager_0.0.8.6-1~webupd8~precise_all.deb && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/yad_0.17.1.1-1~webupd8~precise_amd64.deb && dpkg -i ./*.deb

Note: All commands asked to be run must be run in the terminal, which can be opened by either Ctrl+Alt+T or searching for terminal in the dash.

Is it really broken?

Try running the following command and try to reinstall the software you were trying to install

sudo apt-get update

Pre-Perfomance Steps

Backing up

Back up the following files:

/etc/apt/sources.list
/var/lib/dpkg/status

To do so, use these commands

sudo cp /etc/apt/sources.list /etc/apt/sources.list.original

and

sudo cp /var/lib/dpkg/status /var/lib/dpkg/status.original

Clearing your apt-cache

apt keeps a cache of recently downloaded packages to save bandwidth when it is required to be installed. This can be counter-productive in some cases

Now, to clean it, you have two options

sudo apt-get clean

This will remove all cached packages belonging to the folder /var/cache/apt/archives/ and /var/cache/apt/archives/partial except the .lock files. This is recommended

sudo apt-get autoclean

This scans the folders /var/cache/apt/archives/ and /var/cache/apt/archives/partial and checks if the package is still in the repositories and removes the ones that aren't

Fixing dependencies

Using apt's fix-broken mode

sudo apt-get -f install

This will cause apt to scan for missing dependencies and fix them from the repositories

If the output states that nothing new was installed or upgraded, it has failed.

Checking if all required sources are enabled

Type gksu software-properties-gtk and you'll get this window

Make sure all sources are enabled.

next, go to the Other software tab and check if the required PPAs for the software to be installed are there and are enabled. Also, try disabling some PPAs which might be having broken packages

now, run sudo apt-get update

Try installing the software now

Selecting a better server to download from

Type gksu software-properties-gtk and you'll get this window

Click the Download from the Dropdown box and select other

Click Select Best Server

Run sudo apt-get update

Try installing the software

also, try using sudo apt-get install -f

PPA Purge

This is a tool used to purge broken/unwanted ppa's and their applications along with it

To install it, run

sudo apt-get install ppa-purge

But, Considering the question apt is broken so the above command will fail. So use this command

mkdir ppa-purge && cd ppa-purge && wget http://mirror.pnl.gov/ubuntu/pool/universe/p/ppa-purge/ppa-purge_0.2.8+bzr56_all.deb && wget http://mirror.pnl.gov/ubuntu//pool/main/a/aptitude/aptitude_0.6.6-1ubuntu1_i386.deb && sudo dpkg -i ./*.deb

Now use ppa purge

sudo ppa-purge ppa:someppa/ppa

Y-PPA Manager

Y-PPA Manager is a gui app that helps you manage PPA's and various problems assosiated with it

To install it

sudo add-apt-repository ppa:webupd8team/y-ppa-manager

and

sudo apt-get update

and

sudo apt-get install y-ppa-manager

Considering the question, apt is broken so, use these command instead

sudo su

and

32 Bit:

mkdir y-ppa-manager && cd y-ppa-manager && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/launchpad-getkeys_0.3.2-1~webupd8~oneiric_all.deb && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/y-ppa-manager_0.0.8.6-1~webupd8~precise_all.deb && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/yad_0.17.1.1-1~webupd8~precise_i386.deb && dpkg -i ./*.deb

64 Bit:

mkdir y-ppa-manager && cd y-ppa-manager && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/launchpad-getkeys_0.3.2-1~webupd8~oneiric_all.deb && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/y-ppa-manager_0.0.8.6-1~webupd8~precise_all.deb && wget https://launchpad.net/~webupd8team/+archive/y-ppa-manager/+files/yad_0.17.1.1-1~webupd8~precise_amd64.deb && dpkg -i ./*.deb

Now type in y-ppa-manager

You'll be presented with this window

Double click on advanced, and you'll get this window

Do the following Tasks outlined in black

Prevention is better than cure

It is better to prevent than to search for this question on AskUbuntu :D

So, here are the guidelines to keep you safe

Keep your system up-to-date

always run the following command regularly

sudo apt-get update&&sudo apt-get upgrade

or, you can always use Update Manager with this command

gksu update-manager

Using only trusted PPA's

Only use PPA's meant to be used on Ubuntu also, only use PPA's with trusted sources. Infact, the package might already be in the ubuntu repositories

Backing up when things are good and restoring it later

For this you need Y-PPA-Manager. The steps to install it are given above.

Run this command to open Y-PPA-Manager

y-ppa-manager

You'll be presented with this window

Double click on advanced, and you'll get this window

Run this:

You'll be asked to save a tar.gz file with a dialog similar to the one below. Save it in another partition or a safe place

Later, when you need to restore it again, follow similar steps and when you get to the advanced dialog,Click on this:

You'll be asked to restore from the previous backup which you saved before with a dialog similar to the one below

Still not working?

Package dependency errors are not always generic and depends on the package to be installed.

If following all the steps given to fix the error does not work for you, you can always ask on Ask Ubuntu

Here are some commands which you need to post the output of

sudo apt-get install packagename

and

cat /etc/apt/sources.list

and

cat /etc/apt/sources.list.d/*

(Thanks to Basharat Sial)

There are also other files/commands that you need the output of that might be error specific, and users will probably prompt you in the comments to post the file/command.

These answers so far are focused on how to help the system automagically resolve such an issue, mostly hoping that it resolves itself, but not focused on understanding how to investigate dependencies. It's important to try making sure your package lists are up to date first; always make sure a simple update or clearing of the cache doesn't resolve the issue before digging deeper. If everything behind the scenes is working as it should, however, it's paramount to understand dependencies. Sometimes the layers of dependencies can seem overwhelming, but the concept is simple and key to understanding how software is installed/uninstalled in an Ubuntu system.

I highly recommend aptitude to anyone using a .deb based system, such as Debian, Kali, Ubuntu or any of its derivatives. It provides an interface to browse through layers of dependencies, including recommended and suggested packages. It is an invaluable tool for when apt-get doesn't offer a solution to a conflict and for visualizing dependencies in general. (Synaptic is also available in Ubuntu.)

Aptitude's conflict resolver will walk you through the possible combinations of installed/removed/upgraded/etc packages that satisfy your requested actions without conflict. This can often be successful even when apt is unable to find a solution. As an Ubuntu user, situations don't normally arise that require you to investigate why a particular package is being installed or causing a conflict, but it's almost inevitable as you use the system more.